Dietary Linoleic Acid and Its Oxidized Metabolites Exacerbate Liver Injury Caused by Ethanol via Induction of Hepatic Proinflammatory Response in Mice

Ethanol and unsaturated dietary fat induce unique patterns of hepatic ω-6 and ω-3 PUFA oxylipins in a mouse model of alcoholic liver disease

Alcoholic liver disease (ALD), a significant health problem, progresses through the course of several pathologies including steatosis, steatohepatitis, fibrosis, and cirrhosis. There are no effective FDA-approved medications to prevent or treat any stages of ALD, and the mechanisms involved in ALD pathogenesis are not well understood. Bioactive lipid metabolites play a crucial role in numerous pathological conditions, as well as in the induction and resolution of inflammation. Herein, a hepatic lipidomic analysis was performed on a mouse model of ALD with the objective of identifying novel metabolic pathways and lipid mediators associated with alcoholic steatohepatitis, which might be potential novel biomarkers and therapeutic targets for the disease. We found that ethanol and dietary unsaturated, but not saturated, fat caused elevated plasma ALT levels, hepatic steatosis and inflammation. These pathologies were associated with increased levels of bioactive lipid metabolites generally involved in pro-inflammatory responses, including 13-hydroxy-octadecadienoic acid, 9,10- and 12,13-dihydroxy-octadecenoic acids, 5-, 8-, 9-, 11-, 15-hydroxy-eicosatetraenoic acids, and 8,9- and 11,12-dihydroxy-eicosatrienoic acids, in parallel with an increase in pro-resolving mediators, such as lipoxin A4, 18-hydroxy-eicosapentaenoic acid, and 10S,17S-dihydroxy-docosahexaenoic acid. Elucidation of alterations in these lipid metabolites may shed new light into the molecular mechanisms underlying ALD development/progression, and be potential novel therapeutic targets.

Oxidized linoleic acid metabolites induce liver mitochondrial dysfunction, apoptosis, and NLRP3 activation in mice

Circulating oxidized linoleic acid (LA) metabolites (OXLAMs) are increased in patients with nonalcoholic steatohepatitis (NASH) and their levels correlate with disease severity. However, the mechanisms by which OXLAMs contribute to NASH development are incompletely understood. We tested the hypothesis that LA or OXLAMs provided directly through the diet are involved in the development of hepatic injury. C57BL/6 mice were fed an isocaloric high-fat diet containing low LA, high LA, or OXLAMs for 8 weeks. The livers of OXLAM-fed mice showed lower triglyceride concentrations, but higher FA oxidation and lipid peroxidation in association with increased oxidative stress. OXLAM-induced mitochondrial dysfunction was associated with reduced Complex I protein and hepatic ATP levels, as well as increased mitochondrial biogenesis and cytoplasmic mitochondrial DNA. Oxidative stress increased thioredoxin-interacting protein (TXNIP) in the liver and stimulated the activation of mitochondrial apoptosis signal-regulating kinase 1 (ASK1) leading to apoptosis. We also found increased levels of NOD-like receptor protein 3 (NLRP3) inflammasome components and Caspase-1 activation in the livers of OXLAM-fed mice. In vitro, OXLAMs induced hepatocyte cell death, which was partly dependent on Caspase-1 activation. This study identified key mechanisms by which dietary OXLAMs contribute to NASH development, including mitochondrial dysfunction, hepatocyte cell death, and NLRP3 inflammasome activation.

Leucodin attenuates inflammatory response in macrophages and lipid accumulation in steatotic hepatocytes via P2x7 receptor pathway: A potential role in alcoholic liver disease

The current study was aimed to reveal that leucodin, a sesquiterpene lactone from Artemisia capillaris could inhibit the inflammatory response in macrophages and the lipid accumulation in hepatocytes via P2x7R-NLRP3 inflammasome activation. Several types of macrophages including mouse peritoneal macrophages, mouse bone marrow-derived macrophages and human macrophages THP-1 cells were pretreated with different concentrations of leucodin for 1 h and then stimulated with LPS and ATP. LPS plus ATP initiated IL-1β cleavage and release in mouse peritoneal macrophages and peaked at 4 h. Leucodin did not show significant toxicity within 200 μM and effectively inhibited pro-IL-1β cleavage and release of mature-IL-1β in macrophages. Also, P2x7R antagonist and caspase-1 inhibitor also decreased IL-1β release and cleavage. Additionally, leucodin suppressed P2x7R, TLR4 and NLRP3 expression in LPS/ATP-stimulated macrophages. HepG2 cells were pretreated with different concentrations of leucodin for 1 h and then exposed to ethanol for 24 h. Leucodin suppressed lipid accumulation and enhanced phosphorylation of AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC) in HepG2 cells exposed to ethanol. In addition, leucodin inhibited the expression of sterol regulatory element binding protein-1 (SREBP1) and ACC in ethanol-treated HepG2 cells. Leucodin possessed the capacity for inhibiting inflammatory response in macrophages and suppressing lipid accumulation in hepatocytes, suggesting a promising therapeutic potential targeting inflammation and lipid metabolism in alcoholic liver disease.

