Inhibition of Hepatocyte DNA Synthesis by Transforming Growth Factor β₁ and Cyclic AMP: Effect Immediately before the G₁/S Border¹

Previous studies have shown that both transforming growth factor β (TGF-β) and cyclic AMP (cAMP) inhibit hepatocyte DNA synthesis. While cAMP (in addition to being stimulatory in G₀/early G₁) exerts its inhibition on hepatocytes late in G₁, the point where TGF-β inhibits has not been precisely defined. We have now examined further the inhibitory effects of cAMP and TGF-β₁ on DNA synthesis in primary rat hepatocyte cultures and, in particular, tried to determine where in the prereplicative period the cells are sensitive to these agents. Although a transient exposure to TGF-β₁ (but not glucagon) during the first hours of the cell culturing led to inhibition of DNA synthesis, the cells were more sensitive at a point late in G₁, where they also were inhibited by cAMP. Thus, exposure to TGF-β₁, glucagon, or the cAMP analogue 8-chlorophenylthio-cAMP at a time when there was a continuous recruitment of cells to S phase strongly decreased the rate of S-phase entry. For both TGF-β₁ and cAMP the inhibition was established within 1–2 h, the lag time being indistinguishable for the two agents. No evidence was found for a synergism between TGF-β₁ and cAMP. Treatment with TGF-β₁ did not detectably alter basal or glucagon-stimulated cAMP concentrations. The results suggest that in hepatocytes there is a process immediately before the G₁/S border which is sensitive to both TGF-β₁ and cAMP and which appears to represent a major point of inhibition.