ERYTHROPOIETIN (EPO): CHARACTERISTICS, PRODUCTION, FUNCTIONS - BIOLOGY - 2024

The erythropoietin, haemopoietin or EPO is a glycoprotein hormone functions (cytokine) responsible for the control of proliferation, differentiation and survival of progenitor cells of erythrocytes or red blood cells in bone marrow, ie erythropoiesis.

This protein is one of the various growth factors that control the hematopoietic processes by which, from a small group of pluripotent stem cells, the cells found in the blood are formed: erythrocytes, white blood cells and lymphocytes. That is, the cells of the myeloid and lymphoid lineages.

Diagram representing Hemopoiesis, which includes the process of erythrocyte formation or Erythropoiesis, where erythropoietin acts (Source: OpenStax College via Wikimedia Commons)

Its importance lies in the functional importance of cells that helps to multiply, differentiate and mature, since erythrocytes are responsible for the transport of oxygen from the lungs to the different tissues of the body.

Erythropoietin was the first growth factor to be cloned (in 1985), and its administration for the successful treatment of anemia caused by kidney failure is currently approved by the American Food and Drug Administration (FDA).

The notion that erythropoiesis is controlled by a humoral factor (soluble factor present in the circulation) was proposed more than 100 years ago by Carnot and Deflandre when studying the positive effects on the increase of the percentages of red cells in rabbits treated with serum. of anemic animals.

However, it was not until 1948 that Bonsdorff and Jalavisto introduced the term "erythropoietin" to describe the humoral factor with a specific implication on the production of erythrocytes.

characteristics

Erythropoietin is a protein from the glycoprotein family. It is stable at acidic pHs and has a molecular weight of approximately 34 kDa.

It has about 193 amino acids, including a 27-residue hydrophobic N-terminal region, which is removed by co-translational processing; and an arginine residue at position 166 that is also lost, so the circulating protein has 165 amino acids.

In its structure, the formation of two disulfide bridges between the cysteine ​​residues present in positions 7-161 and 29-33 can be seen, which are linked to its operation. It is made up of more or less 50% alpha helices, which apparently participate in the formation of a globular region or portion.

It has 40% carbohydrates, represented by three oligosaccharide chains N-linked to different aspartic acid residues (Asp), and one O-chain linked to a serine residue (Ser). These oligosaccharides are composed primarily of fucose, mannose, N-acetyl glucosamine, galactose, and N-acetyl neuraminic acid.

The carbohydrate region of EPO fulfills several roles:

- It is essential for its biological activity.

- Protects it from degradation or damage caused by oxygen free radicals.

- The oligosaccharide chains are required for the secretion of the mature protein.

In humans, the gene that codes for this protein is located in the middle of the long arm of chromosome 7, in the region q11-q22; it is found in a single copy in a 5.4kb region and has five exons and four introns. Homology studies indicate that its sequence shares 92% identity with that of other primates and 80% with that of some rodents.

Production

In the fetus

During fetal development, erythropoietin is produced mainly in the liver, but it has been determined that, during this same stage, the gene that codes for this hormone is also abundantly expressed in the middle region of the kidney nephrons.

In the adult

After birth, in what could be considered all the postnatal stages, the hormone is produced essentially in the kidneys. Specifically, by the cells of the cortex and the surface of the renal corpuscles.

The liver also participates in the production of erythropoietin in the postnatal stages, from which about 20% of the total circulating EPO content is excreted.

Other "extrarenal" organs where erythropoietin production has been detected include peripheral endothelial cells, vascular smooth muscle cells, and insulin-producing cells.

Some EPO secretion centers are also known to exist in the central nervous system, including the hippocampus, cortex, brain endothelial cells, and astrocytes.

Regulation of erythropoietin production

The production of erythropoietin is not directly controlled by the number of red blood cells in the blood, but by the supply of oxygen in the tissues. A deficiency of oxygen in the tissues stimulates the production of EPO and its receptors in the liver and kidneys.

This hypoxia-mediated activation of gene expression is the product of the activation of the pathway of a family of transcription factors known as hypoxia-inducible factor 1 (HIF-1).

Hypoxia, then, induces the formation of many protein complexes that fulfill different functions in the activation of erythropoietin expression, and that bind directly or indirectly to factors that translate the activation signal to the promoter of the EPO gene, stimulating its transcription..

Other stressors such as hypoglycemia (low blood sugar), increases in intracellular calcium, or the presence of reactive oxygen species, also trigger the HIF-1 pathway.