Amelioration of Alcoholic Liver Steatosis by Dihydroquercetin through the Modulation of AMPK-Dependent Lipogenesis Mediated by P2X7R-NLRP3-Inflammasome Activation

Dihydroquercetin (TAX) is the most abundant dihydroflavone found in onions, milk thistle, and Douglas fir bark. We investigated whether TAX could inhibit lipid accumulation in alcoholic liver steatosis in vivo and in vitro. An in vivo model was established by intragastrically treating mice with ethanol, and an in vitro model was created by treating HepG2 cells with ethanol. TAX regulated SREBP1 and ACC expression by elevating LKB1 and AMPK phosphorylation. Also, TAX upregulated SIRT1 expression, which was suppressed by ethanol intake. Decreased expression of P2X7R and NLRP3 and suppressed cleavage of caspase-1 by TAX resulted in the inhibition of IL-1β production and release. Additionally, TAX reduced lipogenesis and promoted lipid oxidation via the regulation of AMPK and ACC in ethanol-treated steatotic HepG2 cells. TAX downregulated IL-1β cleavage responses to LPS and ATP stimulation in HepG2 cells. P2X7R deficiency attenuated lipid accumulation, characterized by increased AMPK activity and decreased SREBP1 expression in ethanol-treated HepG2 cells. Our data showed that TAX exhibited the ability to inhibit lipogenesis and a hepatoprotective capacity, indicating that TAX has therapeutic potential for preventing alcoholic liver steatosis.

Anti-inflammatory effects of α-linolenic acid in M1-like macrophages are associated with enhanced production of oxylipins from α-linolenic and linoleic acid

Chronic inflammation, mediated in large part by proinflammatory macrophage populations, contributes directly to the induction and perpetuation of metabolic diseases, including obesity, insulin resistance and type 2 diabetes. Polyunsaturated fatty acids (PUFAs) can have profound effects on inflammation through the formation of bioactive oxygenated metabolites called oxylipins. The objective of this study was to determine if exposure to the dietary omega-3 PUFA α-linolenic acid (ALA) can dampen the inflammatory properties of classically activated (M1-like) macrophages derived from the human THP-1 cell line and to examine the accompanying alterations in oxylipin secretion. We find that ALA treatment leads to a reduction in lipopolysaccharide (LPS)-induced interleukin (IL)-1β, IL-6 and tumor necrosis factor-α production. Although ALA is known to be converted to longer-chain PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), DHA oxylipins were reduced overall by ALA treatment, as was LPS-induced secretion of EPA oxylipins. In contrast, we observed profound increases in oxylipins directly derived from ALA. Lipoxygenase products of linoleic acid were also dramatically increased, and LPS-induced production of AA oxylipins, particularly prostaglandin D2, was reduced. These results suggest that ALA may act to dampen the inflammatory phenotype of M1-like macrophages by a unique set of mechanisms distinct from those used by the long-chain omega-3 fatty acids EPA and DHA. Thus, there is strong rationale for investigating the functions of ALA oxylipins and lesser-known LA oxylipins since they hold promise as anti-inflammatory agents.

Intestinal dysbiosis and permeability: the yin and yang in alcohol dependence and alcoholic liver disease

Alcohol dependence and alcoholic liver disease represent a major public health problem with substantial morbidity and mortality. By yet incompletely understood mechanisms, chronic alcohol abuse is associated with increased intestinal permeability and alterations of the gut microbiota composition, allowing bacterial components, bacteria, and metabolites to reach the portal and the systemic circulation. These gut-derived bacterial products are recognized by immune cells circulating in the blood or residing in remote organs such as the liver leading to the release of pro-inflammatory cytokines which are considered important mediators of the liver-gut-brain communication. Although circulating cytokines are likely not the sole factors involved, they can induce liver inflammation/damage and reach the central nervous system where they favor neuroinflammation which is associated with change in mood, cognition, and drinking behavior. In this review, the authors focus on the current evidence describing the changes that occur in the intestinal microbiota with chronic alcohol consumption in conjunction with intestinal barrier breakdown and inflammatory changes sustaining the concept of a gut-liver-brain axis in the pathophysiology of alcohol dependence and alcoholic liver disease.