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 antibody a profound decrease of the p53 expression to a barely detectable level in PARP knock-out cells. Surprisingly, longer exposure of immunoblots revealed an immunoreactive band at about 75 kD which was stronger in PARP-deficient cells than in wt cells and was not affected upon doxorubicin treatment. The size of the PAb421 immunoreactive protein and the lack of its inducibility in response to DNA damage resembled those of p73, the first described p53 homologue. Therefore, we examined the reactivity of anti-p53 antibodies with in vitro translated p73 protein. Interestingly, p73 was efficiently immunoprecipitated with distinct antibodies recognizing the carboxy-terminus of p53. In Northern blots we observed p73 signals of comparable intensity in controls and PARP-deficient cells. We conclude that elevated expression of p73 may compensate the reduced level of p53 in PARP-deficient cells.