Retinoid X receptor (RXR alpha) gene expression is regulated by fatty acids and dexamethasone in hepatic cells

Nuclear Receptors, Ligands and the Mammalian B Cell

Questions concerning the influences of nuclear receptors and their ligands on mammalian B cells are vast in number. Here, we briefly review the effects of nuclear receptor ligands, including estrogen and vitamins, on immunoglobulin production and protection from infectious diseases. We describe nuclear receptor interactions with the B cell genome and the potential mechanisms of gene regulation. Attention to the nuclear receptor/ligand regulation of B cell function may help optimize B cell responses, improve pathogen clearance, and prevent damaging responses toward inert- and self-antigens.

The effect of the hepatitis C virus (HCV) NS3 protein on the expression of miR-150, miR-199a, miR-335, miR-194 and miR-27a

Hepatitis C virus (HCV) infection is considered one of the most important causes of chronic liver diseases. Many reports have shown that the proteins of the HCV via interactions with gene expression regulatory networks such as cellular pathways and microRNAs can contribute to the development of chronic liver diseases. The present study aimed to investigate the effects of the HCV NS3 protein on the expression of miR-150 miR-199a, miR-335, miR-194, miR-27a in a cell culture model. Plasmids expressing the full length of the HCV NS3 protein were transfected into the LX-2 cell line, while at the same time a plasmid expressing empty GFP (green fluorescent protein) was used as a negative control group. Subsequently, total RNA was extracted and real-time PCR was performed to measure microRNA expression levels. Additionally, the trypan blue exclusion test was performed to examine the effect of the expressing NS3 protein plasmid on cellular viability. The analysis of microRNA gene expression in LX-2 cells indicated that the NS3 protein, which is endogenous to HCV, can significantly upregulate the expression of miR-27a and downregulate the expression of miR-335 and miR-150 in comparison with the control plasmid expressing GFP and normal cells (p < 0.01). These results suggest that the HCV NS3 protein may play a role in the pathogenesis of chronic hepatic diseases such as liver fibrosis via interaction with cellular microRNAs and modulation of microRNA gene expressions.

Myocilin expression is regulated by retinoic acid in the trabecular meshwork-derived cellular environment

Glaucoma is the leading cause of irreversible blindness and is usually classified as angle closure and open angle glaucoma (OAG). Primary open angle glaucoma represents the most frequent clinical presentation leading to ganglion cell death and optic nerve degeneration as a main consequence of an intraocular pressure' (IOP) increase. The mechanisms of this IOP increase in such pathology remain unclear but one protein called Myocilin could be a part of the puzzle in the trabecular meshwork (TM). Previously described to be transcriptionally regulated by glucocorticoids, the comprehension of the trabecular regulation of Myocilin' expression has only weakly progressed since 15 years. Due to the essential molecular and cellular implications of retinoids' pathway in eye development and physiology, we investigate the potential role of the retinoic acid in such regulation and expression. This study demonstrates that the global retinoids signaling machinery is present in immortalized TM cells and that Myocilin (MYOC) expression is upregulated by retinoic acid alone or combined with a glucocorticoid co-treatment. This regulation by retinoic acid acts through the MYOC promoter which contains a critical cluster of four retinoic acid responsive elements (RAREs), with the RARE-DR2 presenting the strongest effect and binding the RARα/RXRα heterodimer. All together, these results open up new perspectives for the molecular understanding glaucoma pathophysiology and provide further actionable clues on Myocilin gene regulation.

Involvement of glucocorticoid receptor activation on anti-inflammatory effect induced by peroxisome proliferator-activated receptor γ agonist in mice

