Zonal differences in ethanol-induced impairments in hepatic receptor binding

Neonatal liver physiology

In the neonate, the liver is relatively immature and undergoes several changes in its functional capacity during the early postnatal period. The essential liver functions can be classified into three categories: metabolism, detoxification, and bile synthesis. In general, the immature liver function has limited consequences on the healthy term neonate. However, preterm neonates are particularly susceptible to the effects of the immature liver function placing them at risk of hypoglycemia, hyperbilirubinemia, cholestasis, bleeding, and impaired drug metabolism. An appreciation of the dynamic changes in liver function during the neonatal period is essential for successful management of neonates who require medical and surgical interventions. This review will focus on the neonatal liver function as well as the changes that the liver undergoes as it matures.

Cyanamide potentiates the ethanol-induced impairment of receptor-mediated endocytosis in a recombinant hepatic cell line expressing alcohol dehydrogenase activity

Ethanol administration has been shown to alter receptor-mediated endocytosis in the liver. We have developed a recombinant hepatic cell line stably transfected with murine alcohol dehydrogenase cDNA to serve as an in vitro model to investigate these ethanol-induced impairments. In the present study, transfected cells were maintained in the absence or presence of 25 mM ethanol for 7 days, and alterations in endocytosis by the asialoglycoprotein receptor were determined. The role of acetaldehyde in this dysfunction was also examined by inclusion of the aldehyde dehydrogenase inhibitor, cyanamide. Our results showed that ethanol metabolism impaired internalization of asialoorosomucoid, a ligand for the asialoglycoprotein receptor. The addition of cyanamide potentiated the ethanol-induced defect in internalization and also impaired degradation of the ligand in the presence of ethanol. These results indicate that the ethanol-induced impairment in endocytosis is exacerbated by the inhibition of aldehyde dehydrogenase, suggesting the involvement of acetaldehyde in this dysfunction.

Fibronectin: Functional character and role in alcoholic liver disease

Fibronectins are adhesive glycoproteins that can be found in tissue matrices and circulating in various fluids of the body. The variable composition of fibronectin molecules facilitates a diversity of interactions with cell surface receptors that suggest a role for these proteins beyond the structural considerations of the extracellular matrix. These interactions implicate fibronectin in the regulation of mechanisms that also determine cell behavior and activity. The two major forms, plasma fibronectin (pFn) and cellular fibronectin (cFn), exist as balanced amounts under normal physiological conditions. However, during injury and/or disease, tissue and circulating levels of cFn become disproportionately elevated. The accumulating cFn, in addition to being a consequence of prolonged tissue damage, may in fact stimulate cellular events that promote further damage. In this review, we summarize what is known regarding such interactions between fibronectin and cells that may influence the biological response to injury. We elaborate on the effects of cFn in the liver, specifically under a condition of chronic alcohol-induced injury. Studies have revealed that chronic alcohol consumption stimulates excess production of cFn by sinusoidal endothelial cells and hepatic stellate cells while impairing its clearance by other cell types resulting in the build up of this glycoprotein throughout the liver and its consequent increased availability to influence cellular activity that could promote the development of alcoholic liver disease. We describe recent findings by our laboratory that support a plausible role for cFn in the promotion of liver injury under a condition of chronic alcohol abuse and the implications of cFn stimulation on the pathogenesis of alcoholic liver disease. These findings suggest an effect of cFn in regulating cell behavior in the alcohol-injured liver that is worth further characterizing not only to gain a more comprehensive understanding of the role this reactive glycoprotein plays in the progression of injury but also for the insight further studies could provide towards the development of novel therapies for alcoholic liver disease.

Cellular fibronectin stimulates hepatocytes to produce factors that promote alcohol-induced liver injury

AIM

To examine the consequences of cellular fibronectin (cFn) accumulation during alcohol-induced injury, and investigate whether increased cFn could have an effect on hepatocytes (HCs) by producing factors that could contribute to alcohol-induced liver injury.

METHODS

HCs were isolated from rats fed a control or ethanol liquid diet for four to six weeks. Exogenous cFn (up to 7.5 μg/mL) was added to cells cultured for 20 h, and viability (lactate dehydrogenase,LDH), apoptosis (caspase activity) and secretion of proinflammatory cytokines (tumor necrosis factor alpha, TNF-α and interleukin 6 IL-6), matrix metalloproteinases (MMPs) and their inhibitors (tissue inhibitors of metalloproteinases, TIMPs) was determined. Degradation of iodinated cFn was determined over a 3 h time period in the preparations.

