Erythropoietin (/ɪˌrɪθrˈpɔɪtən/ or /ɪˌrɪθrpˈɛtɪn, -rə-, -ˈ-, -tən/;[2][3][4] from Greek: ἐρυθρός, erythros 'red' and ποιεῖν, poiein 'make'), additionally known as EPO, hematopoietin, or hemopoietin, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production. It is a cytokine (protein signalling molecule) for erythrocyte (red blood cell) precursors in the bone marrow. Human EPO has a molecular weight of 34 kDa.

Erythropoietin is produced by interstitial fibroblasts in the kidney in close association with peritubular capillary and proximal convoluted tubule. It is additionally produced in perisinusoidal cells in the liver. While liver production predominates in the foetal and perinatal period, renal production is predominant throughout adulthood.

Exogenous erythropoietin, or recombinant human erythropoietin (rhEPO), is produced by recombinant DNA technology in cell culture. Several different pharmaceutical agents are available with a variety of glycosylation patterns, and are collectively called erythropoiesis-stimulating agents (ESA). The specific details for labelled use vary between the package inserts, but ESAs have been used in the treatment of anemia in chronic kidney disease, anaemia in myelodysplasia, and in anaemia from cancer chemotherapy. Boxed warnings include a risk of death, myocardial infarction, stroke, venous thromboembolism, and tumour recurrence. rhEPO has been used illicitly as a performance-enhancing drug;[7] it can most often be detected in blood, due to slight differences from the endogenous protein, for example, in features of posttranslational modification.

Function

Red blood cell production

Erythropoietin is an essential hormone for red blood cell production. Without it, definitive erythropoiesis doesn't take place. Under hypoxic conditions, the kidney will produce and secrete erythropoietin to increase the production of red blood cells by targeting CFU-E, proerythroblast and basophilic erythroblast subsets in the differentiation. Erythropoietin has its primary effect on red blood cell progenitors and precursors (which are found in the bone marrow in humans) by promoting their survival through protecting these cells from apoptosis.

Erythropoietin is the primary erythropoietic factor that cooperates with various additional growth factors (e.g., IL-3, IL-6, glucocorticoids, and SCF) involved in the development of erythroid lineage from multipotent progenitors. The burst-forming unit-erythroid (BFU-E) cells start erythropoietin receptor expression and are sensitive to erythropoietin. Subsequent stage, the colony-forming unit-erythroid (CFU-E), expresses maximal erythropoietin receptor density and is completely dependent on erythropoietin for further differentiation. Precursors of red cells, the proerythroblasts and basophilic erythroblasts additionally express erythropoietin receptor and are therefore affected by it.

Nonhematopoietic roles

Erythropoietin was reported to have a range of actions beyond stimulation of erythropoiesis including vasoconstriction-dependent hypertension, stimulating angiogenesis, and promoting cell survival via activation of Epo receptors resulting in anti-apoptotic effects on ischemic tissues. However this proposal is controversial with numerous studies showing no effect.[9] It is additionally inconsistent with the low levels of Epo receptors on those cells. Clinical trials in humans with ischemic heart, neural and renal tissues haven't demonstrated the same benefits seen in animals. In addition a few research studies have shown its neuroprotective effect on diabetic neuropathy, however these data weren't confirmed in clinical trials that have been conducted on the deep peroneal, superficial peroneal, tibial and sural nerves.[12]

Mechanism of action

Erythropoietin has been shown to exert its effects by binding to the erythropoietin receptor (EpoR).[14][16]

EPO is highly glycosylated (40% of total molecular weight), with half-life in blood around five hours. EPO's half-life might vary between endogenous and various recombinant versions. Additional glycosylation or additional alterations of EPO via recombinant technology have led to the increase of EPO's stability in blood (thus requiring less frequent injections). EPO binds to the erythropoietin receptor on the red cell progenitor surface and activates a JAK2 signalling cascade. High level erythropoietin receptor expression is localised to erythroid progenitor cells. While there are reports that EPO receptors are found in a number of additional tissues, like heart, muscle, kidney and peripheral/central nervous tissue, those results are confounded by nonspecificity of reagents like anti-EpoR antibodies. In controlled experiments, EPO receptor isn't detected in those tissues. In the bloodstream, red cells themselves don't express erythropoietin receptor, so can't respond to EPO. Notwithstanding indirect dependence of red cell longevity in the blood on plasma erythropoietin levels has been reported, a process termed neocytolysis.

