Characterization of erythropoietin and hepcidin in the regulation of persistent injury-associated anemia

Abstract

Introduction The cause of persistent injury-associated anemia is multifactorial and includes acute blood loss, an altered erythropoietin (EPO) response, dysregulation of iron homeostasis, and impaired erythropoiesis in the setting of chronic inflammation/stress. Hepcidin plays a key role in iron homeostasis and is regulated by anemia as well as inflammation. EPO is a main regulator of erythropoiesis induced by hypoxia. A unique rodent model of combined lung injury (LC)/hemorrhagic shock (HS)/chronic restraint stress (CS) was utilized to produce persistent injury-associated anemia to further investigate the roles of EPO, hepcidin, iron, ferritin, and the expression of EPO receptors (EPOr). Methods Male Sprague-Dawley rats were randomly assigned into one of the four groups of rodent models: naïve, CS alone, combined LCHS, or LCHS/CS. Plasma was used to evaluate levels of EPO, hepcidin, iron, and ferritin. RNA was isolated from bone marrow and lung tissue to evaluate expression of EPOr. Comparisons between models were performed by t-tests followed by one-way analysis of variance. Results After seven days, only LCHS/CS was associated with persistent anemia despite significant elevation of plasma EPO. LCHS and LCHS/CS led to a persistent decrease in EPOr expression in bone marrow on day seven. LCHS/CS significantly decreased plasma hepcidin levels by 75% on day one and 84% on day seven as compared to LCHS alone. Hepcidin plasma levels are inversely proportional to EPO plasma levels (Pearson R=-0.362, p<0.05). Conclusion Tissue injury, hemorrhagic shock, and stress stimulate and maintain high levels of plasma EPO while hepcidin levels are decreased. In addition, bone marrow EPOr and plasma iron availability are significantly reduced following LCHS/CS. The combined deficit of reduced iron availability and reduced bone marrow EPOr expression may play a key role in the ineffective EPO response associated with persistent injury-associated anemia.

Severe trauma and chronic stress activates extramedullary erythropoiesis

“…The reduced hemoglobin following LCHS/CS may be the result of a low level of circulating iron. 26 Hepcidin sequesters iron within macrophages preventing the maturation of red blood cells. In addition, while iron deficiency is associated with an increased rate of erythropoiesis, erythroid precursors have been shown to proliferate and then become quiescent without completing the maturation cycle.…”

Impact of Injury Severity on the Inflammatory State and Severe Anemia

“…[8][9][10] In preclinical rodent studies of blunt trauma, hemorrhagic shock and chronic stress, postinjury hypercatecholaminemia is associated with a suppression of bone marrow erythroid progenitor growth, reduced erythroid commitment, an abnormal EPO response, prolonged mobilization of hematopoietic progenitor cells, and dysregulation of iron homeostasis. 11,12 Norepinephrine, a key modulator of hematopoietic progenitor mobilization and an inhibitor of bone marrow erythroid cell growth, also directly stimulates IL-6 release, and several of our prior studies have implicated these cytokines directly in the development of persistent injury-associated anemia. 13,14 Tumor necrosis factor-alpha (TNF-a) and interleukin-6 (IL-6) are both proinflammatory cytokines that influence hematopoiesis and have altered expression following severe trauma.…”

“…19,20 Dysfunctional erythropoiesis in a rodent trauma model can be ameliorated with chemical sympathectomy using either propranolol or clonidine, suggesting that a reduction of hypercatecholaminemia improves erythropoiesis. [11][12][13][21][22][23][24][25][26] Prior research has evaluated the postinjury inflammatory state in severely injured trauma patients and demonstrated their erythropoietic dysfunction and iron dysregulation. 27 To validate our preclinical findings and to expand upon our prior study of the postinjury inflammatory state in humans, the purpose of this study was to evaluate additional severely injured trauma patients and determine if there is a relationship between the ISS in trauma patients, and the degree of neuroendocrine activation and systemic inflammation that is related with persistent injury-associated anemia as well as to further characterize the bone marrow response to injury.…”

Daily propranolol administration reduces persistent injury-associated anemia after severe trauma and chronic stress

“…25 Previous work in severe trauma and chronic stress has demonstrated that EPO suppresses plasma hepcidin and iron levels remain low and anemia persists. 26–27 …”