Roles of Na+/H+ Exchanger Type 1 and Intracellular pH in Angiotensin II-Induced Reactive Oxygen Species Generation and Podocyte Apoptosis

Abstract

A growing body of evidence suggests that podocyte apoptosis is a major cause of decreased podocyte number, which leads to albuminuria and glomerular injury. The aim of this study was to clarify the molecular mechanisms of angiotensin II (Ang II)-induced apoptosis in cultured mouse podocytes. We examined the effects of Ang II (100 nmol/L) on apoptosis, superoxide anions, and cytosol pH in podocytes. For intracellular pH measurements, image analysis was conducted using confocal laser microscopy after incubation with carboxyseminaphthorhodafluor-1. Superoxide anions and intracellular pH were elevated with Ang II treatment. Apoptotic cell numbers, as measured by TUNEL staining and caspase 3 activity, were also augmented in the Ang II–treated group. Pre-treatment with olmesartan (100 nmol/L, an Ang II type 1–receptor blocker), apocynin (50 μmol/L, NADPH oxidase inhibitor), or 5-N,N hexamethylene amiloride [30 μmol/L, Na+/H+ exchanger type 1 (NHE-1) inhibitor] abolished Ang II–induced podocyte apoptosis, whereas NHE-1 mRNA and protein expression was not affected by Ang II treatment. Moreover, Ang II increased NHE-1 phosphorylation. These results suggest that superoxide production, NHE-1 activation, and intracellular alkalization were early features prior to apoptosis in Ang II–treated mouse podocytes, and may offer new insights into the mechanisms responsible for Ang II–induced podocyte injury.