At present, the cell-cell interactions and molecular mechanisms of pancreas fibrogenesis are largely unknown. The purpose of this study was to investigate paracrine stimulatory loops between platelets and pancreatic stellate cells (PSC). Human PSC were obtained by outgrowth from fibrotic human pancreas. Native platelet lysate (nPL) and transiently acidified platelet lysate (aPL) were added to cultured PSC (passage 4 to 7) in the absence of serum. The synthesis of collagen types I and III and c-fibronectin (cFN) was demonstrated on protein (immunofluorescence and quantitative immunoassay) and mRNA (Northern blot) level. Using sections of human pancreas with acute pancreatitis, platelet aggregates in capillaries were demonstrated by transmission electron microscopy. nPL, and to an even greater extent aPL, significantly increased the synthesis of collagen types I and III and of c-FN (120 microl/ml aPL increased collagen type I concentration in PSC supernatants by 1.99 +/- 0.17 times and c-FN of 2.49 +/- 0.28 times, mean +/- SD, n = 3). nPL and aPL also significantly stimulated cell proliferation (increased bromodeoxyuridine (BrdU) incorporation by 6.4 +/- 0.78 times and 10 +/- 0.29 times, respectively). By preincubating aPL with transforming growth factor beta (TGFbeta)- and platelet-derived growth factor (PDGF)-neutralizing antibodies and the TGFbeta-latency associated peptide, respectively, TGFbeta1 was identified as the main mediator stimulating matrix synthesis and PDGF as the responsible mitogen. Our data demonstrate that platelets contain fibrogenic mediators that stimulate proliferation (PDGF) and matrix synthesis (TGFbeta1) of cultured PSC. We suggest that platelets and PSC cooperate in the development of pancreas fibrosis.