Recent insights into the role of the innate immune system in the development of alcoholic liver disease

The innate immune system is responsible for the rapid, initial response of the organism to potentially dangerous stresses, including pathogens, tissue injury, and malignancy. Pattern-recognition receptors of the toll-like receptor (TLR) family expressed by macrophages provide a first line of defense against microbial invasion. Activation of these receptors results in a stimulus-specific expression of genes required to control the infection, including the production of inflammatory cytokines and chemokines, followed by the recruitment of neutrophils to the site of infection. The early stages in the development of alcoholic liver disease (ALD) follow a pattern characteristic of an innate immune response. Kupffer cells, the resident macrophages in the liver, are activated in response to bacterial endotoxins (lipopolysaccharide, LPS), leading to the production of inflammatory and fibrogenic cytokines, reactive oxygen species, as well as the recruitment of neutrophils to the liver. One mechanism by which chronic ethanol can turn the highly regulated innate immune response into a pathway of disease is by disrupting the signal transduction cascades mediating the innate immune response. Recent studies have identified specific modules in the TLR-4 signaling cascade that are disrupted after chronic ethanol exposure, including CD14 and the mitogen-activated protein kinase family members, ERK1/2 and p38. Enhanced activation of these TLR-4 dependent signaling pathways after chronic ethanol likely contributes to the development of alcoholic liver disease.

Endothelin-1-stimulated glucose uptake is desensitized by tumor necrosis factor-alpha in 3T3-L1 adipocytes

Tumor necrosis factor-alpha (TNF-alpha) is a potent inducer of insulin resistance, and increased TNF-alpha expression is associated with impaired glucose disposal. Although insulin is the primary regulator of glucose transport in adipose, endothelin-1, a vasoconstrictor peptide that signals through the heterotrimeric G proteins Galphaq/11, potently stimulates glucose uptake in 3T3-L1 adipocytes by a mechanism independent of phosphatidylinositol (PI) 3-kinase. Here, we report that exposure of 3T3-L1 adipocytes to TNF-alpha for 48 h dose-dependently decreased endothelin-1-stimulated glucose uptake and translocation of GLUT4 to the plasma membrane. TNF-alpha exposure had no effect on endothelin-1 receptor number at the cell surface. In contrast, TNF-alpha treatment reduced the quantity of Galphaq/11 and proline-rich tyrosine kinase 2 (PYK2) and decreased endothelin-1-stimulated PYK2-Tyr402 tyrosine phosphorylation. Taken together, these results suggest that TNF-alpha-induced desensitization of endothelin-1-stimulated GLUT4 translocation and glucose uptake in 3T3-L1 adipocytes is due, at least in part, to a decreased expression of Galphaq/11, leading to a suppression in tyrosine phosphorylation of PYK2.

Chronic ethanol feeding impairs endothelin-1-stimulated glucose uptake via decreased G alpha 11 expression in rat adipocytes

Chronic ethanol feeding decreases insulin-stimulated glucose uptake in rat adipocytes. Here, we show that chronic ethanol also decreases endothelin-stimulated glucose uptake. Endothelin-1 increased uptake of 2-deoxyglucose 2.4-fold in adipocytes isolated from pair-fed rats. However, in adipocytes isolated from rats that had consumed a diet containing 35% ethanol for 4 wk, endothelin-1 did not increase glucose uptake. Although endothelin-1 increased GLUT4 quantity at the plasma membrane in adipocytes from pair-fed rats, there was no increase in GLUT4 after chronic ethanol feeding. Loss of endothelin-1-stimulated glucose uptake after ethanol feeding was associated with a specific decrease in the quantity of Galpha11 in plasma membranes, with no change in Galphaq quantity. Activation of proline-rich tyrosine kinase 2 (PYK2), a downstream target of Galphaq/11 that is required for endothelin-1-stimulated GLUT4 translocation in 3T3-L1 adipocytes, was also suppressed after chronic ethanol feeding. In contrast, activation of p38 MAPK by endothelin-1 was not affected by chronic ethanol exposure. These data demonstrate that chronic ethanol feeding suppresses endothelin-1-stimulated glucose uptake and suggest that decreased expression of Galpha11 coupled to impaired endothelin-1-dependent activation of PYK2 contributes to this response.

