In untreated rodent or human

In untreated rodent or human hepatocytes, PXR or CAR is stabilized by cytoplasmic co-chaperone partners like heat shock protein 90 (HSP90, Fig. 1). Upon ligand binding, the nuclear receptors are freed and translocated into the nucleus, identified as a pivotal step in PXR- or CAR-mediated transactivation. The regulation of PXR on CYP3A transcription can thus follow these steps: the binding of ligand to the receptor, the formation of a heterodimer with the retinoid X receptor α (RXRα), the recruitment of co-activators (for example, the steroid receptor coactivators 1, SRC-1), the binding to PXR-response elements (PXREs), and the transcriptional regulation of target genes. The DNA response elements include direct repeats (DR)-3, DR-4, DR-5 and everted repeats (ER)-6 and ER-8. PXR agonists robustly activated the transcription of Cyp3a genes in different species, while compounds might suppress the constitutive and inductive CYP3A4 expression by interfering the processes in PXR-mediated transcription54, 55. The transactivation process by CAR is similar, except for the binding of CAR-RXRα complex to the phenobarbital responsive enhancer modules (PBREM) of the target genes18, 49. The binding to PBREM was specially discovered in the regulation of human CYP2B6 or mouse Cyp2b10 genes, but it was reported that human CYP2B6 and CY3A4 shared crossed substrate specificity and regulation networks. CAR was also found to transactivate the steroid/RIF-responsive ER6 motif of the human CYP3A4 gene. These facts indicate the versatility of PXR and CAR in the regulation of target genes to some extent. The inductive effects of PXR or CAR ligands depend on the organ-specific expression of the nuclear receptors. PXR is primarily expressed in rodent and human livers, as well as human testis and mouse intestine. CAR is abundant in rodent and human liver and kidney, and is also detected in human Thieno-GTP synthesis or mouse intestine. The regulatory effects of PXR and CAR agonists on human CYP3A4 are overlapping but biased. Although both RIF and CITCO are able to significantly activate the transcription of CYP3A4 and CYP2B6 in human primary hepatocytes, comparatively, RIF is more effective in CYP3A4 induction, and CITCO is a more potent inducer of the latter gene. Treatment with RIF significantly increased CYP3A4 mRNA levels and CYP3A4 catalytic activities in Caco-2 and LS-180 cells, which lead to the elevated catabolism of many active endogenous substances including 1,25-D345. For mouse, PCN was capable of inducing Cyp3a11 transcription in both liver and intestine, while TCPOBOP only induced hepatic Cyp3a11 mRNA levels. These effects were not observed in Pxr- or Car-null mice. In rat hepatic slices, Cyp3a1 was induced by the PXR ligand, PCN, and the PXR/GR ligand, dexamethasone; Cyp3a2 transcription was suppressed by the GR ligand, budesonide; Cyp3a9 was induced by all of the three ligands. On the contrary, the involvement of CAR in the induction of rat CYPs has rarely been studied. Besides the phase I metabolic enzymes, PXR and/or CAR also participates in the regulation of mouse hepatic phase II metabolic enzymes (UDP-glucuronosyltransferases, Ugt1a1; sulfotransferases, Sultn; glutathione S-transferases, Gsta1, Gstm1, Gstm2 and Gstt1) and transporters (multiple drug resistance gene, Mdr1a, Mdr1b; multidrug resistance-associated protein, Mrp2 and Mrp3) in distinct but overlapping manners.
Regulatory roles of VDR on CYP3A in different species
Conclusion The up-regulatory effects of activated VDR on human CYP3A4 are clearly indicated in in vitro and ex vivo experiments, and the molecular mechanism is well elucidated. However, the clinical significance of related VDR polymorphisms and/or VDR-mediated CYP3A4 induction remains to be established. Mice and rats are two kinds of commonly used rodent animal models, in which the organ-specific and isoform-specific CYP3A induction by VDR ligands is also observed. It is noteworthy that most studies only focus on the mRNA level changes of CYP3As, but pay less attention to the changes of the relevant protein levels or metabolic activities, which might influence the pharmacokinetic profiles of CYP3A substrates. In particular, more in vivo evidence could be provided on VDR in the induction of CYP3A based on the Vdr KO mouse or rat models. Despite species discrepancies, the results from mice or rat experiments help to understand better the less focused pathway of CYP3A transcriptional regulation by VDR.