Molecular and cellular pharmacologyInvolvement of endogenous transforming growth factor-α in signal transduction pathway for interleukin-1β-induced hepatocyte proliferation

We studied the effects of interleukin (IL)-1β on DNA synthesis and cell proliferation in primary cultures of adult rat hepatocytes in order to elucidate the mechanisms of its action. Hepatocyte parenchymal cells maintained in a serum-free, defined medium synthesized DNA and proliferated in the presence of IL-1β (3-30 ng/ml), but not IL-1α (0.1-30 ng/ml) in a time- and dose-dependent manner. Specific inhibitors of growth-related signal transducers, such as AG1478, LY294002, PD98059, and rapamycin, completely abolished IL-1β-stimulated hepatocyte DNA synthesis and proliferation. Western blot analysis showed that IL-1β significantly stimulated mitogen-activated protein (MAP) kinase activation within 10 min. Addition of a monoclonal antibody against transforming growth factor (TGF)-α, but not a monoclonal antibody against insulin-like growth factor-I, to the culture dose-dependently inhibited IL-1β-induced hepatocyte mitogenesis. Culture medium TGF-α levels increased significantly within 3 min in response to IL-1β from baseline levels. Peak TGF-α levels (33 pg/ml) were reached at 10 min after IL-1β stimulation. These results indicate that the proliferative mechanism of action of IL-1β is mediated through an increase in autocrine secretion of TGF-α from primary cultured hepatocytes. Secreted TGF-α, in turn, acts as a complete mitogen to induce hepatocyte mitogenesis through the receptor tyrosine kinase/phosphatidylinositol 3-kinase/MAP kinase/mammalian target of rapamycin pathway.