Glucocorticoids increase retinoid-X receptor alpha (RXRalpha) expression and enhance thyroid hormone action in primary cultured rat hepatocytes

We have previously demonstrated that dexamethasone (DEX) enhances the T3-dependent increase in type I 5'-deiodinase (5'DI) mRNA in primary cultured rat hepatocytes grown as spheroids. Here we report that DEX-enhanced T3-responsiveness also occurs in two other T3-regulated hepatic genes, Spot 14 and malic enzyme. This enhancement was inhibited by pretreatment with cycloheximide and the stability of 5'DI and Spot 14 mRNAs was not affected by DEX. We thus hypothesized that a factor(s) that augments T3-responsiveness is induced by DEX. Among the possible candidates examined, retinoid-X receptor alpha (RXRalpha), which is a main heterodimer partner with T3 receptor, appeared to be involved. Whereas DEX increased the amount of RXRalpha mRNA, it did not affect the expression of other possible factors such as steroid receptor coactivator-1 and the binding protein of cAMP response element-binding protein. Northern and Western blot analysis, and electrophoretic mobility shift assay revealed that DEX increased RXRalpha expression at both the mRNA and protein levels. Maximal increase in RXRalpha protein was achieved with the addition of physiological concentrations of DEX (10(-8) M). To test whether the DEX-induced increase in RXRalpha affects ligand-dependent transcriptional activation through other receptors that form heterodimer with RXR, we examined its effect on the retinoic acid (RA)/RA receptor (RAR) system. Indeed, DEX also enhanced the RA-dependent increase in RARbeta mRNA in a cycloheximide-sensitive manner. Increase in the level of RXRalpha in hepatocytes by infection with the RXRalpha-expressing adenovirus resulted in enhancement of the T3-dependent increase in 5'DI mRNA. These results strongly suggest that the DEX-induced augmentation of T3-responsiveness in cultured hepatocytes is mediated, in part, by the increased expression of RXRalpha.

Dose-response relationships between drinking and serum tests in Japanese men aged 40-59 years

Alcohol intake per Japanese adult has been increasing year by year. To show biological effects of drinking, the dose-response relationships between alcohol use and serum indices were analyzed in 5919 Japanese men aged 40-59 years. The subjects were classified into nine groups: a nondrinking (ND, n = 1827) group and eight drinking (1D-8D) groups, by self-reported drinking habit. The 1D (the lightest drinking, n = 699), 5D (n = 942), and 8D (the heaviest drinking, n = 46) groups consumed alcohol less than 30 g per week, 25-30 g alcohol per day, and 100 g alcohol per day or more, respectively. Ten serum indices, total cholesterol, HDL cholesterol, triglyceride, total/HDL cholesterol ratio, LDL cholesterol, gamma-glutamyl transpeptidase, aspartate aminotransferase, alanine aminotransferase, glucose, and uric acid, were used. The dose-response analysis was statistically controlled for age, body mass index, smoking, and habitual exercise, and showed that drinking, even a small amount of alcohol, always had both beneficial and adverse effects on humans. However, alcohol less than 30 g per day may be tolerable for middle-aged Japanese men, because it improved serum lipids profile but did not induce apparent liver cell damage, hyperglycemia, or hyperuricemia.

Cleavage of RXRalpha by a lysosomal enzyme, cathepsin L-type protease

In this study, we characterized a protease responsible for the cleavage of RXRalpha in two human derived cell lines, HepG2 and JEG-3 cells. Electrophoretic mobility shift assay (EMSA) combined with antibody supershift analysis suggested that contamination of cytoplasmic components during nuclear extract preparation could result in complete cleavage of RXRalpha at its N-terminus in JEG-3 cells, while such proteolytic activity was much less evident in HepG2. When the nuclei were purified in the presence of leupeptin, only full-length RXRalpha was found in the extracts prepared from both JEG-3 and HepG2 cells, suggesting a member of cysteine protease family is responsible for the cleavage. The presence of the protease in the cytoplasm, but not in the nucleus, was confirmed by incubating full-length 35S-labeled RXRalpha with each fraction. The cytoplasmic fraction from JEG-3 and HepG2 cells cleaved RXRalpha into smaller sizes with molecular mass of 45, 43, and 31 kD. Immunoprecipitation with antibodies recognizing distinct epitopes indicated that the cleaved RXRalpha with the size of 45 and 43 kD were truncated at N-terminus in which most of the A/B domain was absent. Using a series of protease inhibitors, the enzyme cleaving RXRalpha was characterized as cathepsin L-type protease. The enzyme activity in JEG-3 cells was much higher than that in HepG2 cells. This is the first demonstration that RXRalpha is cleaved by a lysosomal enzyme, cathepsin L-type protease.

