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      Expression of the PXR gene in various types of cancer and drug resistance

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          Abstract

          Pregnane X receptor (PXR) is a member of the nuclear receptor superfamily of ligand-regulated transcription factors. PXR is a key xenobiotic receptor that regulates the expression of genes implicated in drug metabolism, detoxification and clearance, including drug metabolizing enzymes and transporters, suggesting that it is significant in the drug resistance of cancer cells. PXR is expressed in a wide range of tissues in the human body. Studies have demonstrated that PXR is expressed in a variety of tumor types, correlating not only with drug resistance but also with the cell proliferation, apoptosis and prognosis of cancer. The purpose of the present review is to provide a comprehensive review of PXR and its potential roles in multidrug resistance and the biological characteristics of PXR-positive tumors.

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          Most cited references80

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          Ligand binding and co-activator assembly of the peroxisome proliferator-activated receptor-gamma.

          The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a ligand-dependent transcription factor that is important in adipocyte differentiation and glucose homeostasis and which depends on interactions with co-activators, including steroid receptor co-activating factor-1 (SRC-1). Here we present the X-ray crystal structure of the human apo-PPAR-gamma ligand-binding domain (LBD), at 2.2 A resolution; this structure reveals a large binding pocket, which may explain the diversity of ligands for PPAR-gamma. We also describe the ternary complex containing the PPAR-gamma LBD, the antidiabetic ligand rosiglitazone (BRL49653), and 88 amino acids of human SRC-1 at 2.3 A resolution. Glutamate and lysine residues that are highly conserved in LBDs of nuclear receptors form a 'charge clamp' that contacts backbone atoms of the LXXLL helices of SRC-1. These results, together with the observation that two consecutive LXXLL motifs of SRC-1 make identical contacts with both subunits of a PPAR-gamma homodimer, suggest a general mechanism for the assembly of nuclear receptors with co-activators.
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            The nuclear receptor PXR is a lithocholic acid sensor that protects against liver toxicity.

            The pregnane X receptor (PXR) is the molecular target for catatoxic steroids such as pregnenolone 16alpha-carbonitrile (PCN), which induce cytochrome P450 3A (CYP3A) expression and protect the body from harmful chemicals. In this study, we demonstrate that PXR is activated by the toxic bile acid lithocholic acid (LCA) and its 3-keto metabolite. Furthermore, we show that PXR regulates the expression of genes involved in the biosynthesis, transport, and metabolism of bile acids including cholesterol 7alpha-hydroxylase (Cyp7a1) and the Na(+)-independent organic anion transporter 2 (Oatp2). Finally, we demonstrate that activation of PXR protects against severe liver damage induced by LCA. Based on these data, we propose that PXR serves as a physiological sensor of LCA, and coordinately regulates gene expression to reduce the concentrations of this toxic bile acid. These findings suggest that PXR agonists may prove useful in the treatment of human cholestatic liver disease.
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              An orphan nuclear receptor activated by pregnanes defines a novel steroid signaling pathway.

              Steroid hormones exert profound effects on differentiation, development, and homeostasis in higher eukaryotes through interactions with nuclear receptors. We describe a novel orphan nuclear receptor, termed the pregnane X receptor (PXR), that is activated by naturally occurring steroids such as pregnenolone and progesterone, and synthetic glucocorticoids and antiglucocorticoids. PXR exists as two isoforms, PXR.1 and PXR.2, that are differentially activated by steroids. Notably, PXR.1 is efficaciously activated by pregnenolone 16alpha-carbonitrile, a glucocorticoid receptor antagonist that induces the expression of the CYP3A family of steroid hydroxylases and modulates sterol and bile acid biosynthesis in vivo. Our results provide evidence for the existence of a novel steroid hormone signaling pathway with potential implications in the regulation of steroid hormone and sterol homeostasis.
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                Author and article information

                Journal
                Oncol Lett
                Oncol Lett
                OL
                Oncology Letters
                D.A. Spandidos
                1792-1074
                1792-1082
                April 2013
                22 January 2013
                22 January 2013
                : 5
                : 4
                : 1093-1100
                Affiliations
                [1 ]Department of General Surgery, Jiangsu Cancer Hospital, Affiliated to Nanjing Medical University, Nanjing 210009;
                [2 ]Department of Oncology, The First Affiliated Hospital with Nanjing Medical University, Nanjing 210009;
                [3 ]Department of General Surgery, Xuzhou Medical College, Xuzhou 221000, P.R. China
                Author notes
                Correspondence to: Professor Jinhai Tang, Department of General Surgery, Jiangsu Cancer Hospital, Affiliated to Nanjing Medical University, 42 bai zi ting, Nanjing 210009, Jiangsu, P.R. China, E-mail: tangjinhaidoctor@ 123456sina.cn
                Article
                ol-05-04-1093
                10.3892/ol.2013.1149
                3628904
                23599746
                53585774-981e-46e0-99e3-98be61c73576
                Copyright © 2013, Spandidos Publications

                This is an open-access article licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. The article may be redistributed, reproduced, and reused for non-commercial purposes, provided the original source is properly cited.

                History
                : 10 October 2012
                : 02 January 2013
                Categories
                Review

                Oncology & Radiotherapy
                pregnane x receptor,multidrug resistance,cancer,tumor cell apoptosis
                Oncology & Radiotherapy
                pregnane x receptor, multidrug resistance, cancer, tumor cell apoptosis

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