ERYTHROPOIETIN - A MODEL SYSTEM FOR STUDYING OXYGEN-DEPENDENT GENE-REGULATION

The physiological regulation of the red cell mass depends upon enhance d transcription of the erythropoietin (Epo) gene in response to hypoxi a, Studies of Epo gene expression have been useful in investigating th e mechanism by which cells and tissues sense hypoxia and respond with biologically appropriate alterations in gene expression. It is likely that oxygen sensing involves a heme protein in which cobalt and nickel can substitute for iron in the porphyrin ring. Indirect evidence sugg ests that the sensor is present in all cells and is a multi-subunit as sembly containing an NAD(P)H oxidase capable of generating peroxide an d reactive oxygen intermediates, which serve as signaling molecules, T he up-regulation of Epo gene transcription by hypoxia is mediated by a t least two known DNA-binding transcription factors, hypoxia-inducible factor 1 (HIF-1) and hepatic nuclear factor 4 (HNF-4), which bind to cognate response elements in a critical 3' enhancer approximately 50 b p in length. HIF-1 binding is induced by hypoxia as well as by cobalt. The activation of HIF-1 by hypoxia depends upon the selective protect ion of its a subunit from ubiquitin-dependent proteolysis by means of a mechanism that involves redox chemistry and perhaps phosphorylation. HNF-4 is an orphan nuclear receptor that is constitutively expressed in kidney and liver and which cooperates with HIF-1 to give maximal hy poxic induction. In hypoxic cells, p300 or a related family member for ms a macromolecular assembly with HIF-1 and HNF-4, enabling transducti on from the Epo 3' enhancer to the apparatus on the promoter responsib le for the initiation of transcription.