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Recent advances in basic science
Pancreatic ductal adenocarcinoma and acinar cells: a matter of differentiation and development?
  1. Ilse Rooman1,
  2. Francisco X Real2,3
  1. 1Cancer Research Program, Garvan Institute of Medical Research, Darlinghurst-Sydney, NSW, Australia
  2. 2Programa de Patología Molecular, Centro Nacional de Investigaciones Oncológicas, Madrid, Spain
  3. 3Departament de Ciències Experimentals i de la Salut, Universitat Pompeu Fabra, Barcelona, Spain
  1. Correspondence to Francisco X Real, CNIO, Melchor Fernandez Almagro, 3, Madrid 28029, Spain; preal{at}cnio.es andIlse Rooman, Garvan Institute, 384 Victoria St, NSW 2010 Sydney, Australia; i.rooman{at}garvan.org.au

Abstract

Pancreatic ductal adenocarcinoma (PDAC) has long been considered to arise from pancreatic ducts on the basis of its morphology, the occurrence of dysplasia in putative preneoplastic ductal lesions, and the absence of acinar dysplasia in the pancreas of patients with PDAC. However, evidence gathered through both in vitro studies and—more importantly—genetic mouse models of PDAC shows that ductal-type tumours can arise from acinar cells. These findings raise new important questions related to PDAC pathophysiology and call for in-depth studies of acinar cell differentiation in order to better understand PDAC biology. The authors review these issues and discuss how the novel findings should impact on future work aiming at early diagnosis and improved outcome of patients with PDAC.

  • Pancreatic ductal adenocarcinoma
  • chronic pancreatitis
  • acinar cells
  • differentiation
  • metaplasia
  • ras
  • Notch
  • hedgehog
  • Wnt
  • pancreatic cancer
  • pancreatic damage
  • pancreatic physiology

https://doi.org/10.1136/gut.2010.235804

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    Pancreatic ductal adenocarcinoma (PDAC): the problems

    The 5-year survival rate of patients with PDAC is <5%.1 In patients whose tumour is resectable and who receive adjuvant chemotherapy (∼15% of PDAC cases), the 5-year survival rate is up to 20%,2 indicating that early detection can improve outcome. The overall outcomes for PDAC have in fact not changed for almost 50 years.1

    PDAC preneoplastic lesions

    A major advance in the pathological assessment of PDAC has been the consensus on the nomenclature and classification of ductal preneoplastic lesions.3 4 A linear progression model for PDAC has been proposed according to which normal ductal cells evolve to a hyperplastic epithelium without dysplasia (pancreatic intraepithelial neoplasia (PanIN)-1A for a flat epithelium and PanIN-1B for papillary hyperplasia), then acquiring increasing levels of dysplasia (PanIN-2 and PanIN-3). PanIN-3 represents carcinoma in situ and it is the precursor to invasive carcinoma.

    PanIN-1 lesions are common in older people and probably involve a low risk of developing PDAC, whereas high-risk PanIN-3 lesions are almost exclusively found in patients with invasive PDAC.5 Therefore PanIN-2 lesions can be regarded as a turning point in this sequence of progression and as a major target for further study. Other mucinous precursor lesions, such as intraductal papillary mucinous neoplasms (IPMNs) and mucinous cystic neoplasms, are subjects of increasing attention because of their raising incidence and they provide greater opportunities for improved clinical management.6 7

    The PanIN sequence is paralleled genetically by telomere shortening and accumulation of activating point mutations in K-RAS codon 12 (∼85% of cases), p16/CDKN2A inactivation (∼100% of lesions), and—at later stages—TP53 and SMAD4/DPC4 inactivation (60–80% of tumours).4 8 A study by Jones et al showed that PDAC harbours an average of 63 genetic abnormalities (mainly point mutations).9 Genomic-scale data on tumour evolution are rapidly being generated using massive parallel sequencing.10

    The linear progression model: up for review?

    The linear model of PDAC progression is likely to be an oversimplification.11 There is no formal evidence that PanIN-1 lesions are required for the development of PanIN-2 or PanIN-3 or PDAC. It is also conceivable that multiple roads may lead to PDAC and that different PanINs are fates determined by the cell of origin. Alternatively, the type or order of the genetic alterations may determine the final outcome.

    Genomic data reveal the complexity of PDAC and the PanIN–PDAC model is limited by the correlative nature of the morphological–genetic associations. Genetic mouse models of PDAC should allow longitudinal and mechanistic analyses that cannot be carried out in humans. State-of-the-art genomic tools applied to large numbers of tumours should also provide a better understanding of the genetics of progression, the heterogeneity of PDAC, and the existence of tumour subtypes.

    On the origin of PDAC: “Don't ask what a cell can do, ask what it does do!” (quote from G Heppner12)

    Conditional mutant K-Ras knock-in mouse strains have allowed modelling of PDAC. In general, mutant K-Ras is activated in all pancreatic cells by targeting Cre recombinase to pancreatic progenitors at the time of pancreas formation (about E9) using the regulatory elements of pancreatic and duodenal homeobox 1 (Pdx1) or pancreatic transcription factor 1a (Ptf1a), two genes required for pancreas formation.13 14 Most of the mouse models recapitulate the spectrum of PanIN lesions and PDAC. IPMNs, and to a much lesser extent mucinous cystic neoplasms, have been reported to be the most prevalent lesions upon mutant K-RAS expression and concomitant SMAD4 deletion or transforming growth factor α (TGFα) overexpression.15 Because all pancreatic epithelial cells develop from Pdx1+ Ptf1a+ precursors,16 these studies are not informative regarding the cell of origin. Human PDAC has a ductal morphology, and it has long been assumed that PDAC originates in ductal cells. However, genetic mouse models have provided new paradigms on this issue.

    Ductal cells

    Ductal cells are poorly characterised at the molecular level: there is a paucity of embryonic and mature ductal differentiation markers, and little is known about ductal heterogeneity and the mechanisms involved therein. Keratin (Krt)7 and Krt19 are expressed in ducts.17–19 Krt19–K-RasG12V transgenic mice develop hyperplastic ductal lesions with periductal lymphocytic infiltration but no tumours.20 Mice in which mutant K-RAS is activated using Krt19–CreERT develop PanIN-1A lesions.21 Hnf1b–CreERT 22 and CAII–CreERT 23 strains are being used to activate mutant K-Ras in ductal cells at different stages. This will allow us to determine ‘when and how’ duct cells are sensitive to the oncogenic effects of K-Ras.

    Acinar cells: mounting evidence for a role in PDAC

    The notion that acinar cells can give rise to ductal-like cells is not new 24 25: Ela1–TGFα and Ela1–c-myc transgenic mice show acinar to ductal metaplasia and develop acinar-derived ductal tumours.26 27 Selective activation of mutant K-Ras in acinar cells and lineage tracing28–30 has allowed us to conclude that: (1) acinar cells can give rise to PDAC in a K-Ras mutant context; (2) K-Ras activation in embryonic acinar cells leads to acinoductal metaplasia (ADM), abundant PanINs, and PDAC, whereas adult acinar cells are rather refractory; (3) chronic administration of caerulein, which induces a chronic pancreatitis (CP)-like lesion, promotes PanIN and PDAC development when K-Ras is activated in adult acinar cells.29 Multiple episodes of acute pancreatitis have effects similar to those of a chronic insult.31 Therefore recurrent tissue damage creates a permissive context for the oncogenic effects of mutant K-Ras in adult acini, in agreement with the increased risk of PDAC in patients with CP32 33 (figure 1). Signalling events leading to Ras activation beyond a certain threshold (ie, mutation and pancreatitis) may modulate the differentiation state of adult acinar cells and render them more prone to malignant transformation.33

    Figure 1
    Figure 1

    Acinar cells undergoing stress (eg, pancreatitis) dedifferentiate, which implies the loss of activity and expression of transcriptional regulators (Ptf1aHi, Hi refers to high level of expression, Rbpjl and Mist1), their target genes (eg, carboxypeptidase A (CpA), elastase (Ela), amylase (Amy) and chymotrypsin (Ctrb)) and as such the mature phenotype. The dedifferentiated acini have embryonic traits such as Hes1 and low Pdx1 (Pdx1Lo), low Ptf1a levels (Ptf1aLo), and Rbpj in the PTF1 complex replacing Rbpjl. When they evolve into preneoplastic and tumour lesions, Ptf1a expression is lost and a more ‘ductal’ phenotype, with expression of Hes1 and Pdx1, is usually found in tumour lesions. Krt7 and Krt19 are also induced during the acinoductal metaplasia and remain expressed in tumour lesions. The dedifferentiation sensitises the cells to both genetic (ie, mutations, allele loss) and epigenetic (ie, methylation, histone modification) changes that promote PDAC development. Cues from the stroma may affect the acinar differentiation programme, and, vice versa, loss of differentiation may modulate stromal cell function, making it an essential player throughout the course of stress-induced, preneoplastic and neoplastic changes.

    Most studies have reported on the acinar cell contribution to PanINs and PDAC. Concomitant overexpression of mutant K-RAS and TGFα results in IPMN formation when either the Ptf1a or Pdx1 promoter is used, but not when driven by the Ela1 promoter (ie, Ela1–CreERT; LSL–K-RasG12D; Ela1–TGFα mice),15 putting into question whether acinar cells can give rise to IPMNs as well. However, no attempts were made in this study to verify the susceptibility of stressed acinar cells (as in pancreatitis).

