Receptor Mediated Erythropoietin Metabolic Response and Gender Specific Activity

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Erythropoietin, a glycoprotein produced in the interstitial cells of the adult kidney in a hypoxia responsive manner, regulates the daily production of 200 billion red blood cells for oxygen transport from the lungs to the tissues. Erythropoietin acts by binding to the erythropoietin receptor on the surface erythroid progenitor cells in the bone marrow and activates JAK2/STAT5 signal transduction and downstream signaling pathways to promote survival, proliferation and terminal differentiation to form mature erythrocytes. Mice lacking erythropoietin or erythropoietin receptor die in utero due to severe anemia. For more than three decades, recombinant human erythropoietin significantly improved clinical management of anemia in chronic kidney disease resulting in decreased transfusion requirement. Erythropoietin receptor expression extends beyond erythroid tissue and animal models have demonstrated erythropoietin activity in non-erythroid tissue such as brain, heart, and skeletal muscle. In adult brain erythropoietin receptor expression mediates erythropoietin-protective response to brain injury or ischemia in rodents and suggests potential benefit in patients with brain injury or cognitive dysfunction associated with hypoxia and neuroplasticity. Erythropoietin receptor expression in the adult cardiovascular system and skeletal muscle myoblasts provides for erythropoietin protection in rodent models of acute cardiac ischemia and ischemia/reperfusion injury, and skeletal muscle injury, respectively. Endogenous erythropoietin and erythropoietin treatment also regulate mouse metabolism and erythropoietin treatment protects against diet induced obesity mediated by erythropoietin receptor expression in non-erythroid tissues and independent of erythropoietin stimulated erythropoiesis. Mice with erythropoietin receptor restricted to erythroid tissue become obese, glucose intolerant and insulin resistant. Erythropoietin treatment improves glucose tolerance, particularly in young mice, and in male mice during high-fat diet feeding reduces fat mass, increases metabolic activity, and in white adipose tissue increases cellular respiration and mitochondrial biogenesis, and shifts white adipose tissue gene expression toward a brown adipose tissue program with increasing uncoupling protein UCP1. Whole body or fat restricted loss of erythropoietin receptor expression reduced erythropoietin activity to improve glucose tolerance and regulate fat mass. In mammals, white adipose tissue is critical for excess energy storage and for energy mobilization as fatty acids into the blood stream. Dysfunction of lipid metabolism causes a variety of metabolic-related diseases such as obesity and type 2 diabetes mellitus, lipodystrophy, and cachexia. In addition, increase in fat cell mass leads to hormone imbalance with resultant metabolic effects including impaired glucose tolerance and insulin resistance. We used mouse models to assess erythropoietin metabolic regulation of fat mass and lipid metabolism. A transgenic mouse expressing a fluorescent marker gene inserted into the erythropoietin receptor locus indicated erythropoietin receptor expression is highest in erythroid tissue. Among non-erythroid tissue in adult mice, erythropoietin receptor is expressed high in white adipose tissue and to a lesser extent in skeletal muscle, brown adipose tissue and liver. In male mice, erythropoietin treatment and constitutive overexpression of human erythropoietin demonstrated metabolic response to high erythropoietin characterized by decreased fat mass and body weight, and improved glucose tolerance and insulin sensitivity. Elevated erythropoietin regulated lipid metabolism in white and brown adipose tissue, skeletal muscle and liver, decreasing expression of lipogenic genes and increasing expression of lipolytic genes. Erythropoietin regulation of genes associated with lipid metabolism required erythropoietin receptor expression in adipose tissue. In subcutaneous white adipose tissue, erythropoietin activity increased protein stability of transcription factor RUNX1, increased expression of lipolytic genes that contain RUNX1 dependent enhancer motifs, and decreased expression of lipogenic genes that contain RUNX1 dependent silencer motifs. Deletion of erythropoietin receptor in adipose tissue decreased stability of RUNX1 in subcutaneous white adipose tissue and abrogated erythropoietin stimulated increase of lipolytic genes and decreased lipogenic gene expression in white adipose tissue as well as brown adipose tissue, skeletal muscle, and liver. Erythropoietin treatment decreased white adipose tissue expression of ubiquitin ligase FBXW7 and increased RUNX1 stability, providing evidence for erythropoietin regulation of energy metabolism in mice mediated through the erythropoietin-erythropoietin receptor-RUNX1 axis. Erythropoietin receptor expression on immune cells including macrophages contribute to erythropoietin immune modulatory activity. In mice, erythropoietin stimulated erythropoiesis increases macrophages associated with erythroblastic blood islands in the spleen. In obese male mice, in addition to erythropoietin regulation of fat mass, erythropoietin reduced white adipose tissue inflammation associated with diet induced obesity, macrophage infiltration and inflammatory cytokine production, and shifted macrophages toward an anti-inflammatory subtype. Elevated erythropoietin in brain also protects against hypothalamus inflammation in male mice on high fat diet. Erythropoietin regulation of fat mass and inflammation during diet induced obesity observed in male mice, ovariectomized female mice and female mice with disruption of estrogen receptor alpha expression, is detected in aged female mice with associated increased fat mass. We previously showed that nitric oxide synthase and nitric oxide synthase contributes to erythropoietin cardio-protective and neuro-protective activity, and neuronal nitric oxide synthase is required for erythropoietin stimulated erythropoietic response and for maintaining the nitric oxide reservoir in skeletal muscle. We will extend this work to examine the role of nitric oxide pathways in erythropoietin activity, especially in metabolic response and lipid metabolism in skeletal muscle. Our research program on erythropoietin activity mediated by erythropoietin receptor expression in mouse models demonstrated erythropoietin protective activity in brain, the cardiovascular system, skeletal muscle and metabolic regulation of fat mass and inflammation associated with diet induced obesity. These studies add to understanding of erythropoietin treatment including specific non-erythroid response.