Chronic Ethanol Intake Impairs Insulin Signaling in Rats by Disrupting Akt Association with the Cell Membrane
ROLE OF TRB3 IN INHIBITION OF Akt/PROTEIN KINASE B ACTIVATION*
- Ling He ‡ § ,
- Frank A. Simmen ‡ § ,
- Harihara M. Mehendale ¶ ,
- Martin J. J. Ronis ‡ ∥ and
- Thomas M. Badger ‡ § ∥ 1
- ‡Arkansas Children's Nutrition Center and the Departments of §Physiology and Biophysics and ∥Pharmacology and Toxicology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72202 and the Department of ¶Toxicology, University of Louisiana, Monroe, Louisiana 71209
- 1 To whom correspondence should be addressed: Arkansas Children's Nutrition Center, University of Arkansas for Medical Sciences, 1120 Marshall St., Little Rock, AR 72202. Tel.: 501-364-2785; Fax: 501-364-2818; E-mail: badgerthomasm{at}uams.edu.
Abstract
Chronic and excessive alcohol consumption is an important and modifiable risk factor for type 2 diabetes. We previously reported elevations in hepatic Class 1 alcohol dehydrogenase (ADH) expression in ethanol-fed rats correspondent with reduced levels of mature, nuclear sterol-regulatory element-binding protein-1 (SREBP-1), an insulin-induced transcriptional repressor of the ADH gene. In this report, we have studied the effects of insulin and ethanol on ADH gene expression in a highly differentiated rat hepatoma cell line (FGC-4), as well as the in vivo effects of chronic intake of an ethanol-containing diet on hepatic insulin signaling. Insulin inhibited ADH gene expression, and this was abolished by LY294002 (a phosphatidylinositol 3-kinase inhibitor) and small interfering RNA knockdown of SREBP-1. Chronic ethanol intake led to decreased phosphorylation of Akt (protein kinase B) at Thr308, increased phosphorylation of Akt at Ser473, and decreased phosphorylation of glycogen synthase kinase-3β (a downstream effector of Akt). Hepatic membrane-associated Akt content was decreased and cytosolic Akt content was increased in rats fed an ethanol-containing diet. Thus, disruptive effects of ethanol on insulin signaling occurred via impaired phosphorylation of Akt at Thr308. TRB3, a negative regulator of Akt, was induced in liver of ethanol-fed rats. In ethanol-treated FGC-4 cells, small interfering RNA knockdown of TRB3 increased membrane-associated Akt and the phosphorylation of Akt at Thr308. Our results suggest that ethanol induces TRB3, which, through binding to the pleckstrin homology domain of Akt, prevents its plasma membrane association, Akt-Thr308 phosphorylation, and subsequent Akt-mediated signaling. Ethanol inhibition of insulin signaling reduces nuclear SREBP accumulation and results in disinhibition of Class 1 ADH transcription.
Footnotes
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↵2 The abbreviations used are: PI3, phosphatidylinositol 3; PI3k, phosphatidylinositol 3-kinase; SRE, sterol-responsive element; SREBP, SRE-binding protein-1; nSREBP, nuclear SREBP; ADH, alcohol dehydrogenase; siRNA, small interfering RNA; GSK3, glycogen synthase kinase; PH, pleckstrin homology; UEC, urine ethanol concentration; GFP, green fluorescent protein; ADU, arbitrary densitometric units; PIP3, phosphatidylinositol-3, 4, 5-triphosphate; PDK, phosphoinositide-dependent kinase 1; PTEN, phosphatase and tenin homolog deleted on chromosome 10.
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The nucleotide sequence(s) reported in this paper has been submitted to the GenBank™/EBI Data Bank with accession number(s) DQ198085.
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↵* This work was supported by the National Institutes on Alcohol Abuse and Alcoholism Grant AA008645 (to T. M. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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- Received October 3, 2005.
- Revision received January 31, 2006.
- The American Society for Biochemistry and Molecular Biology, Inc.
