Tumour suppressor HKR3 activates tumour suppressor ARF gene expression by interacting with p300 and antagonizing oncoprotein FBI-1 to inhibit cell proliferation

Abstract

Relatively uncharacterized POK family transcription factor HKR3 (Human Krüppel-related 3) has a POZ-domain in the N-terminus and 11 zinc-finger domain in the c-terminus half. The mapped HKR3 gene region is commonly rearranged (leiomyoma, leukemias) or deleted (neuroblastoma, melanoma, Merkel cell carcinomas, pheochromocytoma and breast and colon carcinomas). Correlation of deletion or rearrangement and HKR3 in human cancer suggested the role of HKR3 as a potential tumor suppressor. I investigated whether the HKR3 has a regulatory effect on cell cycle by controlling gene expression of the p53 pathway (ARF-MDM2-TP53-CDKN1A). HKR3 is an activator of the p53 pathway through activating transcription of ARF, p53, and CKDN1A genes. In particular, HKR3 potently activated transcription of tumor suppressor gene ARF by acting on the proximal promoter region (bp, -149 ~ +53) containing Sp1 and FBI-1 binding elements (FREs). HKR3 interacted with co-activator p300 to activate ARF, which increased the acetylation of histone H3 and H4 around the proximal promoter. Oligoucleotide pull-down assays and ChIP assays revealed that HKR3 interferes with FBI-1 binding to the proximal FREs and thereby lifted transcription repression by FBI-1, a proto-oncoprotein known to repress transcription of ARF. HKR3 inhibited cell proliferation, but did not induce apoptosis, suggesting that HKR3 is potentially a tumour suppressor.