Mechanism of action

The mechanism of action of erythropoietin is quite complex and depends mainly on its ability to stimulate different signaling cascades involved in cell proliferation, which are, in turn, related to the activation of other factors and hormones.

In the human body of a healthy adult there is a balance between the production and destruction of red blood cells or erythrocytes, and EPO participates in maintaining this balance by replacing the disappearing erythrocytes.

When the amount of oxygen available in the tissues is very low, the expression of the gene coding for erythropoietin increases in the kidneys and liver. The stimulus can also be given by high altitudes, hemolysis, conditions of severe anemia, hemorrhage or prolonged exposure to carbon monoxide.

These conditions generate a state of hypoxia, which causes the secretion of EPO to increase, to produce a greater number of red cells and the fraction of reticulocytes in circulation, which are one of the progenitor cells of erythrocytes, also increase.

Who does the EPO act on?

In erythropoiesis, EPO is primarily involved in the proliferation and differentiation of progenitor cells involved in the red blood cell lineage (erythrocytic progenitors), but it also activates mitosis in proerythroblasts and basophilic erythroblasts, and also accelerates the release of the reticulocytes of the bone marrow.

The first level at which the protein works is in the prevention of programmed cell death (apoptosis) of the precursor cells formed in the bone marrow, which it achieves by inhibitory interaction with the factors involved in this process.

How does it work?

Cells that respond to erythropoietin have a specific receptor for erythropoietin known as the erythropoietin receptor or EpoR. Once the protein forms a complex with its receptor, the signal is transferred into the cell: towards the nucleus.

The first step for signal transfer is a conformational change that occurs after the protein binds to its receptor, which is, at the same time, bound to other receptor molecules that are activated. Among them is Janus-tyrosine kinase 2 (Jack-2).

Among some of the pathways that are activated downstream, after Jack-2 mediates phosphorylation of tyrosine residues of the EpoR receptor, is the MAP kinase and protein kinase C pathway, which activate transcription factors that increase the expression of specific genes.

Features

Like many hormonal factors in organisms, erythropoietin is not restricted to a single function. This has been elucidated through numerous investigations.

In addition to acting as a proliferation and differentiation factor for erythrocytes, which are essential for the transport of gases through the bloodstream, erythropoietin appears to fulfill some additional functions, not necessarily related to the activation of cell proliferation and differentiation.

In injury prevention

Studies have suggested that EPO prevents cell damage and, although its mechanisms of action are not exactly known, it is believed that it can prevent apoptotic processes produced by reduced or absent oxygen tension, excite toxicity, and exposure to free radicals.

In apoptosis

Its participation in the prevention of apoptosis has been studied by interaction with determining factors in the signaling cascades: Janus-tyrosine kinase 2 (Jak2), caspase 9, caspase 1 and caspase 3, glycogen synthase kinase-3β, activation factor of apoptotic proteases 1 (Apaf-1) and others.

Functions in other systems

It participates in the inhibition of cellular inflammation by inhibiting some pro-inflammatory cytokines such as interleukin 6 (IL-6), tumor necrosis factor alpha (TNF-α) and monocyte chemo-attractant protein 1.

In the vascular system, it has been shown that it collaborates in the maintenance of its integrity and in the formation of new capillaries from existing vessels in areas without vasculature (angiogenesis). In addition, it prevents the permeability of the blood-brain barrier during injuries.

It is believed to stimulate postnatal neovascularization by increasing the mobilization of progenitor cells from the bone marrow to the rest of the body.

It plays an important role in the development of neural progenitor cells through the activation of the nuclear factor KB, which promotes the production of nerve stem cells.

Acting in concert with other cytokines, EPO has a "modulatory" role in controlling the proliferation and differentiation pathways of megakaryocytes and granulocyte-monocytes.

References

1. Despopoulos, A., & Silbernagl, S. (2003). Color Atlas of Physiology (5th ed.). New York: Thieme.
2. Jelkmann, W. (1992). Erythropoietin: Structure, Control of Production, and Function. Physiological Reviews, 72 (2), 449–489.
3. Jelkmann, W. (2004). Molecular Biology of Erythropoietin. Internal Medicine, 43 (8), 649–659.
4. Jelkmann, W. (2011). Regulation of erythropoietin production. J. Physiol., 6, 1251-1258.
5. Lacombe, C., & Mayeux, P. (1998). Biology of Erythropoietin. Haematologica, 83, 724–732.
6. Maiese, K., Li, F., & Zhong, Z. (2005). New Avenues of Exploration for Erythropoietin. JAMA, 293 (1), 1–6.