Effects of nitric oxide on red blood cell development and phenotype

Nitric oxide (NO) is a diffusible free radical generated primarily by NO synthases (NOS), isoenzymes that convert the l-arginine and molecular oxygen to citrulline and NO in cells. Endothelial cells as well as macrophages, components of hematopoietic microenvironment and potent NO producers, play an active role in the modulation of human hematopoietic cell growth and differentiation. A role of NO in erythroid cell differentiation has been postulated based on demonstration that NO inhibits growth, differentiation, and hemoglobinization of erythroid primary cells. Endothelial NOS (eNOS) mRNA and protein levels, as well as bioactivity, decrease during erythroid differentiation, concomitantly with the elevation of hemoglobin levels. Human red blood cells (RBCs) have been reported to contain some eNOS activity; NO appears to affect RBC's deformability. Generally, NO activates cellular soluble guanylyl cyclase (sGC) to produce a second messenger molecule cGMP. NO increases cGMP, gamma-globin, and HbF levels in human erythroid cells whereas inhibition of sGC prevents NO-induced increase in gamma-globin gene expression. Activation of sGC increases gamma-globin gene expression in primary human erythroblasts. High cAMP levels continuously decrease in contrast to steady but low levels of cGMP during erythroid differentiation. The activation of the cAMP pathway has also been reported to induce expression of the gamma-globin gene in human erythroid cells. NO is hydrophobic and accumulates in lipid membranes, and most autoxidation to nitrite in vivo occurs there. The reaction of NO with deoxyhemoglobin produces nitrosylhemoglobin (HbFe(II)NO), while that with oxyhemoglobin produced methemoglobin and nitrate. Nitrite can also react with deoxyhemoglobin to produce NO. This reaction as well as the postulated formation of a thiol-NO derivative of hemoglobin (SNO-Hb) appears to be major mechanisms for the preservation and transport of NO bioactivity by red cells--making NO act as a "hormone." Thus, RBCs and hemoglobin molecules are essential factors in regulating the bioactivity of NO throughout the mammalian body and may be important in the pathophysiology of several circulatory diseases and be the basis for new therapeutic approaches to these diseases.