Synthesis and regulation

Erythropoietin levels in blood are quite low in the absence of anemia, at around 10 mU/ml. Notwithstanding in hypoxic stress, EPO production might increase up to 1000-fold, reaching 10,000 mU/ml of blood. In adults, EPO is synthesised mainly by interstitial cells in the peritubular capillary bed of the renal cortex, with additional amounts being produced in the liver.[18][20] Regulation is believed to rely on a feedback mechanism measuring blood oxygenation and iron availability.[22] Constitutively synthesised transcription factors for EPO, known as hypoxia-inducible factors, are hydroxylated and proteosomally digested in the presence of oxygen and iron.

Medical uses

Erythropoietins available for use as therapeutic agents are produced by recombinant DNA technology in cell culture, and include Epogen/Procrit (epoetin alfa) and Aranesp (darbepoetin alfa); they're used in treating anemia resulting from chronic kidney disease, chemotherapy induced anaemia in patients with cancer, inflammatory bowel disease (Crohn's disease and ulcerative colitis)[2] and myelodysplasia from the treatment of cancer (chemotherapy and radiation). The package inserts include boxed warnings of increased risk of death, myocardial infarction, stroke, venous thromboembolism, and tumour recurrence, particularly when used to increase the haemoglobin levels to more than 11 to 12 g/dl.

History

In 1905, Paul Carnot, a professor of medicine in Paris, and his assistant, Clotilde Deflandre, proposed the idea that hormones regulate the production of red blood cells. After conducting experiments on rabbits subject to bloodletting, Carnot and Deflandre attributed an increase in red blood cells in rabbit subjects to a hemotropic factor called hemopoietin. Eva Bonsdorff and Eeva Jalavisto continued to study red cell production and later called the hemopoietic substance 'erythropoietin'. Further studies investigating the existence of EPO by K.R. Reissman and Allan J. Erslev (Thomas Jefferson Medical College) demonstrated that a certain substance, circulated in the blood, is able to stimulate red blood cell production and increase hematocrit. This substance was finally purified and confirmed as erythropoietin, opening doors to therapeutic uses for EPO in diseases like anemia.[22][2]

Hematologist John Adamson and nephrologist Joseph W. Eschbach looked at various forms of renal failure and the role of the natural hormone EPO in the formation of red blood cells. Studying sheep and additional animals in the 1970s, the two scientists helped establish that EPO stimulates the production of red cells in bone marrow and could lead to a treatment for anaemia in humans. In 1968, Goldwasser and Kung began work to purify human EPO, and managed to purify milligramme quantities of over 95 percent pure material by 1977.[27] Pure EPO allowed the amino acid sequence to be partially identified and the gene to be isolated.[22] Later, a researcher at Columbia University detected a way to synthesise EPO. Columbia University patented the technique and licenced it to Amgen.

In the 1980s, Adamson, Joseph W. Eschbach, Joan C. Egrie, Michael R. Downing and Jeffrey K. Browne conducted a clinical trial at the Northwest Kidney Centers for a synthetic form of the hormone, Epogen, produced by Amgen. The trial was successful, and the results were published in the New England Journal of Medicine in January 1987.[29]

In 1985, Lin et al isolated the human erythropoietin gene from a genomic phage library and were able to characterise it for research and production.[31] Their research demonstrated the gene for erythropoietin encoded the production of EPO in mammalian cells that's biologically active in vitro and in vivo. The industrial production of recombinant human erythropoietin (rhEpo) for treating anaemia patients would begin soon after.

In 1989, the US Food and Drug Administration approved the hormone Epogen, which remains in use today.[2]