    Centroacinar cells (CACs): somewhere in between?

    CACs share many characteristics with ductal cells, including Krt19 expression, and are distinguished by the expression of the Notch target, hairy and enhancer of split 1 (Hes1).34 CACs have recently been attributed progenitor stem cell features,35 and Hes1–CreERT2 mice confirm CACs as the Hes1+ population in the adult pancreas, but it is not clear whether they are true adult stem cells.36 Pancreas-wide Pten-deficient mice develop metaplastic lesions and some PDAC, for which a CAC origin was proposed.37 A definitive role of CACs in PDAC development has not been established, and their selective targeting with mutant genes is awaited.

    Islet cells: me, too?

    It was reported that targeting mutant K-Ras to insulin-expressing cells in combination with caerulein-induced damage results in PanINs.38 However, their origin needs to be firmly established since the RIP–CreERT transgene is also weakly active in acinar cells.39

    Pancreatic stem cells: to be or not to be?

    It has been hypothesised that normal stem/progenitor cells may serve as cancer stem cells.40 Stem cells have been characterised mainly in tissues with self-renewing potential. In ‘quiescent’ tissues, such as the pancreas, the properties of cells with stem cell features may be fundamentally different. Most evidence supports self-replication as the mechanism involved in pancreatic homoeostasis and response to injury.41

    Several pancreatic cancer stem cell markers have been proposed: CD44, CD24, epithelial-specific antigen, CD133 and the genes Bmi1 and Sonic Hedgehog (Shh).40 42 However, all of these markers are also expressed in some differentiated cell types in the adult pancreas,43–45 questioning the identity of cancer stem cells.

    Overall, these studies indicate that oncogenic changes (such as K-Ras activation) in distinct differentiated cells, most convincingly acinar cells, can yield PanINs and PDAC. These findings underline the plasticity of acinar cells under experimental conditions (‘what a cell can do’) (figure 2). Pancreatic injury creates a permissive context favouring tumour development. Whether all that looks like PDAC is molecularly a uniform phenotype remains to be determined. The final question is ‘what a cell does do’—that is, where does human PDAC arise? Massive parallel sequencing at the single cell level will allow clonal cell populations to be traced.

    Figure 2
    Figure 2

    Key transcription factors and genes that govern acinar cell differentiation and plasticity are being unraveled (‘what an acinar cell can do’), and this knowledge will advance our understanding of how this cell type may contribute to pancreatic ductal adenocarcinoma (PDAC) (‘what it does do’). Transcription factors in green drive altered acinar differentiation: Cebp-α promotes hepatocytic differentiation, and c-myc can drive dedifferentiation. Lack of transcription factors depicted in red also affects the acinar differentiation: lack of Rbpjl, (Nr5a2) or hepatic nuclear factor 1a (Hnf1a) turns on a dedifferentiated, even progenitor-like phenotype. If both Rbpjl and Ptf1a are absent, a ductal differentiation is acquired. Acinar cells in the absence of c-myc install an adipocyte-like differentiation, whereas cells from Trp53 null mice undergo epithelial–mesenchymal transition.

    Keeping acinar cells differentiated—development and disease

    Because rodent models strongly support an acinar origin of PDAC, we focus on acinar cell differentiation and the pathways leading to the loss of their differentiated state.

    Differentiation during development

    Mouse acinar cells are derived from multipotent precursors around day E14 and migrate outwards at the epithelial tips, leaving behind the trunk cells, which differentiate into islets and ducts.16 The key transcription factors responsible for acinar differentiation are Ptf1a, which binds a ubiquitous basic helix–loop–helix transcription factor, and recombinant signal binding protein for immunoglobulin kappa J-region-like (Rbpjl). Together they constitute the adult-type PTF1 complex, which is the main activator of genes coding for digestive enzymes.46 Before day E14, Rbpj is part of PTF1 instead of Rbpjl. Rbpj is a homologue of Rbpjl and is known as the DNA-binding transcriptional mediator of the canonical Notch signalling pathway. The embryonic PTF1 complex has distinct target gene specificity including the autoactivation of Ptf1a, of which a certain threshold of expression is needed for acinar cell differentiation. Another target of the PTF1 complex is Rbpjl, which gradually replaces Rbpj, leading to full differentiation of embryonic acinar cells.47 High Ptf1a levels favour acinar differentiation, while low levels favour an endocrine fate during development.48 Additional proteins probably help to refine the acinar transcriptional programme, including Mist1,49 hepatic nuclear factor 1a (Hnf1a) (Molero et al, unpublished) and nuclear receptor subfamily five group A member 2 (Nr5a2).16 Mist1 promotes terminal acinar differentiation by controlling the exocytosis and secretion programme49 and by limiting proliferation through p21,50 a function it has in common with Ptf1a.51 The role of the other players is beginning to be explored. Extrinsic factors also contribute to full acinar differentiation, and the pathways thus activated are discussed later in this review.

    Dedifferentiation: what's in a name?

    Acinar-to-ductal metaplasia (ADM), transdifferentiation and dedifferentiation are terms with a mixed use.44 52–56 ‘Metaplasia’ is a histological term referring to the replacement of one cell type (acinar) by another one (ductal) without any implication for the nature of the change (ie, selective cell death, selective expansion, transdifferentiation or dedifferentiation). ‘Transdifferentiation’ is a cell biology term referring to the switch from one differentiated (acinar) cell to another differentiated/functional (ductal) cell type.57 ‘Dedifferentiation’ is the loss of mature, functional cell features with possible re-establishment of embryonic characteristics. As more acinar and ductal markers and distinct maturation stages are identified, more precision will be achieved in the molecular definition of these processes in acinar cells, especially when lineage tracing is not possible.

    The current use of ‘ductal markers’ is not devoid of pitfalls: Krt20 is expressed by ductal cells in the rat but not in humans or mice; Krt19 and Krt7, Hnf1b and CD133 are expressed in both embryonic and adult ducts 22 43 58; and Sry-box containing gene 9 (Sox9)59 is not restricted to ducts.44 Therefore an improved molecular cartography of ductal cells is needed.

    Mouse acinar cells, identified using Ela1–CreERT2 lineage tracing, ‘dedifferentiate’ when cultured in suspension and acquire an embryonic-like phenotype.44 54 These cells express low levels of Ptf1a, Pdx1 and selected digestive enzyme transcripts, features unique to multipotent pancreatic progenitors.16 They also have the embryonic PTF1 complex sitting on its bona fide target promoters. These features discriminate pancreatic progenitors (dedifferentiation) from ductal cells (transdifferentiation). Transcriptomic analysis further distinguishes these cells from normal adult ductal cells.44 Recent lineage tracing experiments indicate that also human acinar cells under specific culture conditions can acquire a ductal differentiation state.60

    In vivo, the conversion of acinar cells into cells with ductal characteristics has been demonstrated using lineage tracing upon induction of pancreatitis with caerulein,61 pancreatic duct ligation (PDL),62 and in Metallothionein-TGFα (MT–TGFα) transgenic mice.39 Acinar cells undergo transient dedifferentiation, in the 2-day caerulein acute pancreatitis model, with re-activation of pancreatic embryonic characteristics63 and almost complete regeneration of the pancreas within 1 week. Upon prolonged caerulein exposure, chronic damage is induced and acinar dedifferentiation is not followed by acinar cell regeneration.44 61 In other models of damage, such as PDL, acinar cells undergo apoptosis as well as dedifferentiation, similar to suspension cultures.44

    Differentiation in PanIN and PDAC

    Prominent markers displayed by dedifferentiated non-neoplastic acini are shared by PanIN/PDAC lesions. Besides the ductal keratins,64 65 Pdx1 is expressed in dedifferentiated cells, PanIN lesions and PDAC.66 Dedifferentiated acinar cells display low Ptf1a expression,which can be cytoplasmic.67 In human samples, Ptf1a has been reported in some PanIN-A lesions, but it is undetectable in PDAC.67 68 Bmi1, a member of the PRC1 complex, becomes induced on acinar cell dedifferentiation in vitro, in PDL and in caerulein pancreatitis, and during PDAC development in mice.45

    Both in vitro and in vivo models thus support the dedifferentiation of adult acinar cells. We acknowledge that there is still some controversy about the contribution of ADM to PDAC. Telomere shortening and K-Ras mutations occur mainly in PanIN-associated ADM and rarely in isolated ADM in human samples.8 This suggests that the latter lesions are an earlier, and potentially end-stage, lesion and that PanIN-associated metaplasia represent a more advanced stage. So far, little evidence supports their being retrograde extensions of PanINs, as has been hypothesised.8 Mouse models should provide further insights.

    The main pathways involved in acinar differentiation/dedifferentiation are discussed below (figure 3). Other reviews deal with their implications in other aspects of pancreas biology.

    Figure 3
    Figure 3

    Ras and Notch pathway activation can negatively impinge on the differentiation programme of acinar cells. In contrast, Hedgehog (HH), Wnt and transforming growth factor (TGF)β signalling positively impact on maintenance/regeneration of the acinar component. Ras signalling plays a crucial role in this balance, and its overactivation promotes pancreatic ductal adenocarcinoma development, directly but also probably via repression of HH, by interfering with β-catenin signalling, and by activation of Notch signalling. Loss of differentiation results in acquired progenitor features, increased susceptibility to K-Ras, and proneness to ductal and epithelial–mesenchymal transition (EMT) features.

