Suppression of carboxylesterases by imatinib mediated by the down-regulation of pregnane X receptor

Aryl hydrocarbon receptor (AhR) and pregnane X receptor (PXR) play both distinct and common roles in the regulation of colon homeostasis and intestinal carcinogenesis

Both aryl hydrocarbon receptor (AhR) and pregnane X receptor (PXR) belong among key regulators of xenobiotic metabolism in the intestinal tissue. AhR in particular is activated by a wide range of environmental and dietary carcinogens. The data accumulated over the last two decades suggest that both of these transcriptional regulators play a much wider role in the maintenance of gut homeostasis, and that both transcription factors may affect processes linked with intestinal tumorigenesis. Intestinal epithelium is continuously exposed to a wide range of AhR, PXR and dual AhR/PXR ligands formed by intestinal microbiota or originating from diet. Current evidence suggests that specific ligands of both AhR and PXR can protect intestinal epithelium against inflammation and assist in the maintenance of epithelial barrier integrity. AhR, and to a lesser extent also PXR, have been shown to play a protective role against inflammation-induced colon cancer, or, in mouse models employing overactivation of Wnt/β-catenin signaling. In contrast, other evidence suggests that both receptors may contribute to modulation of transformed colon cell behavior, with a potential to promote cancer progression and/or chemoresistance. The review focuses on both overlapping and separate roles of the two receptors in these processes, and on possible implications of their activity within the context of intestinal tissue.

Carbonate-Based Fluorescent Chemical Tool for Uncovering Carboxylesterase 1 (CES1) Activity Variations in Live Cells

Carboxylesterase 1 (CES1) plays a key role in the metabolism of endogenous biomolecules and xenobiotics including a variety of pharmaceuticals. Despite the established importance of CES1 in drug metabolism, methods to study factors that can vary CES1 activity are limited with only a few suitable for use in live cells. Herein, we report the development of FCP1, a new CES1 specific fluorescent probe with a unique carbonate substrate constructed from commercially available reagents. We show that FCP-1 can specifically report on endogenous CES1 activity with a robust fluorescence response in live HepG2 cells through studies with inhibitors and genetic knockdowns. Subsequently, we deployed FCP-1 to develop a live cell fluorescence microscopy-based approach to identify activity differences between CES1 isoforms. To the best of our knowledge, this is the first application of a fluorescent probe to measure the activity of CES1 sequence variants in live cells.

Phosphorylation-Induced Ubiquitination and Degradation of PXR through CDK2-TRIM21 Axis

Pregnane X receptor (PXR) is a member of the nuclear receptor superfamily that is activated by a variety of endogenous metabolites or xenobiotics. Its downstream target genes are involved in metabolism, inflammation and processes closely related to cancer. However, the stability regulation of PXR protein resulting from post-translational modification is still largely undefined. In the present study, primary mouse hepatocytes, hepatoma HepG2 cells and HEK 293T cells were used to investigate gene expression and protein interactions. The role of kinases was evaluated by RNA interference and overexpression constructs with or without PXR phosphorylation site mutations. The activity of CYP3A4 and P-gp was determined by enzymatic and substrate accumulation assays. It was found that E3 ubiquitin ligase TRIM21 mediates the ubiquitination and degradation of PXR and plays an important role in regulating the activity of PXR. On this basis, PXR phosphorylation-associated kinases were evaluated regarding regulation of the stability of PXR. We found cyclin dependent kinase 2 (CDK2) exclusively phosphorylates PXR at Ser350, promotes its disassociation with Hsp90/DNAJC7, and leads to subsequent TRIM21-mediated PXR ubiquitination and degradation. As well-known CDK inhibitors, dinaciclib and kenpaullone stabilize PXR and result in elevated expression and activity of PXR-targeted DMETs, including carboxylesterases, CYP3A4 and P-gp. The suppressed degradation of PXR by CDK2 inhibitors denotes dinaciclib-induced promotion of PXR-targeted genes. The findings of CDK2-mediated PXR degradation indicate a wide range of potential drug–drug interactions during clinical cancer therapy using CDK inhibitors and imply an alternative direction for the development of novel PXR antagonists.

The role of pregnane X receptor (PXR) in substance metabolism

As a member of the nuclear receptor (NR) superfamily, pregnane X receptor (PXR; NR1I2) is a ligand-activated transcription factor that plays a crucial role in the metabolism of xenobiotics and endobiotics in mammals. The tissue distribution of PXR is parallel to its function with high expression in the liver and small intestine and moderate expression in the kidney, stomach, skin, and blood-brain barrier, which are organs and tissues in frequent contact with xenobiotics. PXR was first recognized as an exogenous substance receptor regulating metabolizing enzymes and transporters and functioning in detoxification and drug metabolism in the liver. However, further research revealed that PXR acts as an equally important endogenous substance receptor in the metabolism and homeostasis of endogenous substances. In this review, we summarized the functions of PXR in metabolism of different substances such as glucose, lipid, bile acid, vitamin, minerals, and endocrines, and also included insights of the application of PXR ligands (drugs) in specific diseases.

