Bypass of Abnormal MDM2 Inhibition of p53-dependent Growth Suppression1

Oncoprotein MDM2 inhibits p53-dependent cell cycle arrest and apoptosis. MDM2-overexpressing human cancer cell lines (n = 3) were found to be resistant to growth inhibition after infection by p53-expressing adenovirus (AdPS3), as compared to low MDM2-expressing tumors (n 3), in vitro. The growth of MDM2-overexpressing tumors, however, was inhibited by p21-expressing adenovirus (Ad-p21) infection, and the cyclin-dependent kinase-inhibitory region of p2! was sufficient to bypass the MDM2-p53 feedback loop. The phosphorylation state of Rb correlated with the response to either p53 or p21 gene therapy. MDM2-overexpressing cancer cells infected by Ad-p21 also developed a quiescent barge-cell morphology. The results suggest that MDM2-mediated resistance to p53 may be bypassed by p21 and that the Rb phosphorylation state may predict the effects on growth after Ad-p53 or Ad-p21 infection.