Ruifeng Teng

Survival and proliferative roles of erythropoietin beyond the erythroid lineage.

Since the isolation and purification of erythropoietin (EPO) in 1977, the essential role of EPO for mature red blood cell production has been well established. The cloning of the EPO gene and production of recombinant human EPO led to the widespread use of EPO in treating patients with anaemia. However, the biological activity of EPO is not restricted to regulation of erythropoiesis. EPO receptor (EPOR) expression is also found in endothelial, brain, cardiovascular and other tissues, although at levels considerably lower than that of erythroid progenitor cells. This review discusses the survival and proliferative activity of EPO that extends beyond erythroid progenitor cells. Loss of EpoR expression in mouse models provides evidence for the role of endogenous EPO signalling in nonhaematopoietic tissue during development or for tissue maintenance and/or repair. Determining the extent and distribution of receptor expression provides insights into the potential protective activity of EPO in brain, heart and other nonhaematopoietic tissues.

Disrupted erythropoietin signalling promotes obesity and alters hypothalamus proopiomelanocortin production

While erythropoietin is the cytokine known that regulates erythropoiesis, erythropoietin receptor (EpoR) expression and associated activity beyond hematopoietic tissue remain uncertain. Here we show that mice with EpoR expression restricted to hematopoietic tissues (Tg) develop obesity and insulin resistance. Tg-mice exhibit a decrease in energy expenditure and an increase in white fat mass and adipocyte number. Conversely, erythropoietin treatment of wild-type mice increases energy expenditure and reduces food intake and fat mass accumulation but showed no effect in body weight of Tg-mice. EpoR is expressed at a high level in white adipose tissue and in the proopiomelanocortin neurons of the hypothalamus. While Epo treatment in wild-type mice induces the expression of the polypeptide hormone precursor gene, proopiomelanocortin, mice lacking EpoR show reduced levels of proopiomelanocortin in the hypothalamus. This study provides the first evidence that mice lacking EpoR in nonhematopoietic tissue become obese and insulin resistant with loss of erythropoietin regulation of energy homeostasis.

PPARα and Sirt1 mediate erythropoietin action in increasing metabolic activity and browning of white adipocytes to protect against obesity and metabolic disorders

Erythropoietin (EPO) has shown beneficial effects in the regulation of obesity and metabolic syndrome; however, the detailed mechanism is still largely unknown. Here, we created mice with adipocyte-specific deletion of EPO receptor. These mice exhibited obesity and decreased glucose tolerance and insulin sensitivity, especially when fed a high-fat diet. Moreover, EPO increased oxidative metabolism, fatty acid oxidation, and key metabolic genes in adipocytes and in white adipose tissue from diet-induced obese wild-type mice. Increased metabolic activity by EPO is associated with induction of brown fat–like features in white adipocytes, as demonstrated by increases in brown fat gene expression, mitochondrial content, and uncoupled respiration. Peroxisome proliferator–activated receptor (PPAR)α was found to mediate EPO activity because a PPARα antagonist impaired EPO-mediated induction of brown fat–like gene expression and uncoupled respiration. PPARα also cooperates with Sirt1 activated by EPO through modulating the NAD+ level to regulate metabolic activity. PPARα targets, including PPARγ coactivator 1α, uncoupling protein 1, and carnitine palmitoyltransferase 1α, were increased by EPO but impaired by Sirt1 knockdown. Sirt1 knockdown also attenuated adipose response to EPO. Collectively, EPO, as a novel regulator of adipose energy homeostasis via these metabolism coregulators, provides a potential therapeutic strategy to protect against obesity and metabolic disorders.

Acute erythropoietin cardioprotection is mediated by endothelial response

Increasing evidence indicates that high levels of serum erythropoietin (Epo) can lessen ischemia–reperfusion injury in the heart and multiple cardiac cell types have been suggested to play a role in this Epo effect. To clarify the mechanisms underlying this cardioprotection, we explored Epo treatment of coronary artery endothelial cells and Epo cardioprotection in a Mus musculus model with Epo receptor expression restricted to hematopoietic and endothelial cells (ΔEpoR). Epo stimulation of coronary artery endothelial cells upregulated endothelial nitric oxide synthase (eNOS) activity in vitro and in vivo, and enhanced nitric oxide (NO) production that was determined directly by real-time measurements of gaseous NO release. Epo stimulated phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) and mitogen-activated protein kinase kinase (MEK)/extracellular signal regulated kinase (ERK) signaling pathways, and inhibition of PI3K, but not MEK activity, blocked Epo-induced NO production. To verify the potential of this Epo effect in cardioprotection in vivo, ΔEpoR-mice with Epo response in heart restricted to endothelium were treated with Epo. These mice exhibited a similar increase in eNOS phosphorylation in coronary artery endothelium as that found in wild type (WT) mice. In addition, in both WT- and ΔEpoR-mice, exogenous Epo treatment prior to myocardial ischemia provided comparable protection. These data provide the first evidence that endothelial cell response to Epo is sufficient to achieve an acute cardioprotective effect. The immediate response of coronary artery endothelial cells to Epo stimulation by NO production may be a critical mechanism underlying this Epo cardioprotection.

Erythropoietin action in stress response, tissue maintenance and metabolism.

