Oxidative modification of low-density lipoprotein by the human hepatoma cell line HepG2

Downstream effects on human low density lipoprotein of homocysteine exported from endothelial cells in an in vitro system

A model system is presented using human umbilical vein endothelial cells (HUVECs) to investigate the role of homocysteine (Hcy) in atherosclerosis. HUVECs are shown to export Hcy at a rate determined by the flux through the methionine/Hcy pathway. Additional methionine increases intracellular methionine, decreases intracellular folate, and increases Hcy export, whereas additional folate inhibits export. An inverse relationship exists between intracellular folate and Hcy export. Hcy export may be regulated by intracellular S-adenosyl methionine rather than by Hcy. Human LDLs exposed to HUVECs exporting Hcy undergo time-related lipid oxidation, a process inhibited by the thiol trap dithionitrobenzoate. This is likely to be related to the generation of hydroxyl radicals, which we show are associated with Hcy export. Although Hcy is the major oxidant, cysteine also contributes, as shown by the effect of glutamate. Finally, the LDL oxidized in this system showed a time-dependent increase in uptake by human macrophages, implying an upregulation of the scavenger receptor. These results suggest that continuous export of Hcy from endothelial cells contributes to the generation of extracellular hydroxyl radicals, with associated oxidative modification of LDL and incorporation into macrophages, a key step in atherosclerosis. Factors that regulate intracellular Hcy metabolism modulate these effects.

Selective blockade of mGlu5 metabotropic glutamate receptors protects rat hepatocytes against hypoxic damage

Western blot analysis of protein extracts from rat liver revealed the presence of the mGlu5 receptor, one of the G-protein-coupled receptors activated by glutamate (named "metabotropic glutamate receptors" or mGlu receptors). mGlu5 expression was particularly high in extracts from isolated hepatocytes, where levels were comparable with those seen in the rat cerebral cortex. The presence of mGlu5 receptors in hepatocytes was confirmed by reverse-transcription polymerase chain reaction (RT-PCR) analysis, immunohistochemistry in neonate or adult rat liver, as well as by immunocytochemical analysis in HepG2 hepatoma cells, where the receptor appeared to be preferentially distributed in cell membranes. Interestingly, mGlu1 receptors (which are structurally and functionally homologous to mGlu5 receptors) were never found in rat liver or hepatocytes. In hepatocytes exposed to anoxic conditions for 90 minutes, glutamate, (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid (1S,3R-ACPD) and quisqualate, which all activate mGlu5 receptors, accelerated the onset and increased the extent of cell damage, while 4-carboxy-3-hydroxyphenylglycine (4C3HPG), an agonist of mGlu2/3 receptors, was inactive. 2-methyl-6-(2-phenyl-1-ethynyl)-pyridine (MPEP), a novel, noncompetitive, highly selective mGlu5 receptor antagonist, not only abolished the toxic effect of 1S,3R-ACPD, but, unexpectedly, was protective by itself against anoxic damage. This suggests that hepatocytes express mGlu5 receptors and that activation of these receptors by endogenous glutamate facilitates the development of anoxic damage in hepatocytes.

Suppression of humoral immunization against encapsulated xenogeneic hepatocytes and prolongation of their function by 2-week cyclosporine treatment in the rat

BACKGROUND

Xenogeneic liver transplantation may induce immune reactions not only against the grafted liver but also against the proteins that it synthesizes. We investigated whether 2-week cyclosporine treatment could suppress immunization and improve graft function in a xenogeneic hepatocyte transplantation model.

METHODS

Free or encapsulated human hepatoma cells (HepG2) were cocultured for 28 days with splenocytes from Lewis rats or implanted for 60 days into the peritoneum of Lewis rats.

RESULTS

Anti-HepG2 and antialbumin antibodies were detected in the supernatants of rat splenocytes that were cocultured with HepG2 cells and in the serum of rats that had undergone transplantation with HepG2 cells. Cyclosporine suppressed this antibody production both in vitro and in vivo. Human alpha-GST blood levels, which reflect hepatocyte injury, were low in cyclosporine-treated animals but high when encapsulated HepG2 cells were transplanted without cyclosporine therapy. Western blots revealed human albumin from day 3 to day 60 in the serum of rats treated with cyclosporine, but not after day 30 in untreated rats.

CONCLUSIONS

Xenogeneic hepatocytes induce a humoral response that impairs their viability and function. A 2-week course of cyclosporine suppresses this immune response and improves graft function for up to 60 days.

Cell-mediated oxidation of LDL: comparison of different cell types of the atherosclerotic lesion

The three major cell types of the human atherosclerotic lesion--macrophages (Mø), smooth muscle cells (SMC) and endothelial cells (EC)--were compared for their ability to oxidise low density lipoprotein (LDL) in vitro under identical conditions. Near-confluent cultures were incubated for up to 48 h with 50 microg protein/ml LDL in Ham's F10 medium supplemented with 7 microM Fe2+. All three cell types oxidised LDL readily using our culture conditions. After 24 and 48 h, the degree of LDL oxidation was in the order: Mø > SMC > EC when based on cell growth area and EC > SMC > Mo when based on cellular DNA content. However, LDL oxidation in vitro progressed more slowly between 24 and 48 h, probably due to increasing toxicity to the cells and/or depletion of polyunsaturated fatty acids. We therefore compared the time of onset of LDL oxidation. The earliest increase in LDL oxidation was always apparent with SMC. Gas chromatography revealed that LDL oxidation by all three cell types followed a similar pattern. The polyunsaturated fatty acids linoleic acid (18:2) and arachidonic acid (20:4) were depleted (to 10.3-18.1% and 4.5-24.7% respectively, compared to native LDL), whereas the content of stearic acid (18:0) and oleic acid (18:1) remained unchanged. Cholesterol was depleted (to 54.1-75.6% of native LDL) with a concomitant rise in 7 -hydroxycholesterol (to 60.6-128.1 microg/mg LDL). This corresponds to a conversion of 4.9, 9.5 and 10.4% of LDL cholesterol in EC-, SMC- and Mo-modified LDL respectively. All three cell types showed significant toxicity in the oxidising culture after 24h. The possible relevance to LDL oxidation in atherosclerosis is discussed.

Cellular thiol production and oxidation of low-density lipoprotein

Compelling evidence suggests that low-density lipoprotein (LDL) is oxidized by cells within the arterial intima and that, once oxidized, it is profoundly atherogenic. The precise mechanism(s) by which cells promote the oxidation of LDL in vivo are not known; in vitro, however, oxidation of LDL can be enhanced by a number of differing mechanisms, including reaction with free and protein-bound metal ions, thiols, reactive oxygen species, lipoxygenase, myeloperoxidase and peroxynitrite. This review is concerned with the mechanisms by which cells enhance the oxidation of LDL in the presence of transition metals; in particular, the regulation, pro- and anti-oxidant consequences, and mechanism of action of cellular thiol production are examined, and contrasted with thiol-independent oxidation of LDL in the presence of transition metals.