Erythropoietin

Erythropoietin or exogenous epoetin alfa binds to the erythropoietin receptor (EPO-R) and activates intracellular signal transduction pathways. The affinity (Kd) of EPO for its receptor on human cells is ∼100 to 200 pM. Upon binding to EPO-R on the surface of erythroid progenitor cells, a conformational change is induced which brings EPO-R-associated Janus family tyrosine protein kinase 2 (JAK2) molecules into close proximity. JAK2 molecules are subsequently activated via phosphorylation, then phosphorylate tyrosine residues in the cytoplasmic domain of the EPO-R that serve as docking sites for Src homology 2-domain-containing intracellular signaling proteins. The signalling proteins include STAT5 that once phosphorylated by JAK2, dissociates from the EPO-R, dimerizes, and translocates to the nucleus where they serve as transcription factors to activate target genes involved in cell division or differentiation, including the apoptosis inhibitor Bcl-x. The inhibition of apoptosis by the EPO-activated JAK2/STAT5/Bcl-x pathway is critical in erythroid differentiation. Via JAK2-mediated tyrosine phosphorylation, erythropoietin and epoetin alfa also activates other intracellular proteins involved in erythroid cell proliferation and survival, such as Shc, phosphatidylinositol 3-kinase (PI3K), and phospholipase C-γ1.