Changcheng Zhou

Mutual repression between steroid and xenobiotic receptor and NF-κB signaling pathways links xenobiotic metabolism and inflammation

While it has long been known that inflammation and infection reduce expression of hepatic cytochrome P450 (CYP) genes involved in xenobiotic metabolism and that exposure to xenobiotic chemicals can impair immune function, the molecular mechanisms underlying both of these phenomena have remained largely unknown. Here we show that activation of the nuclear steroid and xenobiotic receptor (SXR) by commonly used drugs in humans inhibits the activity of NF-kappaB, a key regulator of inflammation and the immune response. NF-kappaB target genes are upregulated and small bowel inflammation is significantly increased in mice lacking the SXR ortholog pregnane X receptor (PXR), thereby demonstrating a direct link between SXR and drug-mediated antagonism of NF-kappaB. Interestingly, NF-kappaB activation reciprocally inhibits SXR and its target genes whereas inhibition of NF-kappaB enhances SXR activity. This SXR/PXR-NF-kappaB axis provides a molecular explanation for the suppression of hepatic CYP mRNAs by inflammatory stimuli as well as the immunosuppressant effects of xenobiotics and SXR-responsive drugs. This mechanistic relationship has clinical consequences for individuals undergoing therapeutic exposure to the wide variety of drugs that are also SXR agonists.

Vitamin K2 Regulation of Bone Homeostasis Is Mediated by the Steroid and Xenobiotic Receptor SXR

Vitamin K2 is a critical nutrient required for blood clotting that also plays an important role in bone formation. Vitamin K2 supplementation up-regulates the expression of bone markers, increases bone density in vivo, and is used clinically in the management of osteoporosis. The mechanism of vitamin K2 action in bone formation was thought to involve its normal role as an essential cofactor for γ-carboxylation of bone matrix proteins. However, there is evidence that suggests vitamin K2 also has a transcriptional regulatory function. Vitamin K2 bound to and activated the orphan nuclear receptor SXR and induced expression of the SXR target gene, CYP3A4, identifying it as a bona fide SXR ligand. Vitamin K2 treatment of osteosarcoma cells increased mRNA levels for the osteoblast markers bone alkaline phosphatase, osteoprotegerin, osteopontin, and matrix Gla protein. The known SXR activators rifampicin and hyperforin induced this panel of bone markers to an extent similar to vitamin K2. Vitamin K2 was able to induce bone markers in primary osteocytes isolated from wild-type murine calvaria but not in cells isolated from mice deficient in the SXR ortholog PXR. We infer that vitamin K2 is a transcriptional regulator of bone-specific genes that acts through SXR to favor the expression of osteoblastic markers. Thus, SXR has a novel role as a mediator of bone homeostasis in addition to its role as a xenobiotic sensor. An important implication of this work is that a subset of SXR activators may function as effective therapeutic agents for the management of osteoporosis.

Steroid and xenobiotic receptor and vitamin D receptor crosstalk mediates CYP24 expression and drug-induced osteomalacia

The balance between bioactivation and degradation of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] is critical for ensuring appropriate biological effects of vitamin D. Cytochrome P450, family 24-mediated (CYP24-mediated) 24-hydroxylation of 1,25(OH)2D3 is an important step in the catabolism of 1,25(OH)2D3. The enzyme is directly regulated by vitamin D receptor (VDR), and it is expressed mainly in the kidney, where VDR is also abundant. A recent report suggests that activation of steroid and xenobiotic receptor (SXR) also enhances the expression of CYP24, providing a new molecular mechanism of drug-induced osteomalacia. However, here we showed that activation of SXR did not induce CYP24 expression in vitro and in vivo, nor did it transactivate the CYP24 promoter. Instead, SXR inhibited VDR-mediated CYP24 promoter activity, and CYP24 expression was very low in tissues containing high levels of SXR, including the small intestine. Moreover, 1,25(OH)2D3-induced CYP24 expression was enhanced in mice lacking the SXR ortholog pregnane X receptor, and treatment of humans with the SXR agonist rifampicin had no effect on intestinal CYP24 expression, despite demonstration of marked CYP3A4 induction. Combined with our previous findings that CYP3A4, not CYP24, plays the dominant role in hydroxylation of 1,25(OH)2D3 in human liver and intestine, our results indicate that SXR has a dual role in mediating vitamin D catabolism and drug-induced osteomalacia.

