Comparison of the generation of albumin-associated cytotoxic activity in supernatants from ethanol-containing cultures of human blood monocyte-derived macrophages and of two human hepatoma cell lines

Ethanol metabolism by macrophages: possible role in organ damage

Macrophages and hepatocytes oxidize ethanol to acetate in vitro at comparable rates but by different biochemical pathways. Ethanol metabolism by macrophages is largely ADH-independent and mainly based on cytochrome P450 and on the extracellular release of superoxide anion radicals. There is also evidence that during ethanol metabolism, macrophages release more acetaldehyde extracellularly than hepatocytes; the high concentrations of acetaldehyde around macrophages may damage surrounding tissue cells. Some of this acetaldehyde forms unstable cytotoxic complexes with serum albumin and with erythrocytes. The superoxide anion radicals released by macrophages may not only oxidize ethanol to acetaldehyde but also react with and damage cells in their immediate vicinity. After exposure to ethanol, macrophage-depleted rodents show markedly reduced levels of cytotoxic acetaldehyde-albumin complexes in the blood and reduced levels of hydroxyethyl radicals in the bile compared to control animals, indicating that the generation of such potentially pathogenic molecules is, to a large extent, dependent on macrophage activity. Macrophage-depleted animals also show less early liver damage than control animals. The reduction in ethanol-induced liver damage in macrophage-depleted mice and rats may be due to a reduction or elimination of the generation of various Kupffer-cell-derived hepatotoxic substances, including acetaldehyde and reactive oxygen radicals, in such animals. These data suggest that ethanol metabolism by tissue macrophages may play an important role in mediating ethanol-related tissue damage.

Role of macrophages in acetaminophen (paracetamol)-induced hepatotoxicity

Research into the pathogenesis of acetaminophen (paracetamol)-induced hepatotoxicity has concentrated on the generation of toxic metabolites by the hepatocytes. It has, however, recently been shown that human macrophages cultured with acetaminophen secrete increased quantities of tumour necrosis factor (TNF). This study examines whether macrophages have a direct role in acetaminophen toxicity, using a mouse model in which it is possible to eliminate more that 99 per cent of hepatic macrophages by previously injecting liposomes containing dichloromethylene disphosphonate (DMDP). Acetaminophen-induced liver damage was assessed biochemically and histologically. It was shown that the liver damage occurring 0.5, 1, and 2 h after an intraperitoneal injection of acetaminophen was significantly less in mice previously injected with liposomes containing DMDP than in previously untreated mice, or mice previously injected with empty liposomes. By 4 h there was no difference between the groups. We conclude that macrophages play an early and probably a direct role in mediating the liver damage due to acetaminophen. This is consistent with the role that macrophages have been shown to play in the pathogenesis of alcohol-induced liver damage.

Macrophages are a major source of acetaldehyde in circulating acetaldehyde-albumin complexes formed after exposure of mice to ethanol

C57BL mice were depleted of macrophages by an intravenous injection of liposome-encapsulated dichloromethylene diphosphonate (DCMDP), and control mice were uninjected or injected with empty liposomes. One day after injection, a proportion of the DCMDP-treated and control mice was continuously exposed to ethanol vapor for 4 days. Albumin fractions were separated from the sera of both ethanol-unexposed and ethanol-exposed animals and tested for cytotoxicity against a monolayer of A9 cells using two indicators of cytotoxicity: detachment of adherent cells and a decrease in the ability of cells to reduce tetrazolium. The results show that, in mice exposed to ethanol, macrophages are a major source of the acetaldehyde in circulating cytotoxic acetaldehyde-albumin complexes and presumably also of free acetaldehyde.

Possible role of macrophages in the pathogenesis of ethanol-induced bone marrow damage

Adherent-cell-depleted human marrow cells (MC) were cultured on their own or co-cultured with monolayers of blood-monocyte-derived macrophages or marrow-derived adherent cells with and without 2 mg ethanol/ml for 24 h. The incorporation of 3H-thymidine and 3H-leucine by MC cultured on their own was not significantly influenced by ethanol. By contrast, the incorporation of both radiolabelled compounds was significantly lower in MC from the co-cultures containing ethanol than from those not containing ethanol. This effect was mediated by a diffusible factor produced by macrophages in the ethanol-containing cultures and was independent of intercellular contact. Supernatants from ethanol-containing cultures of marrow-derived adherent cells displayed cytotoxic activity against A9 cells due to the presence of unstable acetaldehyde-albumin complexes. Ethanol inhibited rather than stimulated nitrite production in the MC/marrow-derived adherent cell co-culture system, suggesting that macrophage-derived nitric oxide did not play a role in causing the observed ethanol-related effects. The data indicate that, in the presence of ethanol, macrophages cause inhibition of the incorporation of 3H-thymidine and 3H-leucine into overlying MC, at least partly by oxidizing ethanol to acetaldehyde and releasing some of the potentially cytotoxic acetaldehyde thus formed extracellularly. Bone marrow macrophages may therefore play an important role in the pathogenesis of alcohol-related marrow damage in vivo.