A putative protein inhibitor of activated STAT (PIASy) interacts with p53 and inhibits p53-mediated transactivation but not apoptosis

The p53 protein has recently been reported to be capable of mediating apopt osis through a pathway that is not dependent on its transactivation functio n. We report here that the PIASy member of the protein inhibitor of activat ed STAT family inhibited p53's transactivation function without compromisin g its ability to induce apoptosis of the H1299 nonsmall cell lung carcinoma cell line. The p53 protein bound to PIASy in yeast two-hybrid assays and c oprecipitated in complexes with p53 in immunoprecipitates from mammalian ce lls. PIASy inhibited the DNA-binding activity of p53 in nuclear extracts an d blocked the ability of p53 to induce expression of two of its target gene s, Bax and p21(Waf1/Cip1), in H1299 cells. The block in p53-mediated induct ion of Bax and p21 was determined to be at the level of transactivation, si nce PIASy inhibited p53's ability to transactivate a p21/luciferase reporte r construct. PIASy did not effect the incidence of apoptosis in H1299 cells upregulated for p53. PIASy appears to regulate p53-mediated functions and may direct p53 into a transactivation-independent mode of apoptosis.