Time course of ethanol-induced impairment in fluid-phase endocytosis in isolated rat hepatocytes

The cell biology of the hepatocyte: A membrane trafficking machine

The liver performs numerous vital functions, including the detoxification of blood before access to the brain while simultaneously secreting and internalizing scores of proteins and lipids to maintain appropriate blood chemistry. Furthermore, the liver also synthesizes and secretes bile to enable the digestion of food. These diverse attributes are all performed by hepatocytes, the parenchymal cells of the liver. As predicted, these cells possess a remarkably well-developed and complex membrane trafficking machinery that is dedicated to moving specific cargos to their correct cellular locations. Importantly, while most epithelial cells secrete nascent proteins directionally toward a single lumen, the hepatocyte secretes both proteins and bile concomitantly at its basolateral and apical domains, respectively. In this Beyond the Cell review, we will detail these central features of the hepatocyte and highlight how membrane transport processes play a key role in healthy liver function and how they are affected by disease.

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.

Carrier-mediated uptake of lucifer yellow in skate and rat hepatocytes: a fluid-phase marker revisited

Uptake of lucifer yellow (LY), a fluorescent disulfonic acid anionic dye, was studied in isolated skate (Raja erinacea) perfused livers and primary hepatocytes to evaluate its utility as a fluid-phase marker in these cells. However, our findings demonstrated that LY is transported across the plasma membrane of skate hepatocytes largely via carrier-mediated mechanisms. Isolated perfused skate livers cleared 50% of the LY from the recirculating perfusate within 1 h of addition of either 22 or 220 microM LY, with only 4.5 and 9% of the LY remaining in the perfusate after 7 h, respectively. Most of the LY was excreted into bile, resulting in high biliary LY concentrations (1 and 10 mM at the two doses, respectively), indicating concentrative transport into bile canalicular lumen. LY uptake by freshly isolated skate hepatocytes was temperature sensitive, exhibited saturation kinetics, and was inhibited by other organic anions. Uptake was mediated by both sodium-dependent [Michaelis-Menten constant (K(m)), 125 +/- 57 microM; maximal velocity (V(max)), 1.5 +/- 0.2 pmol. min(-1). mg cells(-1)] and sodium-independent (K(m), 207 +/- 55 microM; V(max), 1.7 +/- 0.2 pmol. min(-1). mg cells(-1)) mechanisms. Both of these uptake mechanisms were inhibited by various organic anions and transport inhibitors, including furosemide, bumetanide, sulfobromophthalein, rose bengal, probenecid, N-ethylmaleimide, taurocholate, and p-aminohippuric acid. Fluorescent imaging techniques showed intracellular vesicular compartmentation of LY in skate hepatocyte clusters. Studies in perfused rat livers also indicated that LY is taken up against a concentration gradient and concentrated in bile. LY uptake in isolated rat hepatocytes was saturable, but only at high concentrations, and demonstrated a K(m) of 3.7 +/- 1.0 mM and a V(max) of 1.75 +/- 0.16 nmol. min(-1). mg wet wt(-1). These results indicate that LY is transported into skate and rat hepatocytes and bile largely by carrier-mediated mechanisms, rather than by fluid-phase endocytosis.

Chronic ethanol consumption impairs receptor-mediated endocytosis of formaldehyde-treated albumin by isolated rat liver endothelial cells

Receptor-mediated endocytosis (RME) by a scavenger receptor on sinusoidal liver endothelial cells (LECs) for formaldehyde-treated bovine serum albumin (f-Alb) has previously been shown to be impaired following chronic ethanol consumption. These studies were initially performed by in situ perfusion, making it difficult to determine the point in the process at which RME is affected. Therefore, it was the purpose of this study to use isolated LECs to begin elucidating at what point in the process chronic ethanol consumption affects RME. Initial studies showed that degradation at the single-cell level were similarly decreased at levels that had been observed for in situ studies, suggesting that the ethanol effects can be repeated using isolated LECs, making them useful for in vitro studies. Binding studies with 125I-formaldehyde-treated bovine serum albumin (125I-f-Alb) demonstrated there was a slight, but significantly different, decrease in binding by LECs from ethanol-fed rats when compared with pair-fed or chow-fed rats. However, the affinity of these receptors was not different between these groups. In contrast, a defect in the initial stages of receptor-ligand internalization was indicated as less surface-bound ligand was internalized and subsequently degraded in cells from the ethanol-treated animals as compared with controls. Additionally, once the data were adjusted for the amount of ligand internalized, the degradation of the internalized ligand was only slightly impaired. These results indicate that chronic ethanol feeding impairs the process of RME by the liver; the major cause of this impairment appears to be caused by a decreased ability of these cells to internalize all of the surface-bound ligand, with a minimal defect in postinternalization events.