Nutritional control of mRNA stability is mediated by a conserved AU-rich element that binds the cytoplasmic shuttling protein HuR

The cationic amino acid transporter, Cat-1, is a high affinity transporter of the essential amino acids, arginine and lysine. Expression of the cat-1 gene increases during nutritional stress as part of the adaptive response to starvation. Amino acid limitation induces coordinate increases in stability and translation of the cat-1 mRNA, at a time when global protein synthesis decreases. It is shown here that increased cat-1 mRNA stability requires an 11 nucleotide AU-rich element within the distal 217 bases of the 3'-untranslated region. When this 217-nucleotide nutrient sensor AU-rich element (NS-ARE) is present in a chimeric mRNA it confers mRNA stabilization during amino acid starvation. HuR is a member of the ELAV family of RNA-binding proteins that has been implicated in regulating the stability of ARE-containing mRNAs. We show here that the cytoplasmic concentration of HuR increases during amino acid starvation, at a time when total cellular HuR levels decrease. In addition, RNA gel shift experiments in vitro demonstrated that HuR binds to the NS-ARE and binding was dependent on the 11 residue AU-rich element. Moreover, HuR binding to the NS-ARE in extracts from amino acid-starved cells increased in parallel with the accumulation of cytoplasmic HuR. It is proposed that an adaptive response of cells to nutritional stress results in increased mRNA stability mediated by HuR binding to the NS-ARE.

Lipopolysaccharide stimulation of ERK1/2 increases TNF-alpha production via Egr-1

Lipopolysaccharide (LPS) is a potent activator of tumor necrosis factor-alpha (TNF-alpha) production by macrophages. LPS stimulates the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 and increases TNF-alpha mRNA and protein accumulation in RAW 264.7 murine macrophages. However, the role of ERK1/2 activation in mediating LPS-stimulated TNF-alpha production is not well understood. Inhibition of ERK1/2 activation with PD-98059 or overexpression of dominant negative ERK1/2 decreased LPS-induced TNF-alpha mRNA quantity. LPS rapidly increased early growth response factor (Egr)-1 binding to the TNF-alpha promoter; this response was blunted in cells treated with PD-98059 or transfected with dominant-negative ERK1/2. Using a chloramphenicol acetyltransferase reporter gene linked to the Egr-1 promoter, we show that LPS increased Egr-1 promoter activity via an ERK1/2-dependent mechanism. These results delineate the role of ERK1/2 activation of Egr-1 activity in mediating LPS-induced increases in TNF-alpha mRNA expression in macrophages.

Chronic ethanol increases lipopolysaccharide-stimulated Egr-1 expression in RAW 264.7 macrophages: contribution to enhanced tumor necrosis factor alpha production

Increased production of tumor necrosis factor alpha (TNFalpha) is associated with the development of alcoholic liver disease. Culture of RAW264.7 macrophages with 25 mm ethanol for 48 h increased lipopolysaccharide (LPS)-stimulated accumulation of tumor necrosis factor alpha (TNFalpha) peptide and mRNA by 2-fold. We investigated whether chronic ethanol-induced increases in the DNA binding and/or promoter activity of the key transcription factors regulating LPS-stimulated TNFalpha promoter activity contribute to increased TNFalpha expression. Binding of Egr-1 to the TNFalpha promoter was increased by 2.5-fold after ethanol exposure, whereas NFkappaB binding was decreased to 30% of control. AP-1 binding was not affected. Changes in binding activity were paralleled by an increased contribution of the Egr-1 binding site and a decreased contribution of the NFkappaB site to LPS-stimulated TNFalpha promoter activity. Overexpression of dominant negative Egr-1 prevented the ethanol-induced increase in LPS-stimulated TNFalpha mRNA accumulation. Chronic ethanol exposure enhanced LPS-stimulated Egr-1 promoter-driven CAT expression and transcription of Egr-1. Induction of Egr-1 is dependent on ERK1/2 activation in other systems. Therefore, we investigated whether the ERK1/2 pathway mediated the chronic ethanol-induced increases in Egr-1 and TNFalpha. Increased Egr-1 promoter activity and TNFalpha mRNA accumulation after chronic ethanol were both prevented by overexpression of dominant negative ERK1/2. LPS-stimulated ERK1/2 phosphorylation was increased 2-fold in cells cultured with ethanol compared with controls. These results demonstrate that enhanced LPS-dependent activation of Egr-1 contributes to increased TNFalpha production after chronic ethanol exposure.

