Regulatory Roles Of CYP3A On Bile Acid Synthesis&Transport And Drug Transporter P-gp In Rats

Cytochrome P450 3 A4(CYP3 A4)is one of the most fundamental human metabolic enzymes.It largely determines the in vivo exposure and bioavailability of drugs.Besides,it helps keep the balance between the synthesis and catabolism of endobiotics like testosterone,vitamin D3,cholesterol and bile acids,and further impacts the binding efficacies of the endobiotics to the respective receptors,which then activate the downstream cellular signaling pathways.Bile acids are amphipathic acids synthesized from cholesterol by the liver,and serve as signal molecules to bind cell membrane receptors(for example,Takeda G-protein receptor 5,TGR5)or nuclear receptors(for example,farnesoid X receptor,FXR).Receptor binding result in either the cascade of signaling pathways or the activation/repression of target gene transcription activities in the nucleus.Rat CYP3A1/2 are the homologues of human CYP3A4.Compared with WT rats,the total serum cholesterol levels of Cyp3al/2 knockout(KO)rats,as well as hepatic Hmgcr and Hmgcsl mRNA levels were significantly decreased,indicating the down-regulation of in vivo cholesterol synthesis.Besides,the higher hepatic Cyp8bl mRNA level of the Cyp3a1/2 KO rats exhibited elevated bile acid synthesis,and together with impaired bile acid elimination by CYP3A,resulted in higher serum bile acid levels.The transcriptional activities of receptors were down-regulated in the liver,while in the small intestine up-regulated.These changes were the possible causes for the transactivation of downstream hepatic genes associated with bile acid metabolism/transportation.Slight alterations of serum hepatic function indicators in Cyp3a1/2 KO rats were observed,such as lower ALP(alkaline phosphatase)levels,due to the continuous hepatic bile acid overload.Hemotoxylin and eosin staining also indicated mild fatty liver disease in Cyp3a1/2 KO rats.What’s more,hepatic mRNA level of rAbcb1b,one of the genes encoding the drug transporter P-glycoprotein(P-gp)in rats,were increased after Cyp3a1/2 KO.Cell-based assays showed that the transactivation of P-gp genes was also associated with bile acid signaling pathway activation.The FXR agonists,chenodeoxycholic acid and GW4064,up-regulated P-gp mRNA and protein levels in rat hepatocarcinoma cell lines and primary hepatocytes in a dose-dependent manner,respectively,whose effects relied on the expression of FXR in the cells.Increased hepatic P-gp levels in Cyp3a1/2 KO rats also changed the pharmacokinetic(PK)profile of the CYP3A1/2 and P-gp co-substrate,docetaxel(DOC).DOC had lower systemic exposure and faster elimination rate in Cyp3a1/2 KO female rats,which arose from its increased organ distribution and biliary excretion mediated by increased P-gp level instead of altered hepatic metabolism.In summary,Cyp3a1/2 KO changed bile acid metabolism/transportation and up-regulated P-gp in rats.As a homeostasis regulator,CYP3 A1/2 played significant roles in the maintenance of rat bile acid pool and the normal functions of bile acid signaling pathway.On the other hand,the regulatory crosslink between CYP3 A and P-gp effectively contributed to the body defense against xenobiotics.