Regulation of CAR and PXR Expression in Health and Disease

Pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are members of the nuclear receptor superfamily that mainly act as ligand-activated transcription factors. Their functions have long been associated with the regulation of drug metabolism and disposition, and it is now well established that they are implicated in physiological and pathological conditions. Considerable efforts have been made to understand the regulation of their activity by their cognate ligand; however, additional regulatory mechanisms, among which the regulation of their expression, modulate their pleiotropic effects. This review summarizes the current knowledge on CAR and PXR expression during development and adult life; tissue distribution; spatial, temporal, and metabolic regulations; as well as in pathological situations, including chronic diseases and cancers. The expression of CAR and PXR is modulated by complex regulatory mechanisms that involve the interplay of transcription factors and also post-transcriptional and epigenetic modifications. Moreover, many environmental stimuli affect CAR and PXR expression through mechanisms that have not been elucidated.

PXR: a center of transcriptional regulation in cancer

Pregnane X receptor (PXR, NR1I2) is a prototypical member of the nuclear receptor superfamily. PXR can be activated by both endobiotics and xenobiotics. As a key xenobiotic receptor, the cellular function of PXR is mostly exerted by its binding to the regulatory gene sequences in a ligand-dependent manner. Classical downstream target genes of PXR participate in xenobiotic responses, such as detoxification, metabolism and inflammation. Emerging evidence also implicates PXR signaling in the processes of apoptosis, cell cycle arrest, proliferation, angiogenesis and oxidative stress, which are closely related to cancer. Here, we discussed, in addition to the characterization of PXR per se, the biological function and regulatory mechanism of PXR signaling in cancer, and its potential for the targeted prevention and therapeutics.

Suppression of carboxylesterases by imatinib mediated by the down-regulation of pregnane X receptor

BACKGROUND AND PURPOSE

Imatinib mesylate (IM) is a first-line treatment for chronic myeloid leukaemia (CML) as a specific inhibitor of BCR-ABL tyrosine kinase. As IM is widely used in CML, in combination with other drugs, the effects of IM on drug-metabolizing enzymes (DMEs) are crucial to the design of rational drug administration. Carboxylesterases (CESs) are enzymes catalysing the hydrolytic biotransformation of several clinically useful drugs. Although IM is known to inhibit cytochromes P450 (CYPs), its effects on DMEs, and CESs in particular, are still largely undefined.

EXPERIMENTAL APPROACH

Hepatoma cell lines (HepG2 and Huh7) and primary mouse hepatocytes were used. mRNA and protein expression were evaluated by quantitative RT-PCR and Western blot analysis. Reporter luciferase activity was determined by transient co-transfection experiment. Pregnane X receptor (PXR) expression was regulated by overexpression and RNA interference. The activity of CESs was determined by enzymic and toxicological assays. Mice were treated with a range of doses of IM to analyse expression of CESs in mouse liver.

KEY RESULTS

The expression and activity of CESs were markedly repressed by IM, along with the down-regulation of PXR and inhibited expression and activity of CYP3A4 and P-gp.

CONCLUSIONS AND IMPLICATIONS

Down-regulation of PXR mediates IM-induced suppression of CESs. IM may inhibit expression of other genes targeted by PXR, thus inducing a wide range of potential drug-drug interactions during treatment of CML. The data deserve further elucidation including clinical trials.

Involvement of pregnane X receptor in the suppression of carboxylesterases by metformin in vivo and in vitro, mediated by the activation of AMPK and JNK signaling pathway

Type 2 diabetes mellitus (T2D) is a complex metabolic disorder requiring polypharmacy treatment in clinic, with metformin being widely used antihyperglycemic drug. However, the mechanisms of metformin as a perpetrator inducing potential drug-drug interactions and adverse drug reactions are scarcely known to date. Carboxylesterases (CESs) are major hydrolytic enzymes highly expressed in the liver, including mouse carboxylesterase 1d (Ces1d) and Ces1e. In the present study, experiments are designed to investigate the effects and mechanisms of metformin on Ces1d and Ces1e in vivo and in vitro. In results, metformin suppresses the expression and activity of Ces1d and Ces1e in a dose- and time-dependent manner. The decreased expression of nuclear receptor PXR and its target gene P-gp indicates the involvements of PXR in the suppressed expression of carboxylesterases by metformin. Furthermore, metformin significantly suppresses the phosphorylation of AMPK and JNK, and the suppression of carboxylesterases induced by metformin is repeatedly abolished by AMPK inhibitor Compound C and JNK inhibitor SP600125. It implies that the activation of AMPK and JNK pathways mediates the suppression of carboxylesterases by metformin. The findings deserve further elucidation including clinical trials and have a potential to make contribution for the rational medication in the treatment of T2D patients.