P53-DEPENDENT REGULATION OF MDR1 GENE-EXPRESSION CAUSES SELECTIVE RESISTANCE TO CHEMOTHERAPEUTIC-AGENTS

Loss of functional p53 paradoxically results in either increased or de creased resistance to chemotherapeutic drugs. The inconsistent relatio nship between p53 status and drug sensitivity may reflect p53's select ive regulation of genes important to cytotoxic response of chemotherap eutic agents, We reasoned that the discrepant effects of p53 on chemot herapeutic cytotoxicity is due to p53-dependent regulation of the mult idrug resistance gene (MDR1) expression in tumors that normally expres s MDR1, To test the hypothesis that wild-type p53 regulates the endoge nous mdr1 gene we stably introduced a trans-dominant negative (TDN) p5 3 into rodent H35 hepatoma cells that express P-glycoprotein (Pgp) and have wild-type p53. Levels of Pgp and mdr1a mRNA were markedly elevat ed in cells expressing TDN p53 and were linked to impaired p53 functio n (both transactivation and transrepression) in these cells. Enhanced mdr1a gene expression in the TDN p53 cells was not secondary to mdr1 g ene amplification and Pgp was functional as demonstrated by the decrea sed uptake of vinblastine. Cytotoxicity assays revealed that the TDN p 53 cell lines were selectively insensitive to Pgp substrates. Sensitiv ity was restored by the Pgp inhibitor reserpine, demonstrating that on ly drug retention was the basis for loss of drug sensitivity. Similar findings were evident in human LS180 colon carcinoma cells engineered to overexpress TDN p53. Therefore, the p53 inactivation seen in cancer s likely leads to selective resistance to chemotherapeutic agents beca use of up-regulation of MDR1 expression.