ABSTRACT: Hypoxia Inducible Factor (HIF-1) is the master regulator of the transcriptional program of mammalian cells in response to hypoxia conditions. Under normoxic conditions, the HIF-1a subunit is a very short lived protein, which is ubiquitinated for proteosomal degradation by the von Hippel-Lindau protein (pVHL)-E3-ubiquitin ligase complex.. During hypoxia HIF1a degradation slows down, increasing the amount of HIF1a and the rate of transcription of the genes regulated by HIF1a. Previous reports have shown that HIF1a deubiquitination is an important regulatory step in this process. USP20 and USP33, two members of the ubiquitin specific protease family have been suggested to play a critical role in pVHL-mediated processes, including stabilization of hypoxia-inducible factor 1a (HIF-1a). Here we demonstrate that in hypoxic cells lowering of the USP33 level reduces the amount of HIF1a protein level and transcription of HIF1a-regulated genes. USP33 level itself was not controlled by the pVHL complex. On the other hand, in normoxic cells the residual level of HIF1a protein was apparently not controlled by the oxygen-regulated pVHL-E3 ligase complex. Perhaps, HIF1a is maintained at a basal level in normoxic cells possibly to support non-hypoxia-related functions of the genes regulated by HIF1a. Knock-down of USP33 reduced proliferation of U2OS and HCT116 cancer cells, but this occurred without changing the concentration of HIF1a. USP33 may, therefore, in normoxic be involved in the regulation of pathways that are not regulated by HIF1.
FEBS Letters 01/2011; · 3.54 Impact Factor