COMPLEX ROLE OF PROTEIN-PHOSPHORYLATION IN GENE ACTIVATION BY HYPOXIA

Mammalian cells are able to sense decreased oxygen tension in their en vironment and turn-on the expression of specific hypoxia responsive ge nes. The best studied of these hypoxia regulated genes is the one that encodes for erythropoietin, the glycoprotein hormone that regulates r ed cell production [1]. The response of the erythropoietin gene to hyp oxia is mediated by an enhancer sequence located at the 3' flanking re gion of the gene [2-4]. A hypoxia inducible DNA-binding protein comple x, termed HIF-1, regulates the transcriptional function of the erythro poietin enhancer [5]. Most interestingly, identical HIF-1 complexes al so control the responses of other hypoxia regulated genes, in what app ears to be a general mechanism of oxygen sensing and response [6, 7]. These other hypoxia regulated genes include vascular endothelial growt h factor, glycolytic enzymes such as pyruvate kinase and aldolase A, g lucose 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 comp onents of the HIF-1 DNA-binding complex [9, 10]. Their biochemical pur ification 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 a nd 94 kDa with a similar tryptic digestion composition. Cloning of the corresponding cDNAs showed that both subunits belong to a subfamily o f basic-helix-loop-helix (b-HLH) transcription factors containing a PA S 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 hydro carbon receptor nuclear translocator (ARNT) protein. These two protein s appear to form a heterodimer complex which then interact with the pu tative hypoxia-enhancer sequences. The mechanisms involved in the hypo xic induction of this DNA-binding complex are still not clear.