Inhibition of protein phosphatase activity induces p53-dependent apoptosis in the absence of p53 transactivation.

Inhibitors of type 1 and type 2A protein phosphatases were used to examine the involvement of protein phosphorylation in regulating the functions of endogenous p53. Exposure of Balb/c 3T3 cells to okadaic acid, an inhibitor of protein phosphatases 1 and 2A, increased the phosphorylation of p53 without changing p53 levels. Okadaic acid treatment enhanced the binding of p53 to a consensus DNA target sequence and caused a 5-8-fold increase in p53 transcriptional activity. Transient expression of SV40 small tumor antigen, a specific inhibitor of protein phosphatase 2A, caused a 4-fold increase in p53 transcriptional activity. Incubation of Balb/c 3T3 cells with okadaic acid also induced programmed cell death in a dose- and time-dependent manner. Decreases in viability, morphological changes, and the appearance of DNA fragmentation were dependent on p53 since cells lacking functional p53 were resistant to okadaic acid-induced apoptosis. The p53-dependent apoptosis induced by okadaic acid was rapid and did not require p53 transcriptional activity. The fact that SV40 small tumor antigen did not induce apoptosis provides additional evidence that p53 transcriptional activity is not sufficient for p53-mediated apoptosis. These results indicate that signaling pathways involving protein phosphorylation play critical roles in controlling the apoptotic activity of p53. Furthermore, a basal level of protein phosphatase 1 or 2A activity is necessary to prevent p53-dependent apoptosis.