Upregulation of mdm-2 expression in Meth A tumor cells tolerating wild-type p53

Overexpression of mouse wild-type p53 (wt p53) in mouse Meth A tumor cells after transfection of wt p53 encoding vectors induced a strong growth-inhibitory response. Cells of only few of randomly selected surviving colonies contained and expressed the transfected wt p53 specific DNA. Despite expressing authentic wt p53, such cells (MethAp53wt) exhibited a similar phenotype as the parental Meth A cells. These cells overexpressed the mdm-2 (mouse double minute-2) gene, both at the RNA and at the protein level. Recently, the MDM-2 protein has been identified as a cellular target of p53, which can abolish its tumor suppressor activity. We, therefore, suggest that MDM-2 has mitigated the growth-inhibitory effect of wt p53 in MethAp53wt cells. Upregulation of mdm-2 expression in MethAp53wt cells was mediated by wt p53, as analysis of Meth A cells carrying a tsp53 (p53Val135) revealed a strict dependence of mdm-2 upregulation upon wt p53 expression. Our results propose that a balanced ratio of MDM-2 and p53 will allow cells to tolerate a limited expression of wt p53. This tolerance is not mediated by a direct inactivation of wt p53 via complex formation with MDM-2, as the majority of both MDM-2 and wt p53 in MethAp53wt cells was not complexed to each other.