Ethanol and membrane protein trafficking: diverse mechanisms of ethanol action

This article represents the proceedings of a symposium at the 2001 RSA Meeting in Montreal, Quebec. The cochairs were Laura E. Nagy and M. Raj Lakshman. The presentations were (1) Ethanol and protein trafficking in liver: modifications of small GTP binding proteins (Rabs), by M. Raj Lakshman; (2) Impaired receptor-mediated endocytosis: its role in alcoholic liver injury, by Carol A. Casey; (3) Ethanol and L1 mediated neurite outgrowth: receptor recycling and L1 signal transduction, by Cynthia F. Bearer; and (4) Ethanol and glucose transport, GLUT4 vesicle trafficking, by Laura E. Nagy.

ERK1/2 and Egr-1 contribute to increased TNF-alpha production in rat Kupffer cells after chronic ethanol feeding

Activation of Kupffer cells by lipopolysaccharide (LPS) is a critical step in the pathogenesis of alcoholic liver disease. Kupffer cells isolated from rats fed ethanol in their diet for 4 wk accumulated 4.3-fold more tumor necrosis factor (TNF)-alpha in response to LPS compared with pair-fed rats. In contrast, LPS-stimulated interleukin (IL)-1 accumulation was 50% lower after ethanol feeding. LPS-stimulated TNF-alpha mRNA accumulation was twofold higher after ethanol feeding, whereas IL-1beta mRNA accumulation was blunted. To understand the mechanisms for this differential response, we investigated the effects of ethanol on LPS-dependent signal transduction. Chronic ethanol feeding increased LPS-stimulated extracellular receptor-activated kinases 1/2 (ERK1/2) activation. Activation of ERK1/2 was required for maximal increases in TNF-alpha and IL-1beta mRNA and was associated with increased binding of early growth response-1 (Egr-1) to the TNF-alpha promoter after ethanol feeding. In contrast, ethanol feeding completely abrogated activation of nuclear factor-kappaB DNA-binding activity by LPS and had no effect on AP-1 binding. Together, these data suggest that enhanced activation of ERK1/2 and Egr-1 contributes to increased TNF-alpha production after chronic ethanol feeding.

Stabilization of tumor necrosis factor alpha mRNA by chronic ethanol: role of A + U-rich elements and p38 mitogen-activated protein kinase signaling pathway

Increased expression of tumor necrosis factor alpha (TNFalpha) in response to chronic ethanol has been implicated in the pathogenesis of alcoholic liver disease. However, the molecular mechanisms by which ethanol increases the levels of TNFalpha are not well characterized. Utilizing Kupffer cells isolated from rats fed an ethanol containing diet and a murine macrophage cell line, RAW264.7, exposed to ethanol in culture, we have demonstrated that exposure to chronic ethanol results in an enhanced expression of lipopolysaccharide (LPS)-induced TNFalpha. While chronic ethanol had no effect on the rate of LPS-induced TNFalpha transcription as measured by nuclear run-on experiments, TNFalpha mRNA half-life was increased by chronic ethanol. Chronic ethanol also potentiated the activation of LPS-induced p38 mitogen-activated protein (MAP) kinase in Kupffer cells, as well as in RAW264.7 cells. Specific inhibition of p38 MAP kinase activation by SB203580 in Kupffer cells or by overexpression of dominant negative p38 MAP kinase in RAW264.7 cells blocked ethanol-mediated TNFalpha mRNA stabilization. Furthermore, using chimeric reporter constructs, we have shown that A + U-rich elements in the 3'-untranslated region of TNFalpha mRNA are not sufficient to impart ethanol-mediated stabilization on TNFalpha mRNA.

