Chronic Renal Hypoxia Leads to Injury and Dysfunction: Role of HIF1alpha

Abstract

Renal hypoxia precedes renal injury and impaired GFR in multiple renal disease models and in hypertension. Further, in imaging studies in humans, renal hypoxia correlates with hypertension and reduced renal function. We propose that renal injury that accompanies hypertension is mediated by events stimulated by hypoxia and responses linked to HIF1a. To test these effects, we developed a pilot study to generate reproducible renal hypoxia in rats by treatment with 2,4‐dinitrophenol (DNP, 30 mg/kg/day). DNP is a mitochondrial uncoupling agent that acts as a proton ionophore, which reduces production of ATP, yet still consumes O2. Renal cortical and medullary pO2 levels were lower in DNP‐treated rats compared to vehicle (Cortex pO2: 34±4 vs 44±4 mmHg; Medulla pO2: 22±3 ve 31±3 mmHg, p<0.001, n=6). Blood pO2 was unchanged by DNP treatment. Expressions of HIF1a and the pro‐oxidant enzyme, NADPH oxidase‐2 (NOX‐2) were increased in DNP‐treated rats. Conversely the antioxidant enzyme, superoxide dismutase‐2 (SOD‐2), was not changed. Further, the renal injury markers TGFb and NGAL were higher in DNP‐treated rats. MAP, basal GFR and RBF were unchanged in response to DNP‐generated renal hypoxia. Separate DNP‐fed rats were treated with direct left renal injections of siRNA directed to HIF1a, 48 hours prior to measuring renal pO2. Expression of HIF1a was reduced by 66±3% in the left kidneys treated with siRNA in DNP‐treated rats and was 32% lower than HIF1a in Veh‐treated rats. Expression of HIF1a in the right kidney was not altered by siRNA injection in the left kidney. Knockdown of HIF1a further reduced pO2 compared to DNP‐treated rats (Cort pO2 = −4±2 mmHg; Med pO2 = −8±1 mmHg, p<0.01), suggesting HIF1a protects the kidney from greater renal hypoxia. Further, expression of renal injury markers was higher in HIF1a reduced kidneys. We conclude that chronic renal hypoxia induces renal injury via oxidant pathways and stimulates HIF1a, which is only partially protective. DNP‐induced renal hypoxia is a viable model to test the long‐term effects of hypertension on renal oxygen metabolism.Support or Funding InformationHL089583; HL068686