IDENTIFICATION OF NOVEL DEUBIQUITINATING ENZYMES (DUBs) THAT REGULATE HYPOXIA INDUCIBLE FACTORS (HIF)

Background: Cancer cells are characterized by rapid proliferation and require adaptive metabolic responses to allow continued biosynthesis and cell growth in the setting of decreased oxygen (hypoxia) and nutrient availability. Hypoxia-inducible factors (HIFs) are a common link between adaptation to hypoxia, changes in cancer metabolism, and malignant progression. HIFs are primarily regulated through post-translational modification and stabilization. The major post-translation regulation of HIFs is ubiquitination and degradation through the ubiquitin proteasome system. Little is known on HIF-α deubiquitination, a process that involves deubiquitinating enzymes (DUBs) which remove ubiquitin from native conjugates. As multiple DUBs have been classified as oncogenes or tumor suppressors, our aim in this study is to identify novel DUBs that regulate HIFs.

Methods: We screened about 140 known DUBs for their effects on HIF transcriptional activity. We selected the most significant DUB and explored its molecular mechanism of regulation on HIF-1. In parallel, we examined the expression pattern of the specific DUB in human normal and tumor tissues by immunohistochemistry.

Results: We identified a new DUB, ubiquitin specific protease 40 (USP40), which increased HIF-1 activity and HIF-1α protein expression levels. Interestingly, USP40 does not influence HIF-1a ubiquitination level and mutations of USP40`s catalytic residues do not abolish its upregulation of HIF-1. Using IHC staining, we observed overexpression of USP40 in bladder, prostate, lung, colon and squamous cell carcinomas tissues.

Conclusions: Exploring the molecular mechanism of how USP40 regulates the HIF-1 pathway and its role in tumor progression may lead to the development of new therapeutic modalities.