GLUT4 vesicle trafficking in rat adipocytes after ethanol feeding: regulation by heterotrimeric G-proteins

Long-term ethanol consumption decreases insulin-stimulated glucose uptake in isolated rat adipocytes. Here we investigate the mechanisms for this decrease. Male Wistar rats were fed for 4 weeks with a liquid diet containing 35% of the calories from ethanol and compared with pair-fed controls. Stimulation of 3-O-methylglucose transport in isolated adipocytes by insulin was decreased by 70% after ethanol feeding. However, stimulation by insulin of the tyrosine phosphorylation of the p85 subunit of phosphoinositide 3-kinase and the phosphorylation of Akt were not affected by ethanol feeding. GLUT4 was mobilized from intracellular light microsomes in response to insulin in both pair-fed and ethanol-fed rats, resulting in 4.3-fold and 3.3-fold increases in GLUT4 associated with plasma membrane in pair-fed and ethanol-fed rats respectively. Surface-accessible GLUT4, assessed by a trypsin cleavage assay or cell-surface labelling with bis-mannose photolabel, was increased 2.3-fold and 1.6-fold respectively, in pair-fed rats after treatment with insulin. In contrast, insulin did not increase surface-accessible GLUT4 in ethanol-fed rats. Treatment of adipocytes with R-phenylisopropyladenosine, an adenosine A1 receptor agonist, increased the transport of 3-O-methylglucose and trypsin-accessible GLUT4, in adipocytes from both pair-fed and ethanol-fed rats. These results demonstrate that whereas the insulin-mediated signalling and translocation of GLUT4 to the plasma membrane is maintained after ethanol feeding, the final fusion of GLUT4 vesicles to the plasma membrane is disrupted, preventing the stimulation of glucose uptake by insulin. Fusion of GLUT4 with the plasma membrane can be stimulated by the activation of adenosine A1 receptors.

Ethanol stimulates glucose uptake and translocation of GLUT-4 in H9c2 myotubes via a Ca(2+)-dependent mechanism

Short-term exposure to ethanol impairs glucose homeostasis, but the effects of ethanol on individual components of the glucose disposal pathway are not known. To understand the mechanisms by which ethanol disrupts glucose homeostasis, we have investigated the direct effects of ethanol on glucose uptake and translocation of GLUT-4 in H9c2 myotubes. Short-term treatment with 12.5-50 mM ethanol increased uptake of 2-deoxyglucose by 1.8-fold in differentiated myotubes. Pretreatment of H9c2 myotubes with 100 nM wortmannin, an inhibitor of phosphatidylinositol 3-kinase, had no effect on ethanol-induced increases in 2-deoxyglucose uptake. In contrast, preincubation with 25 microM dantrolene, an inhibitor of Ca(2+) release from the sarcoplasmic reticulum, blocked the stimulation of 2-deoxyglucose uptake by ethanol. Increased 2-deoxyglucose uptake after ethanol treatment was associated with a decrease in small intracellular GLUT-4 vesicles and an increase in GLUT-4 localized at the cell surface. In contrast, ethanol had no effect on the quantity of GLUT-1 and GLUT-3 at the plasma membrane. These data demonstrate that physiologically relevant concentrations of ethanol disrupt the trafficking of GLUT-4 in H9c2 myotubes resulting in translocation of GLUT-4 to the plasma membrane and increased glucose uptake.

Mobilization of GLUT-4 from intracellular vesicles by insulin and K(+) depolarization in cultured H9c2 myotubes

The insulin-responsive glucose transporter, GLUT-4, moves from an intracellular compartment to the cell surface in response to insulin and/or muscle contraction. Treatment of H9c2 myotubes with insulin significantly increased uptake of 2-deoxyglucose. Depolarization of the myotubes by increasing extracellular [K(+)], which mimics the initial phases of excitation-contraction coupling, also increased 2-deoxyglucose uptake. The K(+)- but not insulin-evoked increase was blocked by dantrolene, an inhibitor of Ca(2+) release from the sarcoplasmic reticulum. In contrast, wortmannin, an inhibitor of phosphatidylinositol 3-kinase, blocked insulin- but not K(+)-stimulated 2-deoxyglucose uptake. Increased glucose uptake in response to insulin or K(+) depolarization was associated with increased GLUT-4 in plasma membranes and depletion of a population of small intracellular GLUT-4-containing vesicles. Similarly, in H9c2 cells transfected with c-myc-tagged GLUT-4, translocation of c-myc GLUT-4 to the cell surface was increased after stimulation with insulin or K(+) depolarization. Taken together, these data demonstrate that insulin and K(+) depolarization increase glucose uptake by recruiting GLUT-4 from intracellular vesicles to the plasma membrane of H9c2 myotubes via distinct signaling mechanisms.

