Received: 21 April 2018 | Revised: 18 June 2018 | Accepted: 6 July 2018 DOI: 10.1002/med.21527 REVIEW ARTICLE Erythropoiesis and chronic kidney disease–related anemia: From physiology to new therapeutic advancements Valeria Cernaro1 | Giuseppe Coppolino2 | Luca Visconti1 | Laura Rivoli3 | Antonio Lacquaniti1 | Domenico Santoro1 | Antoine Buemi4 | Saverio Loddo5 | Michele Buemi1 1Chair of Nephrology, Department of Clinical and Experimental Medicine, University of Abstract Messina, Messina, Italy Erythropoiesis is triggered by hypoxia and is strictly 2Nephrology and Dialysis Unit, Department of regulated by hormones, growth factors, cytokines, and Internal Medicine, “Pugliese‐Ciaccio” Hospital of Catanzaro, Catanzaro, Italy vitamins to ensure an adequate oxygen delivery to all 3Unit of Nephrology, Department of Internal body cells. Abnormalities in one or more of these factors Medicine, Chivasso Hospital, Turin, Italy may induce different kinds of anemia requiring different 4Surgery and Abdominal Transplantation Division, Cliniques Universitaires Saint‐Luc, treatments. A key player in red blood cell production is Université Catholique De Louvain, Brussels, erythropoietin. It is a glycoprotein hormone, mainly Belgium 5Department of Clinical and Experimental produced by the kidneys, that promotes erythroid Medicine, University of Messina, Messina, progenitor cell survival and differentiation in the bone Italy marrow and regulates iron metabolism. A deficit in Correspondence erythropoietin synthesis is the main cause of the normo- Valeria Cernaro, Chair of Nephrology, Department of Clinical and Experimental chromic normocytic anemia frequently observed in pa- Medicine, University of Messina, Via tients with progressive chronic kidney disease. The Consolare Valeria n. 1, Messina 98124, Italy. Email: [email protected] present review summarizes the most recent findings about each step of the erythropoietic process, going from the renal oxygen sensing system to the cascade of events induced by erythropoietin through its own receptor in the bone marrow. The paper also describes the new class of drugs designed to stabilize the hypoxia‐inducible factor by inhibiting prolyl hydroxylase, with a discussion about their metabolism, disposition, efficacy, and safety. According to many trials, these drugs seem able to simulate tissue Valeria Cernaro and Giuseppe Coppolino contributed equally to this study. Saverio Loddo and Michele Buemi contributed equally to this study (senior authors). Med Res Rev. 2019;39:427-460. wileyonlinelibrary.com/journal/med © 2018 Wiley Periodicals, Inc. | 427 428 | CERNARO ET AL. hypoxia and then stimulate erythropoiesis in patients affected by renal impairment. In conclusion, the in‐depth investigation of all events involved in erythropoiesis is crucial to understand anemia pathophysiology and to identify new therapeutic strategies, in an attempt to overcome the potential side effects of the commonly used erythropoiesis‐stimulating agents. KEYWORDS erythropoiesis‐stimulating agents (ESA), erythropoietin, hypoxia‐ inducible factor (HIF), HIF stabilizers, pleiotropic effects 1 | INTRODUCTION Oxygen is the most common chemical element of the earth crust (47% of the mass), while it is present in the percentage of 21% of the volume and 23% of the mass in the atmosphere. It is essential for life. As known, aerobic organisms including humans require oxygen to produce energy through the oxidation of different substrates (for example, carbohydrates and fatty acids). This is the reason why many efforts are physiologically made to ensure an adequate oxygen delivery to all body cells. In the blood, molecular oxygen (O2) is transported by hemoglobin within erythrocytes. The maintenance of normal blood oxygen levels is the result of complex metabolic pathways involving the kidneys, the bone marrow and many molecules (hormones, growth factors, cytokines, vitamins, etc), which act according to a very rigorous “timetable” to regulate erythropoiesis as needed.1 Abnormalities in one or more of these factors may induce different kinds of anemia that require different treatments. Normochromic normocytic anemia is particularly frequent among patients with progressive chronic kidney disease (CKD). The causes are many and include: inadequate production of erythropoietin, erythropoiesis inhibition due to the accumulation of uremic toxins, reduced red blood cell survival, iron deficiency, malnutrition, inflammation, folate and/or vitamin B12 deficiency, dysregulated iron metabolism, oxidative stress, chronic gastrointestinal blood loss, secondary renal hyperparathyroidism, and blood losses during hemodialysis sessions in patients with end‐stage renal disease requiring replacement therapy.2–5 The severity of anemia has been