Prostaglandin E-2 (EP1) receptor agonist-induced DNA synthesis and proliferation in primary cultures of adult rat hepatocytes: The involvement of TGF-alpha

We investigated the effects of prostaglandin (EP) receptor subtype agonists on DNA synthesis and proliferation in primary cultures of adult rat hepato cytes to elucidate their mechanisms of action. Maintained in short-term cul tures (ie. 3.5 h) in a serum-free, defined medium, hepatocyte parenchymal c ells underwent DNA synthesis and proliferation in the presence of sulprosto ne (10(-6) m), PGE(2) (10(-6) m), and 17-phenyl-trinor-PGE(2) (10(-9) ig) i n 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, r espectively, 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 effec ts of these EP1 receptor agonists were potentiated by ionomycin and blocked by verapamil. Hepatocyte mitogenesis was almost completely blocked by spec ific inhibitors of growth-related signal transducers, such as genistein, wo rtmannin, PD98059, and rapamycin. A monoclonal antibody against TGF-alpha d ose-dependently inhibited PGE(2)- and 17-phenyl-trinor-PGE(2)-induced hepat ocyte mitogenesis. Treatment with the EP1 receptor agonists significantly i ncreased the secretion of TGF-alpha, reaching a maximum within 5 min. The i ncrease in TGF-alpha secretion was blocked by SC-51322, U-73122, somatostat in, and verapamil and potentiated by ionomycin. These results indicate that the proliferative mechanisms of action of EP1 receptor agonists are mediat ed through an increase in the autocrine secretion of TGF-alpha, which is de pendent on the EP1 receptor/G-protein involved in PLC regulation/PLC/Ca2+ s ystem. The locally secreted TGF-alpha, in turn, acts as a complete mitogen that stimulates the tyrosine kinase/MAPK pathway in these cells.