Ethanol dissociates hormone-stimulated cAMP production from inhibition of TNF-alpha production in rat Kupffer cells

Ethanol impairs hormone-stimulated cAMP production in a number of cell types, yet the effects of ethanol on downstream responses mediated by cAMP-dependent protein kinase (PKA) are not understood. Here we have investigated the effects of ethanol feeding on cAMP-mediated inhibition of tumor necrosis factor-alpha (TNF-alpha) synthesis in rat Kupffer cells. Male Wistar rats were fed liquid diets containing 36% of calories as ethanol for 4 wk or were pair fed a control diet. Stimulation of cAMP production by the adenosine A2 receptor agonist 5'-(N-ethylcarboxamido)-adenosine (NECA), prostaglandin E2, or forskolin was decreased to 25% of control values in Kupffer cells isolated from ethanol-fed rats. This decrease was associated with a reduction in the quantity of immunoreactive Gsalpha protein in ethanol-fed rats, with no changes observed in Gialpha or Gbeta. TNF-alpha production was higher in ethanol-fed rats in response to stimulation with lipopolysaccharide or latex beads. Despite the profound reduction in the ability of hormone to increase cAMP production, NECA and prostaglandin E2 inhibited TNF-alpha production to an equivalent degree in Kupffer cells from ethanol- and pair-fed rats. Total activity and immuoreactive protein quantity of PKA did not differ between groups. Activation of PKA in response to a 15-min treatment with 1 microM NECA was reduced by 50% in ethanol-fed rats compared with control. Despite this reduction in activation, translocation of the catalytic subunit of PKA to the nucleus and phosphorylation of cAMP response element binding protein in response to activation were observed in Kupffer cells from both ethanol- and pair-fed rats. These data demonstrate that there is a dissociation between ethanol-induced desensitization of hormone-stimulated cAMP production in rat Kupffer cells and the downstream inhibition of TNF-alpha production mediated by cAMP.

Moderate dietary protein and energy restriction modulate cAMP-dependent protein kinase activity in rat liver

Very low protein diets result in a desensitization of hepatic cAMP signaling in rats, which is characterized by a loss of cAMP-dependent protein kinase (PKA) activity and type I regulatory subunit (RI). Here we have tested whether more moderate protein restriction (Trial 1) or energy restriction (Trial 2) also modulates hepatic PKA quantity and activity. In trial 1, weanling rats were allowed free access to diets containing normal protein (15%, AL-NP), moderately restricted protein (12.5%, AL-MP) and low protein (7.5%, AL-LP); in trial 2, rats were allowed free access to diet containing 15% (AL-NP) or 0.5% protein (very low protein, AL-VLP) or were energy restricted by pair-feeding a diet isonitrogenous to AL-NP but at 65% of the energy intake (ER-IN) for 14 d. Body weights were lower (P < 0.05) by d 14 in all restricted groups compared with the AL-NP group. The quantity of cytosolic RI was lower (P < 0.05) in AL-LP and AL-VLP, but not in AL-MP or ER-IN, compared with AL-NP. In contrast, there was no effect of diet on RI in the particulate fraction. RII was not changed by moderate and low protein diets in either the cytosol or particulate fraction. However, type II regulatory subunit (RII) was greater in the cytosol of ER-IN and in the particulate fraction of AL-VLP (P < 0.05) compared with AL-NP. Specific activity of PKA was lower in the cytosol and particulate fraction (P < 0.05) in the AL-VLP and ER-IN groups compared with the AL-NP group. In contrast, specific activity of PKA was maintained in cytosol from AL-LP, but lower in the particulate fraction (P < 0.05) compared with AL-NP. In summary, protein restricted-diets lower RI subunit in the cytosol; however, only in rats fed very low protein diets is this loss of RI associated with lower cytosolic PKA activity. In contrast, energy restriction lowers PKA activity in the cytosol and particulate fractions, independent of signficant reduction in RI or RII subunits. Taken together, these data indicate that moderate protein and energy restrictions have differential effects on activity and quantity of PKA.

