Targeting EPO and EPO receptor pathways in anemia and dysregulated erythropoiesis

Abstract

Introduction: Recombinant human erythropoietin (rhEPO) is a first-line therapeutic for the anemia of chronic kidney disease, cancer chemotherapy, AIDS (Zidovudine therapy), and lower-risk myelodysplastic syndrome. However, rhEPO frequently elevates hypertension, is costly, and may affect cancer progression. Potentially high merit therefore exists for defining new targets for anti-anemia agents within erythropoietin (EPO) and EPO receptor (EPOR) regulatory circuits.

Areas covered: EPO production by renal interstitial fibroblasts is subject to modulation by several regulators of hypoxia-inducible factor 2a (HIF2a) including Iron Response Protein-1, prolyl hydroxylases, and HIF2a acetylases, each of which holds potential as anti-anemia drug targets. The cell surface receptor for EPO (EPOR) preassembles as a homodimer, together with Janus Kinase 2 (JAK2), and therefore it remains attractive to develop novel agents that trigger EPOR complex activation (activating antibodies, mimetics, small-molecule agonists). Additionally, certain downstream transducers of EPOR/JAK2 signaling may be druggable, including Erythroferrone (a hepcidin regulator), a cytoprotective Spi2a serpin, and select EPOR-associated protein tyrosine phosphatases.

Expert opinion: While rhEPO (and biosimilars) are presently important mainstay erythropoiesis-stimulating agents (ESAs), impetus exists for studies of novel ESAs that fortify HIF2a's effects, act as EPOR agonists, and/or bolster select downstream EPOR pathways to erythroid cell formation. Such agents could lessen rhEPO dosing, side effects, and/or costs.

Keywords: Spi2a; anemia; erythroferrone; erythroid models; erythropoiesis; erythropoiesis-stimulating agents; erythropoietin; erythropoietin receptor; hypoxia inducible factors; janus kinase 2; prolyl hydroxylases; protein tyrosine phosphatases.

Figure 1. Approaches to increasing HIF2a levels and/or activity, and heightening endogenous EPO expression

(A) Upon binding to a unique 5’ domain within HIF2a transcripts, Apo-IRP1 inhibits translation. Small molecule inhibitors that interfere with this interaction therefore have the potential to increase HIF2a and EPO expression together with RBC production. (B) Small molecule inhibitors that inhibit PHD2 have been developed that stabilize HIF2a, and likewise increase EPO expression plus erythrocyte formation. This includes HIF2a inhibitors presently in phase-2 and -3 CKD clinical trials. Such inhibitors also will reinforce PHD2 and HIF2a actions on additional targets, and possible consequences of these latter effects will be important to assess. (C) Acetate supplementation recently has been shown to lessen anemia associated with several forms of stress erythropoiesis. This involves Acss2 mediated Acetyl-CoA production as a limiting step in enhancing HIF2a activity, with EPO as one prime target. Recently, this acetate switch also has been indicated to impact on tumor cell growth and metastasis.

Figure 2. Potential targeting of EPOR transmembrane region to activate pre-dimerized EPOR (and JAK2) complexes

(A) Friend virus envelope protein gp55 has been determined to specifically activate EPOR dimers via interactions with an EPOR transmembrane region. In addition, a 5,000 Mr transmembrane protein has been generated via random mutagenesis of bovine papilloma virus E5 protein (which normally binds and activates PDGFR-beta) that chronically activates the EPOR. (B) As proof-of-principle, small molecule agents that target the transmembrane domain of MPL have been successfully developed as a therapeutic for immune thrombocytopenia, with H499 (and T496) as target sites for one such agent. (C) Comparisons of transmembrane regions (and WSXWS plus box-1) domains within the human homodimeric type-1 receptors for EPO, TPO, prolactin, growth hormone and GCSF.

Figure 3. Select signal transducers within erythroid precursor cells, and downstream of the EPOR with druggable potential

(A) One response pathway intrinsic to erythroblasts, and downstream from the EPOR, involves EPO/EPOR/JAK2/Stat5 transcriptional induction of Erythroferrone (ERFE), a C1q-TNF cytokine. As secreted from erythroblasts, ERFE inhibits hepcidin and thereby enhances Ferroportin efflux of systemic iron. ERFE therefore may assist iron deficient anemias, and/or ERFE inhibition may lessen iron overload as associated, for example, with thalassemia. (B) Via studies of an Spi2A Serpin (as an additional novel EPO/EPOR/JAK2/Stat5 target gene) erythroblast lysosomal membrane permeability has been shown to become compromised during stress erythropoiesis, especially in oxidative and elevated ROS contexts. Small molecule inhibitors of leached lysosomal executioner cathepsins B (and L) therefore represent rational new targets as anti-anemia agents and erythroblast cytoprotectants during such stress erythropoiesis (e.g., sickle cell anemia, thalassemia). (C) Among protein tyrosine phosphatases engaged in erythroid precursors via the EPOR, PTPN6 as a negative effector is a potential target in anti-anemia contexts. As a positive effector, PTPN18 might be considered as a new target in a context of lessening EPOR/JAK2 activation or effects for polycythemia and/or myeloproliferative disease.