The mitogen-activated protein (MAP) kinase cascade can either stimulate or inhibit DNA synthesis in primary cultures of rat hepatocytes depending upon whether its activation is acute/phasic or chronic

摘要

pBailie et al. [In Vitro Cell Dev. Biol. (1992) b28A/b, 621-624] reported that primary cultures of rat hepatocytes possess low affinity binding sites for nerve growth factor (NGF). NGF treatment of primary cultures of rat hepatocytes with a maximally effective concentration of NGF (20 ng/ml, 0.8 nM) caused acute phasic activation of Raf-1 and p42supMAPkinase/sup, and a smaller sustained activation of B-Raf. The transient increase in Raf-1 and p42supMAPkinase/sup activity returned to baseline within ~ 30 min. NGF treatment of hepatocytes did not induce expression of cyclin dependent kinase (cdk) inhibitor proteins, but instead stimulated cdk2 activity and increased [sup3/supH]thymidine incorporation into DNA. In contrast to hepatocytes, NGF treatment of PC12 pheochromocytoma cells caused large sustained activations of B-Raf and p42supMAPkinase/sup, and a lower phasic activation of Raf-1. The sustained activations of B-Raf and p42supMAPkinase/sup were for more than 5 h. Treatment of PC12 cells with NGF increased p21supCip1/WAF-1/sup expression, reduced cdk2 activity and inhibited DNA synthesis, the opposite to the effects of NGF treatment of hepatocytes. However when p42supMAPkinase/sup was chronically activated in hepatocytes, via infection with an inducible oestrogen receptor-Raf-1 fusion protein, expression of p21supCip-1/WAF1/sup and p16supINK4a/sup cdk inhibitor proteins increased, cdk2 activity decreased, and DNA synthesis decreased. Equally, treatment of hepatocytes with 50 mM ethanol elevated the basal activity of p42supMAPkinase/sup and temporally extended the ability of NGF treatment to activate p42supMAPkinase/sup. Ethanol and NGF co-treatment increased expression of p21supCip-1/WAF1/sup and p16supINK4a/sup cdk inhibitor proteins and decreased hepatocyte DNA synthesis. These data demonstrate that NGF can cause either acute/phasic or sustained activation of the MAP kinase cascade in different cell types. Acute activation of the MAP kinase cascade correlated with increased DNA synthesis. In contrast, sustained activation of the MAP kinase cascade correlated with increased expression of cdk inhibitor proteins, a reduction in cdk activity, and an inhibition of DNA synthesis. These data suggest a general mechanism exists where acute activation of the MAP kinase cascade promotes G1 progression/S phase entry and that chronic activation of the MAP kinase cascade inhibits this process./p