Intracellular proteolytic cleavage of 9-cis-retinoic acid receptor alpha by cathepsin L-type protease is a potential mechanism for modulating thyroid hormone action

We previously reported that the responsiveness of hepatocytes to thyroid hormone is markedly attenuated when they were cultured as monolayers rather than spheroids. To elucidate the mechanisms underlying the altered responsiveness, thyroid hormone receptor auxiliary proteins in the hepatocytes were analyzed by electrophoretic mobility shift assay. The major thyroid hormone receptor auxiliary protein was identified as 9-cis-retinoic acid receptor alpha (RXRalpha) in the hepatocytes regardless of the culture conditions. The cytoplasmic fraction was shown to contain a protease(s) that cleaves RXRalpha at its amino terminus. The presence of the protease in the cytosol, but not in the nucleus, was ascertained by incubating full-length 35S-labeled RXRalpha with each fraction. Using various protease inhibitors, it was shown that cathepsin L-type protease could participate in the cleavage of the RXRalpha. The enzyme activity was much higher in the monolayers than the spheroids. Inhibition of this enzyme activity in the monolayer hepatocyte resulted in the increase of nuclear RXRalpha protein and the augmentation of T3-dependent induction of spot 14 mRNA. These results suggest that the changes in cathepsin L-type protease activity in the cytosol may alter the turnover of RXRalpha in the nucleus and modify the function of steroid receptor superfamilies that heterodimerize with RXRalpha.

Advanced onset of menarche and higher bone mineral density depending on vitamin D receptor gene polymorphism

Bone mineral density (BMD) at distal forearm, and weight and height of healthy Japanese girls aged 18-19 years were measured and their age at menarche was obtained through a questionnaire. A statistically significant association was found between BMD at distal radius and vitamin D receptor (VDR) gene polymorphism at the ApaI site. The age at menarche in the population with Aa genotype was significantly earlier than that in the aa population. In addition, BMD was significantly dependent on the earlier onset of menarche in the population with genotype Aa but not in the population with genotype aa. BMD was also positively associated with the body mass index (BMI) in the population with genotype Aa. Statistical analysis suggested a stronger effect of VDR genotype on age at menarche than on BMI. Thus, we show that VDR gene polymorphism advances the age at menarche and increases BMD in cooperation with age at menarche.

[Pathogenesis of disorder due to thyroid hormone receptor mutation]

Thyroid hormone action is mediated through its nuclear receptor (thyroid hormone receptor; TR). A mutation in the TR causes resistance to thyroid hormone (RTH) with autosomal dominant inheritance. Using T3 binding assay, gel shift assay and transfection study, the pathogenesis of this disorder was elucidated at a molecular level. Mutant TRs lose their T3 binding activity, and inhibit wild type TR action in a dominant negative manner. For this dominant negative inhibition, the preservation of DNA binding and dimerization activities are required. It is indicated that the mutant TR complex competes with wild type TR complex for the DNA binding site to exert dominant negative inhibition. Thus, the functional analyses of mutant TRs are useful to elucidate the molecular basis of this disorder.

[Functional analysis of nuclear hormone receptors]

Nuclear hormone receptors are transcription factors which regulate gene transcription by lipophilic hormones. The functional domains such as transactivation, DNA binding, ligand binding and dimerization are mapped in the distinct region of the receptors. To analyze these functions, ligand binding assay, gel shift assay, transfection study, GST pull-down method, immunoprecipitation method, two hybrid system and yeast functional assay are employed. These assays have been shown to clarify the molecular basis of the disorders which are due to mutation in the receptors.

[Thyroid hormone receptors and the disorders]

Thyroid hormone is essential for normal development and several metabolic pathways. Its action is mediated through its nuclear receptors (thyroid hormone receptors: TRs), which regulate gene transcriptions in a ligand-dependent manner. The abnormality of TR functions is linked with several disorders including resistance to thyroid hormone (RTH), malignancy and euthyroid sick syndrome. For example, mutant TRs were identified to cause RTH and a certain malignancy. The functionally impaired TRs might be related with euthyroid sick syndrome. The analyses of these TRs will elucidate the molecular mechanism or basis for the disorders, and may provide a new insight to find the way of diagnosis and treatment.