Glucocorticoids are effective anti-inflammatory agents widely used for the treatment of acute and chronic inflammatory diseases. Recent in vitro studies have proposed that glucocorticoid receptor (GR) activation is involved in peroxisome proliferator-activated receptor γ (PPARγ) agonist-induced effects. In this study, to examine the involvement of the GR in PPARγ agonist- and retinoid X receptor (RXR) agonist-mediated anti-inflammatory effects in vivo, we tested the anti-inflammatory effects of dexamethasone (a GR agonist) with pioglitazone (a PPARγ agonist) or 6-[N-ethyl-N-(3-isopropoxy-4-isopropylphenyl)-amino] nicotinic acid (NEt-3IP; an RXR agonist) by using an experimental model of carrageenan-induced inflammation. We also evaluated the effects of a GR antagonist on PPARγ agonist- or RXR agonist-induced anti-inflammatory effects. Results showed that the GR antagonist RU486 reduced the anti-inflammatory effects of GR or PPARγ agonists but not those of the RXR agonist. In addition, combinations of GR and PPARγ agonists or GR and RXR agonists had no effect on carrageenan-induced paw edema. Moreover, the PPARγ antagonist GW9662 and RXR antagonist 6-[N-4-(trifluoromethyl)-benzenesulfonyl-N-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-2-naphthyl)-amino] nicotinic acid (NS-4TF) had no effect on the anti-inflammatory effect of the GR agonist dexamethasone. Therefore, it is suggested that GR activation in vivo does not play a direct role in PPARγ/RXR heterodimer signaling. In contrast, pioglitazone showed a partial anti-inflammatory effect via GR activation. These data provide evidence for the pro-inflammatory activity of pioglitazone.

Thyroid hormone regulation of gene expression in primary cerebrocortical cells: role of thyroid hormone receptor subtypes and interactions with retinoic acid and glucocorticoids

The effects of thyroid hormone on brain development and function are largely mediated by the binding of 3,5,3'-triiodo-L-thyronine (T3) to its nuclear receptors (TR) to regulate positively or negatively gene expression. We have analyzed by quantitative polymerase chain reaction the effect of T3 on primary cultured cells from the embryonic mouse cerebral cortex, on the expression of Hr, Klf9, Shh, Dio3, Aldh1a1, and Aldh1a3. In particular we focused on T3 receptor specificity, and on the crosstalk between T3, retinoic acid and dexamethasone. To check for receptor subtype specificity we used cerebrocortical cells derived from wild type mice and from mice deficient in thyroid hormone receptor subtypes. Receptor subtype specificity was found for Dio3 and Aldh1a1, which were induced by T3 only in cells expressing the T3 receptor alpha 1 subtype. Interactions of T3 with retinoic acid signaling through the control of retinoic acid metabolism are likely to be important during development. T3 had opposing influences on retinoic acid synthesizing enzymes, increasing the expression of Aldh1a1, and decreasing Aldh1a3, while increasing the retinoic acid degrading enzyme Cyp26b1. Dexamethasone increased Klf9 and Aldh1a1 expression. The effects of T3 and dexamethasone on Aldh1a1 were highly synergistic, with mRNA increments of up to 20 fold. The results provide new data on thyroid hormone regulation of gene expression and underscore the importance of thyroid hormone interactions with retinoic acid and glucocorticoids during neural development.

Regulation of drug-metabolizing cytochrome P450 enzymes by glucocorticoids

The regulation of drug-metabolizing cytochrome P450 enzymes (CYP) is a complex process involving multiple mechanisms. Among them, transcriptional regulation through ligand-activated nuclear receptors is the crucial mechanism involved in hormone-controlled and xenobiotic-induced expression of drug-metabolizing CYPs. In this article, we focus, in detail, on the role of the glucocorticoid receptor (GR) in the transcriptional regulation of human drug-metabolizing CYP enzymes and the mechanisms of the regulation. There are at least three distinct transcriptional mechanisms by which GR controls the expression of CYPs: 1) direct binding of GR to a specific gene-promoter sequence called the glucocorticoid responsive element (GRE); 2) indirect binding of GR in the form of a multiprotein complex to gene promoters without a direct contact between GR and promoter DNA; and 3) up- or downregulation of other CYP transcriptional regulators or nuclear receptors (i.e., transcriptional regulatory cross-talk). However, due to the general effect of glucocorticoids on numerous cellular pathways and functions, the net transcriptional effect of glucocorticoids on drug-metabolizing enzymes is usually a combination of several mechanisms. Since synthetic glucocorticoids are widely prescribed in human pharmacotherapy for the treatment of many diseases, comprehensive understanding of the transcriptional regulation of drug-metabolizing CYPs via GR with respect to glucocorticoid therapy or glucocorticoid hormonal status will aid in the development of efficient individualized pharmacotherapy without drug-drug interactions.

