Oxidant Stress in Renal Inflammation: Mechanisms and Remedies

Abstract

Our overall hypothesis was that oxidant stress is a central player in renal inflammation; pharmacological reduction of oxidant stress should therefore relieve renal inflammation. We explored pro- and anti-oxidant mechanisms in three experimental renal injury models. OXIDANT-DEPENDENT RENAL INFLAMMATION IN PROTEIN OVERLOAD: MECHANISMS AND REMEDIES. We investigated the nephropathic effects of albumin-bound fatty acids (ABFA). In cultured tubular (HK-2) cells, oleic acid-rich albumin (OA-BSA) induced much more ROS than albumin alone (BSA). OA-BSA effect was significantly ameliorated by mitochondrial inhibitors (rotenone, CCCP) but not by inhibitors of NADPH oxidase (apocynin), xanthine oxidase (allopurinol), or NOS (L-NAME). An antioxidant mitochondrial SOD mRNA response was observed in BSA-treated but not OA-BSA-treated cells. Thus, ABFA have a dual pro-oxidant mechanism, viz. increased mitochondrial ROS and impaired mitochondrial SOD. Additionally, although BSA and OA-BSA similarly increased IL-6 mRNA expression, OA-BSA induced significantly more IL-6 protein - suggesting that ABFA support transcriptionally-driven pro-inflammatory mechanisms. OA-BSA-induced IL-6 protein was suppressed by SOD augmentation (EUK-8), and by mitochondrial inhibition (rotenone or CCCP), revealing oxidant-dependence. To study this mechanism in vivo, we established a protein-overload model (129S2/Sv mice), with marked proteinuria and tubulointerstitial macrophage infiltration. We probed whether inhibition of mitochondrial ROS, or SOD supplementation, would alleviate ABFA-induced inflammation. Mice received OA-BSA with or without tempol (SOD mimetic) or rotenone (mitochondrial inhibitor). Tempol markedly reduced inflammation, confirming ROS-dependence. Rotenone was also beneficial, supporting mitochondrial involvement. The benefits were not due to anti-proteinuric activity. Separately, niacin significantly reduced OA-BSA-induced proteinuria and inflammation, suggesting that lowering free FA may be beneficial in proteinuric states. ANGIOTENSIN II INFUSION IN MICE: NATURE’S POINTER ON RENAL ANTIOXIDANT STRATEGY. In rats, Angiotensin II (AngII) induces renal inflammation and injury involving NADPH oxidase-dependent oxidant stress. Mice, in contrast, show resistance to hypertensive renal injury. We hypothesised that mice may possess strong intrinsic antioxidant mechanisms that protect against AngII-induced injury. We probed renal redox-related gene responses to AngII in C57BL/6 mice, which are particularly resistant to renal injury. AngII induced hypertension, but not proteinuria or renal injury. AngII induced renal antioxidant genes while pro-oxidant genes (p22phox, Rac2, xanthine oxidase) were suppressed. Expression of HO-1, with protective anti-oxidant and anti-inflammatory properties, was upregulated. Unlike in the kidney, AngII markedly induced aortic mRNA expression of the NADPH oxidase component p47phox, accompanied by aortic hypertrophy. Thus, a kidney-specific antioxidant protective mechanism seems to operate in this model. INFLAMMATION IN UNILATERAL URETERAL OBSTRUCTION (UUO). PROTECTIVE ROLE FOR APOCYNIN? UUO is characterised by rapid tubulointerstitial inflammation, and oxidant stress of unclear source. NADPH oxidase may be involved. We hypothesised that NADPH oxidase inhibition might reduce oxidant-dependent inflammation. The specific inhibitor, apocynin, achieved a moderate but significant 25% reduction in tubulointerstitial macrophage influx during early UUO. Furthermore, apocynin significantly alleviated UUO-induced gene downregulation of BMP-7, a molecule with well-known renoprotective properties. Thus, during UUO, BMP-7 expression may be NADPH oxidase-dependent. Apocynin anti-inflammatory effect may be via enhanced BMP-7 expression. Overall modest benefits of apocynin suggests that other ROS sources or non-oxidant-dependent mechanisms, are involved.