Requirement of corepressor binding of thyroid hormone receptor mutants for dominant negative inhibition

Resistance to thyroid hormone (RTH) is mainly caused by dominant negative inhibition of wild type thyroid hormone receptor (TR) function due to mutations in the ligand binding domain of the TR beta. Because no RTH mutant was identified in the hinge region of the TR, the contribution of this region for dominant negative inhibition was explored. In transient transfection assays, the dominant negative activity of a RTH-type mutant (P453X) was abolished by an introduction of a mutation into the hinge region (P214R). Although this hinge mutation did not alter homo- or heterodimerization with retinoid X receptor (RXR), its association with nuclear receptor corepressor (N-CoR) was impaired. These results indicate that association of corepressor with the RTH mutants through the hinge region is crucial for their dominant negative activity.

Activation of transcriptionally active nuclear factor-kappaB by tumor necrosis factor-alpha and its inhibition by antioxidants in rat thyroid FRTL-5 cells

ABSTRACT Tumor necrosis factor-alpha (TNF-alpha) exerts pleiotropic effects on thyroid follicular cells. However, the intracellular signaling pathway for the TNF-alpha action has not been well elucidated. The present study examined the effects of TNF-alpha on the activation of nuclear factor-kappa B (NF-kappaB) and on the expression of interleukin (IL)-6 gene in rat thyroid FRTL-5 cells. The treatment of the cells with TNF-alpha resulted in the nuclear translocation of p65-p50 heterodimer as well as p50-p50 homodimer NF-kappaBs. The treatment with the antioxidants 20 mM N-acetyl-L-cysteine (NAC) and 10 microM pyrrolidine dithiocarbamate (PDTC) inhibited the TNF-alpha-dependent activation of p65-p50 heterodimer but not the p50-p50 homodimer, indicating that generation of oxidants is required for the activation of the heterodimer NF-kappaB. When the plasmid containing the multimerized NF-kappaB sites upstream of a luciferase reporter gene was transfected into FRTL-5 cells, the treatment with NAC or PDTC prevented the TNF-alpha-dependent increase in the luciferase activities, indicating that the p65-p50 heterodimer is a transcriptionally active NF-kappaB. Accordingly, the TNF-alpha-dependent increase in IL-6 messenger RNA and in secretion of the protein was prevented by the treatment with NAC. These results strongly suggest that TNF-alpha increases the IL-6 gene expression through the activation of NF-kappaB in the thyroid cells, and that antioxidants suppress the TNF-alpha-dependent IL-6 expression by inhibiting the activation of the transcriptionally active NF-kappaB.

Inductions of immediate early genes (IEGS) and ref-1 by human chorionic gonadotropin in murine Leydig cell line (MA-10)

The effect of human chorionic gonadotropin (hCG) on the expression of immediate early genes (IEGs) including all members of fos and jun family, and c-myc was studied using mouse Leydig cell line (MA-10 cells) by Northern blot analyses. In addition, the induction of ref-1 which enhances DNA binding of fos/jun proteins was also analyzed. HCG induced a rapid and transient expression of c-fos, fosB, c-jun, junB, junD and c-myc with a peak at 30 min to 1 h. In contrast, induction of fra-1 mRNA was delayed with a peak at 3 hr. However, fra-2 mRNA was immediately increased by hCG with a peak at 1 h. The ref-1 mRNA was expressed before the stimulation and its level was not altered by hCG at least for 8 hr. The differential induction of IEGs and continuous expression of ref-1 mRNA suggest an important role of their gene products on the regulation of Leydig cell function by hCG.