RESULTS

cFn degradation is impaired in HCs isolated from ethanol-fed animals, leading to its accumulation in the matrix. Addition of exogenous cFn did not affect viability of HCs from control or ethanol-fed animals, and apoptosis was affected only at the higher concentration. Secretion of MMPs, TIMPs, TNF-α and IL-6, however, was increased by exogenously added cFn, with HCs from ethanol-fed animals showing increased susceptibility compared to the controls.

CONCLUSION

These results suggest that the elevated amounts of cFn observed in alcoholic liver injury can stimulate hepatocytes to produce factors which promote further tissue damage.

Effects of ethanol on receptor-mediated endocytosis in the liver

Ethanol administration impairs multiple aspects in the process of receptor-mediated endocytosis (RME) in the liver. Studies from our laboratory over the last 10 years have carefully examined RME by the hepatocyte-specific asialoglycoprotein receptor (ASGP-R). We have identified a time course for ethanol-induced defects in RME and established that many of the impairments occur initially in the centrilobular region of the liver and as early as one week after ethanol administration. Impaired intravesicular acidification in ethanol-fed animals has been identified, and these defects in acidification could alter multiple protein trafficking pathways including RME. In addition to altered acidification, altered receptor function (including receptor inactivation) could also contribute to impaired trafficking. Current studies in our laboratory are aimed at an examination of posttranslational modifications in the receptor (acylation and phosphorylation) that are known to affect its function. A role for the ASGP-R in the process of alcoholic apoptosis is also being examined because proper functioning of the ASGP-R is thought to be important in clearance of apoptotic cells.

Ethanol-impaired hepatic protein trafficking: concepts from the asialoglycoprotein receptor system

OBJECTIVES

Alcohol abuse with its resulting liver injury is a major health problem worldwide. Recent studies have shown that the process of receptor-mediated endocytosis (RME) is especially susceptible to the deleterious effects of ethanol.

DESIGN AND METHODS

In our laboratory, we have shown that after as early as 1 week of ethanol administration, binding, internalization and degradation of asialoorosomucoid, a ligand for the asialoglycoprotein receptor (ASGP-R), is significantly impaired. We have also demonstrated that ethanol administration impairs ATP-dependent acidification of prelysosomal endosomes.

RESULTS

These impairments are seen using ligands internalized by the non-specific process of fluid phase endocytosis as well as those internalized by coated pit endocytosis. In addition, we have identified ethanol-induced alterations in post-translational modifications of the receptor including phosphorylation and fatty acid modification (palmitoylation).

CONCLUSIONS

Impaired function of this receptor could lead to alterations of membrane internalization events after ethanol administration and contribute to ethanol-induced alterations in protein trafficking and signaling in the liver.

Differential effects of monensin on asialoglycoprotein receptor function after short-term ethanol administration

Chronic ethanol consumption is associated with multiple impairments in receptor-mediated endocytosis (RME) by the hepatic asialoglycoprotein receptor (ASGP-R). Previous work on this receptor has shown that its activity can be perturbed by the carboxylic ionophore monensin. This agent has been shown to preferentially affect receptor-ligand dissociation and receptor redistribution of one subset (State 2) of ASGP-R, while receptor function in a second subset (State 1 receptors) is unaffected. In the present study, we examined the effect of monensin on ASGP-R activity and intracellular receptor-ligand dissociation after 7-10 days of ethanol feeding, a time when we have shown altered ASGP-R function in ethanol-fed animals. Hepatocytes from male Wistar rats (fed an ethanol-containing or control diet) were utilized. Ethanol administration decreased total ligand binding by 35-40% (P < 0.01) without a change in receptor protein content. After monensin treatment, surface receptors on cells from control animals were inactivated and redistributed to the cell interior. In cells from ethanol-fed animals, a similar pattern of monensin-induced inactivation was shown, but no redistribution occurred. Intracellular receptor-ligand dissociation was impaired in both cell types, although the monensin-induced effect on dissociation was significantly less dramatic (two-fold) in the hepatocytes from ethanol-fed animals as compared with controls. Thus, although receptors on both cell types were susceptible to monensin, cells from the ethanol-fed animals were less vulnerable to the added effects of this agent. Since monensin affects functioning of State 2, but not State 1 receptors, a very early effect of ethanol may be a preferential impairment in the State 2 receptor population.