Sickle cell disease (SCD) is a recessively inherited hemoglobin disorder; the most common and severe form is a consequence of homozygous βS mutation. High concentration of hemoglobin S damages RBC membranes, leading to hemolysis, vaso-occlusion, and inflammation, which together result in anemia, recurrent painful crises, and multiple end-organ damage (i.e. brain, kidney, lung, and bone). Anemia in SCD is multi-factorial. Causes include hemolysis, ineffective erythropoiesis, impaired iron utilization, insufficient erythropoietin responsiveness, and low oxygen affinity [Sherwood Blood 1986]. RBC transfusions are used to ameliorate symptoms and complications but indications are not clearly defined for adults. We postulate that impaired iron utilization in SCD is a consequence of complex regulation of hepcidin, the main hormone regulating circulating and systemic iron. We aim to explore correlation between hepcidin, inflammation and erythropoiesis to enhance our understanding of iron metabolism, hepcidin regulation, and pathophysiology of anemia in SCD. Ultimately this may yield more specific indications for RBC transfusion, and pave the way for novel therapeutic approaches.
High hepcidin results in the sequestration of iron within macrophages and prevents further iron absorption in the gastrointestinal tract. Hepcidin expression is enhanced by iron and inflammation and suppressed by hypoxia and erythropoiesis. Distinguishing the relative contribution of these competing effects on hepcidin in SCD is complicated. For example, adult SCD patients in steady state have significantly lower hepcidin relative to heterozygous controls and do not correlate with markers of inflammation and erythropoiesis [Kroot Haematologica 2009]. In a randomized, placebo-controlled trial, inhaled steroids (mometasone) were administered to non-asthmatic SCD patients [Glassberg Am J Hematol 2017]. The results demonstrate improved pain scores and reduced hemolysis in the treated group although the mechanism of this effect is not known. In a subsequent analysis, markers of inflammation are suppressed in the treated group, correlating with macrophage activation. We thus evaluate hepcidin expression in stored serum samples from this trial of SCD patients treated with inhaled steroids or placebo. Samples before and after treatment were analyzed. Serum hepcidin quantification was performed with Hepcidin-25 ELISA kit (Intrinsic Lifesciences LLC). In addition, we evaluate if and how hepcidin levels correlate with pro-inflammatory markers measured with olink inflammation assay.
Serum hepcidin concentration is not different in steroid- vs. placebo-treated SCD patients before or after treatment and no significant difference is observed in the hepcidin ratio after:before treatment between the two groups (Fig 1a). However, in the steroid group, hepcidin concentrations are increased in a higher proportion of patients although the difference does not reach statistical significance (Fig 1b). We thus hypothesize that hepcidin levels may be a marker of increased erythropoiesis relative to inflammation in SCD. Both IL-6 (Fig 2a) and IL-10 (Fig 2b), cytokines known to induce hepcidin expression during inflammation, decrease after treatment with steroids relative to placebo, and neither correlates with hepcidin concentration after treatment (Table I). Hepcidin is however inversely correlated with adenosine deaminase (ADA) and hemoglobin S and directly correlative with fibroblast growth factor 23 (FGF23), monocyte chemoattractant protein 1 (MCP1), and hemoglobin F in both placebo- and steroid-treated groups (Table I), potentially suggesting that hepcidin inversely correlates with expanded erythropoiesis. We thus use the newly available human ELISA kit for erythroferrone (ERFE) (Intrinsic Lifesciences LLC), a recently identified erythroid regulator of hepcidin [Kautz Nat Gen 2014], to evaluate correlation with hepcidin. Our results demonstrate that serum ERFE concentration decreases in steroid- (Fig 3a) but not placebo-treated SCD patients (Fig 3b), and no correlation with hepcidin is observed. Taken together, these data for the first time demonstrate a decrease in erythropoiesis and erythroid-regulation-induced hepcidin suppression in inhaled-steroid-treated SCD patients.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.