    Differentiation: signalling cues and pathways

    Ras and downstream from Ras

    Ras proteins integrate signalling by membrane receptors and activate the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK), phosphatidylinositol-3-kinase/protein kinase B (PI3K/Akt) and Ral. The effects of epidermal growth factor receptor (EGFR), Ras and MAPK signalling in adult pancreas are well documented, but less is known of their role in embryonic pancreas development. In EGFR knock-outs, islet differentiation and ductal branching are affected, without acinar changes reported.69 Mice overexpressing TGFα or amphiregulin develop normally, but show altered acinar differentiation and duct cell proliferation.26 70

    Upon acinar cell injury, Ras and its downstream pathways are activated.33 44 Ela1–TGFα transgenic mice develop ADM and carcinomas. In agreement, blocking of EGFR and MAPK signalling inhibits duct formation from cultured acinar cells.26 It is not clear whether EGFR plays a unique role among receptor tyrosine kinases or it stands out because it has been more extensively studied. It has been proposed that Ras activity levels, regardless of whether it emanates from wild-type or mutant protein, determines acinar cell fate.33 In human tissues, ∼30% of CP samples harbour K-RAS mutations: they are generally absent from ADM lesions, but are more common in metaplastic lesions associated with PanINs.71 Patients with PDAC who have wild-type K-RAS tumours are more likely to have a history of CP.72

    Acinar cells overexpressing the human cholecystokinin 2 (CCK2) receptor (ElasCCK2 transgenic mice) undergo dedifferentiation and malignant transformation.73 Src/ERK activation contributes to this effect.74 In other contexts, CCK receptors have been shown to activate Ras proteins.75

    Regarding the PI3K/AKT pathway, overexpression of active Akt in acinar cells by activated Cre under the elastase promoter leads to acinar-derived metaplastic ducts displaying embryonic characteristics.76 IPMN-like tumours—but no PanINs—develop, in line with the occurrence of PIK3CA mutations in 11% of IPMNs.77 Constitutive Akt expression driven by Pdx1–Cre leads to the development of PDAC in old animals.76 Knockout of Pten, a negative regulator of PI3K signalling, in acinar cells results in highly proliferative mucinous ducts expressing pancreatic progenitor markers. Some of these mice also develop tumours,37 supporting a role for PI3K in loss of acinar cell differentiation. However, there is no direct evidence that PI3K inhibition can block acinar cell dedifferentiation.78

    In conclusion, activation of Ras and downstream signalling is linked to acinar cell dedifferentiation and—eventually—tumour generation; whether this association is causal remains to be established.

    β-Catenin signalling

    Wnt-secreted ligands bind membrane receptors, inactivate the axin–glycogen synthase kinase-3–adenomatous polyposis coli (APC) complex, which promotes the proteolytic degradation of cytosolic β-catenin (canonical Wnt signalling), leading to β-catenin accumulation and nuclear translocation and activation of target genes, including c-myc.

    β-Catenin hyperactivation at the time of pancreas specification causes pancreatic agenesis or hypoplasia. At later stages, Wnt signalling blockade reduces proliferation of endocrine and exocrine cells. Conditional β-catenin deletion leads to impaired acinar cell proliferation, with c-myc being a major effector of the β-catenin-induced expansion of the exocrine compartment. The effects on acinar differentiation vary depending on the experimental conditions.79 80

    Cytoplasmic accumulation of β-catenin is often associated with acinar dedifferentiation.63 78 81 β-Catenin signalling determines whether acinar regeneration or persistent dedifferentiation occur upon pancreatic damage.81 Strong and persistent Ras activation interferes with canonical β-catenin signalling, blocks acinar regeneration, and favours ductal metaplasia.81 In vitro studies of isolated acini are consistent with these observations.44 β-Catenin cooperates with activated Ras in the generation of PanIN and PDAC.82 However, activating mutations in β-catenin or other Wnt-related genes are rare in PDAC.82

    Notch signalling

    Notch–ligand interactions lead to proteolytic cleavage of Notch intracellular domain (NICD), which then binds to Rbpj and activates transcription. Mice lacking Notch1/2 develop a quite normal pancreas, but Rbpj inactivation results in loss of pancreatic mass and precocious endocrine differentiation.83–85 Notch overexpression inhibits acinar differentiation.83–85 Inactivation of the Notch target Hes1 results in pancreatic hypoplasia and ectopic pancreas in the gut and the stomach, as well as islet-like clusters and acini in the bile duct.84 85 In the early embryonic pancreas, Hes1-expressing cells are multipotent progenitors in which Notch signalling installs a ductal phenotype. Later in embryogenesis, Hes1 is expressed in exocrine-restricted progenitors, where Notch activation promotes ductal differentiation at the expense of acinar cell fates. In the adult, Hes1 is restricted to a subpopulation of CACs and ducts and marks mainly cells in which the ductal programme is activated.34 36 However, Hes1 is a readout not only of Notch, but of other pathways as well (ie, non-canonical NFκB,86 Jnk pathway87 and Hedgehog88).

    During pancreatitis, Notch signalling is re-activated in both humans and mice,63 89 and it is required for acinar regeneration in cooperation with Wnt signalling.90 Notch activation blocks acinar differentiation, and its inhibition partially blocks acinar dedifferentiation.34 NICD overexpression in mature acini does not result in ductal differentiation; yet, in cooperation with activated Ras, it leads to ADM in mice.91 Therefore Notch contributes to acinar dedifferentiation and ductal metaplasia.

    Matrix metalloproteinase 7 (MMP7) is probably a common effector of Ras, Wnt and Notch.92–94 In acinar cells, MMP7 cleaves Notch and initiates signalling, and NICD inhibits β-catenin-mediated transcriptional activity.90 MMP7 appears to be required for ADM in vitro and in vivo.93 95

    Notch2, rather than Notch1, appears to be the crucial player in pancreatic carcinogenesis.96 Furthermore, Notch1 is tumour suppressive in a K-Ras knock-in tumour model (driven from embryonic stages on).97 A more detailed assessment of the function of each of the Notch proteins is required. Altogether, these studies support the idea that Ras is a crucial determinant of the biological effects of Notch.

    Hedgehog (HH) signalling

    The three hedgehog ligands (Sonic hedgehog (Shh), Desert hedgehog (Dhh) and Indian hedgehog (Ihh)) bind to Patched (Ptch) receptors and release their inhibition of smoothened (Smo). The main HH effectors are Gli transcription factors. Shh downregulation is key for endodermal specification into pancreas. Early work suggested that altered HH signalling in pancreas development affected the mesenchyme, but Gli2 epithelial overexpression is associated with acinar cell loss.98 99

    HH signalling is required for damage-induced acinar regeneration as shown by its pharmacological or genetic inactivation in an acute pancreatitis model; in contrast, ductal metaplasia and proliferation are unaffected.100 101 This effect may be mediated by the hyperactivation of Ras during CP and the resulting inhibition of it on HH signalling.102

    Several HH components, including the receptors Ptch and Smo, are upregulated in PanINs and PDAC.103 104 It has been shown that HH signalling acts in a paracrine manner: epithelial cells produce the ligand leading to pathway activation in the adjacent stroma and fibroblast proliferation/activation.105 The limited autocrine activation of HH in PDAC is thought to result from: (1) Ras inhibition of autocrine HH signalling via DYRK1B kinase102; (2) a HH ligand deregulation of Gli in tumour cells102; (3) primary cilia-mediated attenuation of HH signalling downstream of Smo.99 This mechanism may not operate since the primary cilium is typically lost in PDAC.106

    TGFβ signalling pathway

    The TGFβ pathway plays an important role in normal tissue homoeostasis, and it cross-talks with the MAPK and Wnt pathways. TGFβ restricts endodermal specification to pancreas,107 suppresses proliferation and differentiation towards the endocrine lineage,108 and can also affect embryonic acinar growth and survival.109 110 Transgenic mice expressing a dominant-negative mutant type II TGFβ receptor under control of the metallothionein 1 promoter, which is active in acini but not in ducts, display increased acinar cell proliferation and ADM, suggesting acinar cell dedifferentiation because intermediate cell types were found.111 These mice were less sensitive to caerulein-induced acute pancreatitis.112

    Acinar cells: beyond the epithelium

    Dedifferentiation of epithelial cells has also been linked to epithelial–mesenchymal transition (EMT), a process characterised by the downregulation of epithelial markers and acquisition of mesenchymal features and migratory properties, E-cadherin loss being a hallmark of this process.113 It has recently been suggested that human exocrine pancreatic cells can undergo EMT.114 Dedifferentiation and EMT have been demonstrated by lineage tracing in human endocrine β-cells in culture.115 Murine acinar cells can undergo massive EMT, which is favoured by loss of Trp53.116 These effects may be pertinent to the acquisition of migratory and invasive properties in PDAC.

    Overall, there is extensive evidence that acinar cells display plasticity with the potential to dedifferentiate, transdifferentiate to ductal cells in selected model systems, and acquire mesenchymal properties (figure 2). These processes can thus occur in non-neoplastic cells, and are similar to those occurring in tumours (figure 1).

