Article Text

PDF

Oncogenic transcription factors: cornerstones of inflammation-linked pancreatic carcinogenesis
  1. Sandra Baumgart1,
  2. Volker Ellenrieder1,
  3. Martin E Fernandez-Zapico2
  1. 1Signaling and Transcription Laboratory, Department of Gastroenterology, Philipps-University of Marburg, Marburg, Germany
  2. 2Schulze Center for Novel Therapeutics, Division of Oncology Research, Mayo Clinic, Rochester, Minnesota, USA
  1. Correspondence to Dr Martin E Fernandez-Zapico, Schulze Center for Novel Therapeutics, Mayo Clinic, Gonda 19-216, 200 First St SW, Rochester, MN 55905, USA; fernandezzapico.martin{at}mayo.edu

Abstract

Transcription factors are proteins that regulate gene expression by modulating the synthesis of messenger RNA. Since this process is often one dominant control point in the production of many proteins, transcription factors represent the key regulators of numerous cellular functions, including proliferation, differentiation and apoptosis. Pancreatic cancer progression is characterised by activation of inflammatory signalling pathways converging on a limited set of transcription factors that fine-tune gene expression patterns contributing to the growth and maintenance of these tumours. Thus strategies targeting these transcriptional networks activated in pancreatic cancer cells could block the effects of upstream inflammatory responses participating in pancreatic tumorigenesis. The authors review this field of research and summarise current strategies for targeting oncogenic transcription factors and their activating signalling networks in the treatment of pancreatic cancer.

  • Transcription factors
  • pancreatic cancer
  • inflammation
  • therapy
  • pancreatic cancer
  • carcinogenesis
  • cell signalling
  • gene regulation
  • gene expression
  • pancreatic disease
  • TGF-β
  • molecular oncology
  • pancreatic cancer
  • gastrointestinal cancer
  • pancreas
  • cancer
  • cell signalling
View Full Text

Statistics from Altmetric.com

Oncogenic transcription factors are final effectors of inflammatory signalling pathways during pancreatic carcinogenesis

Pancreatic cancer, one of the most devastating malignancies, is triggered by genetic and epigenetic alterations leading to the aberrant activation of key oncogenes and inactivation of tumour suppressor pathways, and consequently confers transforming cells with a growth and survival advantage over normal cells. The most common genetic abnormality in pancreatic cancer is the activating mutation of KRAS, which is an initial key event in pancreatic carcinogenesis and found in virtually all invasively growing tumours.1 However, mutation of KRAS alone is not sufficient for full neoplastic progression, and additional abnormalities (eg, deletions of tumour suppressor genes) and signals that originate from the tumour microenvironment are required for tumour promotion and progression.2 ,3 The tumour-associated microenvironment comprises stromal cells (mainly fibroblasts and stellate cells), endothelial cells and cells from the innate (eg, macrophages, neutrophils, dendritic cells and natural killer (NK) cells) and adaptive immune system (T and B lymphocytes), which can act in an autocrine and/or paracrine manner to trigger tumour progression. It is now apparent from epidemiological, pharmacological and genetic studies that chronic inflammation can stimulate pancreatic cancer progression through extensive cross-talk interactions between malignant epithelial cells and the surrounding microenvironment.4–6 Accordingly, recent pathological studies suggest that the establishment of pancreatic precursor lesions (PanINs) and the progression to frank adenocarcinoma occurs in most, if not all, cases in the presence of chronic inflammation and is associated with secretion of large amounts of proinflammatory cytokines, growth factors and proteases such as tumour necrosis factor (TNF) α, interleukin (IL) 6, Hedgehog (HH) and transforming growth factor (TGF) β. These cytokines activate an oncogenic network of transcription factors in pancreatic cancer cells leading to tumour growth, survival and invasion4–6 through the regulation of a specific set of target genes (figure 1). Below we will discuss some of the factors regulated by inflammatory cascades, especially those activated in tumorous cells.

