Transferrin Receptor 2 Is a Novel Target to Ameliorate Anemia of Chronic Kidney Disease

Introduction

Progressive renal failure in Chronic Kidney Disease (CKD) decreases erythropoietin (EPO) production and leads to inflammation, reducing EPO responsiveness of erythroid cells. In addition, inflammation stimulates the production of the hepatic hormone hepcidin, that limits circulating iron. Altogether, these pathological alterations lead to anemia.

Current EPO-mimetic/stimulating compounds, supplemented with iron, are effective but often associated to cardio-vascular side effects. Thus, novel and more specific strategies are needed.

Transferrin Receptor 2 (TFR2) is a protein expressed in hepatocytes, where it regulates hepcidin and iron homeostasis, and in erythroid cells, where it binds EPO receptor. Hepatic Tfr2 inactivation causes iron overload, while its deletion in the erythroid compartment enhances erythropoiesis increasing EPO sensitivity of erythroid cells.

We have recently reported that bone marrow (BM) Tfr2 deletion improves anemia in a murine model of β-thalassemia (Artuso et al., Blood 2018). Thus, we aim to investigate whether Tfr2 may be a therapeutic target also for anemia of CKD. Given the TFR2 restricted expression, a TFR2-targeted approach is expected to enhance EPO responsiveness selectively in erythroid cells, reducing the risk of side effects, and to concomitantly correct iron deficiency, because of hepatic Tfr2 downregulation.

Methods

CKD was induced feeding mice an adenine-rich diet for 8 weeks. The protocol consists in a 10-day-long induction phase (0.3% adenine) followed by 46 days of maintenance (0.2% adenine). Mice with BM specific deletion of Tfr2 (Tfr2^BMKO) were generated through BM transplantation (BMT) and CKD induced 10 weeks afterwards.

Antisense oligonucleotides (ASOs) against Tfr2 and control ASOs were injected in wild-type mice twice a week for 6 weeks, starting 2 weeks after renal damage induction.

Results

Wild-type animals fed the adenine diet showed renal damage, increased BUN, anemia, inappropriately low EPO levels and iron restriction, recapitulating human CKD. The same diet was used to induce CKD in mice lacking erythroid Tfr2. Renal damage and iron restriction were comparable between Tfr2^BMKO and control mice, excluding a different effect of the diet in the two groups. Tfr2^BMKO mice had enhanced erythropoiesis, due to the increased EPO responsiveness of erythroid cells lacking Tfr2, as suggested by the over-activation of the EPO-EPOR signaling pathway. Tfr2^BMKO mice maintained higher red blood cell (RBC) count than controls for the entire protocol. Hemoglobin (Hb) levels were higher in Tfr2^BMKO mice for 6 weeks, while reached levels of controls at 8 weeks, concomitant with relative hypoferremia, indicating that BM Tfr2 deletion prevents anemia development until iron availability is adequate to the enhanced erythropoiesis.

Having demonstrated the proof-of-principle that targeting erythroid Tfr2 stimulates erythropoiesis in CKD, we moved to a pharmacologic approach using Tfr2-ASOs, to simultaneously increase iron availability. Tfr2-ASOs increased RBC count and Hb levels during the first 2 weeks of treatment, but Hb reverted to control levels at the end of the protocol. Tfr2-ASO increased the percentage of Ter119+ erythroid cells in the BM and in the spleen, mainly expanding early erythroid precursors, leading to hypothesize that on a long-term iron toxicity impairs erythroid differentiation. Indeed, at the end of treatment, Tfr2-ASOs inhibited Tfr2 expression in the liver, increasing circulating iron over the normal range, while, surprisingly, BM and spleen Tfr2 levels were not reduced. Further studies are currently ongoing to explain this unexpected finding and to understand whether on a long-term the inhibitory effect of Tfr2-ASOs is masked by the concomitant increase in immature erythroid precursors (expressing Tfr2 at high level).

Conclusions

Targeting Tfr2 may be a novel “erythropoiesis-stimulating” approach for anemia of CKD, but Tfr2 inhibition should be tightly controlled to balance Tfr2 downregulation in erythroid cells and in the liver, to the aim of adjusting iron availability according to the enhanced erythropoiesis.