An inhibitory region of the DNA-binding domain of thyroid hormone receptor blocks hormone-dependent transactivation

We have employed a chimeric receptor system in which we cotransfected yeast GAL4 DNA-binding domain/retinoid X receptor beta ligand-binding domain chimeric receptor (GAL4RXR), thyroid hormone receptor-beta (TRbeta), and upstream activating sequence-reporter plasmids into CV-1 cells to study repression, derepression, and transcriptional activation. In the absence of T3, unliganded TR repressed transcription to 20% of basal level, and in the presence of T3, liganded TRbeta derepressed transcription to basal level. Using this system and a battery of TRbeta mutants, we found that TRbeta/RXR heterodimer formation is necessary and sufficient for basal repression and derepression in this system. Additionally, an AF-2 domain mutant (E457A) mediated basal repression but not derepression, suggesting that interaction with a putative coactivator at this site may be critical for derepression. Interestingly, a mutant containing only the TRbeta ligand binding domain (LBD) not only mediated derepression, but also stimulated transcriptional activation 10-fold higher than basal level. Studies using deletion and domain swap mutants localized an inhibitory region to the TRbeta DNA-binding domain. Titration studies further suggested that allosteric changes promoting interaction with coactivators may account for enhanced transcriptional activity by LBD. In summary, our findings suggest that TR heterodimer formation with RXR is important for repression and derepression, and coactivator interaction with the AF-2 domain may be needed for derepression in this chimeric system. Additionally, there may be an inhibitory region in the DNA-binding domain, which reduces TR interaction with coactivators, and prevents full-length wild-type TRbeta from achieving transcriptional activation above basal level in this chimeric receptor system.

No relationship of salivary flow rate or secretory immunoglobulin A to dental caries in children

To investigate the relationship between dental caries and the salivary flow rate, secretory immunoglobulin A (sIgA) or other components in children, nonstimulated whole saliva was collected and teeth status was examined in 138 boys and 134 girls aged 11-12 years. The subjects were apparently healthy. The mean salivary flow rate was faster in boys than in girls (0.29 vs 0.18 ml/min, p < 0.001). In both sexes, secretion of salivary sIgA and three other components (total protein, calcium and amylase activity) was markedly dependent on salivary flow rates. These results suggest that basal components of resting saliva are secondarily secreted with the flow of saliva fluid. The mean erupted permanent teeth was 21.0 teeth (range: 10-28 teeth) in boys, and 23.0 teeth (13-28 teeth) in girls (sex-difference: p < 0.001). The means of DMFT, the DMFT ratio (% of DMFT to erupted permanent teeth) and DT+dt (sum of decayed permanent and milk teeth, an index for active caries) were 3.4 DMFT (range: 0-11 DMFT), 16.0% (0-40.0%) and 0.5 DT+dt (0-7 DT + dt) in boys, and 3.8 DMFT (0-12 DMFT), 16.2% (0-44.4%) and 0.8 DT+dt (0-5 DT+dt) in girls, respectively (sex-differences: p>0.05 in all). The salivary flow rate or the four salivary components (either concentration or secretion rate) used here had no relationship to the DMFT ratio or to DT+dt in either sex. Variation in the flow rate or in the basal components of resting saliva may not influence caries development in healthy children.

NGFI-B, c-fos, and c-jun mRNA expression in mouse brain after acute carbon monoxide intoxication

The expression of immediate early genes (IEG) has been documented in the brain after various kinds of insults such as ischemia and hypoxia. To determine whether acute carbon monoxide intoxication (ACOI) might trigger IEG expression, adult ddY mice were subjected to carbon monoxide exposure at a rate of 30 mL/min for 35 seconds. The levels of NGFI-B, c-fos, and c-jun mRNA were determined by Northern blot analysis. A time-course study in the cerebral cortex indicated that the induction of NGFI-B, c-fos, and c-jun mRNA started as early as 15 minutes, reached a peak at 30 minutes, and returned to the basal level at 1 hour after the ACOI. In addition, the temporal feature of the induction of these IEG mRNA in the hippocampus was very similar to that in the cerebral cortex. Examination of brain regions at 30 minutes after the ACOI revealed a significant induction of NGFI-B mRNA in the cerebellum, thalamus-hypothalamus, brainstem. as well as in the cortex and hippocampus, but not in the striatum or olfactory bulb. Furthermore, the neuroanatomical distribution of c-fos mRNA at 30 minutes after the ACOI was very similar to that of the NGFI-B mRNA. The widespread distribution of these IEG in the brain, especially in the cerebellum and brainstem, indicates that the major cause for the triggering of IEG expression in the brain by the ACOI might be a diffuse hypoxia. These findings show for the first time the temporal and spatial expression of IEG in the brain after ACOI.