Figure 1

Putative cross-talk between oncogenic transcription factors contributing to pancreatic carcinogenesis. Inflammation induces binding of cytokines and inflammatory ligands to their receptors, which in turn activate the corresponding transcription factor signalling pathways. Binding of HH to the transmembrane protein PTC reverses the inhibitory effect on SMO, which again releases GLI transcription factors from their inhibitory complex and finally initiates nuclear translocation. The activation of the pathway can be blocked by inhibitors of SMO such as cyclopamine and its derivatives currently in clinical studies, GDC-0449 and IPI-926. Interactions with the NFκB signalling pathway are illustrated by the dashed arrowhead, which indicates activation of the HH pathway mainly through the regulation of the expression of its ligand SHH. Binding of EGF or IL-6 activates IKKβ kinase, which stimulates degradation of IκBα. Subsequently, NFκB proteins are released to the nucleus, where they mediate gene transcription alone or in cooperation with STAT proteins. Curcumin, quercetin, isoflavone and proteasome or GSK-3 inhibitors interfere with NFκB signalling mainly through inhibition of NFκB activation. Thereby, p-STAT3 binding to p-p65 induces histone acetyltransferase p300 and retains active p65 in the nucleus. Synthetic terpenoids can either block isolated NFκB and STAT3 activation or inhibit the interaction of both pathways. A putative binding of p50 to STAT3 is shown by the dashed circle. The STAT3 signalling pathway can also be activated by the aforementioned inflammatory stimuli. Upon binding of the ligands to the transmembrane receptors, (receptor-bound) kinases, such as JAKs, SRC or ABL, are activated and in turn phosphorylate STAT3 proteins. EGFR inhibitor, erlotinib, as well as tyrosine kinase inhibitors, AZD0530 and dasatinib, have been shown to efficiently block STAT3 activation. The subsequent dimerisation leads to nuclear translocation and target gene transcription, partially by interaction with NFAT transcription factors. The interaction on target promoters is not dependent on STAT and NFAT phosphorylation, illustrated by dashed circles. The canonical NFAT signalling pathway is activated by intracellular Ca2+ increases leading to activation of the phosphatase calcineurin and dephosphorylation of NFAT proteins, which shuttle to the nucleus and bind to their target promoters. Calcineurin inhibitors, CsA and FK506, block NFAT dephosphorylation and nuclear translocation. NFAT phosphorylation does not exclude nuclear localisation and binding to partner proteins, shown by dashed white circles. The dashed arrow indicates a possible interaction with p65, which has been demonstrated for other non-pancreatic tissues. GSK-3 inhibitors successfully disrupt NFAT binding to partner proteins such as STAT3 as well as NFAT transcriptional activity in the nucleus. Finally, the network of inflammatory transcription factors regulates numerous target genes mediating inflammation, growth, invasion, angiogenesis and metastasis, thereby contributing to pancreatic carcinogenesis. Bcl-2/xL, B cell lymphoma 2/xL; bFGF, basic fibroblast growth factor; CDK 4/6, cyclin-dependent kinase 4/6; cIAP 1/2, cellular inhibitor of apoptosis; cFLIP, cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein; COX-2, cyclo-oxygenase-2; CsA, cyclosporine A; CXCR4, CXC-motif chemokine receptor 4; EGF(R), epidermal growth factor receptor; GSK-3, glycogen synthase kinase-3; HGF, hepatocyte growth factor; HH, Hedgehog; ICAM-1, intercellular adhesion molecule 1;IKKβ, IκB kinase β; IκBα, inhibitor of NFκB α; IL-6(R), interleukin-6 (receptor); JAK, janus kinase; 5-LOX, 5-lipo-oxygenase; Mcl-1, myeloid cell leukaemia sequence 1; MMP-9, matrix metaloproteinase 9; NFAT, nuclear factor of activated T cells; PDGF, platelet-derived growth factor; PTC, Patched; SHH, Sonic HH; SMO, Smoothened; STAT3, signal transducer and activator of transcription 3; Sufu, Suppressor of Fused; TGF(R), transforming growth factor (receptor); TNF(R), tumour necrosis factor (receptor); VCAM-1, vascular cell adhesion molecule 1; VEGF, vascular endothelial growth factor; VMP1, vacuole membrane protein 1; XIAP, X-linked inhibitor of apoptosis protein.

