Transferrin receptor 2 is a component of the erythropoietin receptor complex and is required for efficient erythropoiesis

Abstract

IntroductionErythropoiesis is mainly regulated by the kidney-produced hormone erythropoietin (Epo), which is absolutely required for the survival and proliferation of erythroid progenitors and their terminal differentiation to red cells. 1 The Epo receptor (EpoR) is a type 1 transmembrane protein that belongs to the class 1 cytokine receptor family. Its expression level in erythroid cells is low whatever the differentiation stage and even the most Epo-sensitive cells, colony-forming units erythroid (CFU-Es), or proerythroblasts express less than 1000 EpoRs per cell at their surface. 2 In the absence of Epo, EpoR is believed to be homodimeric, in which each dimer protein is constitutively associated with a Janus kinase-2 (Jak-2) tyrosine-kinase molecule. Epo binding modifies the organization of the receptor complex leading to Jak-2 activation. Association between Jak-2 and the EpoR occurs during the receptor maturation process, most likely before EpoR leaves the endoplasmic reticulum, and is essential for expression of the receptor at the cell surface. 3 However, the mechanisms that control the maturation and the transport of the EpoR to the cell surface remain largely unknown. Indeed, the number of cell surface EpoRs is not related to the synthesis level of this protein and most of these molecules accumulate in the endoplasmic reticulum in EpoRoverexpressing cells. 4 Both functional and direct evidence also suggests that unidentified proteins are associated with EpoR on the cell surface. In particular, chemical cross-linking of radiolabeled Epo to cell surface erythroid cells has frequently revealed the presence of 2 additional proteins with apparent molecular masses of 85 and 100 kDa in the EpoR complex. 2,5-8 Unfortunately, because of the low expression level of the EpoR, these proteins could never be identified up to now. The dramatic progress that has been recently introduced by the application of mass spectrometry methods to protein analysis led us to address the question of the identity of the EpoR-associated proteins.Here, we have purified the EpoR complex from UT7 erythroleukemic cells that express endogenous EpoRs to identify EpoRassociated proteins. Mass spectrometry analysis of the purified proteins revealed the presence of the type 2 transferrin receptor (TfR2) in the EpoR complex.Mammals express 2 transferrin receptors that share similar overall structures but possess specific functions. Most cells including erythroid precursors internalize iron from plasma diferric transferrin through the ubiquitously expressed transferrin receptor 1 (TfR1). A Tfr1 gene deletion is lethal in mice with severe anemia and is not compensated by TfR2. 9 In contrast, TfR2 expression is tissue-restricted with high expression in the liver 10 where it plays a key role in iron metabolism regulation. Indeed, TfR2 contributes to the adaptation of hepcidin production to the needs of the body by sensing the circulating iron bound to transferrin. 11 Familial inactivating or non-sense mutations in the TfR2 gene are responsible for...

Hematopoietic deletion of transferrin receptor 2 in mice leads to a block in erythroid differentiation during iron‐deficient anemia

“…3C,D). These results are in agreement with previous studies that suggested a loss of TFR2 results in a delay in the differentiation of erythroblasts [20,23].…”

“…Recent studies using mouse models of anemia have suggested that loss of Tfr2 in the erythroid compartment could be responsible for either increased RBC counts [21,22,24] or a block in the development of erythroblasts [23]. TFR2 was shown to be required for the proper localization and trafficking of EPOR to the surface in an erythroleukemic cell line [20]. These results taken together suggest that TFR2 has a role in erythropoiesis.…”

“…The expression of TFR2 in HEK293 cells appeared to increase the surface localization of the EPOR, indicating that TFR2 could be involved in trafficking of the EPOR. Erythroid progenitors from 4-week-old TfR2 2/2 mice had reduced colony-forming ability in the presence of physiological concentrations of EPO, suggesting that TFR2 is required for the differentiation of erythroid progenitors [20]. The results of these experiments suggested that TFR2 is required for the proper differentiation of erythroid cells.…”

“…The complex could be detected even after treatment with Brefeldin A, suggesting that the complex is formed in the early stages, probably in the endoplasmic reticulum [20]. The expression of TFR2 in HEK293 cells appeared to increase the surface localization of the EPOR, indicating that TFR2 could be involved in trafficking of the EPOR.…”

“…It has been suggested that TFR2 can form a complex with erythropoietin receptor (EPOR) in the UT7 cell line (endogenous protein) and transfected HEK293 cells [20]. The complex could be detected even after treatment with Brefeldin A, suggesting that the complex is formed in the early stages, probably in the endoplasmic reticulum [20].…”