Angiotensin II-induced superoxide and decreased glutathione in proximal tubules: Effect of dietary fructose

Abstract

Angiotensin II exacerbates oxidative stress in part by increasing superoxide (O2–) production by many renal tissues. However, whether it does so in proximal tubules and the source of O2– in this segment are unknown. Dietary fructose enhances the stimulatory effect of angiotensin II on proximal tubule Na+ reabsorption, but whether this is true for oxidative stress is unknown. We hypothesized that angiotensin II causes proximal nephron oxidative stress in part by stimulating NADPH oxidase (NOX)4-dependent O2– production and decreasing the amount of the antioxidant glutathione, and this is exacerbated by dietary fructose. We measured basal and angiotensin II-stimulated O2– production with and without inhibitors, NOX1 and NOX4 expression, and total and reduced glutathione (GSH) in proximal tubules from rats drinking either tap water (control) or 20% fructose. Angiotensin II (10 nM) increased O2– production by 113 ± 42 relative light units·mg protein–1·s–1 in controls and 401 ± 74 relative light units·mg protein–1·s–1 with 20% fructose (n = 11 for each group, P < 0.05 vs. control). Apocynin and the Nox1/4 inhibitor GKT136901 prevented angiotensin II-induced increases in both groups. NOX4 expression was not different between groups. NOX1 expression was undetectable. Angiotensin II decreased GSH by 1.8 ± 0.8 nmol/mg protein in controls and by 4.2 ± 0.9 nmol/mg protein with 20% fructose (n = 18 for each group, P < 0.047 vs. control). We conclude that 1) angiotensin II causes oxidative stress in proximal tubules by increasing O2– production by NOX4 and decreasing GSH and 2) dietary fructose enhances the ability of angiotensin II to stimulate O2– and diminish GSH, thereby exacerbating oxidative stress in this segment.