The Dietary Isothiocyanate Sulforaphane Is an Antagonist of the Human Steroid and Xenobiotic Nuclear Receptor

Sulforaphane (SFN) is a biologically active phytochemical found abundantly in broccoli. SFN has been promoted as a putative chemopreventive agent to reduce cancer, and most studies have associated its anti-cancer effects with the induction of phase II xenobiotic metabolism enzymes via activation of the Keap1/Nrf2 antioxidant response pathway. Interestingly, SFN can significantly down-regulate cytochrome P450 3A4 (CYP3A4) expression in human primary hepatocytes. CYP3A4 is responsible for the hepatic and intestinal metabolism of numerous protoxicants, pharmaceutical compounds, and endogenous sterols. Among the most important mediators of CYP3A4 expression is the nuclear hormone receptor, steroid and xenobiotic receptor (SXR; also called “hPXR”). SXR functions as a xenobiotic sensor to coordinately regulate xenobiotic metabolism via transcriptional regulation of xenobiotic-detoxifying enzymes and transporters. Here, we report that SFN is a specific antagonist of human SXR and that it inhibits SXR-mediated induction of drug clearance. SFN can bind directly to SXR, inhibit SXR coactivator recruitment, and efficiently repress SXR activities. Furthermore, SFN inhibited SXR-mediated CYP3A4 expression and CYP3A4-catalyzed midazolam clearance in human primary hepatocytes. Thus, SFN is the first identified naturally occurring antagonist for SXR (hPXR). Because induction of CYP3A4 can result in adverse drug responses (e.g., lack of efficacy), which are a major public health problem, this discovery could lead to the development of important new therapeutic and dietary approaches to reduce the frequency of undesirable inducer-drug interactions.

Bisphenol A and its analogues activate human pregnane X receptor.

Background: Bisphenol A (BPA) is a base chemical used extensively in many consumer products. BPA and its analogues are present in environmental and human samples. Many endocrine-disrupting chemicals, including BPA, have been shown to activate the pregnane X receptor (PXR), a nuclear receptor that functions as a master regulator of xenobiotic metabolism. However, the detailed mechanism by which these chemicals activate PXR remains unknown. Objective: We investigated the mechanism by which BPA interacts with and activates PXR and examined selected BPA analogues to determine whether they bind to and activate PXR. Methods: Cell-based reporter assays, in silico ligand–PXR docking studies, and site-directed mutagenesis were combined to study the interaction between BPA and PXR. We also investigated the influence of BPA and its analogues on the regulation of PXR target genes in human LS180 cells. Results: We found that BPA and several of its analogues are potent agonists for human PXR (hPXR) but do not affect mouse PXR activity. We identified key residues within hPXR’s ligand-binding pocket that constitute points of interaction with BPA. We also deduced the structural requirements of BPA analogues that activate hPXR. BPA and its analogues can also induce PXR target gene expression in human LS180 cells. Conclusions: The present study advances our understanding of the mechanism by which BPA interacts with and activates human PXR. Activation of PXR by BPA may explain some of the adverse effects of BPA in humans.

The steroid and xenobiotic receptor (SXR), beyond xenobiotic metabolism

The steroid and xenobiotic receptor (SXR) (also known as pregnane X receptor or PXR) is a nuclear hormone receptor activated by a diverse array of endogenous hormones, dietary steroids, pharmaceutical agents, and xenobiotic compounds. SXR has an enlarged, flexible, hydrophobic ligand binding domain (LBD) which is remarkably divergent across mammalian species and SXR exhibits considerable differences in its pharmacology among mammals. The broad response profile of SXR has led to the development of “the steroid and xenobiotic sensor hypothesis”. SXR has been established as a xenobiotic sensor that coordinately regulates xenobiotic clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. In the past few years, research has revealed new and mostly unsuspected roles for SXR in modulating inflammation, bone homeostasis, vitamin D metabolism, lipid homeostasis, energy homeostasis and cancer. The identification of SXR as a xenobiotic sensor has provided an important tool for studying new mechanisms through which diet, chemical exposure, and environment ultimately impact health and disease. The discovery and pharmacological development of new PXR modulators might represent an interesting and innovative therapeutic approach to combat various diseases.

TOCOTRIENOLS ACTIVATE THE STEROID AND XENOBIOTIC RECEPTOR, SXR, AND SELECTIVELY REGULATE EXPRESSION OF ITS TARGET GENES

Vitamin E is an essential nutrient with antioxidant activity. Vitamin E is comprised of eight members, α-, β-, γ-, and δ-tocopherols and α-, β-, γ-, and δ-tocotrienols. All forms of vitamin E are initially metabolized by ω-oxidation, which is catalyzed by cytochrome P450 enzymes. The steroid and xenobiotic receptor (SXR) is a nuclear receptor that regulates drug clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. We show here that all four tocotrienols specifically bind to and activate SXR, whereas tocopherols neither bind nor activate. Surprisingly, tocotrienols show tissue-specific induction of SXR target genes, particularly CYP3A4. Tocotrienols up-regulate expression of CYP3A4 but not UDP-glucuronosyltransferase 1A1 (UGT1A1) or multidrug resistance protein-1 (MDR1) in primary hepatocytes. In contrast, tocotrienols induce MDR1 and UGT1A1 but not CYP3A4 expression in intestinal LS180 cells. We found that nuclear receptor corepressor (NCoR) is expressed at relatively high levels in intestinal LS180 cells compared with primary hepatocytes. The unliganded SXR interacts with NCoR, and this interaction is only partially disrupted by tocotrienols. Expression of a dominant-negative NCoR enhanced the ability of tocotrienols to induce CYP3A4 in LS180 cells, suggesting that NCoR plays an important role in tissue-specific gene regulation by SXR. Our findings provide a molecular mechanism explaining how vitamin supplements affect the absorption and effectiveness of drugs. Knowledge of drug-nutrient interactions may help reduce the incidence of decreased drug efficacy.

