Prostaglandin E2 (EP1) receptor agonist-induced DNA synthesis and proliferation in primary cultures of adult rat hepatocytes:: The involvement of TGF-α

We investigated the effects of prostaglandin (EP) receptor subtype agonists on DNA synthesis and proliferation in primary cultures of adult rat hepatocytes to elucidate their mechanisms of action. Maintained in short-term cultures (ie. 3.5 h) in a serum-free, defined medium, hepatocyte parenchymal cells underwent DNA synthesis and proliferation in the presence of sulprostone (10(-6) m), PGE(2) (10(-6) m), and 17-phenyl-trinor-PGE(2) (10(-9) ig) in a time- and dose-dependent man ner. PGE(2) was less potent than 17-phenyl-trinor-PGE2 in stimulating hepatocyte mitogenesis. Sulprogtone (10(-6) M) and 11-deoxy-PGE(1) (10(-6) M) showed weak and insignificant stimulation, respectively, for hepatocyte mitogenesis. These effects of PGE(2), 17-phenyl-trinor-PGE(2), and sulprostone were abolished by treatment with a specific EP1 receptor antagonist, SC-51322, or the PLC inhibitor U-73122. The effects of these EP1 receptor agonists were potentiated by ionomycin and blocked by verapamil. Hepatocyte mitogenesis was almost completely blocked by specific inhibitors of growth-related signal transducers, such as genistein, wortmannin, PD98059, and rapamycin. A monoclonal antibody against TGF-alpha dose-dependently inhibited PGE(2)- and 17-phenyl-trinor-PGE(2)-induced hepatocyte mitogenesis. Treatment with the EP1 receptor agonists significantly increased the secretion of TGF-alpha, reaching a maximum within 5 min. The increase in TGF-alpha secretion was blocked by SC-51322, U-73122, somatostatin, and verapamil and potentiated by ionomycin. These results indicate that the proliferative mechanisms of action of EP1 receptor agonists are mediated through an increase in the autocrine secretion of TGF-alpha, which is dependent on the EP1 receptor/G-protein involved in PLC regulation/PLC/Ca2+ system. The locally secreted TGF-alpha, in turn, acts as a complete mitogen that stimulates the tyrosine kinase/MAPK pathway in these cells.