The effect of erythropoietin on normal and neoplastic cells

Abstract

Erythropoietin (Epo) is an essential hormone that binds and activates the Epo receptor (EpoR) resident on the surface of erythroid progenitor cells, thereby promoting erythropoiesis. Recombinant human erythropoietin has been used successfully for over 20 years to treat anemia in millions of patients. In addition to erythropoiesis, Epo has also been reported to have other effects, such as tissue protection and promotion of tumor cell growth or survival. This became of significant concern in 2003, when some clinical trials in cancer patients reported increased tumor progression and worse survival outcomes in patients treated with erythropoiesis-stimulating agents (ESAs). One of the potential mechanisms proffered to explain the observed safety issues was that functional EpoR was expressed in tumors and/or endothelial cells, and that ESAs directly stimulated tumor growth and/or antagonized tumor ablative therapies. Since then, numerous groups have performed further research evaluating this potential mechanism with conflicting data and conclusions. Here, we review the biology of endogenous Epo and EpoR expression and function in erythropoiesis, and evaluate the evidence pertaining to the expression of EpoR on normal nonhematopoietic and tumor cells.

Keywords: anemia; angiogenesis; erythropoietin; erythropoietin receptor; tumor.

Figure 1

Erythropoiesis and the expression of stage-specific markers. Notes: Shown is a schematic diagram of the various stages of human erythropoiesis that results in the formation of mature red blood cells (RBCs). Time-dependent expression of various proteins is shown, including erythropoietin receptor (EpoR) and other cell surface markers (green), transcription factors (orange), and effector molecules such as hemoglobin (red).

Figure 2

(A and B) Erythropoietin (Epo) mRNA is expressed in kidney interstitial cells. Mice were made anemic by withdrawing 0.5 mL blood and replacing with 0.5 mL saline 8, 16, and 24 hours prior to sacrifice. Standard in situ hybridization (ISH) on kidney sections was performed with an antisense ³³P-labeled Epo probe. (A) ISH for mouse Epo mRNA in a control mouse; (B) ISH for mouse Epo mRNA in an anemic mouse. Notes: Increased number of cells expressing Epo transcripts in kidney from anemic mice vs normal mice, but with a similar number of grains over renal Epo-producing cells from both normal and anemic kidneys. Data and figure kindly provided by Sheilah Scully, Amgen.

Figure 3

Erythropoietin receptor (EPOR), GATA-1, and SCL/Tal1 have similar transcript profiles in normal human tissue. Notes: Illustrated are levels of transcript (average of n = 2) and standard error obtained through microarray analysis of normal human tissue for EPOR, GATA-1, and SCL/ Tal1. Levels of expression are in mean fluorescence units. Data were obtained from the publicly available database http://biogps.org. Probes shown are: EPOR, 209962_AT; GATA-1, 210446_AT; SCL, 206283-s_AT. Similar intensities were observed with other probes. Note that high-level expression of EPOR mRNA is found primarily in tissue/cell types containing erythroid cells. CD105 (endoglin) is expressed in endothelial cells, but it is also coexpressed with CD71 in erythroid cells.,, Thus the EPOR detected in CD105⁺ cells is likely due to erythroid cell–specific expression.

Figure 4

High-level erythropoietin receptor (EpoR) protein expression is found in erythroid cells but not in other tissues. EpoR expression was analyzed by Western immunoblot analysis with anti-EpoR antibody A82 that was shown to specifically detect human EpoR in erythroid cells. The arrow shows the location of full-length EpoR. Smaller proteins have been shown elsewhere to be EpoR fragments. UT-7/ Epo cells (EpoR positive control) are derived from a megakaryoblastic leukemia and are Epo-dependent. Notes: This research was originally published in Blood. Sinclair AM, Coxon A, McCaffery I, et al. Functional erythropoietin receptor is undetectable in endothelial, cardiac, neuronal, and renal cells. Blood. 2010;115(21):4264–4272. © American Society of Hematology. Abbreviations: HUVEC, human umbilical vein endothelial cells; RPTEC, renal proximal tubule epithelial cells.

Figure 5

Erythropoietin receptor (EpoR) expression in differentiating CD34+ hematopoietic progenitor cells grown with or without Epo. Notes: CD34+ cells were cultured in medium containing 50 ng/mL stem cell factor, 10 ng/mL interleukin (IL)-3, 10 ng/mL IL-6 with (w/) and without (no) 5 U/mL recombinant human erythropoietin (rHuEpo) for the indicated number of days. The arrow denotes the position of full-length EpoR (59 kDa). FLAG-EpoR COS7 is an EpoR positive control cell lysate from COS-7 cells expressing a FLAG-tagged version of EpoR. Data and figure kindly provided by Leigh Busse, Amgen. Abbreviation: MWM, molecular weight marker in kilo Daltons (kDa).

Figure 6

Erythropoietin receptor (EpoR) activation and signaling with Epo in erythroid progenitor cells. Note: Schematic diagram of the signaling cascades and effector responses observed in erythroid progenitor cells when EpoR is activated with erythropoiesis-stimulating agents.