Gfi-1 Represses CDKN2B Encoding p15^INK4B through Interaction with Miz-1

Zinc finger (ZF) transcriptional repressor Gfi-1 plays an important role in hematopoiesis and inner ear development, and also functions as an oncoprotein that cooperates with c-Myc in lymphomagenesis. Gfi-1 represses transcription by directly binding to the consensus DNA sequence in the promoters of its target genes. We report here an alternative mechanism by which Gfi-1 represses CDKN2B encoding the cyclin-dependent kinase inhibitor p15^INK4B. Gfi-1 did not directly bind to CDKN2B, but interacted with Miz-1 and, via Miz-1, was recruited to the core promoter of CDKN2B. The C-terminal zinc finger domains of Gfi-1 and Miz-1 are involved in the interaction. Miz-1 is a POZ-ZF transcription factor that has been shown to mediate transcriptional repression by c-Myc. Like c-Myc, upon recruitment to the CDKN2B promoter, Gfi-1 repressed transcriptional activation of CDKN2B by Miz-1 and in response to TGFb. Notably, Gfi-1 and c-Myc formed a ternary complex with Miz-1 and were both recruited to the CDKN2B core promoter via Miz-1, and acted in collaboration to repress CDKN2B. Consistent with its role in repressing CDKN2B transcription, knockdown of Gfi-1 in human leukemic cells resulted in augmented levels of p15^INK4B, which was associated with attenuated cell proliferation. The expression of p15^INK4B was also significantly higher in Gfi-1^−/− mouse bone marrow (BM) cells than in Gfi-1^+/+ BM cells. Our data reveal a novel mechanism of transcriptional repression by Gfi-1 and also identify CDKN2B as a new Gfi-1 target gene. The findings may have important implications for understanding the role of Gfi-1 in normal development and the cooperation between Gfi-1 and c-Myc in lymphomagenesis.