Regulation of Xenobiotic and Bile Acid Metabolism by the Anti-aging Intervention Calorie Restriction in Mice

Calorie restriction (CR), defined as reduced calorie intake without causing malnutrition, is the best-known intervention to increase life span and slow aging-related diseases in various species. However, current knowledge on the exact mechanisms of aging and how CR exerts its anti-aging effects is still inadequate. The detoxification theory of aging proposes that the up-regulation of xenobiotic processing genes (XPGs) involved in phase-I and phase-II xenobiotic metabolism as well as transport, which renders a wide spectrum of detoxification, is a longevity mechanism. Interestingly, bile acids (BAs), the metabolites of cholesterol, have recently been connected with longevity. Thus, this dissertation aimed to determine the regulation of xenobiotic and BA metabolism by the well-known anti-aging intervention CR. First, the mRNA expression of XPGs in liver during aging was investigated. The age-dependent mRNA profiles of 101 XPGs was determined in livers of male and female mice at 3, 6, 9, 12, 15, 18, 21, 24, and 27 months of age. Gender differences across the lifespan were observed for 52 XPGs. The mRNAs of 40 XPGs were lower in aged than young male mice, and 43 XPGs were lower in aged than young female mice. More XPGs with higher expression with age were observed in female (21 XPGs) than male (4 XPGs) mice. To characterize BA profiles during aging, an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method recently developed by our laboratory was applied to quantify 20 individual BAs in serum and livers of male and female mice from 3 to 27 months of age. Total BAs remained constant with age in liver, but increased 340% from 3 to 27 months in serum of female mice. In contrast, in male mice, BAs did not change in serum or liver. The higher concentrations of BAs in serum of aged female mice were likely due to female-specific increased expression of BA uptake transporters Ntcp and Oatp1b2 in liver as well as the rate-limiting enzyme for BA synthesis Cyp7a1. Previous reports on regulation of gene expression in CR mice were inconsistent, mainly due to the large variation of diets, feeding regimes, as well as age and strain of the mice. Utilizing a "dose-response" model (0, 15, 30, or 40% CR for one month), the present dissertation investigated the regulation of XPG expression in livers by graded CR in male mice under the same study design. In general, CR (30 and 40%) altered the mRNA levels of over half of the 98 XPGs quantified (32 increased and 29 decreased). CR up-regulated some phase-I enzymes (such as Cyp4a14, Nqo1, Fmo2, and Fmo3) and numerous phase-II enzymes (many Sults, some Ugts, and most Gsts), as well as uptake transporter Oatp1a4. Furthermore, over half of the CR-induced alterations of XPG mRNA profiles appeared to be attributable to feminization of the liver. To innovatively investigate the regulation of BA homeostasis by CR, individual BAs were quantified by UPLC-MS/MS in various compartments of the enterohepatic circulation using the "dose-response" model of CR. CR (40%) increased the BA pool size (162%). CR "dose-dependently" increased many individual BAs in serum. Notably, 40% CR increased tauro-deoxycholic acid (TDCA) over 10-fold in serum, liver, and gallbladder. The increase in BAs correlated with increased expression of BA-synthetic (Cyp7a1) and conjugating enzymes (BAL) and the ileal BA-binding protein (Ibabp) by 40% CR. Improved glucose tolerance and lipid parameters in the CR mice correlated with altered BA composition of increased proportion of 12-hydroxylated BAs (cholic acid and DCA) but decreased muricholic acids. In summary, this dissertation identified female-specific age-dependent changes of XPG expression and elevation of total BA concentrations in serum during aging, as well as the feminization effects of the expression of many XPGs by CR, all of which interestingly correlate with the phenomenon that females generally have longer life span than males. The current findings provide new evidence for the detoxification theory of aging and promote a better understanding of BAs as potential longevity signaling molecules, and may provide insight into drug elimination pathways that one should monitor in the elderly as well as people on diets