To directly address whether the PPARα-FGF21 axis is related to the enhanced fatty liver in Cyp2a5−/− mice, we developed Pparα−/−/Cyp2a5−/− mice. In addition to liver, adipose tissue also produce FGF21. In liver, FGF21 is regulated by PPARα, but in adipose tissues, FGF21 is regulated by PPARγ but not by PPARα (Dutchak et al., 2012). Thus, adipose FGF21 should not be affected in Pparα−/−/Cyp2a5−/− mice which maintain PPARγ signaling. Similarly, to exclude the influence of adipose FGF21, liver-specific Fgf21 knockout (Fgf21alb-cre) mice were applied instead of global FGF21 knockout mice to examine whether FGF21 plays a role in alcoholic fatty liver. Fgf21alb-cre mice developed more pronounced alcoholic fatty liver than Fgf21fl/fl mice, and most importantly, Pparα−/−/Cyp2a5−/− mice developed a more severe alcoholic fatty liver than Pparα+/+/Cyp2a5−/− mice. Serum FGF21 was much lower in Pparα−/−/Cyp2a5−/− mice than in Pparα+/+/Cyp2a5−/− mice. These results suggest that the PPARα-FGF21 axis regulates hepatic lipid metabolism, and it is due to the failed induction of the PPARα-FGF21 axis that Cyp2a5−/− mice develop more severe alcoholic fatty liver. In Cyp2a5+/+ mice, ethanol induced a compensatory upregulation of the