Erythropoietin (EPO) regulation of red blood cell production and its induction at reduced oxygen tension provides for the important erythropoietic response to ischemic stress. The cloning and production of recombinant human EPO has led to its clinical use in patients with anemia for two and half decades and has facilitated studies of EPO action. Reports of animal and cell models of ischemic stress in vitro and injury suggest potential EPO benefit beyond red blood cell production including vascular endothelial response to increase nitric oxide production, which facilitates oxygen delivery to brain, heart and other non-hematopoietic tissues. This review discusses these and other reports of EPO action beyond red blood cell production, including EPO response affecting metabolism and obesity in animal models. Observations of EPO activity in cell and animal model systems, including mice with tissue specific deletion of EPO receptor (EpoR), suggest the potential for EPO response in metabolism and disease.

Erythropoietin and hypoxia increase erythropoietin receptor and nitric oxide levels in lung microvascular endothelial cells

Acute lung exposure to low oxygen results in pulmonary vasoconstriction and redistribution of blood flow. We used human microvascular endothelial cells from lung (HMVEC-L) to study the acute response to oxygen stress. We observed that hypoxia and erythropoietin (EPO) increased erythropoietin receptor (EPOR) gene expression and protein level in HMVEC-L. In addition, EPO dose- and time-dependently stimulated nitric oxide (NO) production. This NO stimulation was evident despite hypoxia induced reduction of endothelial NO synthase (eNOS) gene expression. Western blot of phospho-eNOS (serine1177) and eNOS and was significantly induced by hypoxia but not after EPO treatment. However, iNOS increased at hypoxia and with EPO stimulation compared to normal oxygen tension. In accordance with our previous results of NO induction by EPO at low oxygen tension in human umbilical vein endothelial cells and bone marrow endothelial cells, these results provide further evidence in HMVEC-L for EPO regulation of NO production to modify the effects of hypoxia and cause compensatory vasoconstriction.

Erythropoietin signaling: A novel regulator of white adipose tissue inflammation during diet-induced obesity

Obesity-induced white adipose tissue (WAT) inflammation and insulin resistance are associated with macrophage (Mф) infiltration and phenotypic shift from “anti-inflammatory” M2-like to predominantly “proinflammatory” M1-like cells. Erythropoietin (EPO), a glycoprotein hormone indispensable for erythropoiesis, has biological activities that extend to nonerythroid tissues, including antiapoptotic and anti-inflammatory effects. Using comprehensive in vivo and in vitro analyses in mice, EPO treatment inhibited WAT inflammation, normalized insulin sensitivity, and reduced glucose intolerance. We investigated EPO receptor (EPO-R) expression in WAT and characterized the role of its signaling during obesity-induced inflammation. Remarkably, and prior to any detectable changes in body weight or composition, EPO treatment reduced M1-like Mф and increased M2-like Mф in WAT, while decreasing inflammatory monocytes. These anti-inflammatory effects were found to be driven, at least in part, by direct EPO-R response in Mф via Stat3 activation, where EPO effects on M2 but not M1 Mф required interleukin-4 receptor/Stat6. Using obese ∆EpoR mice with EPO-R restricted to erythroid cells, we demonstrated an anti-inflammatory role for endogenous EPO. Collectively, our findings identify EPO-R signaling as a novel regulator of WAT inflammation, extending its nonerythroid activity to encompass effects on both Mф infiltration and subset composition in WAT.

The Effects of Erythropoietin Dose Titration during High-Fat Diet-Induced Obesity

Erythropoietin (Epo) is a pleotropic cytokine with several nonhematopoietic tissue effects. High-dose Epo treatment-mediated effects on body weight, fat mass and glucose tolerance have recently been reported, thus extending its pleotropic effects to fat and glucose metabolism. However, the exact dose range of Epo treatment required for such effects remains unidentified to date. We investigated Epo dosage effect (up to 1000 U/kg) on hematocrit, body weight, body composition, glucose metabolism, food intake, and physical activity, during high-fat diet-induced obesity. We report that Epo doses (1000, 600, 300, and 150 U/kg) significantly reduced body weight gain and fat mass, while, only Epo doses of 300 U/kg and higher significantly affected glucose tolerance. None of the tested Epo doses showed any detectable effects on food intake, and only 1000 U/kg dose significantly increased physical activity, suggesting that these parameters may only be partially responsible for the metabolic effects of Epo treatment.

Erythropoietin regulates POMC expression via STAT3 and potentiates leptin response

The arcuate nucleus of the hypothalamus is essential for metabolic homeostasis and responds to leptin by producing several neuropeptides including proopiomelanocortin (POMC). We previously reported that high-dose erythropoietin (Epo) treatment in mice while increasing hematocrit reduced body weight, fat mass, and food intake and increased energy expenditure. Moreover, we showed that mice with Epo receptor (EpoR) restricted to erythroid cells (ΔEpoRE) became obese and exhibited decreased energy expenditure. Epo/EpoR signaling was found to promote hypothalamus POMC expression independently from leptin. Herein we used WT and ΔEpoRE mice and hypothalamus-derived neural culture system to study the signaling pathways activated by Epo in POMC neurons. We show that Epo stimulation activated STAT3 signaling and upregulated POMC expression in WT neural cultures. ΔEpoRE mice hypothalamus showed reduced POMC levels and lower STAT3 phosphorylation, with and without leptin treatment, compared to in vivo and ex vivo WT controls. Collectively, these data show that Epo regulates hypothalamus POMC expression via STAT3 activation, and provide a previously unrecognized link between Epo and leptin response.