Nuclear receptor corepressors activate rather than suppress basal transcription of genes that are negatively regulated by thyroid hormone

A group of transcriptional cofactors referred to as corepressors (CoRs) were recently shown to play a central role in basal silencing of genes that contain positive triiodothyronine (T3) response elements. In a reciprocal manner, negatively regulated genes are stimulated by unliganded thyroid hormone receptor (TR) and repressed upon the addition of T3. We used a TR beta mutant, called P214R, which fails to interact with CoRs, to examine whether CoRs also play a role in the control of genes that are negatively regulated in response to T3. In studies of three negatively regulated genes (the pituitary thyroid-stimulating hormone alpha-subunit [TSH alpha], TSH beta, and hypothalamic thyrotropin-releasing hormone [TRH] genes), stimulation of basal promoter activity by unliganded TR beta was impaired by introducing the P214R CoR mutation. Coexpression of each of the CoRs SMRT (silencing mediator for retinoid receptors and TRs) and NCoR (nuclear receptor CoR) enhanced basal stimulation of the negatively regulated promoters in a TR-dependent manner, but this effect was not seen with the P214R TR mutant. The mechanism of CoR effects on negatively regulated promoters was explored further with a series of GAL4-TR chimeric receptors and mutants that allowed TR effects to be assessed independently of receptor interactions with DNA. These experiments revealed that, like the negative regulation of genes by wild-type TR, basal activation occurred with GAL4-TR, but not with the GAL4-P214R mutant, and was reversed by the addition of T3. These results suggest that TR interactions with negatively regulated genes may be driven through protein-protein interactions. We conclude that a subset of negatively regulated genes are controlled by a novel mechanism that involves TR-mediated recruitment and basal activation by SMRT and NCoR. Addition of T3 reverses basal activation, perhaps by dissociation of CoRs.

Serum lipid profile in relation to milk consumption in a Japanese population

OBJECTIVE

To investigate relationships between serum lipid profiles, milk consumption and lifestyle, in a Japanese population.

METHODS

A cross-sectional study in 12,610 Japanese men aged 30 to 69 years. Serum total-cholesterol (TC), HDL-cholesterol (HDLC), LDL-cholesterol (LDLC), TC/HDLC ratio, and triglycerides (TG) were determined. Milk consumption was classified into "Yes" (3553 men) and "No" (9057 men) groups by a self-administered questionnaire "Do you drink a glass of cow's milk (180 to 200 mL) or more everyday?". Five variables (age, body mass index, habitual exercise, and smoking and drinking habits) were considered confounding factors for the serum lipids.

RESULTS

Regardless of age, the "Yes" group had higher levels of serum TC, HDLC and LDLC than the "No" group except for one comparison (HDLC in 50 to 54 year old group). The differences in serum TC (p < 0.001), HDLC (p < 0.001) and LDLC (p < 0.001) between the two groups were significant by ANOVA. However, milk consumption had no significant relations to serum TC/HDLC ratio or TG level. Multivariate models including the five confounding factors confirmed these results, and indicated that the "Yes" group had higher adjusted means of serum TC by 0.079 mmol/L (+1.5%, 5.280 vs. 5.201 mmol/L, p < 0.001), HDLC by 0.018 mmol/L (+1.4%, 1.260 vs. 1.242 mmol/L, p < 0.01), and LDLC by 0.066 mmol/L (+2.0%, 3.382 vs. 3.316 mmol/L, p < 0.001) than the "No" group.

CONCLUSION

Milk consumption is part of a lifestyle related to hypercholesterolemia in the Japanese.

Dominant inheritance of resistance to thyroid hormone not linked to defects in the thyroid hormone receptor alpha or beta genes may be due to a defective cofactor

Resistance to thyroid hormone (RTH) is an inherited syndrome of reduced tissue responsiveness to thyroid hormone. To date, all individuals expressing the RTH phenotype have been found to harbor mutations in the thyroid hormone receptor beta (TR beta) gene that impair T3-mediated function. We describe a unique family in which the dominantly inherited RTH is not associated with abnormalities in the TR beta or TR alpha genes, as determined by gene sequencing and linkage analysis. However, affected family members manifest a severe form of RTH, with reduced responses of thyrotrophs and peripheral tissues requiring 8- to 10-fold the normal replacement doses of L-T4 and L-T3. No other endocrine abnormalities were detected. The defect developed de novo in the proposita and was transmitted to her two children of unrelated fathers. As cultured fibroblasts from the proposita responded poorly to T3 despite a normal concentration of TR, other abnormalities in the mediation of T3 action were sought. Nucleotide sequences of the TSH beta promoter, containing thyroid hormone response elements, and TR-interacting protein 1 were normal. Nuclear extracts (NE) of cultured skin fibroblasts from affected individuals of this family were tested for their interaction with normal TR beta and thyroid hormone response elements by the electrophoretic mobility shift assay. NE from the proposita showed a strong additional band compared to NEs from normal individuals and patients with RTH caused by TR beta mutations or deletion. Far Western analysis of NE from the affected daughter hybridized with labeled TR beta demonstrated an additional band that was not seen in NEs from a normal control or patients with TR beta gene defects. It is concluded that the etiology of RTH is not confined to abnormalities in the TR beta gene. An abnormal cofactor with a specific function in the regulation of thyroid hormone action is probably involved in the expression of the RTH phenotype in this family.