Retinoid X receptor alpha participation in dexamethasone-induced rat bile acid coenzyme A-amino acid N-acyltransferase expression in septic liver

To test the hypothesis that dexamethasone (Dex) treatment would restore rat hepatic bile acid coenzyme A-amino acid N-acyltransferase (rBAT) expression in septic rats after cecal ligation and puncture by increasing expression of retinoic acid X receptor alpha (RXRalpha), we assessed survival rate and bile and bile salt concentration in the Dex-treated septic group and compared these results with those for a nontreated septic group, a Dex-treated nonseptic group, and a sham group. Dexamethasone treatment (0.01 mg/kg) significantly improved the survival rate and increased the bile and bile salt concentration in the bile ducts of septic rats (P = <0.05). In our assessment of bile salt-related genes, during sepsis, there were decreases in protein and mRNA expression of rBAT and cholesterol 7 alpha-hydroxylase (CYP7A1). Treatment with Dex restored expression of rBAT and RXR[alpha] but not CYP7A1, bile salt export pump, or multidrug resistance associated protein 2 (MRP2). Na+-taurocholate cotransport protein and organic anion transporting polypeptide 1 were unchanged. In addition, treatment with Dex also restored the DNA-binding activity of RXR/farnesoid-X receptor to rBAT promoter containing inverted repeat 1 sequence. In an experiment to confirm our findings, RXR[alpha] siRNA was found to significantly block Dex-induced increases in expression of rBAT in hepatocytes taken from septic rats (P < 0.01).

CONCLUSION

Dex restored the expression of rBAT in septic rats by enhancing RXR[alpha], a process that might explain the mechanism underlying Dex's anticholestatic effect.

Expression, protein stability and transcriptional activity of retinoic acid receptors are affected by microtubules interfering agents and all-trans-retinoic acid in primary rat hepatocytes

Cellular signaling by glucocorticoid receptor and aryl hydrocarbon receptor is restricted by microtubules interfering agents (MIAs). This leads to down-regulation of drug metabolizing enzymes and drug interactions. Here we investigated the effects of all-trans-retinoic acid (ATRA) and MIAs, i.e. colchicine, nocodazole and taxol on the regulation of retinoic acid receptor (RAR) genes in primary cultures of rat hepatocytes. ATRA (1microM) down-regulated RARalpha and RARgamma mRNAs (decrease 23% and 41%, respectively) whereas it up-regulated RARbeta mRNA (4.3-fold induction). All MIAs diminished the expression of RARs in dose-dependent manner; the potency of MIAs increased in order NOC Collapse

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MESH Headings

• Animals
• Catalysis/drug effects
• Cell Survival/drug effects
• Cells, Cultured
• Colchicine/pharmacology
• Gene Expression Regulation/drug effects
• HeLa Cells
• Hepatocytes/cytology
• Hepatocytes/drug effects
• Hepatocytes/metabolism
• Humans
• Leupeptins/pharmacology
• Microtubules/drug effects
• Proteasome Inhibitors
• RNA, Messenger/genetics
• RNA, Messenger/metabolism
• Rats
• Receptors, Retinoic Acid/genetics
• Receptors, Retinoic Acid/metabolism
• Retinoic Acid Receptor alpha
• Thermodynamics
• Transcription, Genetic/drug effects
• Transglutaminases/metabolism
• Tretinoin/pharmacology
• Tubulin Modulators/pharmacology
• Retinoic Acid Receptor gamma

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Affiliation(s)

Zdenek Dvorák Institute of Medical Chemistry and Biochemistry, Faculty of Medicine, Palacký University Olomouc, Hnevotínská 3, 77515 Olomouc, Czech Republic.
Radim Vrzal
Jitka Ulrichová
Dana Macejová
Slavomíra Ondková
Július Brtko

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Dexamethasone stimulation of retinoic Acid-induced sodium iodide symporter expression and cytotoxicity of 131-I in breast cancer cells

CONTEXT

The sodium iodide symporter (NIS) mediates the active iodide uptake in the thyroid gland as well as lactating breast tissue. Recently induction of functional NIS expression was reported in the estrogen receptor-positive human breast cancer cell line MCF-7 by all-trans retinoic acid (atRA) treatment in vitro and in vivo, which might offer the potential to treat breast cancer with radioiodine.

OBJECTIVE

In the current study, we examined the effect of dexamethasone (Dex) on atRA-induced NIS expression and therapeutic efficacy of 131-I in MCF-7 cells.

