Prolonged activation of the mitogen-activated protein kinase pathway promotes DNA synthesis in primary hepatocytes from p21Cip-1/WAF1-null mice, but not in hepatocytes from p16INK4a-null mice

In primary rat hepatocytes, prolonged activation of the p42/44 mitogen-activated protein kinase (MAPK) pathway is associated with a decrease in DNA synthesis and increased expression of the cyclin-dependent kinase inhibitor (CKI) proteins p21^Cip-1/WAF1 and p16^INK4a. To evaluate the relative importance of these CKIs in mediating this response, we determined the impact of prolonged MAPK activation on DNA synthesis in primary cultures of hepatocytes derived from mice embryonically deleted (null) for either p21^Cip-1/WAF1 or p16^INK4a. When MAPK was activated in wild-type mouse hepatocytes for 24 h, via infection with a construct to express an inducible oestrogen receptor–Raf-1 fusion protein (ΔRaf:ER), the expression of p21^Cip-1/WAF1 and p16^INK4a CKI proteins increased, cyclin-dependent kinase 2 (cdk2) and cdk4 activities decreased, and DNA synthesis decreased. Inhibition of RhoA GTPase function increased the basal expression of p21^Cip-1/WAF1 and p27^Kip-1 but not p16^INK4a, and enhanced the ability of MAPK signalling to decrease DNA synthesis. Ablation of the expression of CCAATT enhancer-binding protein α (C/EBPα), but not of the expression of C/EBPβ, decreased the ability of MAPK signalling to induce p21^Cip-1/WAF1. When MAPK was activated in p16^INK4a-null hepatocytes for 24 h, the expression of p21^Cip-1/WAF1 increased, cdk2 and cdk4 activities decreased and DNA synthesis decreased. In contrast with these findings, prolonged activation of the MAPK pathway in hepatocytes from p21^Cip-1/WAF1-null mice enhanced cdk2 and cdk4 activities and caused a large increase in DNA synthesis, despite elevated expression of p16^INK4a. Inhibition of RhoA GTPase activity in p21^Cip-1/WAF1-null cells partly blunted both the basal levels of DNA synthesis and the ability of prolonged MAPK signalling to increase DNA synthesis. Expression of anti-sense p21^Cip-1/WAF1 in either wild-type or p16^INK4a-null hepatocytes decreased the ability of prolonged MAPK signalling to increase the expression of p21^Cip-1/WAF1, and permitted MAPK signalling to increase both cdk2 and cdk4 activities and DNA synthesis. These results argue that the ability of prolonged MAPK signalling to inhibit DNA synthesis in hepatocytes requires the expression of p21^Cip-1/WAF1, and that the increased expression of p16^INK4a has a smaller role in the ability of this stimulus to mediate growth arrest. Our results also suggest that RhoA function can modulate DNA synthesis in primary hepatocytes via the expression of p21^Cip-1/WAF1 and p27^Kip-1.