Erythropoietin

Abstract

During the past century, few proteins have matched erythropoietin (Epo) in capturing the imagination of physiologists, molecular biologists, and, more recently, physicians and patients. Its appeal rests on its commanding role as the premier erythroid cytokine, the elegant mechanism underlying the regulation of its gene, and its remarkable impact as a therapeutic agent, arguably the most successful drug spawned by the revolution in recombinant DNA technology. This concise review will begin with a synopsis of the colorful history of this protein, culminating in its purification and molecular cloning. It then covers in more detail the contemporary understanding of Epo's physiology as well as its structure and interaction with its receptor. A major part of this article focuses on the regulation of the Epo gene and the discovery of HIF, a transcription factor that plays a cardinal role in molecular adaptation to hypoxia. In the concluding section, a synopsis of Epo's role in disorders of red blood cell production will be followed by an assessment of the remarkable impact of Epo therapy in the treatment of anemias, as well as concerns that provide a strong impetus for the development of even safer and more effective treatment.

Figure 1.

Regulation of red cell production by Epo. (A) Decreased oxygen delivery to specialized cells in the kidney results in increased expression and secretion of Epo, which circulates in the plasma and stimulates marrow progenitors, thereby increasing red cell production. If the increase in red cell mass relieves the hypoxic signal, Epo expression is down-regulated. (B) Plasma Epo levels (milliunits/mL) in patients with different types of and degrees of anemia and in those with primary erythrocytosis and secondary erythrocytosis. HIF, hypoxia inducible factor; PCV, polycythemia vera.

Figure 2.

Epo-dependent signaling. When Epo binds to its dimeric receptor (EpoR) on erythroid progenitor cells, the two receptor monomers are pulled together allowing phosphorylation of JAK2 kinase, which initiates the signal transduction cascade.

Figure 3.

Diagram of the Epo gene. The five exons of the Epo gene are shown as rectangles with the coding regions in black. Far upstream of the Epo promoter is a kidney-inducible element (KIE) that is required for high-level up-regulation of Epo mRNA in the kidney. Just downstream from exon 5 is a critical enhancer that binds to HNF-4 and also to HIF when the Epo-producing cell is hypoxic. These two transcription factors bind to the transcriptional activator p300. This enhanceosome is a powerful inducer of Epo transcription.

Figure 4.

Pathway by which the hypoxia-inducible transcription factor is up-regulated by low intracellular oxygen tension. In normally oxygenated cells, the α subunit (HIF-α) undergoes hydroxylation of two proline residues, one of which is shown here. In the presence of iron (Fe) and α-ketoglutarate, this oxygen-dependent posttranslational modification is catalyzed by an HIF-α-specific prolyl hydroxylase (PH). The von Hippel–Lindau protein (pVHL) binds to hydroxylated HIF-α. Subsequent docking of a ubiquitin ligase (UL) enables HIF-α to be polyubiquitinated.

Figure 5.

Response of an anephric patient to recombinant human erythropoietin (rhEpo) therapy. Note that, before therapy, the patient was severely anemic and transfusion dependent. Treatment with rhEpo resulted in a reticulocytosis followed by a progressive increase in hemoglobin. The dose of rhEpo had to be lowered to prevent the hemoglobin from rising too high. Before rhEpo therapy, the patient was severely iron overloaded. The marked increase in red cell mass following therapy was accompanied by a significant reduction in iron stores. RBC, red blood cells; Fe, iron; TIBC, total iron binding capacity; Sat., saturation (Data from Eschbach et al. 1987.)