    Tumour-suppressive mechanisms

    Other biological processes affecting acinar cells may also contribute to their fate when undergoing dedifferentiation. Senescence and autophagy are briefly discussed here.

    Senescence is an early tumour-suppressive mechanism that results from several types of stress, including telomere erosion, oxidative stress, oncogenic stress and DNA damage. The senescence programme is mainly dependent on the integrity of the Rb and p53 pathways, both of which are altered in PDAC. Consistently, senescence has been reported in PanINs, but not in PDAC.117 There is evidence of increased proliferation in ductal complexes in areas of CP, but also of an activated senescence programme in dedifferentiated acini in experimental CP and in in vitro models.44 51 A role for Twist1 in overcoming mutant K-Ras-induced senescence has been proposed.118 It seems likely that, in pancreatitis, subpopulations of acinar cells distinctly respond to signalling stress, leading to different cellular outputs whereby some acinar cells can overcome this tumour barrier.

    Autophagy aims at preserving cellular integrity in response to stressful conditions (survival) and can also be a tumour-suppressive mechanism. In general, tumour suppressors activate autophagy, whereas oncogenic pathways suppress it.119 It has been suggested that early stages of pancreas tumorigenesis are associated with a downregulation of autophagy-related genes.120 Autophagy is triggered during acute pancreatitis, and autophagy protein 5 (Atg5)-deficient acinar cells are partially protected from the damage induced by caerulein.121 122 Therefore it will be important to further examine the role of autophagy in the context of acinar cell dedifferentiation.

    Inflammation and the stroma: the answer is around?

    An essential question regarding a putative role of acinar cells in PDAC is: What turns off the acinar differentiation programme? We consider here the role of non-cell autonomous mechanisms.

    Cell and tissue architecture: extreme makeover

    Abnormal enzyme secretion can cause oedema and cellular stress and affect epithelial cell–cell contacts with a protumoural effect. These changes are reflected in altered differentiation, underscored by observations using dispersed acinar cells in culture,44 54 including decisive roles for cadherin-mediated cell–cell adhesion, β-catenin and the PI3K pathway.78 Organotypic slice cultures could help to distinguish effects of culture-induced stress and intercellular contact disruption on acinar cell differentiation.

    Inflammation

    Chronic inflammation leads to free radical formation, activation of cyclo-oxygenase-2, NFκB and inducible nitric oxide synthase, and inflammatory cytokines.123 Oxidative stress also causes DNA damage.117

    Interleukin 1β overexpression under the control of the elastase promoter is associated with ADM and inflammation, but no PanINs or tumours develop, even in the setting of mutant Trp53.124 Inflammation can overcome the senescence barrier associated with PanINs, thus favouring PDAC development, in a K-RAS-induced mouse model of PDAC, and similar observations have been made in patients with CP treated with anti-inflammatory drugs.125 Stat3, signalling downstream of interleukin 6, promotes ADM, induces proliferation and blocks apoptosis, through upregulation of chemokines and recruitment of activated macrophages. Ablation of Stat3 in pancreatic epithelial cells in the K-RAS-induced mouse models attenuated PDAC at different levels, with fewer PanINs, lower tumour grade, and reduced metastases.126 127

    The stroma

    Altered activity of mesenchymal cells occurs during acute and chronic pancreatitis and may contribute to acinar dedifferentiation through qualitative or quantitative changes in cellular or matrix composition.128 129 The extensive desmoplastic reaction associated with CP and PDAC may result from positive feedback loops that favour matrix deposition and pancreatic stellate cell activation and further help to block acinar cell regeneration via epigenetic mechanisms.128 Dedifferentiated acini or tumour cells may impinge on the composition of the stroma.128 Hypoxia may also contribute to cell differentiation at the onset of CP and PDAC.130 In relation to the dense stroma of PDAC and the associated hypovascularisation, Shh inhibition causes stromal collapse, increased delivery of chemotherapeutic drugs, and increased antitumour effects.131 Therefore the use of drug combinations that affect stroma and tumour cells may be effective in PDAC.131

    The nerves, last but not least

    The exocrine pancreas is mainly innervated by parasympathetic ganglia and preganglial fibres. Neural signals cooperate with humoral mechanisms (ie, cholecystokinin) to trigger excitation of sensory afferents of the enteropancreatic reflexes.132 Tissue damage is possibly associated with loss of all these homoeostatic mechanisms; nerve fibre activation occurs in CP and PDAC, as has been studied in relationship to acinar cell innervation, pain and tumour cell invasion.133 134

    Where to look ahead?

    The aetiological heterogeneity of PDAC suggests that various routes can lead to PDAC, and a molecular taxonomy of PDAC is necessary. The existence of three PDAC subtypes with major therapeutic implications has been proposed.135

    Our knowledge on acinar cell biology should be transferred to the clinical setting given the evidence that ADM could be a preneoplastic lesion. This is not generally accepted to be the case for PDAC. Yet, metaplasia is well established as a risk for cancer at other sites including the stomach and the oesophagus.

    The molecular analysis of metaplastic lesions may provide insight into new biomarkers of risk and chemopreventive strategies, including epigenetic drugs. If ADM is a risk factor for PDAC, more emphasis should be placed on its early detection, possibly using non-invasive strategies—that is, serum markers.

    The study of PDAC will identify new genes, proteins and pathways that may play roles at different stages of tumour development, including ADM. The International Cancer Genome Consortium (http://www.icgc.org) is an initiative to obtain comprehensive genomic, transcriptomic and epigenomic data on the main human tumour types, including pancreatic. The potential of such data to provide insight into tumour pathogenesis, including ADM and early neoplasia, is revealed by analyses of intratumoural heterogeneity as well as metastatic seeding.9 10 135

    The biology of the pancreatic epithelium is and will be in the next few years a topic of intense research. Understanding cell differentiation may contribute to improved patient care, although—admittedly—this is unlikely to happen ‘tomorrow’.

    Acknowledgments

    We thank P Martinelli, M Flández, L Musgrove and G Samimi for critical reading and suggestions, and X Molero for many valuable discussions. We regret being unable to cite all relevant publications because of space constraints.

