A central role of RLIP76 in regulation of glycemic control.
OBJECTIVE
Pathology 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.


METHODS
Blood 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.


RESULTS
The 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(-/-).


CONCLUSIONS
The 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.
Awasthi
Sanjay
S
Department of Molecular Biology & Immunology, University of North Texas Health Science Center, Fort Worth, Texas, USA. sawasthi@hsc.unt.edu
Singhal
Sharad S
SS
Yadav
Sushma
S
Singhal
Jyotsana
J
Vatsyayan
Rit
R
Zajac
Ewa
E
Luchowski
Rafal
R
Borvak
Jozef
J
Gryczynski
Karol
K
Awasthi
Yogesh C
YC
D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer; D006597:Fructose-Bisphosphatase / Q000378:metabolism; D020690:GTPase-Activating Proteins / Q000235:genetics* / Q000378:metabolism*; D015309:Glucose Clamp Technique; D018149:Glucose Intolerance / Q000378:metabolism / Q000503:physiopathology; D005952:Glucose-6-Phosphatase / Q000378:metabolism; D006854:Hydrocortisone / Q000494:pharmacology; D006943:Hyperglycemia / Q000378:metabolism* / Q000503:physiopathology; D006946:Hyperinsulinism / Q000378:metabolism / Q000503:physiopathology; D007003:Hypoglycemia / Q000378:metabolism / Q000503:physiopathology; D007328:Insulin / Q000097:blood; D007333:Insulin Resistance / Q000502:physiology*; D008055:Lipids / Q000097:blood; D008099:Liver / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018384:Oxidative Stress / Q000502:physiology*; D010729:Phosphoenolpyruvate Carboxykinase (GTP) / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology
D001786:Blood Glucose; D020690:GTPase-Activating Proteins; D007328:Insulin; D008055:Lipids; C490620:Ralbp1 protein, mouse; D006597:Fructose-Bisphosphatase; D005952:Glucose-6-Phosphatase; D010729:Phosphoenolpyruvate Carboxykinase (GTP); D006854:Hydrocortisone