Erythropoietin Signaling and Response in Vascular Smooth Muscle Cells.

Erythopoietin (EPO) and erythropoietin receptor (EPOR) regulate survival, proliferation, differentiation and viability of erythroid progenitor cells. Beyond erythropoiesis, we have observed that human vascular endothelial cells respond to EPO stimulation by inducing EPOR and endothelial nitric oxide synthase (eNOS) expression, increasing NO production and cGMP, particularly under low oxygen tension. In this study, we investigate the response of vascular smooth muscle (VSM) to EPO stimulation and the contribution of blood vessel to relaxation/contraction. We found that VSM cells express EPOR and that treatment with EPO (5 U/ml) at reduced oxygen tension increased EPOR mRNA by 2 fold. This increased EPO responsiveness at low oxygen tension is accompanied by increased cell proliferation at 2% O₂ more than 2 fold. Unlike endothelial cells, EPO did not induce eNOS or NO production in VSM cells. PI-3 kinase was involved in EPO stimulation with no change in MAP kinase. In an isolated blood vessel model system, we checked EPO responsiveness. EPO produced contractions (0.32 ± 0.3 g) of rat renal artery, and pretreatment with LY294002 (10 mM), an inhibitor of PI-3 kinase, statistically significantly inhibited this contraction (58.7 ± 7%). This response was also observed with the endothelium layer removed. In preparations of human internal mammary artery (HIMA) and human saphenous vein (HSV) from patients undergoing coronary artery bypass, EPO did not affect basal vascular tone of HIMA and HSV with the endothelium layer removed, but EPO (5 U/ml) potentated noradrenalin-induced contraction by up to 2 fold in HSV and HIMA. Also, pretreatment with EPO significantly increase angiotensin-evoked contractions of these blood vessels (50 ± 8%, 113 ± 17%, respectively P < 0.01), suggesting that EPO has synergistic effects on angiotensin or noradrenalin-induced [Ca²⁺] mobilization, particularly on intracellular Ca²⁺ release. These data suggest that EPO stimulation of vascular smooth muscle may act to modulate or balance the vasodilatory effects of increased NO production by EPO stimulated endothelium. Under oxygen stress vascular smooth muscle can respond to EPO by proliferation and PI-3 kinase activation as a protective effect and in conjunction with Ca²⁺ release likely contributes to the overall vascular response.