Effect of Short-term Erythropoietin Therapy on Insulin Resistance and Serum Levels of Leptin and Neuropeptide Y in Hemodialysis Patients

Effect of trivalent chromium on erythropoietin production and the prevention of insulin resistance in HepG2 cells

In erythropoietin (EPO)-producing HepG2 cells, we investigated the effect of trivalent chromium (Cr) on the promotion of EPO production and the induction of insulin resistance. Cr increased hypoxia-inducible factor (HIF)-1α protein, EPO mRNA expression and EPO protein levels in HepG2 cells. The effect of Cr on EPO production was inhibited by inhibition of proliferator-activated receptor γ (PPARγ). Insulin resistance was induced by culturing with insulin resistance induction medium supplemented with palmitic acid for 24 h. When Cr was added to the medium, the increase in glucose-6-phosphatase and phosphoenolpyruvate carboxykinase 1 mRNA expression levels and the decrease in the ratio of phosphorylated Akt to Akt protein were suppressed, and the induction of insulin resistance prevented. When a PPARγ inhibitor or siPPARγ was added together with Cr, the inhibitory effect of Cr on the induction of insulin resistance disappeared. In addition, pretreatment with siEPO suppressed the increase in EPO mRNA expression, and the inhibitory effect on the induction of insulin resistance due to the addition of Cr was significantly reduced. These results suggest that the inhibition of insulin resistance induction by Cr in HepG2 cells involves the promotion of EPO production mediated by PPARγ, in addition to other PPARγ-mediated activities.

The Many Facets of Erythropoietin Physiologic and Metabolic Response

In mammals, erythropoietin (EPO), produced in the kidney, is essential for bone marrow erythropoiesis, and hypoxia induction of EPO production provides for the important erythropoietic response to ischemic stress, such as during blood loss and at high altitude. Erythropoietin acts by binding to its cell surface receptor which is expressed at the highest level on erythroid progenitor cells to promote cell survival, proliferation, and differentiation in production of mature red blood cells. In addition to bone marrow erythropoiesis, EPO causes multi-tissue responses associated with erythropoietin receptor (EPOR) expression in non-erythroid cells such neural cells, endothelial cells, and skeletal muscle myoblasts. Animal and cell models of ischemic stress have been useful in elucidating the potential benefit of EPO affecting maintenance and repair of several non-hematopoietic organs including brain, heart and skeletal muscle. Metabolic and glucose homeostasis are affected by endogenous EPO and erythropoietin administration affect, in part via EPOR expression in white adipose tissue. In diet-induced obese mice, EPO is protective for white adipose tissue inflammation and gives rise to a gender specific response in weight control associated with white fat mass accumulation. Erythropoietin regulation of fat mass is masked in female mice due to estrogen production. EPOR is also expressed in bone marrow stromal cells (BMSC) and EPO administration in mice results in reduced bone independent of the increase in hematocrit. Concomitant reduction in bone marrow adipocytes and bone morphogenic protein suggests that high EPO inhibits adipogenesis and osteogenesis. These multi-tissue responses underscore the pleiotropic potential of the EPO response and may contribute to various physiological manifestations accompanying anemia or ischemic response and pharmacological uses of EPO.