Palmitic acid induces production of proinflammatory cytokine interleukin-8 from hepatocytes

Obesity and the metabolic syndrome are closely correlated with hepatic steatosis. Simple hepatic steatosis in nonalcoholic fatty liver disease can progress to nonalcoholic steatohepatitis (NASH), which can be a precursor to more serious liver diseases, such as cirrhosis and hepatocellular carcinoma. The pathogenic mechanisms underlying progression of steatosis to NASH remain unclear; however, inflammation, proinflammatory cytokines, and oxidative stress have been postulated to play key roles. We previously reported that patients with NASH have elevated serum levels of proinflammatory cytokines, such as interleukin-8 (IL-8), which are likely to contribute to hepatic injury. This study specifically examines the effect of hepatic steatosis on IL-8 production. We induced lipid accumulation in hepatocytes (HepG2, rat primary hepatocytes, and human primary hepatocytes) by exposing them to pathophysiologically relevant concentrations of palmitic acid to simulate the excessive influx of fatty acids into hepatocytes. Significant fat accumulation was documented morphologically by Oil Red O staining in cells exposed to palmitic acid, and it was accompanied by an increase in intracellular triglyceride levels. Importantly, palmitic acid was found to induce significantly elevated levels of biologically active neutrophil chemoattractant, IL-8, from steatotic hepatocytes. Incubation of the cells with palmitate led to increased IL-8 gene expression and secretion (both mRNA and protein) through mechanisms involving activation of nuclear factor kappaB (NF-kappaB) and c-Jun N-terminal kinase/activator protein-1.

CONCLUSION

These data demonstrate for the first time that lipid accumulation in hepatocytes can stimulate IL-8 production, thereby potentially contributing to hepatic inflammation and consequent liver injury.

Ethanol inhibits lipid raft-mediated TCR signaling and IL-2 expression: potential mechanism of alcohol-induced immune suppression

BACKGROUND

Alcohol abuse has long-term deleterious effects on the immune system, and results in a depletion and loss of function of CD4(+) T lymphocytes, which regulate both innate and adaptive immunity. T-lymphocyte activation via T-cell receptor (TCR) involves the lipid raft colocalization and aggregation of proteins into the immunological signalosome, which triggers a signaling cascade resulting in the production of interleukin-2 (IL-2). IL-2 regulates the proliferation and clonal expansion of activated T cells and is essential for an effective immune response. The present work examines the mechanisms underlying ethanol-induced dysfunction of CD4(+) T lymphocytes based on the hypothesis that ethanol downregulates lipid raft-mediated TCR signal transduction and resultant IL-2 production.

METHODS

Primary or cultured human T lymphocytes were exposed to ethanol for 24 hours prior to stimulation with anti-CD3/anti-CD28 antibodies or phytohemagglutinin. Effects of ethanol exposure on TCR-signaling (including activation of Lck, ZAP70, LAT, and PLCγ1) and IL-2 gene expression were examined.

RESULTS

Exposure of both primary and cultured human CD4(+) T lymphocytes to physiologically relevant concentrations of ethanol leads to down-regulation of IL-2 mRNA and protein via inhibition of DNA-binding activity of NFAT, the essential transcription factor for IL-2. Ethanol decreases tyrosine phosphorylation and activation of upstream signaling proteins PLCγ1, LAT, ZAP70, and Lck. These effects are prevented by inhibition of metabolism of ethanol. Sucrose density gradient fractionation and confocal microscopy revealed that ethanol inhibited essential upstream lipid raft-mediated TCR-dependent signaling events, namely colocalization of Lck, ZAP70, LAT, and PLCγ1 with plasma membrane lipid rafts.

CONCLUSIONS

Overall, our data demonstrate that ethanol inhibits lipid raft-mediated TCR-signaling in CD4(+) T lymphocytes, resulting in suppression of IL-2 production. These findings may represent a novel mechanism underlying alcohol abuse-associated immune suppression and may be particularly relevant in diseases such as HIV/AIDS and hepatitis C virus infection where alcohol abuse is a known comorbidity.

Interactions of cytokines, S-Adenosylmethionine, and S-Adenosylhomocysteine in alcohol-induced liver disease and immune suppression

Alcoholic liver disease (ALD) remains a leading cause of death in the USA. Defining mechanisms for liver cell death in ALD in order to develop potential new agents for therapeutic intervention is a major focus of the authors' work. Abnormal cytokine metabolism is a major feature of ALD, and a thorough understanding of both mechanisms and interactions of cytokine overproduction and sensitization are critical to developing a possible treatment for ALD. S-Adenosylmethionine has been used in a variety of animal studies and clinical trials and has been reported to improve biochemical parameters of liver function. Last, immunosuppression associated with chronic alcohol abuse is an important predisposing factor to opportunistic infections and cancer. It is the authors' working hypothesis that alcohol consumption leads to chronic activation of the immune system.