    References

    1. ↵
      1. Jemal A,
      2. Siegel R,
      3. Xu J,
      4. et al
      . Cancer Statistics, 2010. CA Cancer J Clin 2010;60:277–300.
      OpenUrlCrossRefPubMedWeb of Science
    2. ↵
      1. Neoptolemos JP,
      2. Stocken DD,
      3. Friess H,
      4. et al
      . A randomized trial of chemoradiotherapy and chemotherapy after resection of pancreatic cancer. N Engl J Med 2004;350:1200–10.
      OpenUrlCrossRefPubMedWeb of Science
    3. ↵
      1. Hruban RH,
      2. Adsay NV,
      3. Albores-Saavedra J,
      4. et al
      . Pancreatic intraepithelial neoplasia: a new nomenclature and classification system for pancreatic duct lesions. Am J Surg Pathol 2001;25:579–86.
      OpenUrlCrossRefPubMedWeb of Science
    4. ↵
      1. Hezel AF,
      2. Kimmelman AC,
      3. Stanger BZ,
      4. et al
      . Genetics and biology of pancreatic ductal adenocarcinoma. Genes Dev 2006;20:1218–49.
      OpenUrlAbstract/FREE Full Text
    5. ↵
      1. Sipos B,
      2. Frank S,
      3. Gress T,
      4. et al
      . Pancreatic intraepithelial neoplasia revisited and updated. Pancreatology 2009;9:45–54.
      OpenUrlCrossRefPubMed
    6. ↵
      1. Sugiyama M,
      2. Suzuki Y,
      3. Abe N,
      4. et al
      . Management of intraductal papillary mucinous neoplasm of the pancreas. J Gastroenterol 2008;43:181–5.
      OpenUrlCrossRefPubMed
    7. ↵
      1. Bj Rk Werner J,
      2. Bartosch-H Rlid A,
      3. Andersson R
      . Cystic pancreatic lesions: Current evidence for diagnosis and treatment. Scand J Gastroenterol 2011;46:773–88.
      OpenUrlCrossRefPubMed
    8. ↵
      1. Hong SM,
      2. Heaphy CM,
      3. Shi C,
      4. et al
      . Telomeres are shortened in acinar-to-ductal metaplasia lesions associated with pancreatic intraepithelial neoplasia but not in isolated acinar-to-ductal metaplasias. Mod Pathol 2011;24:256–66.
      OpenUrlCrossRefPubMed
    9. ↵
      1. Jones S,
      2. Zhang X,
      3. Parsons DW,
      4. et al
      . Core signaling pathways in human pancreatic cancers revealed by global genomic analyses. Science 2008;321:1801–6.
      OpenUrlAbstract/FREE Full Text
    10. ↵
      1. Campbell PJ,
      2. Yachida S,
      3. Mudie LJ,
      4. et al
      . The patterns and dynamics of genomic instability in metastatic pancreatic cancer. Nature 2010;467:1109–13.
      OpenUrlCrossRefPubMedWeb of Science
    11. ↵
      1. Real FX
      . A “catastrophic hypothesis” for pancreas cancer progression. Gastroenterology 2003;124:1958–64.
      OpenUrlCrossRefPubMed
    12. ↵
      1. Short B
      . People & Ideas: Mina Bissell: Context is everything. J Cell Biol 2009;185:374–5.
      OpenUrlFREE Full Text
    13. ↵
      1. Aguirre AJ,
      2. Bardeesy N,
      3. Sinha M,
      4. et al
      . Activated Kras and Ink4a/Arf deficiency cooperate to produce metastatic pancreatic ductal adenocarcinoma. Genes Dev 2003;17:3112–26.
      OpenUrlAbstract/FREE Full Text
    14. ↵
      1. Hingorani SR,
      2. Petricoin EF,
      3. Maitra A,
      4. et al
      . Preinvasive and invasive ductal pancreatic cancer and its early detection in the mouse. Cancer Cell 2003;4:437–50.
      OpenUrlCrossRefPubMedWeb of Science
    15. ↵
      1. Siveke JT,
      2. Einwächter H,
      3. Sipos B,
      4. et al
      . Concomitant Pancreatic Activation of KrasG12D and Tgfa Results in Cystic Papillary Neoplasms Reminiscent of Human IPMN. Cancer Cell 2007;12:266–79.
      OpenUrlCrossRefPubMedWeb of Science
    16. ↵
      1. MacDonald RJ,
      2. Swift GH,
      3. Real FX
      . Transcriptional control of acinar development and homeostasis. In: Kaestner K, ed. Molecular Biology of Digestive Organs. 2010:1–40.
    17. ↵
      1. Pujal J,
      2. Huch M,
      3. Jose A,
      4. et al
      . Keratin 7 promoter selectively targets transgene expression to normal and neoplastic pancreatic ductal cells in vitro and in vivo. FASEB J 2009;23:1366–75.
      OpenUrlAbstract/FREE Full Text
    18. ↵
      1. Brembeck FH,
      2. Rustgi AK
      . The tissue-dependent keratin 19 gene transcription is regulated by GKLF/KLF4 and Sp1. J Biol Chem 2000;275:28230–9.
      OpenUrlAbstract/FREE Full Text
    19. ↵
      1. Means AL,
      2. Xu Y,
      3. Zhao A,
      4. et al
      . A CK19(CreERT) knockin mouse line allows for conditional DNA recombination in epithelial cells in multiple endodermal organs. Genesis 2008;46:318–23.
      OpenUrlCrossRefPubMedWeb of Science
    20. ↵
      1. Brembeck FH,
      2. Schreiber FS,
      3. Deramaudt TB,
      4. et al
      . The mutant K-ras oncogene causes pancreatic periductal lymphocytic infiltration and gastric mucous neck cell hyperplasia in transgenic mice. Cancer Res 2003;63:2005–9.
      OpenUrlAbstract/FREE Full Text
    21. ↵
      1. Ray KC,
      2. Bell KM,
      3. Yan J,
      4. et al
      . Epithelial tissues have varying degrees of susceptibility to Kras(G12D)-initiated tumorigenesis in a mouse model. PLoS One 2011;6:e16786.
      OpenUrlCrossRefPubMed
    22. ↵
      1. Solar M,
      2. Cardalda C,
      3. Houbracken I,
      4. et al
      . Pancreatic exocrine duct cells give rise to insulin-producing beta cells during embryogenesis but not after birth. Dev Cell 2009;17:849–60.
      OpenUrlCrossRefPubMedWeb of Science
    23. ↵
      1. Inada A,
      2. Nienaber C,
      3. Katsuta H,
      4. et al
      . Carbonic anhydrase II-positive pancreatic cells are progenitors for both endocrine and exocrine pancreas after birth. Proc Natl Acad Sci U S A 2008;105:19915–19.
      OpenUrlAbstract/FREE Full Text
    24. ↵
      1. Bockman DE,
      2. Guo J,
      3. Buchler P,
      4. et al
      . Origin and development of the precursor lesions in experimental pancreatic cancer in rats. Lab Invest 2003;83:853–9.
      OpenUrlCrossRefPubMedWeb of Science
    25. ↵
      1. Scarpelli DG,
      2. Rao MS,
      3. Reddy JK
      . Studies of pancreatic carcinogenesis in different animal models. Environ Health Perspect 1984;56:219–27.
      OpenUrlPubMed
    26. ↵
      1. Wagner M,
      2. Luhrs H,
      3. Kloppel G,
      4. et al
      . Malignant transformation of duct-like cells originating from acini in transforming growth factor transgenic mice. Gastroenterology 1998;115:1254–62.
      OpenUrlCrossRefPubMedWeb of Science
    27. ↵
      1. Sandgren EP,
      2. Quaife CJ,
      3. Paulovich AG,
      4. et al
      . Pancreatic tumor pathogenesis reflects the causative genetic lesion. Proc Natl Acad Sci U S A 1991;88:93–7.
      OpenUrlAbstract/FREE Full Text
    28. ↵
      1. Grippo PJ,
      2. Nowlin PS,
      3. Demeure MJ,
      4. et al
      . Preinvasive pancreatic neoplasia of ductal phenotype induced by acinar cell targeting of mutant Kras in transgenic mice. Cancer Res 2003;63:2016–19.
      OpenUrlAbstract/FREE Full Text
    29. ↵
      1. Guerra C,
      2. Schuhmacher AJ,
      3. Cañamero M,
      4. et al
      . Chronic pancreatitis is essential for induction of pancreatic ductal adenocarcinoma by K-ras oncogenes in adult mice. Cancer Cell 2007;11:291–302.
      OpenUrlCrossRefPubMedWeb of Science
    30. ↵
      1. Tuveson DA,
      2. Zhu L,
      3. Gopinathan A,
      4. et al
      . Mist1-KrasG12D knock-in mice develop mixed differentiation metastatic exocrine pancreatic carcinoma and hepatocellular carcinoma. Cancer Res 2006;66:242–7.
      OpenUrlAbstract/FREE Full Text
    31. ↵
      1. Carriere C,
      2. Young AL,
      3. Gunn JR,
      4. et al
      . Acute pancreatitis markedly accelerates pancreatic cancer progression in mice expressing oncogenic Kras. Biochem Biophys Res Commun 2009;382:561–5.
      OpenUrlCrossRefPubMed
    32. ↵
      1. Raimondi S,
      2. Maisonneuve P,
      3. Lowenfels AB
      . Epidemiology of pancreatic cancer: an overview. Nat Rev Gastroenterol Hepatol 2009;6:699–708.
      OpenUrlCrossRefPubMedWeb of Science
    33. ↵
      1. Logsdon CD,
      2. Ji B
      . Ras activity in acinar cells links chronic pancreatitis and pancreatic cancer. Clin Gastroenterol Hepatol 2009;7:S40–3.
      OpenUrlCrossRefPubMed
    34. ↵
      1. Miyamoto Y,
      2. Maitra A,
      3. Ghosh B,
      4. et al
      . Notch mediates TGF alpha-induced changes in epithelial differentiation during pancreatic tumorigenesis. Cancer Cell 2003;3:565–76.
      OpenUrlCrossRefPubMedWeb of Science
    35. ↵
      1. Rovira M,
      2. Scott SG,
