Metabolic Burden of Erythropoietin Stimulated Erythropoiesis in Mice

Erythropoietin (EPO) promotes erythroid differentiation and increases glucose uptake in erythroid progenitor cells in culture. The metabolic burden associated with EPO treatment in adult mice is suggested by a decrease in body weight concomitant with increased hematocrit. Wild type male mice (C57Bl/6, age 8 months) treated with EPO showed the expected increase in hematocrit accompanied by a fall in blood glucose level and a decrease in body weight and fat mass. However, the decrease in body weight is even more evident in obese mice on a high fat diet and has also been linked to non-hematopoietic response, particularly with EPO receptor (EpoR) expression in white adipose tissue. We examined the metabolic burden of EPO treatment (3000U/kg for 3 weeks) in young, lean male mice (3 months) placed on high fat diet at the time of EPO administration. The increase in hematocrit was accompanied by decreased blood glucose level and improved glucose tolerance. However, no difference in body weight was observed between mice treated with EPO and the saline treated group, suggesting that the EPO stimulated decrease in body weight is evident primarily in older animals with greater fat mass. To determine the contribution of EpoR expression in non-hematopoietic tissue to the metabolic EPO response, young male mice with EpoR restricted to erythroid tissue (TgEpoR) were placed on high fat diet and treated with EPO. The expected increased hematocrit was also accompanied by decreased blood glucose level and improved glucose tolerance, and no change in body weight between EPO and saline treatment. The similar responses observed in young wild type and TgEpoR mice suggest that the EPO stimulated increase in glucose metabolism is associated with increased erythropoiesis rather than a direct EPO response in non-hematopoietic tissue. TgEpoR mice exhibit an age dependent increase in fat mass even greater than that observed in wild type mice, and by 8 months TgEpoR mice are obese, glucose intolerant and insulin resistant compared with wild type mice. At 8 months, TgEpoR mice treated with EPO show the increase in hematocrit and, despite the increase in fat mass, there is only a minimal decrease in body weight compared with wild type mice. These data provide evidence that in addition to the age dependent association of EPO stimulated decrease in body weight and fat mass, this decrease in body weight is due largely to EPO response related to EpoR expression in non-hematopoietic tissue. Interestingly, young male mice with targeted deletion of EpoR in adipose tissue placed on a high fat diet and treated with EPO show the increase in hematocrit and improvement in glucose tolerance, and at 8 months, the increase in hematocrit with EPO treatment is accompanied by minimal change in body weight. The similar metabolic response of these mice with targeted deletion of EpoR in adipose tissue to TgEpoR mice indicate the contribution of EpoR expression in adipose tissue to the loss of body weight and fat mass. Therefore, the metabolic burden associated with EPO stimulated erythropoiesis appears to be reflected in improved glucose metabolism and glucose tolerance with minimal or no effect on body weight, is evident in young, lean mice, and is independent of EpoR expression in non-hematopoietic tissue. In older mice, non-hematopoietic metabolic EPO response is more readily apparent as reflected in loss of body weight/fat mass, which overshadows the erythropoietic metabolic response. In combination, the metabolic response to EPO treatment results from EPO stimulated increased erythropoiesis, improved glucose metabolism and glucose tolerance, and an age dependent decrease in body weight and fat mass associated with EpoR expression in non-hematopoietic tissue, particularly in white adipose tissue.