Hypoxic induction of the Epo gene. (A) The Epo gene has 5 exons, represented by rectangles. The coding regions are shown in black. The 3′ enhancer binds to HNF-4 (hepatic nuclear factor 4) and, in hypoxic cells, to HIF (hypoxia inducible factor). These 2 transcription factors bind cooperatively with the adaptor protein p300. This complex interacts with the Epo promoter, thereby enabling initiation of transcription. Upstream kidney inducible elements (KIEs) are required for enhanced renal expression of Epo. (B) The oxygen-sensing mechanism responsible for the oxygen-dependent degradation of HIF-α. In well-oxygenated cells, HIF-1α protein undergoes oxygen- and iron-dependent hydroxylation at proline 564 and proline 402 (not shown). These posttranslational modifications enable the von Hipple Lindau protein (pVHL) to bind specifically to HIF-α, thereby providing a docking site for the E3 ubiquitin ligase (UL). UL is required for ubiquitylation of HIF-α, resulting in uptake by the proteasome and proteolytic degradation. At low oxygen tension, HIF-α is not hydroxylated and therefore escapes degradation. HIF-α can be activated in oxygenated cells by inhibitors of prolyl hydroxylase (PH) or by iron chelation.