Loss Of P53 Function Enhances Diethylnitrosamine-induced Activation Of Activator Protein 1 In Hepatic Cells

Transcription factor p53 is a tumor suppressor that plays essential role in suppression of carcinogenesis. Although hepatocellular carcinoma (HCC) arises from a combination of mutations in oncogenes and tumour suppressor genes including p53, the exact mechanism by which p53 mediates anticancer activity and the relationship between p53 and HCC are not well understood. In this study, we found that pretreatment of human hepatic L02 cells with pifithrin-α(PFTα), an inhibitor for p53-dependent transcriptional activation, resulted in a marked increase in diethylnitrosamine(DEN) -induced activation of activator protein 1 (AP-1) in a dose-and time-dependent manner. Then we found that pre-treatment of L02 cells with c-Jun-NH2-kinases (JNKs) inhibitor, SP600125, resulted in a striking inhibition of AP-1 induction by DEN, implicating the role of JNKs in the induction. However, pre-treatment of L02 cells with p38 inhibitor, SB202190 and extracellular signal-regulated kinases (ERKs) inhibitor, PD98059 did not show obvious decrease in DEN induced AP-1 expression. At last we detect the potential roles of JNKs, p38 kinase and ERKs in AP-1 induction by DEN in L02 cells. Consistent with activation of AP-1, pifithrin-αwas also able to enhance the DEN-induced phosphorylation of c-Jun-NH2-kinases (JNKs), whereas phosphorylation of p38 kinase and ERKs were barely affected. In addition, knockdown of p53 expression by its small interfering RNA also caused the elevation of AP-1 activation and JNKs phosphorylation induced by DEN. All these results show that p53 has a suppressive activity on the cell signaling pathways leading to activation of AP-1 in cell response to DEN. More importantly, since AP-1 cascade and its cofactor network represent a pivotal molecular circuitry participating directly or indirectly in carcinogenesis, taken together with the evidence that p53 mutations are common in HCC, we anticipate that AP-1 transcription factor may play a critical role in the carcinogenesis of p53 deficient cells induced by DEN.