Bioenergetic cues shift FXR splicing towards FXRα2 to modulate hepatic lipolysis and fatty acid metabolism

Farnesoid X receptor (FXR) plays a prominent role in hepatic lipid metabolism. The FXR gene encodes four proteins with structural differences suggestive of discrete biological functions about which little is known. We show that FXR isoforms have specific effects on hepatic metabolism and uncover novel pathways under their control. Gene expression profiling of hepatocytes expressing each FXR variant revealed large differences in their target gene sets. Notably, FXRα2 (but not α1) activates a broad transcriptional program in hepatocytes conducive to lipolysis, fatty acid oxidation, and ketogenesis. Consequently, FXRα2 decreases cellular lipid accumulation and improves insulin sensitivity. FXRα2 expression in Fxr-/- mouse liver activates a similar gene program and robustly decreases hepatic triglyceride levels. On the other hand, FXRα1 reduces hepatic triglyceride content to a lesser extent and does so through regulation of lipogenic gene expression. Bioenergetic cues, such as fasting and exercise, dynamically regulate Fxr splicing in mouse liver to increase Fxrα2 expression. Our results show that the main FXR variants in human liver (α1 and α2) reduce hepatic lipid accumulation through distinct mechanisms and to different degrees. Thus, pharmacological or lifestyle interventions, such as exercise, aimed at modulating hepatic FXR splicing may improve the therapeutic efficacy of FXR agonists. Primary hepatocytes from male C57BL/6J mice were isolated, cultured and transduced as previously described (Estall et al. 2009). To evaluate the gene programs regulated by each FXR isoform, we generated recombinant adenoviruses expressing each human FXR variant (FXRα1, α2, α3, and α4). All adenoviruses also express GFP from an independent CMV promoter. An adenovirus expressing GFP alone was used as a control. Hepatocytes were transduced overnight with each adenovirus and processed for RNA extraction 36 h post-transduction.