      3. Liss AS,
      4. et al
      . Isolation and characterization of centroacinar/terminal ductal progenitor cells in adult mouse pancreas. Proc Natl Acad Sci U S A 2010;107:75–80.
      OpenUrlAbstract/FREE Full Text
    36. ↵
      1. Kopinke D,
      2. Brailsford M,
      3. Shea JE,
      4. et al
      . Lineage tracing reveals the dynamic contribution of Hes1+ cells to the developing and adult pancreas. Development 2011;138:431–41.
      OpenUrlAbstract/FREE Full Text
    37. ↵
      1. Stanger BZ,
      2. Stiles B,
      3. Lauwers GY,
      4. et al
      . Pten constrains centroacinar cell expansion and malignant transformation in the pancreas. Cancer Cell 2005;8:185–95.
      OpenUrlCrossRefPubMedWeb of Science
    38. ↵
      1. Friedlander SY,
      2. Chu GC,
      3. Snyder EL,
      4. et al
      . Context-dependent transformation of adult pancreatic cells by oncogenic K-ras. Cancer Cell 2009;16:379–89.
      OpenUrlCrossRefPubMedWeb of Science
    39. ↵
      1. Blaine SA,
      2. Ray KC,
      3. Anunobi R,
      4. et al
      . Adult pancreatic acinar cells give rise to ducts but not endocrine cells in response to growth factor signaling. Development 2010;137:2289–96.
      OpenUrlAbstract/FREE Full Text
    40. ↵
      1. Lonardo E,
      2. Hermann PC,
      3. Heeschen C
      . Pancreatic cancer stem cells - update and future perspectives. Mol Oncol 2010;4:431–42.
      OpenUrlCrossRefPubMed
    41. ↵
      1. Ku HT
      . Pancreatic Progenitor Cells–Recent Studies. Endocrinology 2008;149:4312–16.
      OpenUrlCrossRefPubMedWeb of Science
    42. ↵
      1. Lee CJ,
      2. Dosch J,
      3. Simeone DM
      . Pancreatic cancer stem cells. J Clin Oncol 2008;26:2806–12.
      OpenUrlAbstract/FREE Full Text
    43. ↵
      1. Lardon J,
      2. Corbeil D,
      3. Huttner WB,
      4. et al
      . Stem cell marker prominin-1/AC133 is expressed in duct cells of the adult human pancreas. Pancreas 2008;36:e1–6.
      OpenUrlCrossRefPubMedWeb of Science
    44. ↵
      1. Pinho AV,
      2. Rooman I,
      3. Reichert M,
      4. et al
      . Adult pancreatic acinar cells dedifferentiate to an embryonic progenitor phenotype with concomitant activation of a senescence programme that is present in chronic pancreatitis. Gut 2011;60:958–66.
      OpenUrlAbstract/FREE Full Text
    45. ↵
      1. Martinez-Romero C,
      2. Rooman I,
      3. Skoudy A,
      4. et al
      . The epigenetic regulators Bmi1 and Ring1B are differentially regulated in pancreatitis and pancreatic ductal adenocarcinoma. J Pathol 2009;219:205–13.
      OpenUrlCrossRefPubMed
    46. ↵
      1. Masui T,
      2. Swift GH,
      3. Deering T,
      4. et al
      . Replacement of Rbpj with Rbpjl in the PTF1 complex controls the final maturation of pancreatic acinar cells. Gastroenterology 2010;139:270–80.
      OpenUrlCrossRefPubMed
    47. ↵
      1. Masui T,
      2. Long Q,
      3. Beres TM,
      4. et al
      . Early pancreatic development requires the vertebrate Suppressor of Hairless (RBPJ) in the PTF1 bHLH complex. Genes Dev 2007;21:2629–43.
      OpenUrlAbstract/FREE Full Text
    48. ↵
      1. Dong PDS,
      2. Provost E,
      3. Leach SD,
      4. et al
      . Graded levels of Ptf1a differentially regulate endocrine and exocrine fates in the developing pancreas. Genes Dev 2008;22:1445–50.
      OpenUrlAbstract/FREE Full Text
    49. ↵
      1. Zhu L,
      2. Tran T,
      3. Rukstalis JM,
      4. et al
      . Inhibition of Mist1 homodimer formation induces pancreatic acinar-to-ductal metaplasia. Mol Cell Biol 2004;24:2673–81.
      OpenUrlAbstract/FREE Full Text
    50. ↵
      1. Jia D,
      2. Sun Y,
      3. Konieczny SF
      . Mist1 regulates pancreatic acinar cell proliferation through p21 CIP1/WAF1. Gastroenterology 2008;135:1687–97.
      OpenUrlCrossRefPubMed
    51. ↵
      1. Rodolosse A,
      2. Chalaux E,
      3. Adell T,
      4. et al
      . PTF1alpha/p48 transcription factor couples proliferation and differentiation in the exocrine pancreas [corrected]. Gastroenterology 2004;127:937–49.
      OpenUrlCrossRefPubMed
    52. ↵
      1. Hall PA,
      2. Lemoine NR
      . Rapid acinar to ductal transdifferentiation in cultured human exocrine pancreas. J Pathol 1992;166:97–103.
      OpenUrlCrossRefPubMedWeb of Science
    53. ↵
      1. Vila MR,
      2. Lloreta J,
      3. Real FX
      . Normal human pancreas cultures display functional ductal characteristics. Lab Invest 1994;71:423–31.
      OpenUrlPubMedWeb of Science
    54. ↵
      1. Rooman I,
      2. Heremans Y,
      3. Heimberg H,
      4. et al
      . Modulation of rat pancreatic acinoductal transdifferentiation and expression of PDX-1 in vitro. Diabetologia 2000;43:907–14.
      OpenUrlCrossRefPubMedWeb of Science
    55. ↵
      1. Minami K,
      2. Okuno M,
      3. Miyawaki K,
      4. et al
      . Lineage tracing and characterization of insulin-secreting cells generated from adult pancreatic acinar cells. Proc Natl Acad Sci U S A 2005;102:15116–21.
      OpenUrlAbstract/FREE Full Text
    56. ↵
      1. Means AL,
      2. Meszoely IM,
      3. Suzuki K,
      4. et al
      . Pancreatic epithelial plasticity mediated by acinar cell transdifferentiation and generation of nestin-positive intermediates. Development 2005;132:3767–76.
      OpenUrlAbstract/FREE Full Text
    57. ↵
      1. Slack JM,
      2. Tosh D
      . Transdifferentiation and metaplasia–switching cell types. Curr Opin Genet Dev 2001;11:581–6.
      OpenUrlCrossRefPubMedWeb of Science
    58. ↵
      1. Bouwens L
      . Cytokeratins and cell differentiation in the pancreas. J Pathol 1998;184:234–9.
      OpenUrlCrossRefPubMedWeb of Science
    59. ↵
      1. Lynn FC,
      2. Smith SB,
      3. Wilson ME,
      4. et al
      . Sox9 coordinates a transcriptional network in pancreatic progenitor cells. Proc Natl Acad Sci U S A 2007;104:10500–5.
      OpenUrlAbstract/FREE Full Text
    60. ↵
      1. Houbracken I,
      2. de Waele E,
      3. Lardon J,
      4. et al
      . Lineage tracing evidence for transdifferentiation of acinar to duct cells and plasticity of human pancreas. Gastroenterology 2011;141:731–41, 741.e1–4.
      OpenUrlCrossRefPubMedWeb of Science
    61. ↵
      1. Strobel O,
      2. Dor Y,
      3. Alsina J,
      4. et al
      . In vivo lineage tracing defines the role of acinar-to-ductal transdifferentiation in inflammatory ductal metaplasia. Gastroenterology 2007;133:1999–2009.
      OpenUrlCrossRefPubMed
    62. ↵
      1. Desai BM
      . Preexisting pancreatic acinar cells contribute to acinar cell, but not islet β cell, regeneration. J Clin Invest 2007;117:971–7.
      OpenUrlCrossRefPubMedWeb of Science
    63. ↵
      1. Jensen JN,
      2. Cameron E,
      3. Garay MV,
      4. et al
      . Recapitulation of elements of embryonic development in adult mouse pancreatic regeneration. Gastroenterology 2005;128:728–41.
      OpenUrlCrossRefPubMedWeb of Science
    64. ↵
      1. Herzig KH,
      2. Altmannsberger M,
      3. Folsch UR
      . Intermediate filaments in rat pancreatic acinar tumors, human ductal carcinomas, and other gastrointestinal malignancies. Gastroenterology 1994;106:1326–32.
      OpenUrlPubMed
    65. ↵
      1. Matros E,
      2. Bailey G,
      3. Clancy T,
      4. et al
      . Cytokeratin 20 expression identifies a subtype of pancreatic adenocarcinoma with decreased overall survival. Cancer 2006;106:693–702.
      OpenUrlCrossRefPubMedWeb of Science
    66. ↵
      1. Park JY,
      2. Hong SM,
      3. Klimstra DS,
      4. et al
      . Pdx1 Expression in Pancreatic Precursor Lesions and Neoplasms. Appl Immunohistochem Mol Morphol 2011;19:444–9.
      OpenUrlCrossRefPubMed
    67. ↵
      1. Dufresne M,
      2. Clerc P,
      3. Dieng M,
      4. et al
      . Id3 modulates cellular localization of bHLH Ptf1-p48 protein. Int J Cancer 2011;129:295–306.
      OpenUrlCrossRefPubMed
    68. ↵
      1. Adell T,
      2. Gomez-Cuadrado A,
      3. Skoudy A,
      4. et al
      . Role of the basic helix-loop-helix transcription factor p48 in the differentiation phenotype of exocrine pancreas cancer cells. Cell Growth Differ 2000;11:137–47.
      OpenUrlAbstract/FREE Full Text
    69. ↵
      1. Miettinen P,
      2. Ormio P,
      3. Hakonen E,
      4. et al
      . EGF receptor in pancreatic β-cell mass regulation. Biochem Soc Trans 2008;036:280–5.
