The D2 dopamine receptor (D2R) is important in the pathogenesis of essential hypertension. We have already reported that systemic deletion of the D2R gene in mice results in reactive oxygen species (ROS)-dependent hypertension, suggesting that the D2R has antioxidant effect. However, the mechanism of this effect is unknown. DJ-1 is a protein which has antioxidant properties. D2R and DJ-1 are expressed in the mouse kidney and colocalize and co-imunoprecipitate in mouse renal proximal tubule cells. We hypothesized that D2Rs regulate renal ROS production in the kidney through regulation of DJ-1 expression or function. Heterozygous D2+/− mice have increased blood pressure, urinary 8-isoprostanes, and renal Nox 4 expression, but decreased renal DJ-1 expression. Silencing D2R expression in mouse renal proximal tubule cells increases ROS production and decreases the expression of DJ-1. Conversely, treatment of these cells with a D2R agonist increases DJ-1 expression and decreases Nox 4 expression and NADPH oxidase activity, effects that are partially blocked by a D2R antagonist. Silencing DJ-1 expression in mouse renal proximal tubule cells increases ROS production and Nox 4 expression. Selective renal DJ-1 silencing by the subcapsular infusion of DJ-1 siRNA in mice increases blood pressure, and renal Nox 4 expression and NADPH oxidase activity. These results suggest that the inhibitory effects of D2R on renal ROS production are at least, in part, mediated by a positive regulation of DJ-1 expression/function and that DJ-1 may have a role in the prevention of hypertension associated with increased ROS production.

The dopamine D2 receptor (D2R) regulates renal reactive oxygen species (ROS) production and impaired D2R function results in ROS-dependent hypertension. Paraoxonase 2 (PON2), which belongs to the paraoxonase gene family, is expressed in various tissues, acting to protect against cellular oxidative stress. We hypothesized that PON2 may be involved in preventing excessive renal ROS production and thus may contribute to maintenance of normal blood pressure. Moreover, the D2R may decrease ROS production, in part, through regulation of PON2. D2R co-localized with PON2 in the brush border of mouse renal proximal tubules. Renal PON2 protein was decreased (-33%±6%) in D2-/- relative to D2+/+ mice. The renal subcapsular infusion of PON2 siRNA decreased PON2 protein expression (-55%), increased renal oxidative stress (2.2-fold), associated with increased renal NADPH oxidase expression (Nox1: 1.9-fold; Nox2: 2.9-fold; and Nox4: 1.6-fold) and activity (1.9-fold), and elevated arterial blood pressure (systolic: 134±5 vs. 93±6 mmHg; diastolic: 97±4 vs. 65±7 mmHg; mean: 113±4 vs. 75±7 mmHg). To determine the relevance of the PON2 and D2R interaction in humans, we studied human renal proximal tubule cells. Both D2R and PON2 were found in non-lipid and lipid rafts and physically interacted with each other. Treatment of these cells with the D2R/D3R agonist quinpirole (1μM, 24h) decreased ROS production (-35%±6%), associated with decreased NADPH oxidase activity (-32%±3%) and expression of Nox2 (-41%±7%) and Nox4 (-47%±8%) protein, and increased expression of PON2 mRNA (2.1-fold) and protein (1.6-fold) at 24h. Silencing PON2 (siRNA, 10nM, 48 h) not only partially prevented the quinpiroleinduced decrease in ROS production by 36%, but also increased basal ROS production (1.3-fold) which was associated with an increase in NADPH oxidase activity (1.4-fold) and expression of Nox2 (2.1-fold) and Nox4 (1.8-fold) protein. Inhibition of NADPH oxidase with diphenylene iodonium (10 μM/30 min) inhibited the increase in ROS production caused by PON2 silencing. Our results suggest that renal PON2 is involved in the inhibition of renal NADPH oxidase activity and ROS production and contributes to the maintenance of normal blood pressure. PON2 is positively regulated by D2R and may, in part, mediate the inhibitory effect of renal D2R on NADPH oxidase activity and ROS production.

Dopamine, which is synthesized in the kidney, independent of renal nerves, plays an important role in the regulation of fluid and electrolyte balance and systemic blood pressure. Lack of any of the five dopamine receptor subtypes (D1R, D2R, D3R, D4R, and D5R) results in hypertension. D1R, D2R, and D5R have been reported to be important in the maintenance of a normal redox balance. In the kidney, the antioxidant effects of these receptors are caused by direct and indirect inhibition of pro-oxidant enzymes, specifically, nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) oxidase, and stimulation of anti-oxidant enzymes, which can also indirectly inhibit NADPH oxidase activity. Thus, stimulation of the D2R increases the expression of endogenous anti-oxidants, such as Parkinson protein 7 (PARK7 or DJ-1), paraoxonase 2 (PON2), and heme oxygenase 2 (HO-2), all of which can inhibit NADPH oxidase activity. The D5R decreases NADPH oxidase activity, via the inhibition of phospholipase D2, and increases the expression of HO-1, another antioxidant. D1R inhibits NADPH oxidase activity via protein kinase A and protein kinase C cross-talk. In this review, we provide an overview of the protective roles of a specific dopamine receptor subtype on renal oxidative stress, the different mechanisms involved in this effect, and the role of oxidative stress and impairment of dopamine receptor function in the hypertension that arises from the genetic ablation of a specific dopamine receptor gene in mice.