Amino acid substitutions of thyroid hormone receptor-beta at codon 435 with resistance to thyroid hormone selectively alter homodimer formation

Thyroid hormone action is mediated through its nuclear receptors (TRs), which bind to target DNA sequences [thyroid hormone response element (TRE)] as a homodimer or a heterodimer with 9-cis-retinoic acid receptors. Mutations of TR beta identified in patients with resistance to thyroid hormone (RTH) cluster primarily at two areas separated by the putative dimerization region. Two TR beta mutations were newly found in patients with RTH at codon 435 histidine (H435L and H435Q) close to the dimerization region. Recent crystallographic study suggested that H435 is critical for direct contact with T3. To study how the side-chain charge of amino acids at this position affects receptor characteristics, T3-binding activity, receptor dimerization, transcriptional activity, and dominant negative action were analyzed in two RTH mutants and two additional artificial mutants (H435R and H435E). The T3 binding affinities of all four mutants were below detection. In electrophoretic mobility shift assay using TRE-DR4 or the inverted palindrome (Lap), heterodimer formation of mutant receptors with 9-cis-retinoic acid receptor was similar to that of wild type receptors. However, homodimer formation varied among mutant receptors, especially using TRE-DR4, with a rank order of wild type = H435R > H435Q > H435L > > H435E. In the presence of a basic amino acid at codon 435, homodimer formation was preserved, whereas substitution to neutral or acidic amino acids resulted in decreased homodimer formation. In transient transfection assays using reporter genes under the control of 2xPal-thymidine kinase (TK), DR4-TK, Lap-TK, or TSH alpha promoter, these four mutants were inactive in T3-dependent transcriptional activation. Dominant negative inhibition was similar for all four mutants. These results indicate that 1) newly found TR beta mutations at codon 435 are responsible for RTH; and 2) codon 435 in TR beta is located at a position that can predominantly alter homodimer formation on certain TREs, such as DR4.

Heterodimerization preferences of thyroid hormone receptor alpha isoforms

Thyroid hormone receptors (TRs) are members of the steroid hormone receptor superfamily and are encoded by two different genes, alpha and beta. Three isoforms (alpha 1, alpha 2, and alpha 3) are created by alternative splicing of the TR alpha gene. In TR alpha 2 and alpha 3, the distal half of the putative dimerization domain is disrupted and the carboxy terminus of the protein is substituted with different amino acids. To evaluate the properties of these alterations in the dimerization region, DNA binding and dimerization of TR alpha isoforms were studied by electrophoretic mobility shift assays. TR alpha 1 formed a monomer or a homodimer on certain thyroid hormone responsive elements (TREs), whereas TR alpha 2 and alpha 3 did not bind effectively to any of the TREs studied. TR alpha 1 formed a heterodimer with 9-cis retinoic acid receptor alpha (RXR alpha) on all TREs studied. Although TR alpha 2 did not bind as a homodimer, it did bind as a heterodimer with RXR alpha to DR4 and MHC-TRE. TR alpha 3 bound as a heterodimer to a broader repertoire of TREs, including DR4, MHC, ME, and F2-TRE. These results indicate that the alterations in the dimerization region in TR alpha 2 and alpha 3 abrogated homodimer binding, but differentially affected heterodimerization with RXR alpha on various TREs.

