1. What are HIF transcriptional factors, and what do they do?
Hypoxia-inducible factors (HIFs) are transcription factors that regulate the expression of genes involved in erythropoiesis, including regulating the production of endogenous erythropoietin (EPO) and regulation of iron transporter proteins.1,2,18
2. Why is the HIF pathway considered the "oxygen-sensing mechanism” of the kidney?
The HIF pathway is considered the oxygen-sensing mechanism of the kidney because its activity is dependent on oxygen levels.1
In the presence of sufficient oxygen (normoxia), HIF-α is hydroxylated by prolyl hydroxylase (PH) and tagged for degradation, and as a result, the HIF pathway is not activated.1
Under hypoxic conditions, hydroxylation of HIF-α does not occur, and the HIF pathway is activated, resulting in increased erythropoiesis.1
HIF pathway activity in REP cells in normoxia and hypoxia1,2
O2=oxygen; PH=prolyl hydroxylase; VHL=von Hippel-Lindau.
3. Why is the oxygen-sensing mechanism impaired in CKD?
In chronic kidney disease (CKD), disease-related structural and functional changes in the kidney lead to an overall decreased oxygen consumption by the kidneys due to progressively reduced tubular sodium reabsorption.16,18
Because the renal erythropoietin-producing (REP) cells sense sufficient oxygen to meet local demand despite low hemoglobin levels, the REP cells do not activate the HIF pathway.2,16,18
As a result, oxygen delivery to the rest of the body may be decreased1,16,18
4. What is pseudonormoxia?
Pseudonormoxia refers to when the REP cells sense sufficient oxygen levels in their immediate environment, which may not reflect the oxygen levels in the whole kidney or the rest of the body.2,16,18
In CKD, the finely tuned oxygen balance of healthy kidneys is impaired, placing the kidneys into a state of pseudonormoxia. REP cells sense normoxia in their immediate environment.2,16,18
5. How does CKD disrupt normal HIF signaling?
In CKD, disease-related changes in the kidney result in sufficient oxygen levels (pseudonormoxia) in the immediate environment of the REP cells. As a result, the HIF pathway in the REP cells is not activated.18
In CKD, oxygen levels in the immediate environment of the REP cells are sufficient to meet the kidney's reduced needs despite low hemoglobin levels.2,18
The HIF signaling is reduced, resulting in decreased EPO production and insufficient erythropoiesis to correct hemoglobin levels; this can lead to worsening anemia and decreased oxygen delivery to the rest of the body.1,2,16,18
6. Can people with CKD still produce EPO?
Yes, patients with CKD still have the ability to produce EPO. Those with end-stage renal disease (ESRD) still experience increases in serum EPO levels following acute hemorrhage and/or blood loss.29
Current evidence indicates that it is the impaired oxygen-sensing mechanism rather than the destroyed EPO-producing capacity that is the primary cause of anemia of CKD.1
7. What are REP cells? Where are they located in the kidney? How are they activated?
The REP cells, or renal erythropoietin-producing cells, do just that—produce EPO. They are primarily located in the outer part of the kidney—called the cortex—and surround the tubules. Because of the REP cells’ environment, they are uniquely qualified to sense oxygen and produce EPO when oxygen is low. In normal oxygen conditions (normoxia), REP cells sense a sufficient oxygen supply and are less active. In low oxygen (hypoxia), REP cells increase EPO production, stimulating the bone marrow to produce red blood cells (erythrocytes). In patients with CKD, the REP cells sense a sufficient oxygen supply (as a result of decreased sodium tubular reabsorption) and thus are less active in producing EPO, leading to anemia.1,2,11-13,16-18,30,31
8. What is the role of inflammation in anemia of CKD?
Inflammation contributes to anemia of CKD through multiple mechanisms including suppression of endogenous EPO production, increased hepcidin levels, impaired proliferation and differentiation of erythroid progenitors, and decreased red blood cell life span.1
9. What is hepcidin, and what is its role in iron metabolism?
Hepcidin is a small peptide made in the liver that regulates iron availability in the body. In CKD, hepcidin is elevated due to inflammation. Hepcidin downregulates ferroportin, preventing the export of iron from inside the cells into the plasma circulation. Therefore, elevated hepcidin results in iron being sequestered inside storage cells such as macrophages, enterocytes, and hepatocytes. The sequestered iron cannot be utilized for erythropoiesis. As a result, functional iron deficiency may develop.1,7,9
10. Does the HIF pathway exist in other organs besides the kidney?
Yes, the HIF pathway also acts as an oxygen-sensing pathway in other organs and, as a result, coordinates EPO production with iron availability. For example, in the gastrointestinal tract, activation of the HIF pathway under low-oxygen conditions leads to increases in proteins that are required for iron absorption. Similarly, in the liver, activation of the HIF pathway under low-oxygen conditions leads to increases of proteins that facilitate iron transport.1,15,18,20,21