EFFECTS OF P53 MUTANTS DERIVED FROM LUNG CARCINOMAS ON THE P53-RESPONSIVE ELEMENT (P53RE) OF THE MDM2 GENE

The present study represents a continuation of previous works in which we observed that lung carcinomas co-expressing MDM2 protein and p53 m utants (mt p53) exhibited more aggressive behaviour. In the above stud ies, we suggested a 'gain of function' mechanism of mt p53 proteins ba sed on the fact that the MDM2 gene possesses a p53-responsive element (MDM2-p53RE). In this study, to prove our hypothesis, we selected 12 c ases from a series of 51 bronchogenic carcinomas. In these 12 cases, w e examined the ability of the expressed mt p53 to bind the MDM2-p53RE and correlated the findings with MDM2 expression. Furthermore, we cons tructed four of these p53 mutants and studied their transactivation pr operties by co-transfecting them with a reporter plasmid carrying MDM2 -p53RE in the p53 null non-small-cell lung carcinoma cell line (NSCLC) H1299. We observed mutant p53 protein DNA-binding activity, which dep ended on the nature and the position of the amino acid substitution. T he fact that the cases with DNA-binding activity were accompanied with MDM2 protein isoforms' overexpression is indicative of a 'gain of fun ction' phenotype. This hypothesis was enforced by the findings of the transfection experiments, which revealed that certain p53 mutants enha nced the expression of the luciferase reporter gene either directly or indirectly via a dominant positive effect on the wild-type p53. In co nclusion, this work is one first attempt to examine if the deregulatio n of the p53/MDM2 autoregulatory feedback loop is due to novel propert ies of certain p53 mutants in the specific environment of a subset of bronchogenic carcinomas.