Inhibition of hepatocyte DNA synthesis by transforming growth factor beta 1 and cyclic AMP: effect immediately before the G1/S border.
Previous studies have shown that both transforming growth factor beta (TGF-beta) 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-beta inhibits has not been precisely defined. We have now examined further the inhibitory effects of cAMP and TGF-beta 1 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-beta 1 (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-beta 1, 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-beta 1 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-beta 1 and cAMP. Treatment with TGF-beta 1 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-beta 1 and cAMP and which appears to represent a major point of inhibition.
Thoresen
G H
GH
Department of Pharmacology, Faculty of Medicine, University of Oslo, Norway.
Refsnes
M
M
Christoffersen
T
T
D000818:Animals; D000242:Cyclic AMP / Q000378:metabolism / Q000494:pharmacology*; D004247:DNA / Q000096:biosynthesis*; D016193:G1 Phase / Q000187:drug effects*; D005934:Glucagon / Q000494:pharmacology; D008099:Liver / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D016196:S Phase / Q000187:drug effects*; D016212:Transforming Growth Factor beta / Q000494:pharmacology*
D016212:Transforming Growth Factor beta; D004247:DNA; D005934:Glucagon; D000242:Cyclic AMP