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 ($O_2^{-}$) production by many renal tissues. However, whether it does so in proximal tubules and the source of $O_2^{-}$ 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 $O_2^{-}$ production and decreasing the amount of the antioxidant glutathione, and this is exacerbated by dietary fructose. We measured basal and angiotensin II-stimulated $O_2^{-}$ 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 $O_2^{-}$ 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 $O_2^{-}$ production by NOX4 and decreasing GSH and 2) dietary fructose enhances the ability of angiotensin II to stimulate $O_2^{-}$ and diminish GSH, thereby exacerbating oxidative stress in this segment.

Keywords: NADPH oxidase; hypertension; kidney; salt.