The pregnane X receptor (PXR) plays an important and diverse role in mediating xenobiotic induction of drug-metabolizing enzymes and transporters. Several protein isoforms of PXR exist, and they have differential transcriptional activity upon target genes; transcript variants 3 (PXR3) and 4 (PXR4) do not induce target gene expression, whereas transcript variants 1 (PXR1) and 2 (PXR2) respond to agonist by activating target gene expression. PXR protein variants also display differences in protein–protein interactions; PXR1 interacts with p53, whereas PXR3 does not. Furthermore, the transcript variants of PXR that encode these protein isoforms are differentially regulated by methylation and deletions in the respective promoters of the variants, and their expression differs in various human cancers and also in cancerous tissue compared to adjacent normal tissues. PXR1 and PXR4 mRNA are downregulated by methylation in cancerous tissue and have divergent effects on cellular proliferation when ectopically overexpressed. Additional detailed and comparative mechanistic studies are required to predict the effect of PXR transcript variant expression on carcinogenesis, therapeutic response, and the development of toxicity.

Introduction The liver plays a central role in transforming and clearing foreign substances. The continuous exposure of the liver to xenobiotics sometimes leads to impaired liver function, referred to as drug-induced liver injury (DILI). The pregnane X receptor (PXR) tightly regulates the expression of genes in the hepatic drug-clearance system and its undesired activation plays a role in DILI. Areas covered This review focuses on the recent progress in understanding PXR-mediated DILI and highlights the efforts made to assess and manage PXR-mediated DILI during drug development. Expert opinion Future efforts are needed to further elucidate the mechanisms of PXR-mediated liver injury, including the epigenetic regulation and polymorphisms of PXR. Novel in vitro models containing functional PXR could improve our ability to predict and assess DILI during drug development. PXR inhibitors may provide chemical tools to validate the potential of PXR as a therapetic target and to develop drugs to be used in the clinic to manage PXR-mediated DILI.

In a mouse model, rifampicin and isoniazid combination treatment results in cholestatic liver injury that is associated with an increase in protoporphyrin IX, the penultimate heme precursor. Both ferrochelatase (FECH/Fech) and aminolevulinic acid synthase 1 (ALAS1/Alas1) are crucial enzymes in regulating heme biosynthesis. Isoniazid has recently been reported to upregulate Alas1 but downregulate Fech protein levels in mice; however, the mechanism by which isoniazid mediates disruption of heme synthesis has been unclear. Two metabolites of isoniazid, pyridoxal isonicotinoyl hydrazone (PIH, the isoniazid-vitamin B6 conjugate) and hydrazine, have been detected in the urine of humans treated with isoniazid. Here we show that, in primary human hepatocytes and the human hepatocellular carcinoma cell line HepG2/C3A, (1) isoniazid treatment increases Alas1 protein levels but decreases Fech levels; (2) hydrazine treatment upregulates Alas1 protein and Alas1 mRNA levels; (3) PIH treatment decreases Fech protein levels, but not Fech mRNA levels; and (4) PIH is detected after isoniazid treatment, with levels increasing further when exogenous vitamin B 6 analogs are coadministered. In addition, the PIH-mediated downregulation of human FECH is associated with iron chelation. Together, these data demonstrate that hydrazine upregulates ALAS1, whereas PIH downregulates FECH, suggesting that the metabolites of isoniazid mediate its disruption of heme biosynthesis by contributing to protoporphyrin IX accumulation.