| Alcohol has been used by human societies since the beginning of recorded history. Today, alcohol abuse or dependence is a worldwide health problem. In the United States, eighteen million people suffer from alcoholism. Excess alcohol consumption is the leading cause of preventable death in United States and the societal costs are estimated at {dollar}185 billion annually. Alcohol metabolism is of fundamental importance in understanding the health effects of alcohol. For years, our laboratory has used a total enteral nutrition (TEN) model to study the effects of chronic and excessive alcohol intake on the liver (alcoholic liver disease, ALD) and alcohol metabolism in adult male Sprague-Dawley rats. In this intragastric infusion model where alcohol-containing diets are infused nearly 23 hours per day, we observe regular pulses of blood ethanol concentrations (BECs) that fluctuate between 0 and 500 mg/dl. Our laboratory has previously reported that the pulsatile BECs are caused by the cyclic induction of the principal ethanol metabolizing liver enzyme, hepatic class I alcohol dehydrogenase (ADH). Alcohol-induced ADH induction occurs via transcription and translation in a dose-dependent fashion. However, prior to the present studies, the molecular mechanisms underlying the transcriptional induction of ADH were unknown. Using the TEN model and a highly differentiated rat hepatoma cell line (FGC-4), I demonstrated that ethanol transcriptionally induces ADH via complex interactions of hepatic transcription factors: reduced levels of C/BEPbeta (LIP), increased levels of C/BEPbeta (LAP), and reduced mature nuclear SREBP-1 (an insulin-induced transcriptional repressor of the ADH gene). Chronic ethanol consumption in human subjects is associated with a higher incidence of type II diabetes. Since I observed ethanol effects on a mediator of insulin signaling, SREBP-1c, I monitored changes in hepatic insulin signaling. Insulin inhibited ADH gene expression, an effect abolished by the PI3K inhibitor LY294002 and by siRNA knockdown of SREBP-1. Chronic ethanol consumption led to decreased phosphorylation of Akt (PKB) at Thr 308, increased phosphorylation of Akt at Ser473, and decreased phosphorylation of GSK3beta (a downstream effector of Akt). Hepatic membrane-associated Akt content was decreased and cytosolic Akt content increased in rats fed ethanol-containing diets (13 g/kg/d, high dose). TRB3, a negative regulator of Akt, was induced in the livers of ethanol-fed rats (high dose). In ethanol-treated FGC-4 cells, siRNA knockdown of TRB3 increased membrane-associated Akt and the phosphorylation of Akt at Thr308. Thus, disruptive effects of ethanol on insulin signaling occurred via impaired phosphorylation of Akt at Thr308. These results suggest that chronic ethanol feeding in the TEN rat model impairs insulin signaling and results in insulin resistance by inducing TRB3, which through binding to the PH domain of Akt, prevents its plasma membrane association, Akt-Thr308 phosphorylation and subsequent Akt-mediated signaling. Inhibition of insulin signaling by ethanol reduces nSREBP-1 accumulation and results in disinhibition of class I ADH transcription. High dose ethanol caused endoplasmic reticulum (ER) stress in livers and in FGC-4 cells. Ethanol-induced ER stress in FGC-4 cells was reduced by the small chemical chaperones 4-phenyl butyric acid and taurine-ursodeoxycholic acid, as was the induction of TRB3. Thus, ethanol-induced ER stress is suggested to be a major factor in alcohol-induced TRB3 induction. Prospective cohort studies have shown that light to moderate drinking is associated with a reduced risk of diabetes. I have demonstrated that feeding low dose ethanol (4 g/kg/d) increased insulin signaling by suppressing the PI3K regulatory subunit-p55gamma, which leads to the increased association of PI3K catalytic subunits (p110s) with IRS1 and subsequent activation of the downstream effectors such as nSREBP-1. In FGC-4 cells, siRNA knock... |
