The STAT-1 transcription factor has been implicated as a tumor suppressor by virtue of its ability to inhibit cell growth and promoting apoptosis. However, the mechanisms by which STAT-1 mediates these effects remain unclear. Using human and mouse STAT-1-deficient cells, we show here that STAT-1 is required for optimal DNA damage-induced apoptosis. The basal level of the p53 inhibitor Mdm2 is increased in STAT-1(-/-) cells, suggesting that STAT-1 is a negative regulator of Mdm2 expression. Correspondingly, both basal p53 levels, and those induced by DNA damage were lower in STAT-1(-/-) cells. In agreement with this lower p53 response to DNA damage in cells lacking STAT-1, the induction of p53 responsive genes, such as Bax, Noxa, and Fas, was reduced in STAT-1-deficient cells. Conversely, STAT-1 overexpression enhances transcription of these genes, an effect that is abolished if the p53 response element in their promoters is mutated. Moreover, STAT-1 interacts directly with p53, an association, which is enhanced following DNA damage. Therefore, in addition to negatively regulating Mdm2, STAT-1 also acts as a coactivator for p53. Hence STAT-1 is another member of a growing family of protein partners able to modulate the p53-activated apoptotic pathway.
This work was supported by the British Heart Foundation (British Heart Foundation Intermediate Fellowship; to A. S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
