Transforming growth factor-β1 inhibits the growth of primary adult rat hepatocyte cultures by increasing cAMP levels

We investigated the mechanisms of transforming growth factor-beta 1 (TGF-beta 1) inhibition on transforming growth factor-alpha (TGF-alpha)-induced DNA synthesis and proliferation in primary cultures of adult rat hepatocytes. TGF-alpha (1.0 ng/ml) produced a 4.2-fold elevation of DNA synthesis during 3 h of culture and a 1.2-fold increase in nucleus number (proliferation) during 4 h of culture. TGF-beta 1 dose dependently inhibited the TGF-cr-induced hepatocyte DNA synthesis and proliferation: half-maximal inhibition occurred at a TGF-beta 1 concentration of 0.08 ng/ml. The inhibitory effects of 1.0 ng/ml TGF-beta 1 were almost completely reversed by adenylate cyclase inhibitors, 2,4-dideoxyadenosine (10(-6) M), and somatostatin (3 x 10(-7) M): or by a specific inhibitor of protein kinase A, H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride; 10(-7) M). In addition, while TGF-alpha did not affect the basal cellular adenosine 3',5'-monophosphate (cAMP) levels, TGF-beta 1 was found to produce dose-dependent increases in cellular cAMP levels. The cAMP-elevating effects of TGF-beta 1 were also reversed by 2,4-dideoxyadenosine (10-6 M), and somatostatin (3 x 10(-7) M), but not by H-89 (10(-7) M). The present results suggest that the specific mechanisms involved in the growth inhibitory effect of TGF-beta 1 on TGF-alpha-induced hepatocyte DNA synthesis and proliferation are via stimulation of adenylate cyclase, which increases intracellular cAMP and subsequently activates protein kinase A. (C) 1999 Elsevier Science B.V. All rights reserved.