Deposition of cellular fibronectin increases before stellate cell activation in rat liver during ethanol feeding

Cellular fibronectin (cFN)-a structural extracellular matrix protein-facilitates cell adhesion, migration, and differentiation during organ development; wound healing; tissue regeneration; and fibrogenic processes. cFN is deposited early in various fibrotic diseases and seems to function as a template for deposition of other extracellular matrix proteins, such as collagen type I and laminin, in the injured area. We have compared the relative changes in cFN levels with other pathogenic markers of alcoholic liver injury over time of ethanol feeding in the rat. Male Wistar rats were allowed free access to a liquid diet containing 36% of total energy as ethanol or pair-fed an isocaloric control diet for 4, 8, and 12 weeks. Serum alanine aminotransferase activity and total liver lipid were increased in ethanol-fed animals, compared with pair-fed controls after 4, 8, and 12 weeks of feeding. Liver lipid content was higher in ethanol-fed rats as early as 4 weeks and was further increased by 12 weeks of feeding. Total fibronectin and cFN protein quantity was greater in liver from ethanol-fed rats after 8 and 12 weeks (fibronectin: 2.3-fold and 2.6-fold; cFN: 4.3-fold and 2.6-fold higher than pair-fed at 8 and 12 weeks, respectively). alpha-Smooth muscle actin, an indicator of hepatic stellate cell activation, was increased in the liver of ethanol-fed rats after 12 weeks of feeding (344% higher compared with pair-fed), with no differences observed at any earlier time points. In summary, increases in hepatic immunoreactive cFN content were observed subsequent to increased liver lipid concentration, but before hepatic stellate cell activation in rats fed the ethanol-based diet. These data suggest that deposition of cFN in the liver during long-term ethanol consumption may represent an early response to injury similar to that observed in other models of liver injury and wound healing.

Chronic ethanol feeding in a high-fat diet decreases insulin-stimulated glucose transport in rat adipocytes

Ethanol consumption has been associated with glucose intolerance and insulin resistance and is suggested to be an independent risk factor in the development of non-insulin-dependent diabetes mellitus. We have investigated the long-term effects of ethanol consumption on insulin-regulated glucose transport in rat adipocytes. Male Wistar rats were fed a high-fat liquid diet containing 35% ethanol (ethanol fed) or a control diet that isocalorically substituted maltose dextrin for ethanol (ad libitum). A third group was pair fed the control diet. Basal rates of 2-deoxyglucose uptake were similar in adipocytes from all three groups. Treatment with insulin increased 2-deoxyglucose uptake in ad libitum- and pair-fed rats but did not stimulate uptake in ethanol-fed rats. Similarly, although okadaic acid increased 2-deoxyglucose uptake in pair-fed rats, it had no effect in ethanol-fed rats. GLUT-1 quantity was greater in pair-fed and ethanol-fed rats compared with ad libitum controls. GLUT-4 was decreased in ethanol-fed compared with pair-fed rats but was not different from ad libitum controls. In ad libitum- and pair-fed rats, insulin increased the translocation of GLUT-4 to the cell surface by 2.0-fold. In contrast, translocation of GLUT-4 was not observed after insulin stimulation of ethanol-fed rats, paralleling the loss of insulin-stimulated glucose uptake. In ethanol-fed rats, GLUT-4 protein quantity was negatively associated with increased Gs alpha protein and isoproterenol-stimulated adenosine 3',5'-cyclic monophosphate production. These data suggest that loss of insulin-stimulated glucose uptake in rat adipocytes after chronic ethanol feeding is at least partially due to decreased movement of GLUT-4 to the cell surface after insulin stimulation.

Chronic ethanol feeding impairs glucose tolerance but does not produce skeletal muscle insulin resistance in rat epitrochlearis muscle

