Altered mitochondrial function, calcium signaling, and catecholamine release in chromaffin cells of diabetic and SHR rats.
A recent study shows that angiotensin II receptor type 2 is expressed and localized with angiotensin II to mitochondrial inner membrane in various cell types and regulates NO generation and mitochondrial functions. The findings might have important implications in various inflammatory conditions and cell senescence. The renin-angiotensin system (RAS) is a dominant regulator of the cardiovascular and renal function. Ang II is the key effector molecule of this system. Most studies have focused on the effect of extracellular Ang II on its receptors on the plasma membrane. However, Ang II and renin are also co-localized intracellularly in many types of cells including juxtaglomerular cells,3 cultured neuroblastoma,4,5 and various endocrine cells. Intracellular Ang II is generated within the cells and secreted for local paracrine function. It is also transported from the extracellular space via an intracrine mechanism. Induction of Ang II expression in stretched cardiac myocyte and consequent myocyte hypertrophy may represent an intracrine mechanism.6 Likewise, microinjection of Ang II in vascular smooth muscle cells results in an increase in [Ca ]i via intracellular AT1. Neonatal cardiac myocytes transfected with an adenoviral Ang II expression vector shows elevated rate of cellular hypertrophy independent of cell surface AT1. Adult mice injected with an Ang II expression plasmid vector develop significant cardiac hypertrophy without an increase in blood pressure or plasma Ang II levels.7 These effects were not suppressed by losartan administration indicating an intracellular mechanism of the intracrine Ang II.9 These observations suggest possible functions of intracellular Ang II via intracellular AT1 if Ang II gains access to the intracellular space either by internalization or possibly by local biosynthesis. The presence of AT2 also has been reported in the nucleus of sheep kidney cortical cells, which is functionally linked to endothelial nitric oxide synthetase (eNOS) and soluble guanylyl cyclase.10 AT2 on plasma membrane could also stimulate NO synthesis with or without the bradykinin B2 receptor participation.11–13 The studies discussed above have suggested the presence of an intracellaular RAS. Likewise, previous studies have implicated biological effects of Ang II in mitochondrial size and proliferation. However, the presence of mitochondrial angiotensin system had remained unrecognized until a recent report by Abadir et al.1These investigators showed that Ang II and its receptor AT2co-localize on mitochondrial inner membrane in a variety of human and mouse tissues including human skeletal muscle cells, monocytes, mouse cardiac myocytes, renal tubular cells, neuronal cells, vascular endothelial cells, and hepatocytes by immunogold electron microscopy. Mitochondrial localization of AT2 was further confirmed by fluorescence overlay in human fibroblast transfected with AT2-green fluorescent protein fusion protein and fluorescence labeling of mitochondria by MITO Tracker Red. These findings suggest that Ang II and AT2 may be common general components of the inner membrane of the mitochondria. An interesting observation was that AT2 expression in the mitochondria of mouse renal tubular cells was decreased with age. In contrast, type 1 Ang II receptor AT1 level, which is very low in mitochondria of young mice, was markedly elevated with age (70 wk of age). Long-term treatment of animals with the AT1 receptor blocker losartan maintained the mitochondrial (mt) AT2 at a high level in aged animals, whereas the elevated AT1 in aged animals was not affected by losartan. Functionally, stimulation of isolated mitochondria by the AT2-specific agonist CGP421140 resulted in a dosedependent increase in nitric oxide (NO) production. Respiratory oxygen consumption by the isolated mitochondria with glutamate/malate-ADP–support was suppressed by NO produced by CGP stimulation, but the suppression of respiration was reversed by the inhibition of NO synthetase (NOS) by AT2 specific antagonist PD123319 or by NOS inhibitor L-nitroarginine methyl ester. Inhibition of respiration by CGP was not accompanied by changes in the membrane potential indicating that the bioenergetic properties of the mitochondria were not affected. The suppression of respiration by NO was presumably due to its competition with O2 for binding to cytochrome C oxidase (COX IV), which is also located in the inner membrane of the mitochondria. These observations clearly demonstrate intra mitochondrial localization and function of AT2 and its role in activating mtNOS to produce NO and inhibit respiration. The inverse relationship between intramitochondrial levels of AT1 and AT2 with aging is particularly intriguing. The tissue protective NO production mediated by mtAT2-mtNOS decreased with aging and harmful reactive oxygen species producing mtAT1 increased in 70-wk-old mice. Long-term administration of AT1 receptor blocker has been known to have beneficial effects against The opinions expressed in this Commentary are not necessarily those of the editors or of the American Heart Association. Commentaries serve as a forum in which experts highlight and discuss articles (published elsewhere) that the editors of Circulation Research feel are of particular significance to cardiovascular medicine. Commentaries are edited by Aruni Bhatnagar and Ali J. Marian. From the Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN. Correspondence to Tadashi Inagami, Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN 37232. E-mail tadashi.inagami@Vanderbilt.edu (Circ Res. 2011;109:1323-1324.) © 2011 American Heart Association, Inc.