Influence of farnesoid X receptor (FXR) on lipid metabolism in calf hepatocytes exposed to high fatty acid levels

IntroductionFatty liver is a common metabolic disease in dairy cows during early postpartum period, which is characterized by excessive hepatic triacylglycerol (TAG) accumulation. However, the mechanisms of bile acid (BA) metabolism in dairy cows experiencing fatty liver remain poorly elucidated. The farnesoid X receptor (FXR) plays a critical role in the regulation of BA homeostasis. Consequently, the aim of this study was to investigate the effect of FXR-mediated BA metabolism following stimulation with high concentrations of free fatty acids (FFA).MethodsIn vivo, liver tissue from healthy control cows (n = 6; with hepatic TAG < 1%) and fatty liver cows (n = 6; with hepatic TAG > 2%) were used to evaluate the factors related to BA metabolism. In vitro, hepatocytes isolated from three healthy female calves were exposed to either 1.2 mM FFAs or kept as controls to simulate metabolic distress. Subsequently, hepatocytes were treated with either 5 µM of the FXR activator GW4064 or 5 µM of the FXR inhibitor (Z)-guggulsterone with or without the addition of 1.2 mM FFAs.ResultsOur findings indicate that both in vivo and in vitro exposure to FFAs was associated with increased mRNA and protein abundance of bile acid synthesis-related factors (CYP7A1, CYP8B1) and mRNA expression of CYP7B1. Conversely, the expression of BA synthesis-related factors (FXR, CYP27A1) and BA transporters (ABCC2, ABCB11) were diminished in fatty liver cows compared to controls. Furthermore, compared to the control group, fatty acid synthesis-related factors (SREBF1, ACC1, FASN), a mitochondrial dysfunction marker (VDAC1), and oxidative stress indicators (ROS, H2O2) were upregulated in the FFA group. Additionally, cholesterol synthesis-related genes (SREBF2, HMGCR) were lower in the FFA group compared to the control group. Notably, compared to the FFA group, the GW4064 + FFA group showed reduced expression of CYP7A1, CYP8B1, CYP27A1, CYP7B1, SREBF1, ACC1, FASN, and VDAC1, along with decreased TAG, ROS, and H2O2 in hepatocytes. Conversely, the expression of FXR, SREBF2, HMGCR, ABCC2, and ABCB11 was higher in the GW4064 + FFA group compared to the FFA group. Furthermore, the application of the FXR inhibitor (Z)-guggulsterone yielded results that were contrary those observed with GW4064.DiscussionOverall, our data suggest that FXR activation by GW4064 effectively attenuated high FFA-induced BA and lipid accumulation in calf hepatocytes, which ultimately alleviated hepatocyte oxidative damage.