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

Attila Brunyanszki, Katalin Erdelyi, Bartosz Szczesny, Gabor Olah, Reinaldo Salomao, David N. Herndon, Csaba Szabo

The classical role of hemoglobin in the erythrocytes is to carry oxygen from the lungs to the tissues via the circulation. However, hemoglobins also act as redox regulators and as scavengers of the gaseous mediators nitric oxide (NO) and hydrogen sulfide (H2S). Here we show that upregulation of hemoglobins (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 localized to the mitochondria, where it interacts with Complex I and, on one hand, increases mitochondrial respiratory rate and mitochondrial membrane potential, and on the other hand, protects from H2O2-induced cytotoxicity and mitochondrial DNA damage. Both burn injury and sepsis were associated with increased plasma levels of H2S. Incubation of mononuclear cells with H2S induced hemoglobin mRNA upregulation in PBMCs in vitro. Intracellular hemoglobin upregulation conferred a protective effect against cell dysfunction elicited by H2S. Hemoglobin uptake was also associated with a protection from and induced the upregulation of HIF-1α and Nrf2 mRNA. In conclusion, PBMC 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 H2S and other gaseous transmitters, oxidants and free radicals produced in critically ill patients.