Erythropoietin (EPO) is of immense practical clinical utility in the treatment of anemia, and insight into EPO-regulated signaling pathways may reveal new targets for the treatment of dysregulated erythropoiesis. While significant headway has been made in understanding the intracellular signals mediating the effects of EPO through the homodimeric EPO receptor (EPOR), we clearly don’t know all the intracellular mediators involved. In this issue, Verma et al. report a novel mediator of EPO-dependent expansion of human erythroid progenitor cells and development of erythroblasts, which they have named regulator of human erythroid cell expansion (RHEX).
Applying a global phosphotyrosine (PY) phosphoproteomic approach, the authors identified an uncharacterized 26K open reading frame that was rapidly and highly tyrosine-phosphorylated at two sites after EPO stimulation. RHEX has no significant homology with known proteins and, although well-conserved in humans and other primates, it is not found in the genomes of rats, mice, or lower vertebrates. RHEX knockdown in an EPO-dependent human cell line attenuated cell growth and activation of ERK1 and ERK2, with no effect on cell death. In purified populations of human erythroid progenitor cells, RHEX knockdown delayed the formation of maturing erythroblasts. Coimmunoprecipitation experiments indicated that RHEX associates with the versatile adaptor protein GRB2 following EPO stimulation.
This outstanding paper opens up a number of important questions regarding the function of RHEX and the regulation of EPOR signaling. Is RHEX involved in the intracellular signaling of other cytokines? If so, does EPO signaling compete with other cytokine signaling pathways for GRB2 binding to RHEX? In addition to the homodimeric EPOR, a heterodimeric EPOR (EPOR chain and CD131, the common β-chain of GM-CSFR and IL-3R), which is present on some normal and cancer cell types, mediates nonerythropoietic effects of EPO. Is RHEX also an intermediary in signaling through the heterodimeric EPOR? Finally, it is of interest that RHEX was not detected in rats, mice, and lower vertebrates. Why is this? And how does EPO–EPOR signaling differ in different species?
Although the investigators chose to focus on RHEX, their PY-phosphoproteomic analysis identified other EPO-induced candidate targets, including molecular adaptors, serine/threonine kinases, tyrosine phosphatases, ubiquitin factors, and cell cycle regulators, that should provide for continued productive analysis of EPO-induced signaling. The EPO–EPOR signaling gray box is opening up, and with it will come greater insight into EPO’s actions and its modulation for clinical advantage.