Kidney EPO production | HIF-PHD Inhibitors in Renal Anemia

Acta Physiol (Oxf)2022 May 1;e13826. doi: 10.1111/apha.13826. Online ahead of print.

EPO synthesis induced by HIF-PHD inhibition is dependent on myofibroblast transdifferentiation and colocalizes with non-injured nephron segments in murine kidney fibrosis.

Kobayashi H, Davidoff O, Pujari-Palmer S, Drevin M, Haase VH.

This study from the Haase lab investigates to what degree HIF-PHD inhibitors are capable to induce kidney EPO production in CKD. Aims, experimental approach, results and conclusion are outlined in the following abstract.

Aims: Kidney EPO production is regulated by hypoxia-inducible factor (HIF)-2. In the kidney, it is produced by cortico-medullary perivascular interstitial cells, which transdifferentiate into collagen-producing myofibroblasts in response to injury. Inhibitors of prolyl hydroxylase domain (PHD) dioxygenases (HIF-PHIs) activate HIF-2 and stimulate kidney and liver EPO synthesis in patients with anemia of chronic kidney disease (CKD). We examined whether HIF-PHIs can reactivate EPO synthesis in interstitial cells that have undergone myofibroblast transdifferentiation in established kidney fibrosis.
Methods: We investigated Epo transcription in myofibroblasts and character- ized the histological distribution of kidney Epo transcripts by RNA in situ hybrid- ization combined with immunofluorescence in mice with adenine nephropathy (AN) treated with HIF-PHI molidustat. Lectin absorption chromatography was used to assess liver-derived EPO. In addition, we examined kidney Epo transcription in Phd2 knockout mice with obstructive nephropathy.
Results: In AN, molidustat-induced Epo transcripts were not found in areas of fibrosis and did not colocalize with interstitial cells that expressed α-smooth muscle actin, a marker of myofibroblast transdifferentiation. Epo transcription was associated with megalin-expressing, kidney injury molecule 1-negative nephron segments and contingent on residual renal function. Liver-derived EPO did not contribute to serum EPO in molidustat-treated mice. Epo transcription was not associated with myofibroblasts in Phd2 knockout mice with obstructive nephropathy.
Conclusions: Our studies suggest that HIF-PHIs do not reactivate Epo transcription in interstitial myofibroblasts and that their efficacy in inducing kidney EPO in CKD is dependent on the degree of myofibroblast formation, the preservation of renal parenchyma and the level of residual renal function.

Keywords: anemia, chronic kidney disease, erythropoietin, hypoxia-inducible factor, molidustat, prolyl hydroxylase domain.

Anemia is a major complication of advanced chronic kidney disease (CKD), as its prevalence increases with CKD stage. A major contributing factor to the pathogenesis of anemia of CKD is relative erythropoietin (EPO) deficiency, that is, the inadequate kidney EPO production in response to a given degree of hypoxia or anemia, which decreases oxygen delivery in blood and is from here on referred to as anemic hypoxia. “Pseudo-normoxia,” the notion that renal EPO-producing cells (REPC) sense “sufficient oxygen” due to reduced epithelial oxygen demand when glomerular filtration rate (GFR) is decreased, and transdifferentiation of REPC into collagen-producing myofibroblasts in response to injury have been proposed as the two major underlying mechanisms. The latter results in a reduction of the number of interstitial cells that have capacity to produce EPO, and has been associated with a suppression of EPO transcription by inflammatory cytokines, and with repressive epigenetic marks in regulatory regions of the EPO gene.

HIF-PHD inhibitors (HIF-PHIs) belong to a new class of anemia drugs that activate HIF-2 signaling and stimulate the production of endogenous EPO in kidney and liver. Although HIF-PHIs have been shown to induce kidney Epo transcription in models of CKD or inflammation, the cellular phenotype and the histologic distribution of renal interstitial cells that respond to pharmacologic HIF- PHD inhibition with EPO synthesis remain ill-defined in CKD. Genetic studies have suggested that PHD inactivation may preserve EPO production in interstitial cells that have undergone transition to myofibroblasts. However, it is not clear whether pharmacologic HIF-PHD inhibition can reactivate EPO production in myofibroblasts once fibrosis is established. Here, we took advantage of two different models of fibrotic kidney injury, adenine-induced nephropathy (AN), and unilateral ureteral obstruction (UUO), to examine whether HIF-PHIs are capable of activating EPO synthesis in myofibroblasts.

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see commentary: The prolyl hydroxylase inhibitor molidustat fails to restore erythropoietin production in the fibrotic kidney [Acta Physiologica 2022].