The role of thrombocytes in liver fibrogenesis: effects of platelet lysate and thrombocyte-derived growth factors on the mitogenic activity and glycosaminoglycan synthesis of cultured rat liver fat storing cells.

A central problem in the study of the pathogenesis of liver fibrosis (fibrogenesis) is the identification of the cellular sources of the extracellular matrix and the dissection of the molecular mediators stimulating connective tissue synthesis in certain hepatic target cells. In the present study the role of platelets and of some platelet-derived polypeptide growth factors in the proliferation and proteoglycan synthesis of rat liver fat storing cells in culture (the principle connective tissue-producing cell type in liver) was determined. Fat storing cell proliferation was determined by measurement of the DNA-content, and [3H]thymidine- and bromodeoxyuridine-incorporation. Glycosaminoglycan synthesis was determined by the measurement of [35S]sulphate incorporation. Human platelet lysate (0.3 to 2.6 g protein per litre medium) stimulated, in a dose-dependent manner, both the proliferation and glycosaminoglycan synthesis of rat liver fat storing cells kept as a primary culture in Dulbecco's modification of Eagle's medium in the absence of foetal calf serum. More than 70% of the newly synthesized glycosaminoglycans were found in the medium. Among the various thrombocyte-derived polypeptides tested as candidate mediators of the platelet-derived fibrogenic activity, platelet derived growth factor was not effective in enhancing glycosaminoglycan synthesis, and it stimulated the proliferation of fat storing cells only about 2 fold. On the other hand, epidermal growth factor proved to be a stimulus of both processes. Transforming growth factor beta (greater than 10 pmol/l) inhibited foetal calf serum (Dulbecco's modification of Eagle's medium with a fraction of foetal calf serum of 0.1) and epidermal growth factor stimulated proliferation but enhanced the synthesis of sulphated glycosaminoglycans about 2-fold. These results suggest the possible role of transforming growth factor beta as a negative modulator for fat storing cells proliferation but a positive modulator for fat storing cell transformation and extracellular glycosaminoglycan matrix synthesis. Furthermore, our results indicate a cooperation between different hepatic and extrahepatic cell types by paracrine stimulation of fat storing cells. Transforming growth factor beta in combination with epidermal growth factor appear to be candidate mediators of the platelet-derived fibrogenic activity, which stimulates fat storing cells in culture, and might also be effective in vivo during hepatic repair processes following liver injury.