Hepatic Farnesoid X-Receptor Isoforms alpha 2 and alpha 4 Differentially Modulate Bile Salt and Lipoprotein Metabolism in Mice

The nuclear receptor FXR acts as an intracellular bile salt sensor that regulates synthesis and transport of bile salts within their enterohepatic circulation. In addition, FXR is involved in control of a variety of crucial metabolic pathways. Four FXR splice variants are known, i.e. FXR alpha 1-4. Although these isoforms show differences in spatial and temporal expression patterns as well as in transcriptional activity, the physiological relevance hereof has remained elusive. We have evaluated specific roles of hepatic FXR alpha 2 and FXR alpha 4 by stably expressing these isoforms using liver-specific self-complementary adeno-associated viral vectors in total body FXR knock-out mice. The hepatic gene expression profile of the FXR knock-out mice was largely normalized by both isoforms. Yet, differential effects were also apparent; FXR alpha 2 was more effective in reducing elevated HDL levels and transrepressed hepatic expression of Cyp8b1, the regulator of cholate synthesis. The latter coincided with a switch in hydrophobicity of the bile salt pool. Furthermore, FXR alpha 2-transduction caused an increased neutral sterol excretion compared to FXR alpha 4 without affecting intestinal cholesterol absorption. Our data show, for the first time, that hepatic FXR alpha 2 and FXR alpha 4 differentially modulate bile salt and lipoprotein metabolism in mice