Effects of Farnesoid X Receptor Activation on Arachidonic Acid Metabolism, NF-kB Signaling, and Hepatic Inflammation

Abstract

Inflammation has a recognized role in nonalcoholic fatty liver disease (NAFLD) progression. In the present work, we studied the effect of high-fat diet (HFD) on arachidonic acid metabolism in the liver and investigated the role of the farnesoid X receptor (FXR, NR1H4) in eicosanoid biosynthetic pathways and nuclear factor light-chain enhancer of activated B cells (NF-kB) signaling, major modulators of the inflammatory cascade. Mice were fed an HFD to induce NAFLD and then treated with the FXR ligand obeticholic acid (OCA). Histology and gene expression analyses were performed on liver tissue. Eicosanoid levels were measured from serum and urine samples. The molecular mechanism underlying the effect of FXR activation on arachidonic acid metabolism and NF-kB signaling was studied in human liver Huh7 cells and primary cultured hepatocytes. NAFLD was characterized by higher (∼25%) proinflammatory [leukotrienes (LTB)] and lower (∼3-fold) anti-inflammatory [epoxyeicosatrienoic acids (EETs)] eicosanoid levels than in chow mice. OCA induced the expression of several hepatic cytochrome P450 (P450) epoxygenases, the enzymes responsible for EET synthesis, and mitigated HFD-induced hepatic injury. In vitro, induction of CYP450 epoxygenases was sufficient to inhibit NF-kB signaling and cell migration. The CYP450 epoxygenase pan-inhibitor gemfibrozil fully abolished the protective effect of OCA, indicating that OCA-mediated inhibition of NF-kB signaling was EET-dependent. In summary, NAFLD was characterized by an imbalance in arachidonate metabolism. FXR activation reprogramed arachidonate metabolism by inducing P450 epoxygenase expression and EET production. In vitro, FXR-mediated NF-kB inhibition required active P450 epoxygenases.

Farnesoid X receptor activation induces the degradation of hepatotoxic 1‐deoxysphingolipids in non‐alcoholic fatty liver disease

“…A similar pattern was observed in our mouse model of NAFLD. Mice fed a HFD over 16 weeks gained up to 50% more weight than the mice on chow diet (Figure ), and showed signs of steatohepatitis, as previously described . A time‐dependent increase in the circulating levels of 1‐deoxysphingolipids, especially 1‐deoxySO, in mice fed the HFD compared with the normal chow group was observed (Figure D‐G), suggesting that 1‐deoxysphingolipids may be involved in the chronic, progressive hepatic damage that occurs in NAFLD.…”

“…FXR has been associated with various effects on energy homoeostasis, including regulation of lipid metabolism and insulin sensitivity, which in turn can contribute to liver protection. 14,22,23 In the present study, we showed that mice fed a HFD exhibited characteristics of NAFLD and had increased levels of 1-deoxysphingolipids, both in the blood and in the liver. Accumulation of 1-deoxysphingolipids has been shown to promote hepatocyte apoptosis and mitochondrial damage both in vivo and in vitro.…”

“…We previously reported that FXR activation protects the liver and kidneys from obesity‐induced inflammation and damage in mice fed a HFD . To study the role of FXR in sphingolipid metabolism, mice fed either a chow diet or a HFD were treated without/with OCA, a semi‐synthetic FXR agonist, for 16 weeks.…”

“…Hepatocytes were isolated from female C57/BJ mice, as described previously . A sterile cannula was inserted through the right ventricle to preperfuse the liver with a prewarmed (37°C) preperfusion solution (0.5 mM EGTA, 20 mM Hepes in Hank's Balanced Salt [HBS] solution, pH 7.4) for 10 minutes.…”

“…We previously reported that FXR activation protects the liver and kidneys from obesity-induced inflammation and damage in mice fed a HFD. 14,20 To study the role of FXR in sphingolipid metabolism, mice fed either a chow diet or a HFD were treated without/with OCA, a semisynthetic FXR agonist, for 16 weeks. As expected, the mice that received OCA expressed higher levels of genuine FXR target gene mRNA, both at the hepatic and intestinal levels ( Figure S4).…”