Role of the Ah receptor in homeostatic control of fatty acid synthesis in the liver.

We have previously demonstrated a role for the aryl hydrocarbon receptor (AHR) in the attenuation of the cholesterol biosynthesis pathway. This regulation did not require that the AHR binds to its cognate response element. Based on these observations and other reports depicting a role for AHR in lipid metabolism, we chose to investigate the involvement of the receptor in the regulation of the fatty acid synthesis pathway in mice and humans. For this purpose, C57BL/6J, liver-specific transgenic DRE-binding mutant AhR (A78D-AhrTtr CreAlb Ahrfx/fx) and CreAlb Ahrfx/fx mice were treated with an AHR ligand, and hepatic mRNA expression levels of key fatty acid genes (e.g., Acaca, Fasn, Scd1) were measured. The basal levels of those genes were also compared between C57BL6/J and hepatic AHR-deficient mice, as well as between Ahb and Ahd congenic mice. To extend these results to humans, fatty acid gene expression in human cells were compared with AHR-silenced cells. In addition, primary human hepatocytes were treated with an AHR ligand to assess alterations in gene expression and fatty acid synthesis. These studies indicated that the AHR constitutively attenuates the expression of key fatty acid synthesis genes in the absence of binding to its cognate response element. In addition, activation of AHR led to further repression of the expression of these genes and a decrease in overall fatty acid synthesis and secretion in human hepatocytes. Based on our results, we can conclude that increased AHR activity represses fatty acid synthesis, suggesting it may be a future therapeutic target.