The MDM2 oncoprotein is overexpressed in rhabdomyosarcoma cell lines and stabilizes wild-type p53 protein

MDM2 gene overexpression has been implicated in the pathogenesis of human neoplasia via inhibition of the p53 tumor-suppressor function. To investigate the potential involvement of the MDM2 oncogene in the pathogenesis of childhood rhabdomyosarcoma (RMS), we studied MDM2 abnormalities in six RMS cell lines in correlation with the p53 status. Three showed overexpression of MDM2 mRNA and protein, one with concomitant MDM2 mRNA and protein, one with concomitant MDM2 gene amplification. All three lacked p53 mutation and expressed low levels of p53 mRNA but exhibited elevated p53 proteins. Double immunostaining revealed that the overexpressed MDM2 and p53 proteins were co-localized to the same cell nuclei. Furthermore, the two proteins were physically associated, as shown by co-immunoprecipitation and Western blot analysis. The half-life of the p53 protein was prolonged in the MDM2-overexpressing RMS cells. The extended half-life of the wild-type p53 protein and its complex formation with the elevated MDM2 suggest that the underlying mechanism for p53 protein accumulation in these cell lines is p53 stabilization by an overabundant MDM2 protein. The overexpressed MDM2 protein had a short half-life. The three remaining RMS cell lines exhibited low MDM2 mRNA and protein levels and carried p53 mutations. This study suggests that MDM2 overexpression represents an alternative mechanisms for p53 inactivation in a subset of childhood RMS without p53 mutations. The results further indicate that the elevated MDM2 protein is responsible for wild-type p53 protein accumulation via stabilization.