Lack of expression and function of erythropoietin receptors in the kidney

Erythropoiesis-stimulating agents (ESAs) stimulate formation of red blood cells by binding to and activating Epo receptors (EpoR) on erythroid progenitor cells. Beyond successful treatment of anemia, ESAs have been reported to reduce damage following insult to organs, including the kidney, possibly via direct activation of EpoR. However, data on ESA effects outside hematopoietic functions are conflicting. Furthermore, limited use of appropriate EpoR-positive and EpoR-negative controls and lack of specific anti-EpoR antibodies make interpretation of data difficult. Recently positive and negative control cell types were validated and a sensitive and specific anti-EpoR antibody (A82) that detects low levels of EpoR protein was described.

A82 was used to measure EpoR protein levels in tissues, human renal cells and human cell lines by western blot analysis. Surface EpoR was examined on renal cells by measuring binding of [125I]-rHuEpo or antibodies. Renal cells and cell lines were treated with rHuEpo to see if phosphorylation of signaling proteins or proliferation/survival could be induced. Small inhibitory RNA (siRNA) were used to determine if EpoR knockdown affected cell viability.

Total EpoR protein was low to undetectable in tissues and renal cells with no detectable EpoR on cell surfaces. EpoR knockdown had no effect on viability of renal cell lines. RHuEpo had no detectable effect on intracellular signaling on renal cell lines with no growth-promoting or survival effect on primary human renal cells.

These results suggest that functional EpoR protein is absent on renal cells and that non-EpoR mechanisms should be explored to explain non-hematopoietic effects of ESAs.