Dynamic Regulation of Erythropoietin Gene Expression in Mammals

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Anemia, a common condition in Veteran patients, is a potent risk factor for increased morbidity and mortality when accompanying other disease states. Current therapies for anemia involve non-physiological bolus administration of erythropoietin (Epo), an endocrine factor produced in the adult kidney and liver with severe anemia. Bolus Epo administration has untoward effects, including an increased thrombotic risk and a stimulatory effect on cancer. Understanding how Epo is regulated will allow investigators to develop rational therapies besides Epo replacement. Our current focus is defining molecular mechanisms regulating Epo expression in mammals. Hypoxia Inducible Factor (HIF) transcription factors are a family of three molecular mediators that induce a protective cellular response to hypoxia. The second HIF member, HIF-2α, is critical for in vivo Epo gene expression. While an essential role of HIF-2α in Epo regulation is now recognized, the factors responsible for temporal Epo gene expression in vivo, or for abnormal repression of Epo gene expression in anemia patients, remain poorly understood. Although HIF-2α undergoes oxygen-dependent post-translational modifications, this mechanism is not the sole or even major mechanism for controlling HIF-2 signaling. Activity of HIF-1α, the founding HIF member, is controlled predominantly by oxygen-dependent post-translational modifications of the HIF-1α protein, which result in marked changes in HIF-1α protein levels. However, this mechanism is not the sole or even predominant mechanism for regulation of HIF-2 signaling. Instead, HIF-2α activity is controlled by two opposing and post-translational modifications, acetylation and deacetylation, that operate in a cyclical manner. Our central hypothesis is that hypoxia triggers changes in intermediary metabolism to effect acetylation of HIF-2α. The biochemical basis for control of HIF-2α acetylation is via an entity termed the acetate switch, which couples changes in intermediary metabolism with acetyl CoA generation. The molecular mediator for the acetate switch is acetyl CoA synthetase 2 (Acss2), an acetate-dependent enzyme that generates a specific pool of acetyl CoA used to induce coupled acetylation and activation of HIF-2α. Because a subset of Acss2 transits to the nucleus during hypoxia, the ability of Acss2 to augment HIF-2α activity may be conferred by its localization to the nuclear compartment during stress. Deciphering precisely how Acss2 regulates HIF-2 signaling will provide insights into normal Epo regulation, identify potential pathophysiological mechanisms responsible for anemia, and stimulate development of novel treatments for anemia patients.