Ethanol increases receptor-dependent cyclic AMP production in cultured hepatocytes by decreasing Gi-mediated inhibition

Abstract

Increasing evidence suggests that ethanol-induced changes in cyclic AMP (cAMP) signal transduction play a critical role in the acute and chronic effects of ethanol. Here we have investigated the effects of ethanol on cAMP signal transduction in primary cultures of rat hepatocytes. Acute exposure to ethanol had a biphasic effect on glucagon-receptor-dependent cAMP production in intact cells: 25-50 mM-ethanol decreased cAMP, whereas treatment with 100-200 mM-ethanol increased cAMP. After chronic exposure to 50-200 mM-ethanol for 48 h in culture, glucagon-receptor-dependent cAMP levels were increased, but no change in glucagon receptor number was observed. These effects of ethanol were independent of ethanol oxidation. Chronic ethanol treatment also increased adenosine-receptor- and forskolin-stimulated cAMP production. Increased cAMP production was also observed upon stimulation of adenylate cyclase with glucagon, forskolin and F- in membranes isolated from cells cultured with 100 mM-ethanol for 48 h. However, no differences were observed in basal and MnCl2-stimulated adenylate cyclase activity. The quantity of alpha i protein was decreased by 35% after chronic ethanol treatment, but no change in the quantity of alpha s protein was detected. Decreased alpha i protein was associated with a decrease in G(i) function, as assessed by the ability of 0.1 nM-guanosine 5'-[beta gamma-imido]triphosphate and 1 microM-somatostatin to inhibit forskolin-stimulated adenylate cyclase activity. Taken together, these results suggest that chronic exposure to ethanol increases receptor-dependent cAMP production in hepatocytes by decreasing the quantity of alpha i protein at the plasma membrane and thereby decreasing the inhibitory effects of G(i) on adenylate cyclase activity.

Activation of Gαs Mediates Induction of Tissue-type Plasminogen Activator Gene Transcription by Epoxyeicosatrienoic Acids

“…Although the consensus sequence for CRE is not present in the t-PA promoter, a functional CRE-like element has been identified, which acts synergistically with a putative AP-2 binding site to induced t-PA gene transcription by phorbol 12-myristate 13-acetate (22). Indeed, previous studies have shown that cholera toxin and dibutyryl-cAMP can directly induce t-PA gene transcription in rat hepatocytes (30,31). Furthermore, we have previously shown that low concentrations of alcohol enhance isoproterenol-induced t-PA gene transcription via increase in cAMP levels and induction of CRE promoter activity in vascular endothelial cells (32).…”

Effects of acute and chronic ethanol on cyclic AMP accumulation in NG108‐15 cells: differential dependence of changes on extracellular adenosine

“…It is possible that the increase in iloprost stimulation is due to a chronic ethanol-induced increase in IP receptor number, since chronic ethanol has recently been reported to increase expression of b-opioid receptor, a2-adrenoceptor and muscarinic acetylcholine receptor expression in NG108-15 cells Hu et al, 1993 (Nagy & DeSilva, 1992) and human placental trophoblasts (Karl et al, 1994). Thus the chronic ethanol-induced increase in cyclic AMP accumulation is a characteristic of different cell types.…”

Mechanism of Liver Tyrosine Aminotransferase Increase in Ethanol-Treated Mice and Its Effect on Serum Tyrosine Level

“…Nagy reported that acute exposure of primary rat hepatocyte cultures to ethanol increases the extracellular adenosine concentration ( 19 ), and that ethanol increases cAMP production in cultured hepatocytes via an adenosine receptor-dependent mechanism ( 23 ). Adenosine receptors have been divided into three major subclasses, A1, A2, and A3.…”