The transcription factor NFκB, a master regulator of innate immunity and inflammation, represents a molecular bridge between chronic inflammation and cancer development.7–9 In fact, there has been increasing interest in its role in both inflammation-induced carcinogenesis and maintenance of established cancer, where it is constitutively activated.10 The NFκB family comprises five members, which form homodimeric and heterodimeric complexes: RelA (p65), RelB, Rel (cRel), NFκB1 (p50 and its precursor p105) and NFκB2 (p52 and its precursor p100). Activation of these proteins is regulated by the canonical NFκB activation pathway which applies to RelA/p50 dimers, which are sequestered in the cytosol through interactions with inhibitory proteins of the IκB family. After stimulation by inflammatory cytokines such as TNFα, IκB is phosphorylated by the IκB kinase (IKK) complex, leading to their ubiquitylation and subsequent degradation by the proteasome pathway. RelA/p50 dimers are then translocated into the nucleus, where they activate the transcription of growth-promoting genes (eg, cyclin D1, cyclin E, CDK2 and c-Myc) as well as cytokines (eg, IL-6) (figure 1). NFκB has been shown to be constitutively activated in pancreatic cancer, where its inhibition enhances the sensitivity of cancer cells to chemotherapeutic agents and death receptor-mediated apoptosis. In addition, NFκB helps to control proliferation, cell survival and invasion of pancreatic cancer cells11 ,12 induced by inflammatory stimuli originated from the microenvironment.

The functions mediated by NFκB are at least partially performed in cooperation with other factors such as the signal transducer and activator of transcription 3 (STAT3).13–15 This interaction occurs at several subcellular levels and in a highly context-dependent manner. NFκB controls the expression levels of STAT3 induced by tumour micronenvironment cytokines and growth factors, most notably the proinflammatory cytokine IL-6.16 ,17 Interestingly, active STAT3, in turn can feed back on NFκB signalling leading to the accumulation of RelA in the nucleus of cancer cells.18 The increased activation levels of STAT3 in cancers are almost exclusively secondary to the activation of upstream signalling and, similarly to NFκB, do not result from genetic alterations. STAT3 is turned on by IL-6 binding to the extracellular domain of transmembrane cytokine receptors and subsequent activation of receptor-associated tyrosine kinases, such as Janus kinases (JAKs). Phosphorylation of key tyrosine (Tyr705) and serine (Ser727) residues on STAT3 induces formation of transcriptional active STAT3 homodimers associated through reciprocal SH2 domain–phosphotyrosine-705 (pTyr705) interactions. Dimeric STAT3 complexes translocate to the nucleus, where they bind to DNA and promote specific gene transcription. In the nucleus of tumour cells and infiltrating immune cells present within the tumour microenvironment, STAT3 and NFκB co-regulate numerous genes controlling tumour cell proliferation and survival, including induction of c-Myc, cyclin D1 and Bcl-2, as well as epithelial-to-mesenchymal transdifferentiation (EMT) and migration genes such as E-cadherin, Twist and Snail14 ,18–20 (figure 1). Accumulating evidence suggests a crucial role for IL-6 in constitutive STAT3 activation in gastrointestinal cancers including pancreatic cancer, which ultimately determines tumour burden and prognosis. Notably, active STAT3 is often found at the invasive edge of tumours, adjacent to inflammatory cells linking the inflammatory processes to cancer development.16 ,21 Recent reports by Lesina et al 22 and Fukuda et al 23 established that the aberrant activation of IL-6/STAT3 axis promotes PanIN progression and pancreatic cancer development. Moreover, STAT3 represents a critical component of pancreatitis-accelerated PanIN formation and supports cell growth and metaplasia-associated inflammation.22 ,23

