Compensatory expression of p73 in PARP-deficient mouse fibroblasts as response to a reduced level of regularly spliced wild-type p53 protein

We have investigated the effect of PARP gene inactivation on the expression of wild-type (wt) p53 protein. Using immortalized fibroblasts from control and PARP knock-out mice we have found by immunoblotting with the PAb421 an tibody a profound decrease of the p53 expression to a barely detectable lev el in PARP knock-out cells. Surprisingly, longer exposure of immunoblots re vealed an immunoreactive band at about 75 ED which was stronger in PARP-def icient cells than in wt cells and was not affected upon doxorubicin treatme nt. The size of the PAb421 immunoreactive protein and the lack of its induc ibility in response to DNA damage resembled those of p73, the first describ ed p53 homologue. Therefore, we examined the reactivity of anti-p53 antibod ies with in vitro translated p73 protein. Interestingly, p73 was efficientl y immunoprecipitated with distinct antibodies recognizing the carboxy-termi nus of p53. In Northern blots we observed p73 signals of comparable intensi ty in controls and PARP-deficient cells. We conclude that elevated expressi on of p73 may compensate the reduced level of p53 in PARP-deficient cells, (C) 1999 Academic Press.