Chronic Ethanol Intake Impairs Insulin Signaling in Rats by Disrupting Akt Association with the Cell Membrane

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 Writings from as early as the 17th century, as well as modern epidemiological studies, suggest that chronic and excessive alcohol consumption is positively associated with the onset of type 2 diabetes (1-5). Ethanol intake has been reported to decrease glucose uptake and utilization consistent with the development of insulin resistance, a central component of diabetes (6). Results from these previous studies suggest that the ethanol impairment of insulin action is likely to be downstream from PI3 2 kinase; however, the molecular mechanisms underlying the effects of alcohol on insulin resistance and type-2 diabetes remain to be determined (7).We previously reported that chronic intragastric infusion of an ethanol-containing diet to rats results in unique and predictably recurring cyclic fluctuations in plasma ethanol concentrations (8) as a consequence of cyclic expression of the major alcohol metabolizing enzyme, hepatic Class 1 alcohol dehydrogenase (ADH). Further studies from our laboratory demonstrated that alcohol induces hepatic ADH gene transcription via decreased levels of nuclear SREBP-1c protein, a negative regulator of the ADH gene (9). SREBP-1c is encoded by an insulinresponsive gene (10) and is an important early mediator in the pathway of insulin action in the liver (11). These observations led to the current hypothesis under study, namely that ethanol may suppress nuclear SREBP-1c via disruption of insulin signaling, which may be a potential link between alcohol consumption and insulin resistance. In this study, we elucidate the mechanism by which chronic ethanol intake inhibits insulin action and identify TRB3, a previously identified modulator of Akt signaling, as a primary ethanol-responsive modifier of insulin signaling (12). EXPERIMENTAL PROCEDURESMaterials-All chemicals, unless otherwise specified, were purchased from Sigma. Antibodies were purchased from commercial suppliers: SREBP-1 was from Santa Cruz Biotechnology Inc. (Santa Cruz, CA); GSK3, and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) were from Upstate Biotechnology (Lake Placid, NY); phospho-GSK3, Akt, phospho-Akt 473 , phospho-Akt 30...

Aberrant hepatic TRIB3 gene expression in insulin-resistant obese humans

“…TRIB3 is an inhibitor of the insulin-responsive AKT1 kinase and has been implicated in insulin resistance in several studies using cell lines and animal models [1,2,5,6]. In the present study, we provide initial evidence for an aberrant hepatic expression of TRIB3 in insulin-resistant human study participants.…”

“…An inhibitory role for TRIB3 in insulin signal transduction has been suggested, as it was shown to bind to the pleckstrin homology domain of the insulinresponsive Ser-Thr kinase, v-akt murine thymoma viral oncogene homologue 1 (AKT1), thereby preventing its membrane association, Thr 308 phosphorylation and activation via upstream kinases [1,2]. Although, in mice, deletion of Trib3 did not produce major disturbances in insulin signalling and glucose homeostasis [3], and in cultured rat hepatocytes adenoviral Trib3 overexpression failed to affect insulin mediated AKT1 phosphorylation [4], several other studies have reported data consistent with a role for Trib3 in insulin resistance.…”

“…In the fasted state, TRIB3 was increased, thereby preventing residual insulin signalling and promoting glucose output by the liver [1]. Furthermore, increased expression of Trib3 has been implicated in ethanol-induced hepatic insulin resistance [2]. Most recently, a protein termed adaptor protein, phosphotyrosine interaction, PH domain and leucin zipper containing 1, which mediates adiponectin signalling, was shown to enhance insulin-stimulated activation of AKT1 and suppression of gluconeogenesis by blocking the interaction of AKT1 with TRIB3 through direct competition [6].…”

Mitochondrial dysfunction and oxidative stress in the pathogenesis of alcohol- and obesity-induced fatty liver diseases

“…While emerging data indicates that alcohol dependent insulin resistance may contribute to the accumulation of fat in hepatocytes, the evidence in support of this is less clear; primarily due to observations that alcohol has "dose-dependent" effects on insulin signaling [24]. Studies by Badger and colleagues have demonstrated that while high dose chronic alcohol consumption impairs hepatic insulin signaling, low dose ethanol exposure activates insulin signaling in hepatocytes [24,25]. Importantly, for the first time, these findings provide biochemical support for epidemiologic evidence suggesting that while low dose alcohol consumption may be cardioprotective and decrease the risk of developing type 2 diabetes; heavy, prolonged alcohol consumption increases the risk for type 2 diabetes.…”

Differentiation-dependent expression of Adhfe1 in adipogenesis

“…Ample evidence exists confirming the function of one of the downstream effectors of PI 3-kinase, Akt/ PKB [68], in the metabolic effects of insulin [76]. In the specific case of ethanol metabolism, studies in rat liver indicate that chronic ethanol intake impairs insulin signaling by disrupting Akt/PKB membrane association [20]. As such, it may be of interest that our studies demonstrate that PI 3-kinase signaling is key to maintaining basal Adhfe1 transcript level when assessed in 3T3-L1 adipocytes.…”