Hyperphosphorylation of the asialoglycoprotein receptor in isolated rat hepatocytes following ethanol administration

Susceptibility to T cell-mediated liver injury is enhanced in asialoglycoprotein receptor-deficient mice

T cell activation and associated pro-inflammatory cytokine production is a pathological feature of inflammatory liver disease. It is also known that liver injury is associated with marked impairments in the function of many hepatic proteins including a hepatocyte-specific binding protein, the asialoglycoprotein receptor (ASGPR). Recently, it has been suggested that hepatic ASGPRs may play an important role in the physiological regulation of T lymphocytes, leading to our hypothesis that ASGPR defects correlate with inflammatory-mediated events in liver diseases. Therefore, in this study we investigated whether changes in hepatocellular ASGPR expression were related to the dysregulation of intrahepatic T lymphocytes and correlate with the development of T-cell mediated hepatitis. Mice lacking functional ASGPRs (receptor-deficient, RD), and wild-type (WT) controls were intravenously injected with T-cell mitogens, Concanavalin A (Con A) or anti-CD3 antibody. As a result of T cell mitogen treatment, RD mice lacking hepatic ASGPRs displayed enhancements in liver pathology, transaminase activities, proinflammatory cytokine expression, and caspase activation compared to that observed in normal WT mice. Furthermore, FACS analysis demonstrated that T-cell mitogen administration resulted in a significant rise in the percentage of CD8+ lymphocytes present in the livers of RD animals versus WT mice. Since these two mouse strains differ only in whether they express the hepatic ASGPR, it can be concluded that proper ASGPR function exerts a protective effect against T cell mediated hepatitis and that impairments to this hepatic receptor could be related to the accumulation of cytotoxic T cells that are observed in inflammatory liver diseases.

Impact of asialoglycoprotein receptor deficiency on the development of liver injury

The asialoglycoprotein (ASGP) receptor is a well-characterized hepatic receptor that is recycled via the common cellular process of receptor-mediated endocytosis (RME). The RME process plays an integral part in the proper trafficking and routing of receptors and ligands in the healthy cell. Thus, the mis-sorting or altered transport of proteins during RME is thought to play a role in several diseases associated with hepatocyte and liver dysfunction. Previously, we examined in detail alterations that occur in hepatocellular RME and associated receptor functions as a result of one particular liver injury, alcoholic liver disease (ALD). The studies revealed profound ethanol-mediated impairments to the ASGP receptor and the RME process, indicating the importance of this receptor and the maintenance of proper endocytic events in normal tissue. To further clarify these observations, studies were performed utilizing knockout mice (lacking a functional ASGP receptor) to which were administered several liver toxicants. In addition to alcohol, we examined the effects following administration of anti-Fas (CD95) antibody, carbon tetrachloride (CCl(4)) and lipopolysaccharide (LPS)/galactosamine. The results of these studies demonstrated that the knockout mice sustained enhanced liver injury in response to all of the treatments, as shown by increased indices of liver damage, such as enhancement of serum enzyme levels, histopathological scores, as well as hepatocellular death. Overall, the work completed to date suggests a possible link between hepatic receptors and liver injury. In particular, adequate function and content of the ASGP receptor may provide protection against various toxin-mediated liver diseases.

Impaired receptor-mediated endocytosis: its role in alcohol-induced apoptosis

Hepatocyte apoptosis, inflammation, and fibrosis are prominent features of liver disease in general and of alcoholic liver injury in particular. Although the link between these processes remains unclear, one universal characteristic of liver injury is the induction of hepatocellular damage, which results in the generation of apoptotic bodies. Work from our laboratory over the last several years has studied the effect of ethanol administration on the process of apoptosis and a role for altered endocytosis in alcoholic apoptosis. We initially focused our research on the hepatocyte by examining endocytosis using the asialoglycoprotein receptor (ASGP-R) pathway as a model and we identified multiple ethanol-induced impairments in receptor function. We also showed that uptake of apoptotic bodies is impaired in hepatocytes isolated from ethanol-fed animals compared to controls, and that this impairment is linked to altered ASGP-R function. Recent work from our laboratory is examining a link between ethanol-impaired ASGP-R function, apoptotic body accumulation, and inflammation in the liver. We are particularly interested in data showing that factors produced by Kupffer cells incubated with apoptotic bodies can lead to production of tumor necrosis factor-alpha and interleukin-6, and that this effect is exacerbated in the setting of alcohol administration. In addition, we have preliminary data showing that media from Kupffer cell cultures incubated with apoptotic bodies can induce hepatocyte killing. The goal of our future work is to show that inadequate removal of apoptotic cells, in part via altered receptor-mediated endocytosis, plays a role in the course of pathogenesis of alcoholic liver injury.

