| Nuclear hormone receptors are ligand-modulated transcription factors that are involved in a variety of physiological, developmental and toxicological processes. This dissertation contributes to the molecular understanding of the regulation of pregnane X receptor (PXR) and constitutive androstane receptor (CAR), two closely related members of the nuclear receptor superfamily. These two nuclear receptors are activated by many xenobiotics including some clinically used drugs and regulate a large number of genes involved in the xenobiotic response. Thus, the activation of PXR or CAR protects cells from toxic insults. However, PXR and CAR are also important in regulating drug-drug interactions. The results presented in this dissertation were primarily obtained from reporter gene assays, cultured mouse hepatocytes studies and in vivo mouse experiments. In this dissertation, several novel agonists for PXR were identified including the herbal compound forskolin and the antipsychotic clozapine, which suggests that clozapine and the herb Coleus forskohlii should be used cautiously in patients on combination therapy due to their potential to promote metabolism of co-administrated medications. In addition, data presented here show that PXR and CAR are not simply functionally redundant xenobiotic sensors, but rather the interaction between PXR and CAR is an important determinant in the regulation of the overall xenobiotic response. This is illustrated by results obtained using guggulsterone, an herbal compound previously known as a PXR agonist. However, guggulsterone was found to behave as a CAR inverse agonist as well. Thus, the cellular ratio of CAR to PXR determines the effect of guggulsterone or guggulsterone-like compound on the expression of shared PXR and CAR target genes involved in the xenobiotic response. It is further revealed that besides its commonly recognized role in the xenobiotic response, CAR also senses metabolic stress. The data show that fasting increases CAR expression and activity, while PXR expression and activity remains unchanged. Taken together, the work presented in this dissertation supports the notion that CAR and PXR have overlapping yet distinct functions in that both nuclear receptors sense xenobiotic stress but only CAR senses metabolic stress. |