Activation of PXR induces hypercholesterolemia in wild-type and accelerates atherosclerosis in apoE deficient mice

The nuclear hormone receptor pregnane X receptor (PXR; also called SXR) functions as a xenobiotic sensor to coordinately regulate xenobiotic metabolism via transcriptional regulation of xenobiotic-detoxifying enzymes and transporters. Although many clinically relevant PXR ligands have been shown to affect cholesterol levels, the role of PXR in cholesterol homeostasis and atherosclerosis has not been thoroughly investigated. Here, we report that activation of PXR by feeding the PXR agonist pregnenolone 16&agr;-carbonitrile (0.02%) for 2 weeks to wild-type (WT) mice significantly increased total cholesterol levels and atherogenic lipoproteins VLDL and LDL levels, but had no effect in PXR knockout (PXR−/−) mice. Chronic PXR activation in atherosclerosis prone apolipoprotein E deficient (ApoE−/−) mice was found to decrease HDL levels and increase atherosclerotic cross-sectional lesion area at both the aortic root and in the brachiocephalic artery by 54% (P < 0.001) and 116% (P < 0.01), respectively. PXR activation significantly regulated genes in the liver involved in lipoprotein transportation and cholesterol metabolism, including CD36, ApoA-IV, and CYP39A1, in both WT and ApoE−/− mice. Furthermore, PXR activation can increase CD36 expression and lipid accumulation in peritoneal macrophages of ApoE−/− mice. In summary, PXR activation in WT mice increases levels of the atherogenic lipoproteins VLDL and LDL, whereas in ApoE−/− mice, PXR increases atherosclerosis, perhaps by diminishing levels of the antiatherogenic ApoA-IV and increasing lipid accumulation in macrophages.—Zhou, C., N. King, K. Y. Chen, and J. L. Breslow. Activation of PXR induces hypercholesterolemia in wild-type and accelerates atherosclerosis in apoE deficient mice.

Identification of a Chromosome-Targeting Domain in the Human Condensin Subunit CNAP1/hCAP-D2/Eg7

ABSTRACT CNAP1 (hCAP-D2/Eg7) is an essential component of the human condensin complex required for mitotic chromosome condensation. This conserved complex contains a structural maintenance of chromosomes (SMC) family protein heterodimer and three non-SMC subunits. The mechanism underlying condensin targeting to mitotic chromosomes and the role played by the individual condensin components, particularly the non-SMC subunits, are not well understood. We report here characterization of the non-SMC condensin component CNAP1. CNAP1 contains two separate domains required for its stable incorporation into the complex. We found that the carboxyl terminus of CNAP1 possesses a mitotic chromosome-targeting domain that does not require the other condensin components. The same region also contains a functional bipartite nuclear localization signal. A mutant CNAP1 missing this domain, although still incorporated into condensin, was unable to associate with mitotic chromosomes. Successful chromosome targeting of deletion mutants correlated with their ability to directly bind to histones H1 and H3 in vitro. The H3 interaction appears to be mediated through the H3 histone tail, and a subfragment containing the targeting domain was found to interact with histone H3 in vivo. Thus, the CNAP1 C-terminal region defines a novel histone-binding domain that is responsible for targeting CNAP1, and possibly condensin, to mitotic chromosomes.

Deficiency of PXR decreases atherosclerosis in apoE-deficient mice

The pregnane X receptor (PXR, also known as SXR) is a nuclear hormone receptor activated by xenobiotics as well as diverse sterols and their metabolites. PXR functions as a xenobiotic sensor to coordinately regulate xenobiotic metabolism via transcriptional regulation of xenobiotic-detoxifying enzymes and transporters. Recent evidence indicates that PXR may also play an important role in lipid homeostasis and atherosclerosis. To define the role of PXR in atherosclerosis, we generated PXR and apoE double knockout (PXR−/−apoE−/−) mice. Here we show that deficiency of PXR did not alter plasma triglyceride and cholesterol levels in apoE−/− mice. However, PXR−/−apoE−/− mice had significantly decreased atherosclerotic cross-sectional lesion area in both the aortic root and brachiocephalic artery by 40% (P < 0.01) and 60% (P < 0.001), respectively. Interestingly, deficiency of PXR reduced the expression levels of CD36, lipid accumulation, and CD36-mediated oxidized LDL uptake in peritoneal macrophages of PXR−/−apoE−/− mice. Furthermore, immunofluorescence staining showed that PXR and CD36 were expressed in the atherosclerotic lesions of apoE−/− mice, and the expression levels of PXR and CD36 were diminished in the lesions of PXR−/−apoE−/− mice. Our findings indicate that deficiency of PXR attenuates atherosclerosis development, which may result from decreased CD36 expression and reduced lipid uptake in macrophages.