Acute ethanol exposure impairs angiogenesis and the proliferative phase of wound healing

Acute ethanol exposure represents an increased risk factor for morbidity and mortality associated with surgical or traumatic injury. Despite clinical observations suggesting that ethanol exposure before injury alters tissue repair processes, little direct evidence about the mechanism by which ethanol affects the wound healing process is available. In this study, excisional wounds from female BALB/c mice with or without circulating ethanol levels of 100 mg/dl were used to assess wound closure, angiogenesis, and collagen content. Ethanol exposure resulted in a significant but transient delay in wound closure at day 2 postwounding (28 +/- 4% vs. 17 +/- 1%). In addition, total collagen content was significantly reduced by up to 37% in wounds from ethanol-treated mice compared with controls. The most significant effect of ethanol exposure on wounds was on vascularity because angiogenesis was reduced by up to 61% in wounds from ethanol-treated mice. The reduction in vessel density occurred despite near-normal levels of proangiogenic factors VEGF and FGF-2, suggesting a direct effect of ethanol exposure on endothelial cell function. Further evidence for a direct effect was observed in an in vitro angiogenesis assay because the exposure of endothelial cells to ethanol reduced angiogenic responsiveness to just 8.33% of control in a cord-forming assay. These studies provide novel information regarding the effect of a single dose of ethanol on multiple parameters of the wound healing process in vivo and suggest a potential mechanism by which ethanol impairs healing after traumatic injury.

The lobular expression of the rat asialoglycoprotein receptor is regulated at the posttranscriptional level

The purpose of this study was to define the distribution of the asialoglycoprotein receptor (ASGP-R) main peptide, rat hepatic lectin (RHL)-1, within the rat liver lobule and to investigate its possible modulation in physiological states characterised by marked changes of receptorial expression. In particular, we chose livers from rats partially hepatectomised or at the end of pregnancy, as models, respectively, of decreased or increased expression of the ASGP-R, and used the in situ hybridisation and immunocytochemistry techniques to analyse in parallel the lobular distributions of RHL-1 mRNA and protein. In normal rat liver, although the RHL-1 mRNA was homogeneously distributed, the RHL-1 peptide was predominantly localised on the surface of pericentral hepatocytes with a gradient of expression towards the periportal zone. This gradient of expression of RHL-1 peptide was reduced in regenerating livers, in which the positive stain was restricted to a few layers of cells around the central vein. In contrast, livers at the end of pregnancy showed an overall increase of the peptide with a concomitant flattening of the gradient across the liver plate. In all the conditions, we never observed important changes in the pattern of expression of the specific mRNA. These findings indicate that the distribution of ASGP-R is heterogeneous across the liver lobule, with a pattern of expression prevalently modulated at the posttranscriptional level.

Receptor-mediated endocytosis by the asialoglycoprotein receptor: effect of ethanol administration on endosomal distribution of receptor and ligand

Using the asialoglycoprotein receptor (ASGP-R) and a representative ligand, asialoorosomucoid (ASOR), we have previously shown ethanol-induced impairment of endosomal acidification, receptor recycling and ligand binding, internalization, and degradation. In the current study, we further investigated ethanol-induced alterations in receptor/ligand trafficking by labeling endosomes in vivo with either Texas-Red-ASOR or 125I-ASOR, and then assessing the receptor/ligand content of endosomes. We assessed two fractions after both 5 and 25 min of labeling: 'early endosomes' (EEs; endosomes from the cell periphery) and 'late endosomes' (LEs; endosomes farther into the cell interior). At both time points, significantly more ligand was found in EE fractions isolated from chow- and pair-fed controls (3:1, EE to LE, respectively). However, endosomes isolated from ethanol-fed animals showed a shift over time toward a more equal ligand distribution between endosome fractions (P < or = 0.05). Analysis of the ASGP-R content revealed a distribution pattern between the endosome fractions similar to that observed for ligand distribution. Impairment of receptor-ligand dissociation was assessed in endosome fractions by determining bound/free ligand ratios. Analysis showed that most of the ligand present in both endosome fractions was free (56-99%), although more was bound to receptor in EE vs LE of both control and ethanol animals (P < or = 0.05). At 5 min, more ligand remained bound in endosomes from ethanol-fed animals compared with control endosomes (P < or = 0.05), and the same pattern was observed at the latter time point. These results suggest that delayed dissociation may cause the receptor ligand complexes to travel farther into the cell interior, which may impair proper trafficking of the ligand to lysosomes and alter the receptor recycling.

Alcoholic liver disease and apoptosis

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Carol A. Casey and Amin Nanji. The presentations were (1) Mechanisms of apoptosis in alcoholic liver disease, by Amin A. Nanji; (2) Impaired receptor-mediated endocytosis: Its role in alcoholic apoptosis, by Carol A. Casey; (3) Toxicity of ethanol in HepG2 cells that express CYP2E1, by Arthur I. Cederbaum; (4) Mitochondrial regulation of ethanol-induced hepatocyte apoptosis, by M. Adachi; and (5) Apoptosis in alcoholic hepatitis, by T. Takahashi.

Alcoholic liver disease and apoptosis

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Carol A. Casey and Amin Nanji. The presentations were (1) Mechanisms of apoptosis in alcoholic liver disease, by Amin A. Nanji; (2) Impaired receptor-mediated endocytosis: Its role in alcoholic apoptosis, by Carol A. Casey; (3) Toxicity of ethanol in HepG2 cells that express CYP2E1, by Arthur I. Cederbaum; (4) Mitochondrial regulation of ethanol-induced hepatocyte apoptosis, by M. Adachi; and (5) Apoptosis in alcoholic hepatitis, by T. Takahashi.