Recently, it has been reported that tyrosine phosphorylation and activation of the STAT3 transcription factor is stimulated by the transcription factor NFATc1, another oncogenic factor with key roles in gene regulation during pancreatic carcinogenesis.24 NFAT factors together with NFκB belong to the extended NFκB/Rel family of transcription factors and thus share structural similarities of their DNA-binding domains, also known as Rel homology regions (RHR). The canonical mode of DNA binding by the RHR has been well characterised in the structures of several NFκB–DNA complexes, and it becomes clear that NFAT can bind to several promoters with κB sites.25 NFAT proteins modulate inflammatory processes and participate in the regulation of genes influencing cell growth and differentiation. In resting cells, NFAT resides in the cytosol and in a hyperphosphorylated form. A rise in intracellular calmodulin-bound Ca2+ levels activates the heterodimeric phosphatase, calcineurin, which dephosphorylates multiple serine residues in the NFAT N-terminal region, resulting in rapid translocation of the protein from the cytoplasm to the nucleus, where it enhances local chromatin acetylation and promotes de novo gene transcription.26 Similarly to NFκB and STAT3, ectopic activation of individual NFAT members is now recognised as an important aspect of oncogenic transformation in many different human malignancies including pancreatic cancer. In these tumours, sustained activation of the Ca2+/calcineurin/NFAT signalling and transcription pathway has emerged as a multifunctional and powerful regulatory principle governing growth and survival of transformed cells.26–28 Expression and constitutive activation of two members of the family, NFATc1 and NFATc2, is found in PanIN-2 lesions and in the nuclei of most of the invasive tumours. NFAT-dependent cell growth and transformation through c-Myc promoter activation in pancreatic cancer requires interaction with the ETS-like transcription factor, ELK-1.26 ,29 ,30 Similarly, NFAT forms promoter-bound transcription complexes with NFκB and STAT3 in pancreatic cancer cells in a signalling-dependent manner, promoting cellular functions associated with pancreatic cancer development and progression (unpublished work).

Other key oncogenic factors implicated in pancreatic cancer and activated by inflammatory cytokines are the members of the GLI family (GLI1, 2 and 3),31–43 which regulate the expression of genes important for tumour microenvironment (IL-6),33 cell proliferation (CDK2),34 apoptosis (bcl-2) and autophagy (VMP-1) (Fernandez-Zapico ME, personal communication, 2011.). These zing finger proteins were originally identified as downstream effectors of the HH signalling pathway31–37 (figure 1), a cascade that is activated during pancreatic inflammatory processes and carcinogenesis. As downstream effectors of HH, the GLI factors are activated via two multi-transmembrane proteins: Patched (PTC) and Smoothened (SMO). In this receptor complex, PTC is the ligand-binding subunit, while SMO represents the signalling component. Upon binding of HH to its receptor (PTC), the inhibitory effect of PTC on SMO is released, and signal is triggered leading to the activation of GLI proteins.31 ,32 HH-driven pancreatic tumours are mainly characterised by inappropriate ligand expression, in particular Sonic HH and Indian HH, which is overexpressed in pancreatic tumours, both in mice models and human tumours.34–37 Interestingly, GLI activation by the HH ligands seems to be exclusively increased in stromal cells, suggesting an important contribution of the tumour microenvironment to paracrine GLI signalling in pancreatic carcinogenesis.31 ,36 ,37 These observations may be of therapeutic relevance since pharmacological inhibition of HH–GLI signalling in a mouse model of pancreatic cancer enhances chemotherapy delivery.38 There is an emerging body of experimental work highlighting the role of GLI in pancreatic carcinogenesis independently of HH.38 ,39 Recent reports demonstrate that KRAS regulates GLI activity by increasing GLI1 expression and protein stability and cooperates with GLI2 during pancreatic carcinogenesis.39–41 Other studies have found that GLI factors are regulated in pancreatic cancer cells through a SMO-independent mechanism by TGFβ.39 Activation of this signalling pathway leads to an increase in expression of both GLI1 and GLI2 in pancreatic cancer cells.42 Hence, numerous lines of evidence support a role for GLI proteins in pancreatic cancer and suggest that this family of transcription factors is a suitable therapeutic target for this dismal disease.

Current strategies to target oncogenic transcription factor in pancreatic cancer

Surgical resection to date represents the best therapy for long-term survival in pancreatic cancer. At diagnosis, however, only 15% of the patients are amenable to surgical resection43 and alternative treatment options mostly fail because of resistance to chemotherapy or radiation.44 Currently, the only approved treatments are gemcitabine and erlotinib, both of which are not effective in the majority of patients and, most notably, the benefit on survival is measured in weeks.45 On the basis of the central role and intensive cross-talk of inflammatory transcription factors in signalling networks aimed at maintaining the transformed phenotype in pancreatic cancer, these nuclear molecules represent suitable nodes for therapeutic intervention. In the following paragraph, we discuss a few examples of current studies aimed at inhibiting the activity of these oncogenic transcription factors and their target genes in pancreatic carcinogenesis (figure 1 and table 1). To date, there are no effective strategies to directly target a transcription factor,46 rather the discussed compounds target upstream signalling pathways to block the activation of the respective transcription factor and its signalling.

