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/

Bailie et al, [In Vitro Cell Dev, Biol, (1992) 28A, 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 (2 0 ng/ml, 0.8 nM) caused acute phasic activation of Raf-1 and p42(MAPki nase), and a smaller sustained activation of B-Raf. The transient incr ease in Raf-1 and p42(MAPkinase) activity returned to baseline within similar to 30 min. NGF treatment of hepatocytes did not induce express ion of cyclin dependent kinase (cdk) inhibitor proteins, but instead s timulated cdk2 activity and increased [H-3]thymidine incorporation int o DNA, In contrast to hepatocytes, NGF treatment of PC12 pheochromocyt oma cells caused large sustained activations of B-Raf and p42(MAPkinas e), and a lower phasic activation of Raf-1, The sustained activations of B-Raf and p42(MAPkinase) were for more than 5 h, Treatment of PC12 cells with NGF increased p21(Cip/WAF-1) expression, reduced cdk2 activ ity and inhibited DNA synthesis, the opposite to the effects of NGF tr eatment of hepatocytes, However when p42(MAPkinase) was chronically ac tivated in hepatocytes, via infection with an inducible oestrogen rece ptor-Raf-1 fusion protein, expression of p21(Cip-1/WAF1) and p16(INK4a ) cdk inhibitor proteins increased, cdk2 activity decreased, and DNA s ynthesis decreased. Equally, treatment of hepatocytes with 50 mM ethan ol elevated the basal activity of p42(MAPkinase) and temporally extend ed the ability of NGF treatment to activate p42(MAPkinase). Ethanol an d NGF co-treatment increased expression of p21(Cip-1/WAF1) and p16(INK 4a) cdk inhibitor proteins and decreased hepatocyte DNA synthesis. The se data demonstrate that NGF can cause either acute/phasic or sustaine d activation of the MAP kinase cascade in different cell types. Acute activation of the MAP kinase cascade correlated with increased DNA syn thesis. In contrast, sustained activation of the MAP kinase cascade co rrelated with increased expression of cdk inhibitor proteins, a reduct ion in cdk activity, and an inhibition of DNA synthesis. These data su ggest a general mechanism exists where acute activation of the MAP kin ase cascade promotes G1 progression/S phase entry and that chronic act ivation of the MAP kinase cascade inhibits this process.