Cyclin kinase inhibitor p21 potentiates bile acid-induced apoptosis in hepatocytes that is dependent on p53

Prolonged activation of the mitogen-activated protein kinase (MAPK) pathway enhances expression of the cyclin kinase inhibitor p21 that can promote growth arrest and cell survival in response to cytotoxic insults. Bile acids can also cause prolonged MAPK activation that is cytoprotective against bile acid-induced cell death. Here, we examined the impact of bile acid-induced MAPK signaling and p21 expression on the survival of primary mouse hepatocytes. Deoxycholic acid (DCA) caused prolonged activation of the MAPK pathway that weakly enhanced p21 protein expression. When DCA-induced MAPK activation was blocked using MEK1/2 inhibitors, both hepatocyte viability and expression of p21 were reduced. Surprisingly, constitutive overexpression of p21 in p21+/+ hepatocytes enhanced DCA-induced cell killing. In agreement with these findings, treatment of p21-/- hepatocytes with DCA and MEK1/2 inhibitors also caused less apoptosis than observed in wild-type p21+/+ cells. Expression of p21 in p21-/- hepatocytes did not modify basal levels of apoptosis but restored the apoptotic response of p21-/- cells to those of p21+/+ cells overexpressing p21. These findings suggest that basal expression of p21 plays a facilitating, proapoptotic role in DCA-induced apoptosis. Overexpression of p21 enhanced p53 protein levels. In agreement with a role for p53 in the enhanced apoptotic response, overexpression of p21 did not potentiate apoptosis in p53-/- hepatocytes but, instead, attenuated the death response in these cells. In conclusion, our data suggest that overexpression of p21 can promote apoptosis, leading to elevated sensitivity to proapoptotic stimuli.