Complex role of protein phosphorylation in gene activation by hypoxia

Mammalian cells are able to sense decreased oxygen tension in their environment and turn-on the expression of specific hypoxia responsive genes. The best studied of these hypoxia regulated genes is the one that encodes for erythropoietin, the glycoprotein hormone that regulates red cell production [1]. The response of the erythropoietin gene to hypoxia is mediated by an enhancer sequence located at the 3' flanking region of the gene [2-4]. A hypoxia inducible DNA-binding protein complex, termed HIF-1, regulates the transcriptional function of the erythropoietin enhancer [5]. Most interestingly, identical HIF-1 complexes also control the responses of other hypoxia regulated genes, in what appears to be a general mechanism of oxygen sensing and response [6, 7]. These other hypoxia regulated genes include vascular endothelial growth factor, glycolytic enzymes such as pyruvate kinase and aldolase A, glucose transporter 1 and endothelin, among others. All these genes are transcriptionally activated by hypoxia and also by transition metals such as cobalt (Go) and by iron chelators such as desferrioxamine (Dfx) [reviewed in 8]. Recently Wang and Semenza purified the protein components of the HIF-1 DNA-binding complex [9, 10]. Their biochemical purification revealed the presence of one subunit of about 120 kDa (HIF-1 alpha) and a second subunit, HIF-1 beta with polypeptides of 91, 93 and 94 kDa with a similar tryptic digestion composition. Cloning of the corresponding cDNAs showed that both subunits belong to a subfamily of basic-helix-loop-helix (b-HLH) transcription factors containing a PAS domain. HIF-1 alpha resulted to be a newly recognized member of the group while HIF-1 beta turn out to be the already described aryl hydrocarbon receptor nuclear translocator (ARNT) protein. These two proteins appear to form a heterodimer complex which then interact with the putative hypoxia-enhancer sequences. The mechanisms involved in the hypoxic induction of this DNA-binding complex are still not clear.