| The uptake of cardiac glycosides by the liver of rats is low at birth and increases with age, as well as by treatment of rats with phenobarbital or pregnenolone-16α-carbonitrile (PCN). The rat hepatic uptake transporter, organic anion transporting polypeptide 2 (oatp2; Slc21a5) has been cloned and transports cardiac glycosides with high affinity. Therefore, the purpose of this dissertation was to determine the regulation of rat hepatic oatp2 at both translational and transcriptional levels. The short-term regulation of the uptake functions mediated by rat oatp1 and oatp2 expressed in Xenopus laevis oocytes was not affected by a Protein Kinase A (PKA) activator, but was suppressed by a protein Kinase C (PKC) activator. Second, the levels of oatp2 protein and mRNA were low at birth and gradually increased during postnatal development. In addition, treatment of newborn rats with PCN dramatically increased the amount of oatp2 proton and mRNA at all age groups. The rat oatp2 gene was composed of 16 exons, and alternative splicing of the second non-coding exon generates two forms of oatp2 cDna. The mechanism by which PCN induces rat hepatic oatp2 appears to be due to its binding to the pregnene-x-receptor-retinoid-x-receptor α (PXR-RXRα) heterodimer, which interacts with the PXR response elements, identified −8 kb upstream, of the transcription start site of the rat oatp2. Furthermore, levels of oatp2 protein and mRNA were determined after treatment with prototypical microsomal enzyme inducers that act through ligand-activated transcription factor pathways. The amounts of oatp2 protein were decreased by ligands for the aryl hydrocarbon receptor (AhR), moderately increased by ligands for constitutive androstane receptor (CAR), and dramatically increased by ligands of pregnane x receptor (PXR), however, it was not affected by ligands for peroxisome proliferator activated receptor (PPAR), as well as activators for antioxidant/electrophile response element (ARE/EpRE). The ability of these inducers in regulating oatp2 mRNA level did not correlate with their ability to regulate levels of oatp2 protein, suggesting that post-translational modification of oatp2 is involved. In conclusion, this dissertation shows that there is transcriptional and post-translational regulation of rat hepatic oatp2, which indicates potential age-drug and drug-drug interactions at the level of hepatic uptake. |
