Hemoglobin in the Kidney
Authors: Volker H. Haase
Journal: Journal of the American Society of Nephrology (2008)

Hemoglobins (Hb) in both prokaryotes and eukaryotes belong to an ancient family of heme-associated, iron-containing globin proteins. Mammalian Hb are tetrameric metalloproteins (approximately 68 kD) that are present in millimolar concentrations within red blood cells, where they function as O2-carrier proteins, facilitate CO2 transport in deoxygenated blood, and react with nitric oxide. Tetramers are composed of identical heterodimers that consist of α and β chains encoded by gene families located on separate chromosomes. The expression of various α and β genes in erythroid cells is tightly coordinated during development and in adulthood, HbA (α2β2 tetramer) being the most abundant adult Hb. Although Hb have been traditionally thought to be exclusively expressed in cells of erythroid lineage, reports now suggest expression in nonerythroid cells, such as activated macrophages, respiratory epithelial cells, and Clara cells.1,2 As part of a genome-wide microarray and proteomics screen to identify differentially regulated genes in chronically hypoxic kidneys, Nishi et al.3 unexpectedly found increased expression of α and β globin in kidney homogenates and investigated the cellular source of renal Hb in this issue of JASN. Using manual and laser capture microdissection of nephron segments, as well as sieving and in situ hybridization techniques, Nishi et al. demonstrate that renal Hb are specifically expressed in mesangial cells, where they may exist in dimeric form. Although the biologic functions of nonerythroid Hb are poorly understood, Nishi et al. propose that mesangial Hb may function as a scavenger for reactive oxygen species and thus protect cells from oxidative stress. This notion is supported by overexpression studies in cultured mesangial cells demonstrating that intracellular generation of reactive oxygen species is decreased in cells transfected with α- and β-Hb and that increased Hb expression is associated with improved cell viability, albeit modest, as measured by lactate dehydrogenase release.3 Although Nishi et al. establish that both α- and β-Hb are expressed in mesangial cells, several questions regarding expression levels and biochemical structure remain unanswered. For a comprehensive understanding of the biologic functions of mesangial Hb, it will be important to determine the ratio of α- and β-polypeptide chains in mesangial cells under physiologic conditions, establish whether mesangial Hb are predominantly expressed as individual polypeptide subunits or whether they form heterotetramers, and clarify whether the heme prosthetic group is efficiently incorporated. The heme group contains a porphyrin ring, which provides four nitrogen ligands to the central iron atom (Fe), whereas the fifth nitrogen ligand is provided from a histidine residue in the globin component of Hb. The sixth coordination site of iron binds external ligands, such as O2, nitric oxide, and carbon monoxide. The heme iron is normally carried in a ferrous state [Fe(II)] and is oxidized to ferric [Fe(III)] or ferryl [Fe(IV)] forms under conditions of oxidative stress or as a result of auto-oxidation that is accelerated with hypoxia. Ferric and ferryl Hb species initiate undesired oxidation reactions under pathophysiologic conditions such as hemolysis and ischemia-reperfusion injury4 and may blunt the cytoprotective effects observed by Nishi et al.3 Aside from its central role in O2 transport, erythroid Hb has a significant role in nitric oxide metabolism.5 Endothelial nitric oxide, which is generated from L-arginine, is a potent vasodilator and diffuses into the blood, where it chemically interacts with Hb in erythrocytes. There, it can either react with oxyhemoglobin to form methemoglobin [Fe(III)] and nitrate, with deoxyhemoglobin to form iron-nitrosyl Hb, or with cysteine at position 93 of β-Hb to form S-nitroso Hb5,6; therefore, Hb may function as both nitric oxide scavengers and nitric oxide carriers and participate in the regulation of vascular tone.5,6 Whether the chemical reactions of nitric oxide with Hb also occur in mesangial cells is unclear and certainly warrants further investigation because the nitric oxide–Hb chemical interaction may be relevant to the regulation of glomerular hemodynamics. Although not formally examined in mesangial cells, Nishi et al. observed transient induction of kidney Hb by hypoxia in a model of chronic hypoxic injury generated by placement of a left main renal artery stenosis.3 Hypoxic regulation of Hb synthesis has been observed for certain Hb chains in several species in vitro and in vivo and in some cases may involve the hypoxia-inducible factor signaling pathway.7,8 Neuroglobin and cytoglobin, two more recently described globin family members, are also hypoxically induced and confer cytoprotection during ischemia.9,10 Whether hypoxia modulates the biologic function of mesangial Hb remains to be investigated. Nishi et al.3 propose that increased Hb gene expression in mesangial cells protects cells from oxidative stress, at least under normoxic conditions. Although the underlying mechanisms by which cytoprotection is afforded are not known, their findings are consistent with the cytoprotective role of other heme-containing globin family members4,11 and may depend on the structural configuration of mesangial Hb (monomeric versus tetrameric) or the redox status of heme iron. Neuroglobin is a recently discovered monomeric globin that is expressed in neurons and in pancreatic islets, where it is converted to its ferric form under oxidative stress. Ferric neuroglobin regulates signal transduction through G-protein–coupled receptors and promotes cell survival under hypoxia.11 Knockdown studies using small interfering RNA or cell lines from genetic mouse models of defective Hb expression will be useful in the future to elucidate further the molecular mechanism that underlies the cytoprotective effects of mesangial Hb under various physiologic and pathologic conditions. In conclusion, the study by Nishi et al together with previous reports of nontraditional functions for Hb, such as its role in redox regulation, nitric oxide metabolism, and regulation of vascular tone and protection against oxidative stress, provokes interesting speculations with regard to renal physiology and pathology and will certainly stimulate further investigations into the biology of mesangial Hb in the setting of acute and chronic renal injury. DISCLOSURES None.