Table 1

Clinical trials targeting inflammatory signalling pathways in pancreatic cancer

Dietary ingredients such as curcumin, quercetin and isoflavone genistein have been shown to efficiently suppress NFκB machinery and in turn exert antitumour and proapoptotic effects in numerous murine tumour models and in phase II trials where they potentiated antitumour activity of gemcitabine.47–51 Synthetic terpenoids have proven to efficiently block NFκB as well as STAT3 activation and subsequently prolonged survival in a transgenic mouse model of pancreatic cancer. Interestingly, one proposed mechanism mediating its antitumour properties is blockade of NFκB–STAT3 cross-talk.52 Anti-inflammatory compounds such as aspirin and sulindac further inhibited NFκB in a genetically engineered mouse model and thus delayed the progression of PanINs and cancer formation, highlighting the significance of inflammation-induced transcription factors and their networks in pancreatic cancer progression.53 ,54 The effects of aspirin on tumorigenesis remain to be further elucidated, as other studies could not demonstrate inhibitory effects on cancer growth,55 rather suggesting a tumour-preventive function. Finally, several proteasome inhibitors (eg, bortezomib), which prevent the degradation of IκB and in turn NFκB release, could efficiently block tumour growth in preclinical models with and without chemotherapy.56

Genetic silencing of STAT3 suppresses tumour growth, invasion and metastasis in a pancreatic cancer xenograft model and therefore underscores the requirement of STAT3 for pancreatic cancer growth.57 Several compounds inhibiting the activation of STAT3 in pancreatic cancer cells have been evaluated in preclinical models. As the mechanism of persistent STAT3 activation in tumours has been attributed to phosphorylation by tyrosine kinases such as JAK and SRC family kinases, or epidermal growth factor receptor (EGFR), most studies aim to assess inhibitors of upstream activating pathways. Treatment with AZD0530, a SRC inhibitor, thus resulted in downregulation of p-STAT3 and significantly inhibited tumour growth in pancreatic cancer mouse models.58 These findings were confirmed by Nagaraj et al, who used dasatinib to block SRC kinase in vivo. In addition, combined inhibition of SRC and EGFR with dasatinib, erlotinib and gemcitabine synergistically blocked constitutively activated STAT3 and consequently growth in a mouse model of pancreatic cancer.59

The genetic knockdown of NFAT remarkably reduced in vivo tumour growth of pancreatic cancer cells.29 ,30 Two widely used immunosuppressive agents, cyclosporine A (CsA) and FK506, act as potent calcineurin–NFAT inhibitors. Their use in cancer therapy is limited, however, because of large side effects and the fact that long-term application is accompanied by increased incidence of cancer. Consequently, a tumour-targeted therapy is required. Several inhibitors structurally and functionally related to CsA and FK506 that exhibit fewer side effects have been developed, but remain to be tested in terms of efficacy, stability and bioavailability in preclinical tumour models.27 Nonetheless, small-molecule inhibitors of NFAT are preferred for clinical use. Further strategies to block NFAT activity in pancreatic cancer include targeting of endogenous activators of NFAT, such as glycogen synthase kinase-3 (GSK-3) (unpublished work). Recent studies revealed that the inhibition of GSK-3 kinase activity disrupts STAT3–NFAT interaction and consequently reduces NFAT transcriptional activity and cell growth in pancreatic xenograft mouse models. Moreover, it acts to stabilise NFAT protein and accordingly enhances oncogenic NFAT functions (unpublished work). GSK-3 is also a potent modulator of NFκB activity in gastrointestinal cancers. Several authors have confirmed the abrogation of NFκB activity and target gene transcription upon pharmacological GSK-3 depletion in mouse models of pancreatic cancer.60 ,61 Since chronic inflammation represents a risk factor for the development of pancreatic cancer, inflammatory pathways should be considered as critical therapeutic targets. Notably, GSK-3 regulates the transcription factors, NFκB, STAT, NFAT and possibly GLI, in different cells present within the tumour microenvironment.62 As these transcription factors also correlate with inflammatory responses, they emerged as potential drug targets in inflammatory diseases. Inflammation is often linked to carcinogenesis, and therefore these proteins primarily represent key modulators of inflammatory processes promoting cancer development. This renders GSK-3 an interesting tool for repressing (inflammation-mediated) carcinogenesis.

