Inhibition of rat hepatocyte proliferation by transforming growth factor β and glucagon is associated with inhibition of ERK2 and p70 S6 kinase



Stimulation of hepatocyte proliferation by epidermal growth factor (EGF) and insulin is inhibited by transforming growth factor β (TGF-β) and by glucagon. It is also suppressed by inhibitors of various protein kinases, including rapamycin, which blocks activation of p70 S6 kinase (p70S6k), PD98059, which inhibits the activation of extracellular-regulated kinase (ERK), and SB 203580, an inhibitor of the p38 mitogen-activated protein kinase (p38 MAPK). In this study, we investigated whether the inhibition of proliferation by TGF-β involves these protein kinase cascades. Culture of hepatocytes with TGF-β for 16 hours decreased the stimulation by EGF of ERK2 and p70S6k (by 50% and 35%, respectively), but did not affect the stimulation of either p38 MAPK, c-jun NH2 -terminal kinase (JNK), or protein kinase B (PKB). Culture of hepatocytes with glucagon for 16 hours also inhibited the stimulation by EGF of activation of ERK2 and p70S6k (by ≈50%). The inhibitory effects of glucagon were observed when the hormone was added either 10 minutes or 60 minutes before EGF addition, whereas no effects of TGF-β were observed after 10-minute or 60-minute incubation. These results suggest that the inhibition of hepatocyte proliferation by TGF-β may be in part mediated by inhibition of ERK2 and p70S6k, but does not involve PKB, JNK, or p38 MAPK. Unlike glucagon, the effects of TGF-β are not elicited in response to short-term treatment