High Transferrin Saturation Is Associated with Lower Monocytic Ferritin Heavy Chain Expression in Sickle Cell Anemia Patients

The pathophysiology of sickle cell anemia (SCA) involves hemolysis, vaso-occlusion and a chronic inflammatory state. Iron overload secondary to blood transfusions is a frequent complication in these patients, but cannot be adequately estimated by serum ferritin levels, because ferritin is also an acute phase reactant. Although excess iron elevates both ferritin levels and transferrin saturation (TSAT) in SCA patients, there is notorious discrepancy between these parameters. Ferritin is composed of heavy (FHC) and light chains (FLC), and ferroxidase activity by FHC is an important cytoprotective mechanism against redox-iron, a product of heme breakdown and largely present in overt iron overload. Previous studies have shown that overexpression of FHC in sickle cell mice prevented free hemoglobin-induced vaso-occlusion. Since ferritin is also highly expressed in circulating monocytes, and these cells have been shown to interact with other cellular types in the sickle cell vaso-occlusive process, we aimed to characterize ferritin chains in monocytes and investigate the relationship with biomarkers of iron metabolism, inflammation and hemolysis.

Peripheral blood monocytes from sixteen adult sickle cell anemia patients in steady state were isolated using a double Ficoll-Percoll density gradient to separate monocytes from neutrophils and lymphocytes. FHC, FLC, TLR4 (toll-like receptor 4), and SLC40A1(ferroportin) gene expressions were determined by RT-qPCR. Blood samples were also collected to determine serum ferritin, iron, and TSAT, and plasma levels of lactate dehydrogenase, soluble transferrin receptor, erythropoietin, and C reactive protein.

We found that the expression of TLR4, a receptor known to be activated by heme, correlated with FLC, but not FHC expression. Higher TLR4 expression was also associated with higher serum iron, but not with ferritin, TSAT, or LDH. Interestingly, we did not find a correlation between C reactive protein levels and ferritin in this group of patients. As expected, the expressions of both ferritin chains were correlated with each other (P=0.027, r=0.55), but we found the strongest correlation between FHC and TSAT (P=0.0008, r=-0.652). Patients with a TSAT over 40% had significantly lower expression of monocytic FTH (P=0.003). This suggests that either excessive iron can lead to FHC downregulation in monocytes, or that a decrease in monocytic ferritin ferroxidase activity in some SCA patients may impair safe iron storage in ferritin and contribute to the development of higher TSAT, independently from ferritin levels. Our data support that human monocyte regulation of ferritin chains in SCA patients mirrors what has been described in hepatic cells in a sickle cell mouse model. Patients with increased TSAT may be relatively deprived of the cytoprotective ferroxidase activity of FHC, and a relationship between FHC deficiency and complications in SCA remains to be investigated. Further studies should also address whether FHC in monocytes influences cell adhesion, thus supporting an important role for iron trafficking in cells involved in sickle cell vaso-occlusion, and corroborating other studies associating organ damage in SCA with iron metabolism dysregulation.