In this study, we found that serum nephrin and podocalyxin levels and their corresponding plasma mRNA levels were significant reduced after rHuEPO treatment in DKD patients. After rHuEPO treatment for 36 weeks, serum nephrin and podocalyxin levels, but not their corresponding plasma mRNA levels, had significant inverse correlation with the change in hemoglobin level.
Several previous studies suggested that erythropoietin plays an important role in podocyte function in normal and diseased kidneys [19]. Echigoya et al [16] reported that erythropoietin receptor is expressed in developing and mature podocytes of mouse kidney, suggesting a possible role for erythropoietin in podocyte biology. Erythropoietin prevents podocyte injury and apoptosis in vivo and vitro [13]. In our present study, we found that serum nephrin and podocalyxin levels, and their corresponding plasma mRNA levels, were significant reduced after rHuEPO treatment, which is in line of a podocyte protection effect. Although we did not explore the mechanism of podocyte protection, three previous studies found that erythropoietin prevents diabetes-induced podocyte damage in diabetic mice [20, 21] as well as aminonucleoside-induced kidney injury [22]. Specifically, erythropoietin protects podocytes from the damage by advanced glycation end-products [23], and Eto et al [14] reported that rHuEPO treatment ameliorates podocyte injury by a direct effect on podocytes by the maintenance of the actin cytoskeleton and nephrin expression. In the last mentioned study, there was a concomitant decrease in proteinuria following rHuEPO treatment [14], which was not observed in our patients. The reason for this discrepancy is not clear.
We observed a significant inverse correlation between the change in serum podocyte marker levels and the corresponding change in hemoglobin level following rHuEPO treatment. The explanation of this finding is unknown. The correction of anemia and intra-renal hypoxia per se may ameliorate podocyte injury [24, 25], but rHuEPO may have direct off-target effects on podocyte [13, 14]. It should be noted that baseline serum podocyte marker levels did not correlate with hemoglobin level, suggesting that anemia or intra-renal hypoxia is less likely to entirely account for the podocyte damage. A previous study on kidney allograft recipients showed that erythropoietin, but not the correction of anemia alone, protects from chronic allograft injury [15].
Our study have several inadequacies. First, this was a single-center observational study with limited sample size and no control group. The natural course of circulating podocyte marker levels in untreated DKD patient is unknown, and elaborated multivariate analysis to identify predictors of change in podocyte marker level was not possible. In this study, we recruited only CKD 3b or 4 stage patients. Further studies are required to define the effect of rHuEPO treatment on podocyte biomarker levels in diabetic patients with early stages. Similarly, further studies are needed to determine whether hypoxia inducible factor (HIF) prolyl hydroxylase (PH) inhibitor, a new class of agents for the treatment of anemia in CKD [26, 27], may have a similar effect on circulating podocyte marker. In this study, we did not quantify glomerular podocyte number, intra-renal podocyte-related molecules levels, or urinary podocyte markers before and after rHuEPO treatment. Monitoring of urinary and intra-renal podocyte-related molecules, either at mRNA or protein level, may shed extra light on the effect of rHuEPO treatment on podocyte dysfunction [28, 29]. Unfortunately, because of the limitations in our original study design, assay of these markers was not possible.
In conclusion, we found that serum nephrin and podocalyxin levels and their corresponding plasma mRNA levels were significant reduced after rHuEPO treatment in DKD patients, and the change in serum nephrin and podocalyxin levels correlated with the change in hemoglobin level. The effect of rHuEPO on podocyte injury deserves further study.