The activation of the AKT/protein kinase B kinases by mutation of the PTEN lipid phosphatase results in enhanced survival of a diversity of tumors. This resistance to apoptosis is partly accomplished by the inhibition of genetic programs induced by a subfamily of forkhead transcription factors including AFX. Here we describe an AFX-regulated pathway that appears to account for at least part of this apoptotic regulatory system. Cells induced to synthesize an active form of AFX die by activating the apoptotic death pathway. An analysis of genes regulated by AFX demonstrated that BCL-6, a transcriptional repressor, is up-regulated ∼4–7-fold. An examination of the BCL-6 promoter demonstrated that AFX bound to specific target sites that could activate transcription. BCL-XL, an anti-apoptotic protein, contains potential BCL-6 target sites in its promoter. An analysis of endogenous BCL-XL levels in AFX-expressing cells revealed enhanced down-regulation of the transcript (∼1.3–1.7-fold) and protein, and BCL-6 directly binds to and suppresses theBCL-XL promoter. Finally, macrophages isolated from BCL-6−/− mice show enhanced survivalin vitro. These results suggest that AFX regulates apoptosis in part by suppressing the levels of anti-apoptotic BCL-XL through the transcriptional repressor BCL-6.
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Published, JBC Papers in Press, January 2, 2002, DOI 10.1074/jbc.M110901200
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