The use of chemical HH–GLI pathway inhibitors, such as cyclopamine and its derivatives, in clinical trials (GDC-0449 and IPI-926) demonstrates a reduced tumour burden and metastasis in mouse models.36–39 ,63 Furthermore, the effect of an SMO antagonist on the efficacy of chemotherapy was recently analysed in a genetic mouse model of pancreatic cancer. Interestingly, delivery and efficacy of gemcitabine could be improved, which was mainly mediated through depletion of tumour-associated stromal tissue. These findings most notably indicate a contribution of the HH–GLI pathway to inflammation-linked chemoresistance.36 It has also been shown that NFκB directly regulates HH expression in a model of inducible NFκB activation in pancreatic acinar cells. Therein, genetic silencing of HH significantly reduced the IKK2-mediated increase in in vivo tumour growth, which suggests an intense interaction between NFκB and HH signalling pathways in oncogenesis64 and supports the rationale of a combination therapy targeting these two oncogenic pathways in pancreatic cancer.

In summary, oncogenic transcription factor proteins are powerful molecules involved in the regulation of cell proliferation, differentiation and apoptosis. Furthermore, these factors have been demonstrated to play an important role in the pathogenesis of pancreatic cancer. Although direct interference with the activity of transcription factors would represent the most specific strategy, it remains a challenging and, moreover, a unidirectional task46 because of intensive cross-talk of inflammatory transcription factors. As numerous oncogenic and inflammatory pathways converge on these factors, the targeting of central upstream regulators, and subsequently the complete network, appears feasible. This, in contrast with isolated chemotherapy, could block the effects of multiple pathways that contribute to malignancy. Targeting transcription factors such as NFκB or STAT3 that moreover mediate resistance to apoptosis13 and chemoresistance,65 a common feature of pancreatic cancer, would offer a favourable option. Nonetheless, there are still limitations for the application of the aforementioned drugs. At this time, some display low bioavailability and, as a consequence, low tumour tissue levels in vivo.50 As STAT3, NFκB or NFAT factors exert global effects on functions and development of immune cells, efforts to suppress inflammatory signalling could nevertheless affect immunsurveillance and in turn tumour-suppressive responses on long-term application. Thus, understanding the role of transcription factors in modulating gene expression and pancreatic carcinogenesis mainly in terms of inflammation will lead to more effective clinical tools for diagnosis, prognosis, treatment and prevention strategies for this dismal disease. Table 1 summarises current and closed clinical trials evaluating inhibitors of NFκB, STAT3 and GLI transcription factors in the treatment of pancreatic cancer.

Acknowledgments

We thank Angela McCleary-Wheeler and Gaurav Aggarwal for critical reading of the manuscript and helpful advice. This work was generously supported by the Deutsche Forschungsgemeinschaft (to VE: KFO210, SFB-TR17), the LOEWE-Schwerpunkt ‘Tumour and Inflammation’ (to VE) and the Max-Eder program of the German Cancer Research Foundation (to VE: 70-3022-El I), Novartis Foundation (to VE), Schulze Center for Novel Therapeutics (to MEF-Z), Mayo Clinic Cancer Center (to MEF-Z), the National Institutes of Health CA136526 (to MEF-Z), Mayo Clinic Pancreatic SPORE P50 CA102701 (to MEF-Z), and Mayo Clinic Center for Cell Signalling in Gastroenterology P30 DK84567 (to MEF-Z).

References

View Abstract

Footnotes

  • Competing interests None.

  • Provenance and peer review Commissioned; externally peer reviewed.

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.