Actualités du métabolisme du fer

Iron in the General Population and Specificities in Older Adults: Metabolism, Causes and Consequences of Decrease or Overload, and Biological Assessment

Iron is involved in many types of metabolism, including oxygen transport in hemoglobin. Iron deficiency (ID), ie a decrease in circulating iron, can have severe consequences. We provide an update on iron metabolism and ID, highlighting the particularities in older adults (OAs). There are three iron compartments in the human body: 1) the functional compartment, which consists of heme proteins including hemoglobin, myoglobin and respiratory enzymes; 2) iron reserves (IR), which consist mainly of liver stocks and are stored as ferritin; and 3) transferrin. There are two types of ID. Absolute ID is characterized by a decrease in IR. Its main pathophysiological mechanism is bleeding, which is often digestive and can be due to neoplasia, frequent in OAs. Biological assessment shows low serum ferritin and transferrin saturation (TS) levels. Furthermore, hypochromic microcytic anemia is frequent, and the serum-soluble transferrin receptor (sTfR) level is high. Functional ID, in which IR are high or normal, is due to inflammation, which is also frequent in OAs, particularly in its chronic form. Biological assessments show high serum ferritin, normal or low TS, and normal sTfR levels. Moreover, C-reactive protein is elevated, and there is moderate non-regenerative non-macrocytic anemia. The main characteristics of iron metabolism anomalies in the elderly are the high frequency of ID (20% of ID with anemia in adults ≥85 years) and the severity of its consequences, which include cognitive impairment in case of ID or iron overload and decrease of physical activity in case of ID. In conclusion, causes of ID are frequently intertwined in OAs as a result of the polymorbidity that characterizes them. ID can have dramatic consequences, especially in frail OAs. Thus, measuring the appropriate biological markers prevents errors in the positive diagnosis of ID type, clarifies etiology, and informs treatment-related decision-making.

Rheumatoid anemia

Rheumatoid anemia is a typical example of anemia of chronic disease. It differs from other forms of anemia, such as iron deficiency anemia or iatrogenic anemia. Rheumatoid anemia is normochromic, normocytic or, less often, microcytic, aregenerative, and accompanied with thrombocytosis. Serum transferrin levels are normal or low, transferrin saturation is decreased, serum ferritin levels are normal or high, the soluble transferrin receptor (sTfR) is not increased (a distinguishing feature with iron deficiency anemia), and the sTfR/log ferritin ratio is lower than 1. This review discusses the prevalence and impact of rheumatoid anemia based on a review of the literature. Iron metabolism, absorption, diffusion, storage, and use by the bone marrow are described using published data on transferrin, ferritin, and hepcidin. Hepcidin is now recognized as a key factor in rheumatoid anemia, in conjunction with the cytokine interleukin-6 (IL-6). Hepcidin is a hormone that lowers serum iron levels and regulates iron transport across membranes, preventing iron from exiting the enterocytes, macrophages, and hepatocytes. In addition, hepcidin inhibits intestinal iron absorption and iron release from macrophages and hepatocytes. The action of hepcidin is mediated by binding to the iron exporter ferroportin. Hepcidin expression in the liver is dependent on the protein hemojuvelin. Inflammation leads to increased hepcidin production via IL-6, whereas iron deficiency and factors associated with increased erythropoiesis (hypoxia, bleeding, hemolysis, dyserythropoiesis) suppress the production of hepcidin. Data from oncology studies and the effects of recombinant human IL-6 support a causal link between IL-6 production and the development of anemia in patients with chronic disease. IL-6 diminishes the proportion of nucleated erythroid cells in the bone marrow and lowers the serum iron level, and these abnormalities can be corrected by administering an IL-6 antagonist. IL-6 stimulates hepcidin gene transcription, most notably in the hepatocytes. Studies involving human hepatocyte exposure to a panel of cytokines showed that IL-6, but not TNFα or IL-1, induced the production of hepcidin mRNA. Recent data on hepcidin level variations in patients with rheumatoid arthritis are reviewed. Rheumatoid anemia is best corrected by ensuring optimal control of systemic disease activity. The role for iron supplementation (per os or intravenously) and erythropoietin in the treatment of rheumatoid anemia is discussed. Given the cascade of interactions linking IL-6, hepcidin, and anemia, IL-6 antagonists hold considerable promise for the management of rheumatoid anemia.