Erythropoietin (EPO) is best known as the hormone that regulates red blood cell production. A body of work beginning in 2004 proposed that it also acts through a second receptor arrangement, later named the innate repair receptor, and a short synthetic peptide, ARA-290 (cibinetide), was characterized against that proposal. The receptor itself is disputed. This overview sets out what three primary papers report: the one that proposed the receptor, the one that derived a short peptide from the structure of EPO, and one that tested the proposed complex directly and did not find it.
The 2004 Proposal
Brines et al. (2004) started from an observation about EPO derivatives. They had characterized derivatives that do not bind the EPO receptor (EPOR) yet are tissue-protective. Carbamylated EPO (CEpo), for example, does not stimulate erythropoiesis, yet it prevented tissue injury in a wide variety of in vivo and in vitro models. The authors took this to suggest that another receptor is responsible for tissue protection, and hypothesized that EPOR combines with the common beta receptor (βcR, also called CD131), the signal-transducing subunit shared by the receptors for granulocyte-macrophage colony-stimulating factor, IL-3 and IL-5.
The paper reports four lines of evidence. Membrane proteins prepared from rat brain, heart, liver or kidney were greatly enriched in EPOR after passage over EPO or CEpo columns, and the EPOR was found in a complex with βcR. Antibodies against EPOR co-immunoprecipitated βcR from membranes of neuronal-like P-19 cells. Immunocytochemistry showed the two receptors colocalized in spinal cord neurons and cardiomyocytes. Neither EPO nor CEpo was active in cardiomyocyte or spinal cord injury models performed in the βcR knockout mouse. The authors also note that βcR knockout mice show normal erythrocyte maturation, so βcR is not required for erythropoiesis.
From Helix B to an Eleven-Residue Peptide
Brines et al. (2008) asked which part of EPO carries the tissue-protective activity. They delimited it to helix B (amino acid residues 58–82), which faces the aqueous medium when EPO is bound to the receptor homodimer, and reported that helix B was neuroprotective in vitro and tissue-protective in vivo in a variety of injury models. An 11-amino-acid peptide composed of adjacent amino acids forming the aqueous face of helix B was also tissue-protective. Neither helix B nor the 11-residue peptide was erythropoietic in vitro or in vivo. The summary given by the authors is that small, nonerythropoietic peptides that simulate a portion of the three-dimensional structure of EPO mimic its tissue-protective activities.
The ARA-290 record in the research library traces the peptide to this work. It describes cibinetide as eleven residues corresponding to a surface region of helix B of erythropoietin, with pyroglutamate at position 1.
The 2018 Test
Cheung Tung Shing et al. (2018) open by calling a direct interaction between EPOR and the beta-common (βc) receptor to form an innate repair receptor controversial. In their account some studies show a functional link between the two receptors in tissue protection, others show no involvement of the βc receptor in tissue repair, and there was no biophysical evidence confirming a direct association either in vitro or in vivo. They examined the extracellular regions of the two receptors in silico and in vitro, in the presence and in the absence of EPO or of the EPO-derived peptide ARA290. Their computational and genomic studies suggested a possible interaction, but their in vitro biophysical analysis showed that the extracellular regions do not specifically associate. They also examined the βc receptor gene (Csf2rb) under anaemic stress and found no requirement for it in mice. Their conclusion is that the extracellular regions of EPOR and the βc receptor do not directly interact, and that the innate repair receptor is not involved in anaemic stress.
Reading the Three Papers Together
- Different kinds of evidence. The 2004 paper reports co-purification, co-immunoprecipitation and colocalization from tissue membranes and cells, plus a knockout result. The 2018 paper reports an in vitro biophysical analysis of the extracellular regions of the two receptors.
- Function and association are separate questions. The absence of EPO activity in βcR knockout mice is a functional result. Whether the two receptors physically associate is a different question, and the 2018 study answers it for the extracellular regions.
- Who did the work. The 2004 and 2008 papers share a first author and several co-authors. The 2018 paper comes from a different set of authors.
- Which molecule. EPO, carbamylated EPO, helix B and the 11-residue peptide are four different molecules. A result for one is not a result for another.
Related Library Records
The research library holds an identity monograph for ARA-290 (cibinetide), and the receptor has its own page at innate repair receptor, where it is recorded as proposed and disputed.
References
- Brines M, Grasso G, et al. Erythropoietin mediates tissue protection through an erythropoietin and common beta-subunit heteroreceptor. Proc Natl Acad Sci U S A 2004. PMID: 15456912.
- Brines M, Patel NS, et al. Nonerythropoietic, tissue-protective peptides derived from the tertiary structure of erythropoietin. Proc Natl Acad Sci U S A 2008. PMID: 18676614.
- Cheung Tung Shing KS, Broughton SE, et al. EPO does not promote interaction between the erythropoietin and beta-common receptors. Sci Rep 2018. PMID: 30127368.
This article summarizes publicly available research for educational purposes and does not constitute medical advice, a therapeutic claim, or a recommendation for human use. Products referenced are sold for laboratory research use only.