Herein, we have investigated whether male Wistar rats develop impaired glucose tolerance after ethanol feeding. Rats were fed a liquid diet providing 35% calories from ethanol (EF) or a control diet that isocalorically replaced ethanol with maltose-dextrins for 4 weeks. Intravenous glucose tolerance was impaired in EF rats compared with pair-fed (PF), but not ad libitum (AL) controls. Areas under the intravenous glucose tolerance test curve were 5476 +/- 516 mm2, 3056 +/- 421 mm2, and 4199 +/- 613 mm2 (p < 0.05) for AL, PF, and EF rats, respectively. Initial plasma insulin concentrations in EF rats were comparable with PF rats; however, 15 min after a dextrose challenge, plasma insulin levels in EF rats were 39% lower than PF rats. Because skeletal muscle is the primary sink for insulin-mediated glucose disposal, the development of skeletal muscle insulin resistance after ethanol feeding could contribute to impaired glucose tolerance. Total GLUT1 was not affected by diet in either red or white muscle. No difference in the total quantity of insulin-responsive glucose transporter, GLUT4, was observed in red muscle. In contrast, GLUT4 was 20% lower in white muscle from EF rats, compared with PF and AL rats. However, insulin-stimulated glucose transport into the epitrochlearis, a white muscle group, was not impaired with ethanol feeding. These data demonstrate that chronic ethanol feeding impairs glucose tolerance; impaired glucose tolerance was associated with an inability to maintain plasma insulin levels, rather than the development of skeletal muscle insulin resistance.

Very low protein diets induce a rapid decrease in hepatic cAMP-dependent protein kinase followed by a lower increase in adenylyl cyclase activity in rats

Recent evidence indicates that cAMP-mediated responses are desensitized in liver during malnutrition. While receptor-stimulated production of cAMP is increased in hepatocytes from rats fed very low protein diets for 14 d, activity of cAMP-dependent protein kinase (PKA) is decreased in liver cytosol. The present study investigated the time course for this desensitization. Weanling rats were fed either a 0.5 (malnourished) or 15% protein (control) diet for 1, 3, 7 or 14 d. Total PKA activity decreased after only 3 d of feeding the low protein diet. This decrease was confined to the cytosolic compartment and was associated with a lower quantity of immunoreactive RI regulatory subunit of PKA, with no difference in the quantity of immunoreactive RII regulatory subunit. In contrast, basal-, MnCl2- and guanine nucleotide regulatory protein-stimulated adenylyl cyclase activities were not greater in liver membranes of malnourished rats than in those of the control rats until the 2nd wk of feeding. Greater activity was paralleled by an increase in the quantity of the stimulatory guanine nucleotide regulatory protein at d 14. The inhibitory guanine nucleotide regulatory protein quantity did not differ between dietary groups. Greater cAMP production was not mediated by changes in PKA phosphorylation of adenylyl cyclase because preincubation of membranes with purified PKA catalytic subunit decreased MnCl2-stimulated cAMP production equally in liver membranes of both control and malnourished rats. Similarly, treatment with alkaline phosphatase decreased adenylyl cyclase activity but did not eliminate the difference in adenylyl cyclase activity between control and malnourished rats. These data demonstrate that loss of PKA activity is an early response to a low protein diet and that, subsequently, a number of molecular adaptations occur which increase cAMP production. These changes may be adaptive responses to malnutrition that maintain essential cAMP-dependent functions.

Equilibrative adenosine transport in rat hepatocytes after chronic ethanol feeding

Acute treatment of cells with ethanol in vitro inhibits adenosine uptake via equilibrative nucleoside transporters. After longer periods of exposure to ethanol in culture, rechallenge with ethanol no longer inhibits adenosine uptake. Herein, we have investigated the long-term effects of ethanol consumption in vivo on equilibrative nucleoside transport. Rats were fed a liquid diet containing 35% of calories as ethanol (ethanol-fed). Control rats were pair-fed a liquid diet that isocalorically substituted maltose dextrins for ethanol. After 4 weeks of ethanol consumption, nucleoside transport was measured in isolated hepatocytes. Uptake of [3H]adenosine was lower in ethanol-fed rats compared with control. Influx of the nonmetabolizable nucleoside analog, [3H]formycin B, was also decreased after ethanol feeding. However, neither the number of nitrobenzylthioinosine (NBMPR) binding sites or inhibition of adenosine uptake by NBMPR were affected by ethanol feeding. In controls, acute treatment of isolated hepatocytes with 100 mM ethanol inhibited [3H]adenosine uptake by 30-40%. However, in ethanol-fed rats, acute challenge with ethanol did not inhibit [3H]adenosine uptake. These data demonstrate that long-term ethanol feeding decreases equilibrative nucleoside transport in hepatocytes independent of a change in the number of nucleoside transporters and renders adenosine uptake insensitive to inhibition by ethanol.