IFN regulatory factor 8 regulates MDM2 in germinal center B cells.

IFN regulatory factor 8 (IRF8) is a transcription factor that affects the differentiation and function of myeloid, dendritic, and B cells. Herein we report that IRF8 regulates the expression of Mdm2, a suppressor of p53-dependent and -independent apoptosis pathways, in germinal center (GC) B cells. In GC B cells of IRF8-deficient mice, Mdm2 transcripts were greatly down-regulated, and MDM2 protein was poorly expressed in GC of Irf8(-/-) mice. Small interfering RNA-induced repression of IRF8 in a GC-derived B cell line resulted in decreased expression of MDM2 at the protein level but increased expression of p53 and p21. We found that IRF8 binds to the Mdm2 P2 promoter, and that cotransfection of an IRF8 expression vector with an Mdm2 reporter construct stimulated significant increases in reporter activity. Additionally, transcripts of the p53 target Pmaip1 (Noxa) were significantly increased in IRF8-deficient GC B cells as well as in the IRF8 knockdown B cell line. Finally, cells deficient in IRF8 exhibited growth suppression and increased sensitivity to apoptosis induced by etoposide or IL-21. These results suggest that by regulating MDM2, IRF8 might allow GC B cells to tolerate physiological DNA breaks that otherwise would trigger apoptosis.