Erythropoietin: sites of synthesis and regulation of secretion.

Erythropoietin (Epo) is a glycoprotein that promotes the proliferation and differentiation of erythrocyte precursors. The major site of Epo production is the kidney, while the liver is the main extrarenal site of Epo production. Within these organs, the cells synthesizing Epo were identified by using in situ hybridization in hypoxic animals with an increased Epo mRNA expression. Epo-producing cells in the kidney were peritubular cells, most likely endothelial cells of the cortex and outer medulla. Glomerular and tubular cells were not labeled. In three patients with renal adenocarcinomas associated with polycythemia, in situ hybridization showed a strong labeling of the tumor cells. Epo secretion is stimulated by hypoxia, which is detected by an oxygen sensor located in the kidney. This oxygen sensor has been recently shown to be an heme protein. At the Epo gene level, studies to identify cis-acting DNA sequences, and trans-activation factors for inducible kidney and liver Epo expression are being pursued.