Mitogen-activated protein kinase upregulation reduces renal D1 receptor affinity and G-protein coupling in obese rats

Abstract

Reactive oxygen species play a key role in pathophysiology of cardiovascular diseases by modulating G-protein-coupled receptor signaling. We have shown that treatment of animal models of diabetes and aging with tempol decreases oxidative stress and restores renal dopamine D1 receptor (D1R) function. In present study, we determined whether oxidation of D1R and upregulation of mitogen-activated protein kinases (MAPK) were responsible for decreased D1R signaling in obese animals. Male lean and obese Zucker rats were supplemented with antioxidants tempol or lipoic acid for 2 weeks. Compared to lean, obese animals were hyperglycemic and hyperinsulinemic with increased oxidative stress, D1R oxidation and decreased glutathione levels. These animals had decreased renal D1R affinity and basal coupling to G-proteins. SKF-38393, a D1R agonist failed to stimulate G-proteins and adenylyl cyclase. Obese animals showed marked increase in renal MAPK activities. Treatment of obese rats with tempol or lipoic acid decreased blood glucose, reduced oxidative stress, and restored the basal D1R G-protein coupling. Antioxidants also normalized MAPK activities and restored D1R affinity and SKF-38393 induced D1R G-protein coupling and adenylyl cyclase stimulation. These studies show that D1R oxidation and MAPK upregulation contribute to D1R dysfunction in obese animals. Consequently, antioxidants while reducing the oxidative stress normalize the MAPK activities and restore D1R signaling.

Oxidative stress reduces renal dopamine D1 receptor-Gq/11α G protein-phospholipase C signaling involving G protein-coupled receptor kinase 2

“…8, 19). In agreement with our previous studies, we found that obese animals exhibit oxidative stress as evidenced by increased lipid peroxidation and advanced glycation end products along with decreased levels of glutathione and SOD activity, indicating a compromised antioxidant mechanism (6,7). Diabetes can induce oxidative stress via several mechanisms (8,19).…”

“…Other circulating factors, such as free fatty acid and leptin, also contribute to increased reactive oxygen species generation (8,19). In agreement with these findings, our group has previously shown that obese Zucker rats are hyperinsulinemic, hyperlipidemic, and hyperglycemic, a hallmark of type 2 diabetes-associated obesity (6,7). Because the pathogenesis of diabetes as well as its complications involves oxidative stress, the use of antioxidant is an appealing concept (8,19).…”

“…In addition, we found that treatment of these animals with antioxidants, while reducing oxidative stress, normalized GRK2 sequestration and receptor phosphorylation and restored D1R signaling (7,12,27). Although these reports provide substantial evidence related to the role of GRKs in D1R hyperphosphorylation and subsequent desensitization, most of these studies were focused on D1R coupling to G s ␣ and AC activation (6,7,12,27). Numerous studies have demonstrated that renal D1R also mediates its functional effects through activation of the G q/11 ␣ protein and its downstream effector, phospholipase C (PLC) (13,14,40,48).…”

“…Renal proximal tubular carboxymethyllysine was measured using enzyme-linked immunosorbent assay as described by Koo and Vaziri (24), and malondialdehyde was determined using the method of Mihara and Uchiyama (28). Glutathione was determined colorimetrically as described previously (6).…”

“…Therefore, the present study was designed to investigate whether uncoupling of D1R from G q/11 ␣ and failure to stimulate PLC activity are contributing to receptor dysfunction in renal proximal tubules of obese rats. Since obese Zucker rats exhibit increased oxidative stress (1,6,7,25), we wondered whether antioxidant supplementation could restore the D1R coupling to G q/11 ␣ and, subsequently, receptor functional response. Both lean and obese Zucker rats were provided with lipoic acid, an antioxidant-supplemented diet, for 2 wk.…”