      OpenUrlCrossRef
    70. ↵
      1. Wagner M,
      2. Weber CK,
      3. Bressau F,
      4. et al
      . Transgenic overexpression of amphiregulin induces a mitogenic response selectively in pancreatic duct cells. Gastroenterology 2002;122:1898–912.
      OpenUrlCrossRefPubMedWeb of Science
    71. ↵
      1. Shi C,
      2. Hong SM,
      3. Lim P,
      4. et al
      . KRAS2 mutations in human pancreatic acinar-ductal metaplastic lesions are limited to those with PanIN: implications for the human pancreatic cancer cell of origin. Mol Cancer Res 2009;7:230–6.
      OpenUrlAbstract/FREE Full Text
    72. ↵
      1. Crous-Bou M,
      2. Porta M,
      3. Morales E,
      4. et al
      . Past medical conditions and K-ras mutations in pancreatic ductal adenocarcinoma: a hypothesis-generating study. Cancer Causes Control 2009;20:591–9.
      OpenUrlCrossRefPubMedWeb of Science
    73. ↵
      1. Clerc P,
      2. Leung-Theung-Long S,
      3. Wang TC,
      4. et al
      . Expression of CCK2 receptors in the murine pancreas: proliferation, transdifferentiation of acinar cells, and neoplasia. Gastroenterology 2002;122:428–37.
      OpenUrlCrossRefPubMedWeb of Science
    74. ↵
      1. Ferrand A,
      2. Vatinel S,
      3. Kowalski-Chauvel A,
      4. et al
      . Mechanism for Src activation by the CCK2 receptor: Patho-physiological functions of this receptor in pancreas. World J Gastroenterol 2006;12:4498–503.
      OpenUrlPubMed
    75. ↵
      1. Duan RD,
      2. Zheng CF,
      3. Guan KL,
      4. et al
      . Activation of MAP kinase kinase (MEK) and Ras by cholecystokinin in rat pancreatic acini. Am J Physiol 1995;268:G1060–5.
      OpenUrlPubMed
    76. ↵
      1. Elghazi L,
      2. Weiss AJ,
      3. Barker DJ,
      4. et al
      . Regulation of pancreas plasticity and malignant transformation by Akt signaling. Gastroenterology 2009;136:1091–103.
      OpenUrlCrossRefPubMed
    77. ↵
      1. Schonleben F,
      2. Qiu W,
      3. Ciau NT,
      4. et al
      . PIK3CA mutations in intraductal papillary mucinous neoplasm/carcinoma of the pancreas. Clin Cancer Res 2006;12:3851–5.
      OpenUrlAbstract/FREE Full Text
    78. ↵
      1. Minami K,
      2. Okano H,
      3. Okumachi A,
      4. et al
      . Role of Cadherin-mediated Cell-Cell Adhesion in Pancreatic Exocrine-to-Endocrine Transdifferentiation. J Biol Chem 2008;283:13753–61.
      OpenUrlAbstract/FREE Full Text
    79. ↵
      1. Murtaugh LC
      . The what, where, when and how of Wnt/beta-catenin signaling in pancreas development. Organogenesis 2008;4:81–6.
      OpenUrlCrossRefPubMed
    80. ↵
      1. Dessimoz J,
      2. Grapin-Botton A
      . Pancreas development and cancer: Wnt/beta-catenin at issue. Cell Cycle 2006;5:7–10.
      OpenUrlCrossRefPubMed
    81. ↵
      1. Morris JPt,
      2. Cano DA,
      3. Sekine S,
      4. et al
      . Beta-catenin blocks Kras-dependent reprogramming of acini into pancreatic cancer precursor lesions in mice. J Clin Invest 2010;120:508–20.
      OpenUrlCrossRefPubMedWeb of Science
    82. ↵
      1. Heiser PW,
      2. Cano DA,
      3. Landsman L,
      4. et al
      . Stabilization of beta-catenin induces pancreas tumor formation. Gastroenterology 2008;135:1288–300.
      OpenUrlCrossRefPubMedWeb of Science
    83. ↵
      1. Kim W,
      2. Shin YK,
      3. Kim BJ,
      4. et al
      . Notch signaling in pancreatic endocrine cell and diabetes. Biochem Biophys Res Commun 2010;392:247–51.
      OpenUrlCrossRefPubMed
    84. ↵
      1. De La OJ,
      2. Murtaugh LC
      . Notch and Kras in pancreatic cancer: at the crossroads of mutation, differentiation and signaling. Cell Cycle 2009;8:1860–4.
      OpenUrlCrossRefPubMedWeb of Science
    85. ↵
      1. Murtaugh LC,
      2. Stanger BZ,
      3. Kwan KM,
      4. et al
      . Notch signaling controls multiple steps of pancreatic differentiation. Proc Natl Acad Sci U S A 2003;100:14920–5.
      OpenUrlAbstract/FREE Full Text
    86. ↵
      1. Aguilera C,
      2. Hoya-Arias R,
      3. Haegeman G,
      4. et al
      . Recruitment of IkappaBalpha to the hes1 promoter is associated with transcriptional repression. Proc Natl Acad Sci U S A 2004;101:16537–42.
      OpenUrlAbstract/FREE Full Text
    87. ↵
      1. Curry CL,
      2. Reed LL,
      3. Nickoloff BJ,
      4. et al
      . Notch-independent regulation of Hes-1 expression by c-Jun N-terminal kinase signaling in human endothelial cells. Lab Invest 2006;86:842–52.
      OpenUrlPubMedWeb of Science
    88. ↵
      1. Ingram WJ,
      2. McCue KI,
      3. Tran TH,
      4. et al
      . Sonic Hedgehog regulates Hes1 through a novel mechanism that is independent of canonical Notch pathway signalling. Oncogene 2008;27:1489–500.
      OpenUrlCrossRefPubMedWeb of Science
    89. ↵
      1. Bhanot U,
      2. Kohntop R,
      3. Hasel C,
      4. et al
      . Evidence of Notch pathway activation in the ectatic ducts of chronic pancreatitis. J Pathol 2008;214:312–19.
      OpenUrlCrossRefPubMedWeb of Science
    90. ↵
      1. Siveke JT,
      2. Lubeseder-Martellato C,
      3. Lee M,
      4. et al
      . Notch signaling is required for exocrine regeneration after acute pancreatitis. Gastroenterology 2008;134:544–55.
      OpenUrlCrossRefPubMedWeb of Science
    91. ↵
      1. De La OJ,
      2. Emerson LL,
      3. Goodman JL,
      4. et al
      . Notch and Kras reprogram pancreatic acinar cells to ductal intraepithelial neoplasia. Proc Natl Acad Sci U S A 2008;105:18907–12.
      OpenUrlAbstract/FREE Full Text
    92. ↵
      1. Fukushima H,
      2. Yamamoto H,
      3. Itoh F,
      4. et al
      . Association of matrilysin mRNA expression with K-ras mutations and progression in pancreatic ductal adenocarcinomas. Carcinogenesis 2001;22:1049–52.
      OpenUrlAbstract/FREE Full Text
    93. ↵
      1. Sawey ET,
      2. Johnson JA,
      3. Crawford HC
      . Matrix metalloproteinase 7 controls pancreatic acinar cell transdifferentiation by activating the Notch signaling pathway. Proc Natl Acad Sci U S A 2007;104:19327–32.
      OpenUrlAbstract/FREE Full Text
    94. ↵
      1. Crawford HC,
      2. Fingleton BM,
      3. Rudolph-Owen LA,
      4. et al
      . The metalloproteinase matrilysin is a target of beta-catenin transactivation in intestinal tumors. Oncogene 1999;18:2883–91.
      OpenUrlCrossRefPubMedWeb of Science
    95. ↵
      1. Crawford HC,
      2. Scoggins CR,
      3. Washington MK,
      4. et al
      . Matrix metalloproteinase-7 is expressed by pancreatic cancer precursors and regulates acinar-to-ductal metaplasia in exocrine pancreas. J Clin Invest 2002;109:1437–44.
      OpenUrlCrossRefPubMedWeb of Science
    96. ↵
      1. Mazur PK,
      2. Einwachter H,
      3. Lee M,
      4. et al
      . Notch2 is required for progression of pancreatic intraepithelial neoplasia and development of pancreatic ductal adenocarcinoma. Proc Natl Acad Sci U S A 2010;107:13438–43.
      OpenUrlAbstract/FREE Full Text
    97. ↵
      1. Hanlon L,
      2. Avila JL,
      3. Demarest RM,
      4. et al
      . Notch1 Functions as a Tumor Suppressor in a Model of K-ras–Induced Pancreatic Ductal Adenocarcinoma. Cancer Res 2010;70:4280–6.
      OpenUrlAbstract/FREE Full Text
    98. ↵
      1. Hebrok M
      . Hedgehog signaling in pancreas development. Mech Dev 2003;120:45–57.
      OpenUrlCrossRefPubMedWeb of Science
    99. ↵
      1. Cervantes S,
      2. Lau J,
      3. Cano DA,
      4. et al
      . Primary cilia regulate Gli/Hedgehog activation in pancreas. Proc Natl Acad Sci U S A 2010;107:10109–14.
      OpenUrlAbstract/FREE Full Text
    100. ↵
      1. Kayed H,
      2. Kleeff J,
      3. Osman T,
      4. et al
      . Hedgehog signaling in the normal and diseased pancreas. Pancreas 2006;32:119–29.
      OpenUrlCrossRefPubMedWeb of Science
    101. ↵
      1. Feldmann G,
      2. Habbe N,
      3. Dhara S,
      4. et al
      . Hedgehog inhibition prolongs survival in a genetically engineered mouse model of pancreatic cancer. Gut 2008;57:1420–30.
      OpenUrlAbstract/FREE Full Text
    102. ↵
      1. Lauth M,
      2. Bergstrom A,
      3. Shimokawa T,
      4. et al
      . DYRK1B-dependent autocrine-to-paracrine shift of Hedgehog signaling by mutant RAS. Nat Struct Mol Biol 2010;17:718–25.
      OpenUrlCrossRefPubMedWeb of Science
    103. ↵
      1. Thayer SP,
      2. di Magliano MP,
      3. Heiser PW,
      4. et al
