The proto-oncoprotein KR-POK represses transcriptional activation of CDKN1A by MIZ-1 through competitive binding

Abstract

The BTB/POZ family of proteins has been implicated in multiple biological processes, including tumorigenesis, DNA damage responses and cell cycle progression and development. MIZ-1 (Myc-interacting zinc-finger protein 1) is known to activate transcription of CDKN1A. We recently found that a kidney cancer-related POK transcription factor, KR-POK, is highly expressed in kidney, brain, and bone marrow cancer tissues and is a potential proto-oncoprotein. Mouse Kr-pok represses transcription of the CDKN1A by acting on the proximal promoter. The BiFC/FRET assay, co-immunoprecipitation and GST-fusion protein pull-down assay indicate that MIZ-1 and Kr-pok interact via their POZ domains. Oligoucleotide pull-down assays and ChIP assays revealed that MIZ-1 binds to the proximal GC-box#3 (bp, -55 to -63) and the MIZ-1 binding elements, MRE-A (bp, -90 to -64) and MRE-B (bp, -27 to -17). Interestingly, MIZ-1 also binds to the distal p53 binding elements. Kr-pok binds to the proximal GC-box#1 (bp, -95 to -100) and #3 (bp, -55 to -63) relatively strongly. It also shows weak binding to the MREs and the distal p53 binding elements. Kr-pok competes with MIZ-1 in binding to these elements and represses transcription by inhibiting MIZ-1/p300 recruitment, which decreases the acetylation of histones H3 and H4. Our data indicate that Kr-pok stimulates cell proliferation by interfering with the function of MIZ-1 in CDKN1A gene transcription using a mechanism that is radically different from other MIZ-1 interacting proteins, such as BCL6, c-Myc, and Gfi-1.