Modification of cultured Madin-Darby canine kidney cells with dietary unsaturated fatty acids and regulation of arachidonate cascade reaction

Madin-Darby canine kidney (MDCK) cells were modified with dietary unsaturated fatty acids. The effects on the fatty acid composition in each phospholipid class and the formation of prostanoids upon stimulation were studied, from which the specificity of metabolism of individual unsaturated fatty acids and the regulation of arachidonate cascades in the modified cells were discussed. C18 unsaturated fatty acids were preferentially incorporated into phosphatidylcholine (PC) over phosphatidylethanolamine (PE), but arachidonic acid (20:4(n-6)) derived from gamma-linolenic acid (18:3(n-6)) was much more predominant in PE than PC. The fatty acid level in PE ranged from about 26-28% when the cells were modified with 20:4(n-6) or 5,8,11,14,17-eicosapentaenoic acid (20:5(n-3)), indicating the limitation of the storage of the eicosapolyenoic acids. The extra amounts appeared to be stored in PC. 18:3(n-6) was comparable to 20:4(n-6) to raise the level of 20:4(n-6) in PE, but not in PC which had half of 20:4(n-6) in PE. The supplementation of linoleic acid (18:2(n-6)). 18:3(n-6), and 20:4(n-6) caused significant increases in the synthesis of prostaglandin (PG)E2 up to almost the same levels when the modified cells were stimulated with 50 nM PMA and 100 nM A23187 for 24h. The cultured cells modified with eicosapolyenoic acids including 20:3(n-6), 20:4(n-6), and 20:5(n-3) were found to be inhibitory for the induction of PGE2 synthetic activity involving de nova synthesis of PG endoperoxide synthase, suggesting negative feedback regulation of the modified cells.

Second zinc finger mutants of thyroid hormone receptor selectively preserve DNA binding and heterodimerization but eliminate transcriptional activation

Transcriptional regulation by thyroid hormone is mediated through its nuclear receptors (TRs), which bind to target responsive elements as homodimers or as heterodimers with 9-cis retinoic acid receptors (RXRs). We examined the dimerization and functional properties of TRs containing mutations in the first and second zinc finger regions of the DNA binding domain. Interestingly, a mutation (R158G) in the loop of second zinc finger, or a chimeric mutant in which the second zinc finger of the glucocorticoid receptor (GR) was substituted for that of the TR, did not form homodimer, but still bound as a heterodimer with RXR alpha. Despite the presence of heterodimer formation, these mutants were functionally inactive in transfection assays. We conclude that sequences within the loop of the second zinc finger may play an important role in stability in vivo or transcriptional activation of the TR.

Antioxidants inhibit tumor necrosis factor-alpha mediated stimulation of interleukin-8, monocyte chemoattractant protein-1, and collagenase expression in cultured human synovial cells

OBJECTIVE

To study whether the induction of mRNA for interleukin-8 (IL-8), monocyte chemoattractant protein-1 (MCP-1), and collagenase by tumor necrosis factor-alpha (TNF-alpha) is suppressed by antioxidants in human synovial cells. TNF-alpha has been shown to exert some of its effects by stimulating production of reactive oxygen intermediates in some cell lines other than synovial cells.

METHODS

Amounts of mRNA for IL-8, MCP-1, and collagenase were determined by Northern blot analysis. Electrophoretic mobility shift assays were performed for the detection of a transcription factor, nuclear factor-kappa B(NF-kappa B). The concentration of IL-8 in the medium was determined by ELISA.

RESULTS

TNF-alpha increased the expression of IL-8, MCP-1, and collagenase mRNA in human synovial cells. NF-KB known to induce IL-8 gene transcription was also increased in nuclear extracts from the synovial cells treated with TNF-alpha. Prior addition of antioxidant, N-acetyl-L-cysteine (NAC) or 2-oxothiazolidine-4-carboxylate (OTC), suppressed TNF-alpha stimulated expressions of IL-8, MCP-1, and collagenase mRNA in a dose dependent manner. Treatment with NAC also suppressed TNF-alpha induced increase in NF-kappa B. The changes of IL-8 in the medium reflected the mRNA levels. Hydrogen peroxide (H2O2) induced the expression of mRNA for the cytokines but not collagenase mRNA, and NAC suppressed the effect of H2O2.

CONCLUSION

Our data suggest that TNF-alpha induces expression of proinflammatory cytokines such as IL-8 and MCP-1 through generation of reactive oxygen intermediates and subsequent activation of NF-kappa B in human synovial cells, and the antioxidants may inhibit, at least in part, the activation of NF-kappa B by TNF-alpha. These results indicate that antioxidants such as NAC may be useful in treating rheumatoid arthritis.