The elucidation of potential binding sites and transport of EPO across the placental barrier has clinical significance in terms of assessing safety to the fetus with rhEPO administration to anemic pregnant women (refer to ). In addition, measurement of circulating EPO may serve as a marker of fetal distress since increased nucleated red blood cell counts can be measured in the neonate from complicated pregnancies. Previous studies have resulted in contrasting results with no demonstrable binding of radiolabeled EPO by the human placenta and no transport between the maternal and fetal compartments in a model of perfused cotyledons. Interestingly, in the latter study the authors measured a 50% loss of EPO from the perfusate after 5 h, suggesting that the administered rhEPO was being bound without placental transfer. Our laboratory has also attempted to identify specific EPO binding sites on isolated trophoblast cells using 125I-labeled ligand without success (data not shown). However, since these cells coexpress EPO, prior occupancy of the receptors may mask binding.
A proximal event in the signal transduction of EPO in erythroid precursor cells is the phosphorylation of the EPO-R on tyrosine residues (see, and citations therein). Like other members of the cytokine receptor superfamily, EPO-R itself does not contain a tyrosine kinase domain. Rather, interaction with EPO results in EPO-R homodimerization and activation of the Janus tyrosine kinase, JAK 2, which is bound to the proximal cytoplasmic region of EPO-R resulting in tyrosine phosphorylation of several proteins including EPO-R itself.