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 (HS). 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 extra-cellular 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 HO-induced cytotoxicity and mitochondrial DNA damage. Both burn injury and sepsis were associated with increased plasma levels of HS. Incubation of mononuclear cells with HS induced hemoglobin mRNA upregulation in PBMCs . Intracellular hemoglobin upregulation conferred a protective effect against cell dysfunction elicited by HS. 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 HS and other gaseous transmitters, oxidants and free radicals produced in critically ill patients.