DESIGN

For this purpose, NIS mRNA and protein expression levels in MCF-7 cells were examined by Northern and Western blot analysis after incubation with Dex (10(-9) to 10(-7) m) in the presence of atRA (10(-6) m) as well as immunostaining using a mouse monoclonal human NIS-specific antibody. In addition, NIS functional activity was measured by iodide uptake and efflux assay, and in vitro cytotoxicity of 131-I was examined by in vitro clonogenic assay.

RESULTS

After incubation with Dex in the presence of atRA, NIS mRNA levels in MCF-7 cells were stimulated up to 11-fold in a concentration-dependent manner, whereas NIS protein levels increased up to 16-fold and iodide accumulation was stimulated up to 3- to 4-fold. Furthermore, iodide efflux was modestly decreased after stimulation with Dex in the presence of atRA. Furthermore, in the in vitro clonogenic assay, selective cytotoxicity of 131-I was significantly increased from approximately 17% in MCF-7 cells treated with atRA alone to 80% in MCF-7 cells treated with Dex in the presence of atRA.

CONCLUSION

Treatment with Dex in the presence of atRA significantly increases functional NIS expression levels in addition to inhibiting iodide efflux, resulting in an enhanced selective killing effect of 131-I in MCF-7 breast cancer cells.

Unliganded RXR acts as an inhibitory factor on troglitazone-induced activation

Troglitazone (TZ), a thiazolidinedione derivative, is a specific ligand for the peroxisome proliferator-activated receptor (PPAR) gamma and improves insulin sensitivity. PPARgamma regulates the expression of genes by binding to PPAR response element in promoter regions of regulator genes as heterodimers with a retinoid X receptor (RXR). We report here that PPARgamma activation by TZ depends on the expression levels of RXR. A transient transfection study in CV-1 cells revealed that the activation by TZ was suppressed by increasing amounts of expression of RXR, but not PPARgamma. Northern blot analysis revealed that PPARgamma and RXR were not expressed in CV-1 cells, and TZ did not induce PPARgamma or RXR mRNA in CV-1 cells indicating that RXR suppression is not related to these endogenous receptor expressions. Electrophoretic mobility shift assay revealed that the increasing amount of RXR did not compete with the DNA binding of the PPARgamma/RXR heterodimer in the presence or absence of TZ. Transfected co-activators enhanced the TZ-dependent gene transcription, and this activation was inhibited by excessive amounts of RXR, indicating that unliganded RXR may recruit the specific coactivators from the PPARgamma/RXR heterodimer.

Cross-talk between fatty acid and cholesterol metabolism mediated by liver X receptor-alpha

LXR alpha (liver X receptor, also called RLD-1) is a nuclear receptor, highly expressed in tissues that play a role in lipid homeostasis. In this report we show that fatty acids are positive regulators of LXR alpha gene expression and we investigate the molecular mechanisms underlying this regulation. In cultured rat hepatoma and primary hepatocyte cells, fatty acids and the sulfur-substituted fatty acid analog, tetradecylthioacetic acid, robustly induce LXR alpha (up to 3.5- and 7-fold, respectively) but not LXR beta (also called OR-1) mRNA steady state levels, with unsaturated fatty acids being more effective than saturated fatty acids. RNA stability and nuclear run-on studies demonstrate that changes in the transcription rate of the LXR alpha gene account for the major part of the induction of LXR alpha mRNA levels. A similar induction of protein level was also seen after treatment of primary hepatocytes with the same fatty acids. Consistent with such a transcriptional effect, transient transfection studies with a luciferase reporter gene, driven by 1.5 kb of the 5'-flanking region of the mouse (m)LXR alpha gene, show a peroxisome proliferator-activated receptor-alpha-dependent increase in luciferase activity upon treatment with tetradecylthioacetic acid and the synthetic peroxisome proliferator-activated receptor-alpha activator, Wy 14.643, suggesting that the mLXR alpha 5'-flanking region contains the necessary sequence elements for fatty acid responsiveness. In addition, in vivo LXR alpha expression was induced by fatty acids, consistent with the in vitro cell culture data. These observations demonstrate that LXR alpha expression is controlled by fatty acid signaling pathways and suggest an important cross-talk between fatty acid and cholesterol regulation of lipid metabolism.