Intestine is responsible for the biotransformation of many orally-exposed chemicals. The nuclear receptor constitutive androstane receptor (CAR/Nr1i3) can be activated by various drugs and environmental chemicals, and in turn it is known to up-regulate the expression of many drug-processing genes (DPGs) in liver to facilitate xenobiotic biotransformation. However, relatively less is known regarding the effect of CAR on the basal and inducible expression of DPGs in various sections of intestine. To fill this critical knowledge gap, sixty-day-old wild-type and CAR-null mice were administered the CAR-ligand TCPOBOP (3mg/kg, i.p.) or vehicle once daily for 4-days. CAR-null mice had altered basal expression of many DPGs in a section-specific manner in intestine. Consistent with the liver data (Aleksunes and Klaassen, 2012), TCPOBOP up-regulated many DPGs in specific sections of small intestine in a CAR-dependent manner. In contrast, the mRNAs of certain DPGs were previously known to be up-regulated by TCPOBOP in liver but were not altered in intestine. Interestingly, many known CAR-target genes were highest expressed in colon where CAR is minimally expressed, suggesting that additional regulators are involved in regulating their expression. In conclusion, the present study has shown that CAR is critical in both maintaining the basal expression as well as the pharmacological regulation of certain DPGs in a section-specific manner of the intestine, and the CAR activation in liver and intestine produces overlapping but not identical results. Therefore, Identification of the xenobiotic responses to CAR activation in intestine is critical for further understanding the adverse drug-drug interactions for many orally-exposed chemicals.

Xenobiotic biotransformation is a vital process in the body that detoxifies various drugs, dietary factors, and environmental toxicants. This process is mediated by various drug-metabolizing enzymes in Phase-I (oxidation, reduction, and hydrolysis) and Phase-II, as well as transporters. The hepatic nuclear factor 4alpha (HNF4α) is a master regulator that transcriptionally up-regulates a wide spectrum of drug-metabolizing enzymes and transporters in liver. HNF4α also plays an important role in liver development and nutrient homeostasis. Previously, it has been shown that HNF4α can trans-activate the constitutive androstane receptor (CAR), which is a major ligand-activated xenobiotic-sensing nuclear receptor. However, very little is known about the effect of CAR activation by xenobiotics on the regulation of HNF4α in liver. In the present study, male mice at either 5-day-old neonatal age or 60-day-old adult age in wild-type, CAR-null, or humanized CAR-transgenic (hCAR-TG) genotypes were treated with vehicle (corn oil, which served as the control), or a species-appropriate CAR ligand (TCPOBOP for mouse CAR and CITCO for hCAR), once daily for 4 days. A novel truncated coding exon 9 was observed for the HNF4α transcript variant in mouse liver (RNA-Seq). Isoform-specific RT-PCR and gel electrophoresis confirmed the presence of the novel HNF4α isoform. During development, both the known and the novel isoform had increased expression with age. The mouse CAR ligand, TCPOBOP, down-regulated HNF4α in a mouse CAR-dependent manner only at mice at age 60, whereas hCAR did not appear to regulate the expression of HNF4α at any ages. In conclusion, the present study has identified a novel HNF4α transcript variant in the mouse liver, and discovered that both the known and novel isoforms were regulated by CAR in a species and age-specific manner.