Effect of ethanol on adenosine triphosphate, cytosolic free calcium, and cell injury in rat hepatocytes. Time course and effect of nutritional status

The events implicated in the early phases of acute ethanol-induced hepatocyte injury and their relation with the nutritional status of the liver are not clearly defined. We aimed to determine the effect of ethanol on ATP and cytosolic free Ca2+ in hepatocytes isolated from fed or fasted rats. Cell injury was assessed by LDH release and trypan blue uptake, ATP by [31P]NMR spectroscopy, and cytosolic free Ca2+ with aequorin. In control conditions, cells from fasted animals had a lower ATP level (-52%) and a higher cytosolic free Ca2+ (+101%) than did those isolated from fed animals. Ethanol caused a dose-dependent cell injury in both groups. At all ethanol doses, greater, damage occurred when using hepatocytes isolated from fasted rats. In both groups, a dose-dependent decrease in ATP content and a rise in cytosolic free Ca2+ were seen. The magnitude of these changes were significantly greater in the fasted group. In conclusion, these data showed that fasting affects the energy status and cytosolic free calcium level in hepatocytes; ethanol causes a dose-dependent cell injury that occurs in association with a fall in ATP and a rise in cytosolic free Ca2+ levels. The nutritional status of an animals is an important determinant of the severity of ethanol-induced damage to liver cells.

Zonal differences in ethanol-induced impairments in fluid-phase endocytosis in rat hepatocytes

We have previously shown that ethanol administration impairs the processes of fluid-phase endocytosis (FPE) and receptor-mediated endocytosis (RME) in isolated rat hepatocytes after as early as 1 week of ethanol administration. The defects in RME were most prominent in the perivenule (PV) region of the liver lobule, the area wherein alcoholic liver disease has been shown to start and predominate. We undertook the present study to see if changes in FPE were likewise more apparent in the PV versus the periportal (PP) region of the liver. For these studies, we fed male Sprague-Dawley rats with an ethanol-supplemented liquid diet or an isocaloric control diet for 1 or 5 weeks. PV and PP hepatocytes were isolated using a digitonin-collagenase perfusion method. Internalization and efflux of the marker dye, Lucifer Yellow, was then examined in the cell populations. After as early as 1 week of feeding, cells from the PV region in ethanol-fed animals showed dramatic impairments in the net internalization of dye, compared with PV cells from the pair-fed controls, and these changes persisted throughout the 5-week feeding period. In contrast, internalization of Lucifer Yellow in cells from the PP region of the liver were not different between control and ethanol animals. Because net internalization represents the balance between uptake into the cells versus efflux from the cells, we examined these components individually. Early uptake of the dye into the cell was not altered by ethanol treatment. The decreased net internalization seemed to be caused by enhanced efflux of the dye, which was significantly increased in PV cells, compared with the same cell type in control animals. Cells from the PP region of the ethanol-fed animals did not exhibit altered efflux after either 1 or 5 weeks of feeding. These results indicate that ethanol-induced impairments in FPE are more dramatic in the PV region of the liver, and these impairments seem to result from an ethanol-induced enhancement of efflux.

Effect of hyperosmolarity on both receptor-mediated and fluid-phase endocytosis in ethanol-fed animals

We have shown previously that chronic ethanol administration impairs hepatic receptor-mediated endocytosis (RME) of asialoorosomucoid (ASOR), epidermal growth factor and insulin, whereas early uptake by fluid-phase endocytosis (FPE) of a fluorescent dye, Lucifer Yellow (LY), is not altered. Results of these studies suggested that ethanol-induced injury was primarily affecting endocytosis in coated pit areas of the plasma membrane while internalization in noncoated membrane areas was unaffected. In the present study, we investigated the effects of blocking clathrin-coated pit mediated endocytosis by hyperosmolarity on FPE of LY and on RME of ASOR. We also examined the effects of hyperosmolarity on the binding and internalization of insulin, a ligand endocytosed by both RME and FPE. Uptake of LY by noncoated regions of the membrane was not altered in control animals, whereas in hepatocytes from ethanol-fed animals uptake of LY was decreased by 35-40% in the presence of 0.12 M sucrose (P < 0.05). These hyperosmolar conditions almost completely inhibited (> 85%) the endocytosis of 125I-ASOR by RME in both ethanol and control cells. Results with insulin showed slight effects (20-30% impairment) on uptake of the ligand in the presence of sucrose. These results are consistent with previous reports that in normal cells the coated pit pathway is impaired by hyperosmolarity, whereas endocytosis in noncoated regions is unaltered. It appears, however, that both FPE and RME in hepatocytes from ethanol-fed animals are susceptible to perturbation by hyperosmolarity. These results indicate that the noncoated pit pathway may be sensitive to stressful conditions such as hyperosmolarity after ethanol treatment.