associated with poor outcomes in patients with CKD. Foley and colleagues demonstrated that mean hemoglobin was an independent risk factor for left ventricular dilatation, development of cardiac failure, and mortality in a cohort of 432 patients on dialysis (hemodialysis or peritoneal dialysis) followed prospectively for about 41 months.6 Moreover, anemia seems to accelerate the progression of renal damage through different mechanisms. Firstly, low hemoglobin levels reduce oxygen delivery to the kidneys with resulting medullary hypoxia that favors interstitial fibrosis. Anemia may also stimulate renal sympathetic nerve activity and then induce an increase in glomerular pressure and proteinuria, which is another factor contributing to CKD progression.7 A complete understanding of the pathophysiology of anemia as well as the correct management of this disorder makes necessary a thorough knowledge of the physiological cascade of events triggered by hypoxia, from the renal oxygen sensor system to the formation of the red blood cell in the bone marrow. The aim of the present review has been therefore to summarize the most recent findings about each step of this process, also describing all drugs, already available or still experimental, able to promote erythropoiesis in CKD, with a discussion about their metabolism and disposition. CERNARO ET AL. | 429 2 | THE OXYGEN SENSING SYSTEM AND THE ROLE OF HIF IN THE KIDNEY Erythropoietin is a glycoprotein hormone playing a key role in the production of erythrocytes. The starting point of the erythropoietic process is an oxygen sensing system that is located in the kidney at the level of cells known as renal erythropoietin‐producing (REP) cells. The discovery and identification of REP cells are the result of many experimental studies. At first, it was thought that cells producing erythropoietin were endothelial cells of the renal cortex and external medulla.8 Later studies showed that erythropoietin is produced by interstitial fibroblasts localized between tubules and capillaries in the kidney mid‐cortical region.9–11 These spaces are characterized by low oxygen delivery and high oxygen consumption; it follows that REP cells detect hypoxia with great sensitivity.12 Interestingly, REP cells have shown remarkable plasticity that makes them capable of changing into myofibroblasts under experimental conditions of kidney injury. In such circumstances, REP cells lose their ability to produce erythropoietin and switch to a fibrogenic cellular phenotype, through a mechanism probably involving nuclear factor κB signaling as observed by Souma et al13 in a mouse model of unilateral ureteral obstruction. These authors also demonstrated that the reversion of inflammation induced by unilateral ureteral obstruction resulted in the recovery of REP cell physiologic phenotype and erythropoietin‐producing potential. Then, they concluded that phenotypic transition FIGURE 1 Regulation of HIF‐1 activity by oxygen. HIF‐1 beta subunit is constitutively synthesized whereas alpha subunit expression is regulated by oxygen levels. Under normoxia conditions, HIF‐1 alpha is rather unstable and is rapidly degraded (within 5 minutes) in the cytoplasmic proteasomes through a process triggered by prolyl hydroxylase domain (PHD)‐containing proteins (activated by O2 and other factors) and involving the von Hippel‐Lindau protein (VHL). When blood oxygen concentration is low, prolyl hydroxylase activity is reduced. Consequently, HIF‐1 alpha is not degraded and moves into the nucleus where it binds to the beta subunit to form HIF‐1, which in turn stimulates the expression of genes responsible for the adaptation of cells to hypoxia, including the gene codifying for erythropoietin. After oxygen concentration normalization, HIF‐1 alpha is degraded at the level of nuclear proteasomes. HIF‐1, hypoxia‐inducible factor 430 | CERNARO ET AL. of REP cells to myofibroblasts could represent the link between anemia and tubulointerstitial fibrosis, which is the pathophysiological event responsible for the histological and functional changes leading to progressive CKD.14 Hypoxia induces a complex adaptive response that aims at facilitating cellular metabolism and restoring normal oxygen blood levels through a process involving the transcription factor known as hypoxia‐inducible factor‐1(HIF‐1). The compensatory mechanisms stimulated by hypoxia include erythropoiesis through increased erythropoietin production, angiogenesis, anaerobic glycolysis, glucose transport, and regulation of cell cycle and apoptosis.15 HIF‐1 is a heterodimeric transcription factor consisting of two subunits known as alpha and beta. While HIF‐1beta is constitutively synthesized, HIF‐1 alpha expression is tightly regulated by oxygen concentrations. When