Curcumin induces ABCA1 expression and apolipoprotein A-I-mediated cholesterol transmembrane in the chronic cerebral hypoperfusion aging rats.

Cerebral hypoperfusion or aging often results in the disturbances of cholesterol and lipoprotein, which have been well depicted as a common pathological status contributing to neurodegenerative diseases such as vascular dementia (VaD) and Alzheimer's dementia (AD). The pathway of the liver X receptor-ß (LXR-ß)/retinoic X receptor-α (RXR-α)/ABCA1 plays a vital role in lipoprotein metabolism. Curcumin, a kind of phenolic compound, has been widely used. It has been reported that curcumin can reduce the levels of cholesterol in serum, but the underlying mechanisms are poorly understood. In this study, we evaluated the effects of curcumin on the cholesterol level in brain, vascular cognitive impairment and explored whether the mechanisms for those effects are through activating LXR-ß/RXR-α and ABCA1 expression and apoA-I. With a Morris water test, we found that curcumin treatment could attenuate cognitive impairment. With HE and Nissl staining, we found that curcumin could significantly ameliorate the abnormal changes of pyramidal neurons. Meanwhile, the expression of LXR-ß, RXR-α, ABCA1 and apoA-I mRNA and protein were increased in a dose-dependent manner after curcumin treatment. Interestingly, both serum HDL cholesterol and total cholesterol levels were statistically higher in the curcumin treatment group than those other groups. We conclude that curcumin has the ability to activate permissive LXR-ß/RXR-α signaling and thereby modulate ABCA1 and apoA-I-mediated cholesterol transmembrane transportation, which is a new preventive and therapeutic strategy for cerevascular diseases.

Isoform-selective activation of human constitutive androstane receptor by Ginkgo biloba extract: functional analysis of the SV23, SV24, and SV25 splice variants.

Naturally occurring splice variants of human constitutive androstane receptor (hCAR) exist, including hCAR-SV23 (insertion of amino acids SPTV), hCAR-SV24 (APYLT), and hCAR-SV25 (SPTV and APYLT). An extract of Ginkgo biloba was reported to activate hCAR-SV24 and the wild type (hCAR-WT). However, it is not known whether it selectively affects hCAR splice variants, how it activates hCAR isoforms, and which chemical is responsible for the effects of the extract. Therefore, we evaluated the impact of G. biloba extract on the functionality of hCAR-SV23, hCAR-SV24, hCAR-SV25, and hCAR-WT and compared it with that of phenobarbital, di-(2-ethylhexyl)phthalate (DEHP), 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO), and 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) in cell-based reporter gene assays. Among the hCAR splice variants investigated, only hCAR-SV23 was activated by G. biloba extract, and this required cotransfection of a retinoid X receptor α (RXRα) expression plasmid. The extract activated hCAR-SV23 to a lesser extent than hCAR-WT, but ginkgolide A, ginkgolide B, ginkgolide C, ginkgolide J, and bilobalide were not responsible for the effects of the extract. CITCO activated hCAR-SV23, hCAR-SV24, and hCAR-WT. By comparison, phenobarbital activated hCAR-WT, whereas DEHP activated hCAR-SV23, hCAR-SV24 (with exogenous RXRα supplementation), and hCAR-WT. TCPOBOP did not affect the activity of any of the isoforms. G. biloba extract and phenobarbital did not bind or recruit coactivators to the ligand-binding domains of hCAR-WT and hCAR-SV23, whereas positive results were obtained with the controls (CITCO for hCAR-WT and DEHP for hCAR-SV23). In conclusion, G. biloba extract activates hCAR in an isoform-selective manner, and hCAR-SV23, hCAR-SV24, and hCAR-WT have overlapping, but distinct, sets of ligands.

Nuclear hormone retinoid X receptor (RXR) negatively regulates the glucose-stimulated insulin secretion of pancreatic ß-cells.