      . Hedgehog is an early and late mediator of pancreatic cancer tumorigenesis. Nature 2003;425:851–6.
      OpenUrlCrossRefPubMedWeb of Science
    104. ↵
      1. Pasca di Magliano M,
      2. Sekine S,
      3. Ermilov A,
      4. et al
      . Hedgehog/Ras interactions regulate early stages of pancreatic cancer. Genes Dev 2006;20:3161–73.
      OpenUrlAbstract/FREE Full Text
    105. ↵
      1. Yauch RL,
      2. Gould SE,
      3. Scales SJ,
      4. et al
      . A paracrine requirement for hedgehog signalling in cancer. Nature 2008;455:406–10.
      OpenUrlCrossRefPubMedWeb of Science
    106. ↵
      1. Seeley ES,
      2. Carriere C,
      3. Goetze T,
      4. et al
      . Pancreatic cancer and precursor pancreatic intraepithelial neoplasia lesions are devoid of primary cilia. Cancer Res 2009;69:422–30.
      OpenUrlAbstract/FREE Full Text
    107. ↵
      1. Wandzioch E,
      2. Zaret KS
      . Dynamic signaling network for the specification of embryonic pancreas and liver progenitors. Science 2009;324:1707–10.
      OpenUrlAbstract/FREE Full Text
    108. ↵
      1. Tulachan SS,
      2. Tei E,
      3. Hembree M,
      4. et al
      . TGF-beta isoform signaling regulates secondary transition and mesenchymal-induced endocrine development in the embryonic mouse pancreas. Dev Biol 2007;305:508–21.
      OpenUrlCrossRefPubMedWeb of Science
    109. ↵
      1. Lee MS,
      2. Gu D,
      3. Feng L,
      4. et al
      . Accumulation of extracellular matrix and developmental dysregulation in the pancreas by transgenic production of transforming growth factor-beta 1. Am J Pathol 1995;147:42–52.
      OpenUrlPubMedWeb of Science
    110. ↵
      1. Sanvito F,
      2. Herrera PL,
      3. Huarte J,
      4. et al
      . TGF-beta 1 influences the relative development of the exocrine and endocrine pancreas in vitro. Development 1994;120:3451–62.
      OpenUrlAbstract
    111. ↵
      1. Bottinger EP,
      2. Jakubczak JL,
      3. Roberts IS,
      4. et al
      . Expression of a dominant-negative mutant TGF-beta type II receptor in transgenic mice reveals essential roles for TGF-beta in regulation of growth and differentiation in the exocrine pancreas. EMBO J 1997;16:2621–33.
      OpenUrlAbstract
    112. ↵
      1. Wildi S,
      2. Kleeff J,
      3. Mayerle J,
      4. et al
      . Suppression of transforming growth factor beta signalling aborts caerulein induced pancreatitis and eliminates restricted stimulation at high caerulein concentrations. Gut 2007;56:685–92.
      OpenUrlAbstract/FREE Full Text
    113. ↵
      1. Peinado H,
      2. Olmeda D,
      3. Cano A
      . Snail, Zeb and bHLH factors in tumour progression: an alliance against the epithelial phenotype? Nat Rev Cancer 2007;7:415–28.
      OpenUrlCrossRefPubMedWeb of Science
    114. ↵
      1. Fanjul M,
      2. Gmyr V,
      3. Sengenes C,
      4. et al
      . Evidence for Epithelial-Mesenchymal Transition in Human Adult Pancreatic Exocrine Cells. J Histochem Cytochem 2010;58:807–23.
      OpenUrlAbstract/FREE Full Text
    115. ↵
      1. Russ HA,
      2. Bar Y,
      3. Ravassard P,
      4. et al
      . In vitro proliferation of cells derived from adult human beta-cells revealed by cell-lineage tracing. Diabetes 2008;57:1575–83.
      OpenUrlAbstract/FREE Full Text
    116. ↵
      1. Pinho AV,
      2. Rooman I,
      3. Real F
      . p53-dependent regulation of growth, epithelial-mesenchymal transition and stemness in normal pancreatic epithelial cells. Cell Cycle 2011;10:1312–21.
      OpenUrlCrossRefPubMedWeb of Science
    117. ↵
      1. Collado M,
      2. Gil J,
      3. Efeyan A,
      4. et al
      . Tumour biology: Senescence in premalignant tumours. Nature 2005;436:642.
      OpenUrlCrossRefPubMedWeb of Science
    118. ↵
      1. Lee KE,
      2. Bar-Sagi D
      . Oncogenic KRas Suppresses Inflammation-Associated Senescence of Pancreatic Ductal Cells. Cancer Cell 2010;18:448–58.
      OpenUrlCrossRefPubMedWeb of Science
    119. ↵
      1. Chen N,
      2. Debnath J
      . Autophagy and tumorigenesis. FEBS Lett 2010;584:1427–35.
      OpenUrlCrossRefPubMedWeb of Science
    120. ↵
      1. Kim HS,
      2. Lee SH,
      3. Do SI,
      4. et al
      . Clinicopathologic correlation of beclin-1 expression in pancreatic ductal adenocarcinoma. Pathol Res Pract 2011;207:247–52.
      OpenUrlCrossRefPubMed
    121. ↵
      1. Ohmuraya M,
      2. Yamamura K
      . Autophagy and acute pancreatitis: a novel autophagy theory for trypsinogen activation. Autophagy 2008;4:1060–2.
      OpenUrlPubMedWeb of Science
    122. ↵
      1. Mareninova OA,
      2. Hermann K,
      3. French SW,
      4. et al
      . Impaired autophagic flux mediates acinar cell vacuole formation and trypsinogen activation in rodent models of acute pancreatitis. J Clin Invest 2009;119:3340–55.
      OpenUrlPubMedWeb of Science
    123. ↵
      1. Farrow B,
      2. Sugiyama Y,
      3. Chen A,
      4. et al
      . Inflammatory mechanisms contributing to pancreatic cancer development. Ann Surg 2004;239:763–9, discussion 9–71.
      OpenUrlCrossRefPubMed
    124. ↵
      1. Marrache F,
      2. Tu SP,
      3. Bhagat G,
      4. et al
      . Overexpression of Interleukin-1[beta] in the Murine Pancreas Results in Chronic Pancreatitis. Gastroenterology 2008;135:1277–87.
      OpenUrlCrossRefPubMedWeb of Science
    125. ↵
      1. Guerra C,
      2. Collado M,
      3. Navas C,
      4. et al
      . Pancreatitis-induced Inflammation Contributes to Pancreatic Cancer by Inhibiting Oncogene-Induced Senescence. Cancer Cell 2011;19:728–39.
      OpenUrlCrossRefPubMedWeb of Science
    126. ↵
      1. Lesina M,
      2. Kurkowski MU,
      3. Ludes K,
      4. et al
      . Stat3/Socs3 activation by IL-6 transsignaling promotes progression of pancreatic intraepithelial neoplasia and development of pancreatic cancer. Cancer Cell 2011;19:456–69.
      OpenUrlCrossRefPubMedWeb of Science
    127. ↵
      1. Fukuda A,
      2. Wang SC,
      3. Morris JPt,
      4. et al
      . Stat3 and MMP7 contribute to pancreatic ductal adenocarcinoma initiation and progression. Cancer Cell 2011;19:441–55.
      OpenUrlCrossRefPubMedWeb of Science
    128. ↵
      1. Hernandez-Munoz I,
      2. Skoudy A,
      3. Real FX,
      4. et al
      . Pancreatic ductal adenocarcinoma: cellular origin, signaling pathways and stroma contribution. Pancreatology 2008;8:462–9.
      OpenUrlCrossRefPubMedWeb of Science
    129. ↵
      1. Chu GC,
      2. Kimmelman AC,
      3. Hezel AF,
      4. et al
      . Stromal biology of pancreatic cancer. J Cell Biochem 2007;101:887–907.
      OpenUrlCrossRefPubMedWeb of Science
    130. ↵
      1. Erkan M,
      2. Reiser-Erkan C,
      3. Michalski CW,
      4. et al
      . Cancer-stellate cell interactions perpetuate the hypoxia-fibrosis cycle in pancreatic ductal adenocarcinoma. Neoplasia 2009;11:497–508.
      OpenUrlPubMedWeb of Science
    131. ↵
      1. Kelleher FC
      . Hedgehog signaling and therapeutics in pancreatic cancer. Carcinogenesis 2011;32:445–51.
      OpenUrlAbstract/FREE Full Text
    132. ↵
      1. Chandra R,
      2. Liddle RA
      . Neural and hormonal regulation of pancreatic secretion. Curr Opin Gastroenterol 2009;25:441–6.
      OpenUrlCrossRefPubMedWeb of Science
    133. ↵
      1. Samkharadze T,
      2. Erkan M,
      3. Reiser-Erkan C,
      4. et al
      . Pigment Epithelium-Derived Factor Associates With Neuropathy and Fibrosis in Pancreatic Cancer. Am J Gastroenterol 2011;106:968–80.
      OpenUrlCrossRefPubMed
    134. ↵
      1. Michalski CW,
      2. Oti FE,
      3. Erkan M,
      4. et al
      . Cannabinoids in pancreatic cancer: correlation with survival and pain. Int J Cancer 2008;122:742–50.
      OpenUrlCrossRefPubMed
    135. ↵
      1. Collisson EA,
      2. Sadanandam A,
      3. Olson P,
      4. et al
      . Subtypes of pancreatic ductal adenocarcinoma and their differing responses to therapy. Nat Med 2011;17:500–3.
      OpenUrlCrossRefPubMed

    Footnotes

    • Funding This work was partially funded by grant 10/FRL/2-03 from the Cancer Institute, NSW (to IR) and grants SAF2007-60860 and ONCOBIO Consolider from Ministerio de Ciencia e Innovación (Madrid, Spain) and grant EPC-TM-NET from the EU 7th Framework Programme (to FXR).

    • Competing interests None.

    • Provenance and peer review Commissioned; externally peer reviewed.