Ultimately we decided to investigate the
Ultimately, we decided to investigate the role of Cyp3a in obesity because Hepatic P450 Reductase-null (HRN) mice with no P450 activity show hepatic steatosis with increased Cyp2b10 and Cyp3a11 expression, perturbations in liver size, lipid homeostasis, and increased liver polyunsaturated fatty acids (PUFAs) . In addition, activation of PXR, the key regulator of Cyp3a members, increases obesity in females but not males [14,39]. This implicates detoxification CYPs in obesity, and Cyp3a members are the most prominent hepatic CYPs. However, the potential role of Cyp3a on obesity and diet-induced fatty liver disease has not been investigated. We used mice lacking seven of the eight Cyp3a genes on a B6 background to monitor the effects of a high-fat diet (HFD) (diet-induced obesity) on weight gain, glucose and insulin tolerance, fatty liver, and alterations in hepatic gene expression consistent with metabolic disorders.
Materials and methods
Discussion Cyp3a-null female mice gained 50% less weight than WT female mice and Cyp3a-null male mice are slightly heavier than their WT counterparts following treatment with a HFD (diet-induced obesity) (Fig. 1). This indicates gender specific responses to the HFD and demonstrates a role of Cyp3a in the metabolism and utilization of fatty acids. Diverse gender responses might be expected as several Cyp3a members are sexually dimorphic in the liver. For example, Cyp3a41 and Cyp3a44 are both female specific [44,62,64] and human CYP3A4 is expressed slightly more in females than males [8,65]. Cyp3a-null female mice reacted to a glucose challenge more rapidly than WT mice, which is consistent with their reduced weight gain and 30% lower WAT weight. In addition, plasma adiponectin concentrations are higher and serum B-OHB concentrations are lower in Cyp3a-null female mice, which is consistent with lower WAT, faster recovery from glucose, and rapid β-oxidation [, , ]. Adiponectin plays a central role as it increases hepatic glucose and fatty gpr120 agonist utilization, and in turn protects the liver from fatty liver disease . Interestingly, resveratrol protects from alcohol-induced fatty liver disease in part by increasing fatty acid oxidation as observed through greater B-OHB coupled with enhanced adiponectin, and increased expression of Cpt1a as well as other genes associated fatty acid oxidation [70,71]. Adiponectin also decreases lipid uptake by the liver in part by decreasing CD36 and this could provide a mechanism for compensating for a diet rich in fatty acids . In addition, the Cyp3a inhibitor naringin found in grapefruit and other citrus also reduces serum B-OHB concentrations [73,74], is associated with reduced adiposity, weight gain in mice and humans, and increased Cpt1a expression through Pparα activation coupled with Srebp-1 inhibition [75,76]. However, Cpt1a expression was much greater in Cyp3a-null mice than WT mice prior to the HFD, but no significant difference in Cpt1a or other energy-related genes was found after a HFD in female mice. It is also possible that the liver is not the only organ in which Cyp3a plays a role in the metabolism and utilization of fatty acids in HFD-fed female mice as several of the perturbed parameters are regulated by WAT, kidney and skeletal muscle [66,77,78]. In contrast to females, Cyp3a-null male mice gained weight relative to WT mice; albeit a relatively small amount. In addition, Cyp3a-null male mice have increased liver lipids including a 1.5X increase in liver triglycerides (Fig. 5B) and 2X increase in total polar lipids (Table 3). Specific lipid groups (PI, PS, SM) and long chain fatty acids that are 38–40 carbons are increased in the livers of Cyp3a-null male mice relative to WT mice (Fig. 6). An increase in long-chain fatty acids is consistent with increased storage and increased metabolism , perturbations in the transport of long-chain fatty acids is associated with liver disease , and increased SM is associated with obesity [81,82]. However, while SM is associated with obesity, PI is associated with anti-obesity effects by regulating hepatic lipid metabolism genes . Therefore, most but not all lipid markers are consistent with fatty liver disease and obesity.