A Central Role of RLIP76 in Regulation of Glycemic Control

Abstract

OBJECTIVEPathology associated with oxidative stress frequently results in insulin resistance. Glutathione (GSH) and GSH-linked metabolism is a primary defense against oxidative stress. Electrophilic lipid alkenals, such as 4-hydroxy-t-2-nonenal (4HNE), generated during oxidative stress are metabolized primarily to glutathione electrophile (GS-E) conjugates. Recent studies show that RLIP76 is the primary GS-E conjugate transporter in cells, and a regulator of oxidative-stress response. Because RLIP76−/− mice are hypoglycemic, we studied the role of RLIP76 in insulin resistance.RESEARCH DESIGN AND METHODSBlood glucose, insulin, lipid measurements, and hyperinsulinemic-euglycemic and hyperglycemic clamp experiments were performed in RLIP76+/+ and RLIP76−/− C57B mice, using Institutional Animal Care and Use Committee–approved protocols. Time-resolved three-dimensional confocal fluorescence microscopy was used to study insulin endocytosis.RESULTSThe plasma insulin/glucose ratio was ordered RLIP76−/− < RLIP76+/− < RLIP76+/+; administration of purified RLIP76 in proteoliposomes to RLIP76+/+ animals further increased this ratio. RLIP76 was induced by oxidative or hyperglycemic stress; the concomitant increase in insulin endocytosis was completely abrogated by inhibiting the transport activity of RLIP76. Hydrocortisone could transiently correct hypoglycemia in RLIP76−/− animals, despite inhibited activity of key glucocorticoid-regulated hepatic gluconeogenic enzymes, phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and fructose 1,6-bisphosphatase, in RLIP76−/−.CONCLUSIONSThe GS-E conjugate transport activity of RLIP76 mediates insulin resistance by enhancing the rate of clathrin-dependent endocytosis of insulin. Because RLIP76 is induced by oxidative stress, it could play a role in insulin resistance seen in pathological conditions characterized by increased oxidative stress.

RLIP76 Protein Knockdown Attenuates Obesity Due to a High-fat Diet

“…In contrast, the insulin/ glucose ratio was lower and did not change significantly from baseline in the RLIP76 Ϫ/Ϫ mice fed a HFD. These results confirm peripheral and hepatic insulin sensitivity of the knock-out mice in glucose-clamp studies (42), and indicate that genetic loss of RLIP76 nearly completely prevents HFD-induced obesity and insulin resistance. Glucose tolerance was significantly impaired in RLIP76 ϩ/ϩ mice given a HFD as compared with RLIP76 Ϫ/Ϫ mice (Fig.…”

“…RLIP76 Ϫ/Ϫ Mice Are Resistant to HFD-induced Hyperlipidemia-Hyperlipidemia is a key pathological component of MetS as well as obesity. We have previously shown that, in addition to BG, both serum cholesterol and triglycerides are also considerably lower in RLIP76 Ϫ/Ϫ mice (42,43). Results of the present studies (Fig.…”

“…The present studies demonstrate that RLIP76 Ϫ/Ϫ mice, known to have greater insulin sensitivity and lower baseline weight (42), are resistant to HFD-mediated obesity because the inflammation-mediated effects of HFD on obesity are impaired in these mice. This is remarkable because RLIP76 Ϫ/Ϫ mice are known to have very high levels of tissue oxidative stress as compared with RLIP76 ϩ/ϩ mice (45,46), and imply that oxidative stress alone is insufficient to mediate obesity caused by HFD.…”

“…Indeed, drugs that are used to treat MetS including rosiglitazone, metformin, atorvastatin, or gemfibrozil failed to lower already low lipid and glucose levels in RLIP76 Ϫ/Ϫ mice (42,43). Our previous studies also showed that AMPK, a key regulator of carbohydrate and lipid metabolism, as well as transcription factors PPAR␣ and PPAR␥, which are targets of hypolipidemic and insulin-sensitizing drugs, are already maximally activated in RLIP76 Ϫ/Ϫ mice tissues (43).…”

“…Twelve mice (6 RLIP76 ϩ/ϩ and 6 RLIP76 Ϫ/Ϫ ) were fed with NC and the other 12 mice (6 RLIP76 ϩ/ϩ and 6 RLIP76 Ϫ/Ϫ ) with a HFD for 12 weeks. Thereafter these mice were fasted for 16 h followed by intraperitoneal administration of 2 g/kg body weight of glucose (42).…”