Upregulation and Mitochondrial Sequestration of Hemoglobin Occur in Circulating Leukocytes during Critical Illness, Conferring a Cytoprotective Phenotype

The classical role of hemoglobin in the erythrocytes is to carry oxygen from the lungs to the tissues via the circulation. However, hemoglobin also acts as a redox regulator and as a scavenger of the gaseous mediators nitric oxide (NO) and hydrogen sulfide (H 2 S). Here we show that upregulation of hemoglobin (α, β and δ variants of globin proteins) occurs in human peripheral blood mononuclear cells (PBMCs) in critical illness (patients with severe third-degree burn injury and patients with sepsis). The increase in intracellular hemoglobin concentration is a result of a combination of enhanced protein expression and uptake from the extracellular space via a CD163-dependent mechanism. Intracellular hemoglobin preferentially localizes to the mitochondria, where it interacts with complex I and, on the one hand, increases mitochondrial respiratory rate and mitochondrial membrane potential, and on the other hand, protects from H 2 O 2 -induced cytotoxicity and mitochondrial DNA damage. Both burn injury and sepsis were associated with increased plasma levels of H 2 S. Incubation of mononuclear cells with H 2 S induced hemoglobin mRNA upregulation in PBMCs in vitro. Intracellular hemoglobin upregulation conferred a protective effect against cell dysfunction elicited by H 2 S. Hemoglobin uptake also was associated with a protection from, and induced the upregulation of, HIF-1α and Nrf2 mRNA. In conclusion, PBMCs in critical illness upregulate their intracellular hemoglobin levels by a combination of active synthesis and uptake from the extracellular medium. We propose that this process serves as a defense mechanism protecting the cell against cytotoxic concentrations of H 2 S and other gaseous transmitters, oxidants and free radicals produced in critically ill patients.