Age‐ and species‐specific activation of CAR on bile acid homeostasis in mice
Authors: Cheng Sunny Lihua , Buckley Kyle , Rizzolo Dan , Buckley Brian , Guo Grace L. , Cui Julia Yue 
Published: November 2023

Pharmacological activation of the constitutive androstane receptor (CAR) has been shown to protect against bile acid (BA)‐induced liver injury in adults (Beilke et al., 2009). During development, targeting CAR by agonists has been suggested to be one of the ultimate goals in the management and treatment of total parenteral nutrition related cholestasis in newborns (Hendaus, 2013). However, very little is known regarding the age‐ and species‐specific effects of CAR activation on BA homeostasis during development. The goal of this study was to use RNA sequencing and BA metabolomics to investigate the effects of CAR activation on the expression of genes involved in BA homeostasis and the BA profiles in 5‐ and 60‐day‐old wild‐type (WT) and humanized CAR transgenic (hCAR‐TG) mice. Male mice were i.p. administered corn oil or a species‐appropriate CAR ligand (TCPOBOP at 3mg/kg for mCAR, and CITCO at 30mg/kg for hCAR) once daily for 4 days. In control mice of both genotypes, from newborns to adulthood, there was a marked decrease in total serum BAs likely due to the maturation of the enterohepatic system. Specifically, serum total primary BAs and conjugated BAs were down‐regulated with age in both WT and hCAR‐TG mice. Conversely, there was a marked increase in serum total secondary BAs and unconjugated BAs, likely due to an ontogenic increase in intestinal bacteria for BA‐dehydroxylation and deconjugation. Following mCAR‐activation, there was an increase in serum total BAs, primary BAs, secondary BAs, and conjugated BAs at both Day 5 and 60. However, the increase in serum BAs was not observed following hCAR activation. In fact, serum cholic acid was decreased by CITCO in 60‐day‐old hCAR‐TG mice. Regarding hepatic BA synthesis, Cyp7a1 mRNA was minimally influenced by CAR activation regardless of the age or species. Conversely, the regulation of Cyp8b1 mRNA seems age‐specific, because it was up‐regulated by CAR of both species at Day 5, but was down‐regulated at Day 60. The regulation of many other BA‐synthetic enzymes at mRNA level was mCAR‐specific, evidenced by an induction of Akr1d1, but a reduction of Amacr, Hsd17b4, Hsd3b7, and Scp2 at both ages only by activation of mCAR but not hCAR. Age‐ and/or species‐specific regulatory effects of CAR activation were also observed for genes involved in BA conjugation and transport. However, several BA‐processing genes appeared to be “universal targets” of CAR of both species and at both ages, including a consistent up‐regulation of Cyp39a1, Akr1c14, Mrp3, and Mrp4. However, in general, mCAR activation produced greater degree of induction as compared to hCAR activation at the same age. In conclusion, the present study suggests that hCAR activation has less effects on serum BA metabolites as compared to mCAR, whereas mCAR and hCAR activation produce both age‐ and species‐specific effects on the hepatic expression of genes involved in BA homeostasis. Understanding the age‐ and species‐differences in the pharmacodynamics of CAR activation is essential in precision medicine for pediatric patients.Support or Funding InformationGrace Liejun Guo glg48@eohsi.rutgers.edu