OBJECTIVE: Retinoid X receptors (RXRs) are members of the nuclear hormone receptor superfamily and are thought to be key regulators in differentiation, cellular growth, and gene expression. Although several experiments using pancreatic ß-cell lines have shown that the ligands of nuclear hormone receptors modulate insulin secretion, it is not clear whether RXRs have any role in insulin secretion. RESEARCH DESIGN AND METHODS: To elucidate the function of RXRs in pancreatic ß-cells, we generated a double-transgenic mouse in which a dominant-negative form of RXRß was inducibly expressed in pancreatic ß-cells using the Tet-On system. We also established a pancreatic ß-cell line from an insulinoma caused by the ß-cell-specific expression of simian virus 40 T antigen in the above transgenic mouse. RESULTS: In the transgenic mouse, expression of the dominant-negative RXR enhanced the insulin secretion with high glucose stimulation. In the pancreatic ß-cell line, the suppression of RXRs also enhanced glucose-stimulated insulin secretion at a high glucose concentration, while 9-cis-retinoic acid, an RXR agonist, repressed it. High-density oligonucleotide microarray analysis showed that expression of the dominant-negative RXR affected the expression levels of a number of genes, some of which have been implicated in the function and/or differentiation of ß-cells. CONCLUSIONS: These results suggest that endogenous RXR negatively regulates the glucose-stimulated insulin secretion. Given these findings, we propose that the modulation of endogenous RXR in ß-cells may be a new therapeutic approach for improving impaired insulin secretion in type 2 diabetes.

Low retinol levels differentially modulate bile salt-induced expression of human and mouse hepatic bile salt transporters.

UNLABELLED: The farnesoid X receptor/retinoid X receptor-alpha (FXR/RXRalpha) complex regulates bile salt homeostasis, in part by modulating transcription of the bile salt export pump (BSEP/ABCB11) and small heterodimer partner (SHP/NR0B2). FXR is activated by bile salts, RXRalpha by the vitamin A derivative 9-cis retinoic acid (9cRA). Cholestasis is associated with vitamin A malabsorption. Therefore, we evaluated the role of vitamin A/9cRA in the expression of human and mouse bile salt export pump (hBSEP/mBsep), small heterodimer partner (hSHP/mShp), and mouse sodium-dependent taurocholate co-transporting polypeptide (mNtcp). HBSEP and hSHP transcription were analyzed in FXR/RXRalpha-transfected HepG2 cells exposed to chenodeoxycholic acid (CDCA) and/or 9cRA. BSEP promoter activity was determined by luciferase reporter assays, DNA-binding of FXR and RXRalpha by pull-down assays. Serum bile salt levels and hepatic expression of Bsep, Shp, and Ntcp were determined in vitamin A-deficient (VAD)/cholic acid (CA)-fed C57BL/6J mice. Results indicated that 9cRA strongly repressed the CDCA-induced BSEP transcription in HepG2 cells, whereas it super-induced SHP transcription; 9cRA reduced DNA-binding of FXR and RXRalpha. The 9cRA repressed the CDCA-induced BSEP promoter activity irrespective of the exact sequence of the FXR-binding site. In vivo, highest Bsep messenger RNA (mRNA), and protein expression was observed in CA-fed VAD mice. Shp transcription was highest in CA-fed vitamin A-sufficient mice. Ntcp protein expression was strongly reduced in CA-fed VAD mice, whereas mRNA levels were normal. CA-fed control and VAD mice had similarly increased serum bile salt levels. CONCLUSION: We showed that 9cRA has opposite effects on bile salt-activated transcription of FXR/RXRalpha target genes. Vitamin A deficiency in CA-fed mice leads to high BSEP expression. Clearance of serum bile salts may, however, be limited because of post-transcriptional reduction of Ntcp. The molecular effects of vitamin A supplementation during cholestasis need further analysis to predict a therapeutic effect.

Expression of retinoid X receptor transcripts and their significance for developmental competence in in vitro-produced pre-implantation-stage bovine embryos.

Retinoid X receptors are frequently implicated in their role in embryonic development. However, there is no report regarding their expression in embryos with different qualities. In this study, expression pattern and levels of retinoid X receptor transcripts in different qualities and stages of in vitro-produced pre-implantation bovine embryos were examined using real-time polymerase chain reaction (PCR) techniques. Four independent cDNA pools per sample were prepared from mRNAs isolated from different developmental-stage embryos, after classifying them to be of good and bad quality. Primers were also designed to amplify products with PCR. Based on analysis, except retinoid X receptor gamma (RXRG) transcripts, the remaining two types (alpha and beta forms) were expressed in all samples with significantly higher (p < or = 0.05) levels until the eight-cell stage compared with the stages thereafter. However, the transcript levels for bad-quality embryos were consistently lower in all cases in comparison with good-quality embryos. RXRG was identified in 39% of the samples, but all in the pre-embryonic genome activation development stage. Therefore, strong expression patterns of these transcripts in earlier stages indicates their possible role during the maternal phase of embryo development, and the variations of copy number in embryos with different qualities substantiates their possible candidacy as potential quality markers. Moreover, identifying the transcript variations in different qualities and expression of RXRG at these stages is a novel input to the pre-implantation-stage knowledge.