Stat1-Dependent, p53-Independent Expression of p21waf1 Modulates Oxysterol-Induced Apoptosis

Abstract

7-Ketocholesterol (7kchol) is prominent in atherosclerotic lesions where apoptosis occurs. Using mouse fibroblasts lacking p53, p21^waf1, or Stat1, we found that optimal 7kchol-induced apoptosis requires p21^waf1 and Stat1 but not p53. Findings were analogous in a human cell system. Apoptosis was restored in Stat1-null human cells when wild-type Stat1 was restored. Phosphorylation of Stat1 on Ser⁷²⁷ but not Tyr⁷⁰¹ was essential for optimum apoptosis. A neutralizing antibody against beta interferon (IFN-β) blunted Ser⁷²⁷ phosphorylation and apoptosis after 7kchol treatment; cells deficient in an IFN-β receptor subunit exhibited blunted apoptosis. IFN-β alone did not induce apoptosis; thus, 7kchol-induced release of IFN-β was necessary but not sufficient for optimal apoptosis. In Stat1-null cells, expression of p21^waf1 was much less than in wild-type cells; introducing transient expression of p21^waf1 restored apoptosis. Stat1 and p21^waf1 were essential for downstream apoptotic events, including cytochrome c release from mitochondria and activation of caspases 9 and 3. Our data reveal key elements of the cellular pathway through which an important oxysterol induces apoptosis. Identification of the essential signaling events that may pertain in vivo could suggest targets for therapeutic intervention.