Pancreatic stellate cells (PSCs) are implicated as key mediators of pancreatic fibrogenesis and are found in increased numbers in areas of pancreatic injury. This increase in number may be due to increased local proliferation and/or migration of PSCs to affected areas from surrounding tissue. We have recently shown that PSCs can migrate and that this migration is stimulated by PDGF in a predominantly chemotactic manner [Gut 52 (2003) 677]. However, the signalling mechanisms responsible for PDGF-induced PSC migration are not known.
Aims: (i) To determine whether PDGF-induced PSC migration is mediated by the PI3-kinase pathway. (ii) To investigate whether cell migration is influenced by cell proliferation and whether an interaction exists between the PI3-kinase pathway and the ERK1/2 pathway (known to mediate cell proliferation) in PSCs exposed to PDGF.
Methods: (i) PI3-kinase activity was assessed by measuring the activation (phosphorylation) of its downstream substrate Akt in rat PSCs incubated with PDGF (10ng/mL) for 5min, 15min, 60min, and 24hr in the presence or absence of the specific PI3-kinase inhibitor wortmannin. (ii) The role of the PI3-kinase pathway in PSC migration was examined by assessing PSC migration through a porous membrane after exposure to PDGF in the presence and absence of wortmannin for 24hr. (iii) The relationship between migration and proliferation was assessed by examining migration of PSCs exposed to PDGF in the presence and absence of mitomycin C, an inhibitor of cell proliferation. (iv) The interaction between PI3-kinase and ERK1/2 was examined by incubation of PSCs with PDGF in the presence and absence of wortmannin, followed by assessment of ERK1/2 activation by western blot.
Results: PDGF increased Akt activation in PSCs as early as at 5min of incubation and this increase was sustained for 24hr. Inhibition of PI3-kinase by wortmannin decreased basal as well as PDGF-induced migration and also inhibited ERK1/2 activation. Inhibition of PSC proliferation with mitomycin C significantly reduced (but did not abolish) basal and PDGF-induced PSC migration.
Conclusions: (i) The PI3-kinase pathway is induced in PSCs after exposure to PDGF and this induction is sustained for at least 24hr. (ii) The PI3-kinase pathway plays a role in PDGF-induced PSC migration and is partially involved in mediating ERK1/2 activation. (iii) PSC migration is dependent, at least in part, on cell proliferation.