In this study, we demonstrated the unexpected results that the co-administration of even a single dose of intravenous SFO attenuated the EPO response 3 weeks after CERA administration. In accordance with the changes in Hb, ERFE and Ret levels were lower with CERA + SFO than with CERA. In addition, the regression analysis of the relationship among Ret, ERFE, and HEPC led to the hypothesis that ESA administration increases erythroblasts, which release ERFE to suppress hepcidin expression in hepatocytes and favour iron acquisition for haemoglobin synthesis .
It has already been established that iron is indispensable for erythropoiesis and will accelerate the synthesis of haem and haemoglobin. . Erythroblasts require large amounts of iron to facilitate high rates of haem synthesis. The final step in the haem biosynthetic pathway occurs on the inner mitochondrial membrane, where iron is inserted into protoporphyrin IX by ferrochelatase (FECH). Iron is not only a substrate of haem biosynthesis, but is also utilized in the formation of the Fe-S cluster proteins in the mitochondria, which regulate the activity of enzymes associated with haem biosynthesis. These enzymes are aconitase, erythroid delta-aminolevulinate synthase, and FECH [23-25]. Thus, it is reasonable to assume that iron would accelerate erythropoiesis. However, administration of intravenous iron (40 mg of SFO) attenuated the EPO response to CERA for 4 weeks.
We recently proposed that excessive iron storage could even hamper erythropoiesis or the differentiation of erythroid in the therapy for renal anaemia . In this review, we hypothesised that two possible mechanisms could affect erythropoiesis, that is, an increase in hepcidin levels and the bone marrow environment with oxidative stress.
Iron administration inevitably increases hepcidin levels, which hampers iron recycling from senescent erythrocytes and diminishes the iron supply to erythroblasts sequentially. The role of hepcidin-mediated iron restriction in the development of renal anaemia has been demonstrated in several recent animal studies. In these observations, mice with adenine-induced CKD did not show anaemia or iron deficiency if hepcidin expression was deleted [4, 26]. In the present study, hepcidin levels were significantly higher with CERA + SFO administration than with CERA alone throughout the study period. Therefore, it is reasonable to suspect that the higher hepcidin level was related to the effect of co-administration of iron on erythropoiesis.
Second, a large body of in vitro and in vivo evidence shows that bone marrow iron overload inhibits erythroid differentiation. However, this mechanism may not affect the results of the present study, because decreases in serum ferritin and TSAT after CERA administration may show a decline in the iron storage of bone marrow.
In addition, we should take into account FGF23 metabolism, above all else, from recent observations. Although FGF23 has been established as a phosphaturic hormone, it has been demonstrated to have a negative effect on erythropoiesis, as FGF23 knockout mice showed increased erythropoiesis with reduced erythroid cell apoptosis. FGF23 is formed as an intact, biologically active protein (iFGF23) and proteolytically cleaved into cFGF23 . Cleavage of iFGF23 is regulated by several factors, including EPO, iron, CKD, and inflammation [15, 16]. Although cFGF23 is believed to be inactive, recent animal studies showed that cFGF23 even reduces the suppressive effect of iFGF23 on erythropoiesis by competitively inhibiting FGF23 receptor signalling [17, 18, 28]. In the present study, cFGF23 level was significantly higher in the CERA period than in the CERA+SFO period, whereas iFGF23 level did not differ between CERA and CERA + SFO periods. From these observations, we can speculate that the difference in cFGF23 level between the CERA and CERA+SFO periods may affect erythropoiesis. A single administration of intravenous iron may affect FGF23 cleavage, and consequently, lower cFGF23 levels.
In this study, we also examined the effects of CERA and SFO on inflammatory markers, such as hsCRP and IL-6. Interestingly, a single administration of intravenous iron increased serum levels of hsCRP, but not IL-6. ESA has been reported to have the potential to ameliorate uremic inflammation [29, 30]. Conversely, IL-6 and tumour necrosis factor-alpha levels reportedly increase temporarily after the administration of intravenous iron preparations . Iron is taken up by monocytes, resulting in the activation of the transcription factor, nuclear factor-kappa B, which plays a central role in the immune response and increases inflammatory cytokines . In this study, hsCRP and IL-6 levels did not significantly change after administration of CERA. However, they became higher after administration of CERA + SFO compared with administration of CERA alone. Although the anti-inflammatory effects of CERA were not documented, the inflammatory effects of SFO were observed. The inflammation induced by SFO could, at least in part, account for the attenuation of haematopoiesis.
First, the sample size of this study was small, which could have influenced the results. Second, the measurement of each biomarker is limited to patients undergoing haemodialysis after the administration of CERA and CERA + SFO. The effects of other ESA preparations or oral iron preparations are unknown. Similarly, the effects of patients with CKD not undergoing haemodialysis are also unknown.