Selective antagonism of thyroid hormone action by retinoic acid

Polychlorinated biphenyls disturb differentiation of normal human neural progenitor cells: clue for involvement of thyroid hormone receptors

Polychlorinated biphenyls (PCBs) are ubiquitous environmental chemicals that accumulate in adipose tissues over the food chain. Epidemiologic studies have indicated that PCBs influence brain development. Children who are exposed to PCBs during development suffer from neuropsychologic deficits such as a lower full-scale IQ (intelligence quotient), reduced visual recognition memory, and attention and motor deficits. The mechanisms leading to these effects are not fully understood. It has been speculated that PCBs may affect brain development by interfering with thyroid hormone (TH) signaling. Because most of the data are from animal studies, we established a model using primary normal human neural progenitor (NHNP) cells to determine if PCBs interfere with TH-dependent neural differentiation. NHNP cells differentiate into neurons, astrocytes, and oligodendrocytes in culture, and they express a variety of drug metabolism enzymes and nuclear receptors. Like triiodothyronine (T3), treatment with the mono-ortho-substituted PCB-118 (2,3',4,4 ,5-pentachlorobiphenyl; 0.01-1 microM) leads to a dose-dependent increase of oligodendrocyte formation. This effect was congener specific, because the coplanar PCB-126 (3,3',4,4 ,5-pentachlorobiphenyl) had no effect. Similar to the T3 response, the PCB-mediated effect on oligodendrocyte formation was blocked by retinoic acid and the thyroid hormone receptor antagonist NH-3. These results suggest that PCB-118 mimics T3 action via the TH pathway.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Differential expression of thyroid hormone receptor isoforms by thyroid hormone and lithium in rat GH3 and B103 cells

BACKGROUND

The interaction between lithium, a mood stabilizer, and the thyroid axis has been extensively studied; however, the regulation of thyroid hormone receptors by lithium is yet to be investigated.

METHODS

To test whether lithium affects thyroid hormones at the receptor level, we examined the effects of lithium in combination with triiodothyronine (T3) on gene expression of thyroid hormone receptor isoforms in GH3 and B103 cells.

RESULTS

The pattern of expression as well as the magnitude of regulation of the different thyroid hormone receptor isoforms appeared to be cell line specific. Whereas T3 regulated all four isoforms in GH3 cells at both time points, T3 did not alter thyroid hormone receptor TR alpha 1 and TR alpha 2 mRNA in B103 cells. Addition of lithium to thyroid hormone-deficient GH3 cells decreased TR alpha 1, alpha 2, and beta 2 expression without affecting TR beta 1 expression at 2 but not 5 days. Addition of lithium to T3-treated GH3 cells did not further modulate gene expression of TR alpha 1, alpha 2, beta 1, or beta 2 when compared to cells treated with T3 alone. The effects of lithium in B103 cells appeared to be isoform specific as well as time dependent, since TR alpha 1 expression was selectively decreased in B103 cells, when treated with T3 in the presence of lithium.

CONCLUSIONS

The present study provides direct evidence that T3 and/or lithium regulate TR gene expression in vitro in a both time-dependent and cell line-specific manner.

Characterization of TSH-positive cells in foetal rat pars tuberalis that fail to express Pit-1 factor and thyroid hormone beta2 receptors

The thyroid stimulating hormone (TSH)-immunoreactive cells (TSH cells) in the pars tuberalis (PT-TSH cell) of the male rat pituitary gland show an intense spot-like TSH immunoreaction in the paranuclear cytoplasm. However, the ontogenic origin and characteristics of these spot-like stained PT-TSH cells remain to be elucidated. The present study was designed to investigate the distribution and characteristics of PT-TSH cells in the foetal and adult rat pituitary by immunostaining for Pit-1 factor and thyroid hormone receptors (TRs) and reverse transcriptase-polymerase chain reaction (RT-PCR). TSH cells first appeared in the PT at 15.5 days of gestation and were either stained diffusely throughout the cytoplasm or displayed a strongly stained, spotty appearance in the paranuclear region. By 15.5 days of gestation, the rostral part of the PT consisted of columnar epithelium, in which TSH immunoreactivity was spot-like in the apical region of cytoplasm corresponding to the Golgi apparatus. At the 16.5 days of gestation, TSH cells were present in the pars distalis (PD); however, the cells were mostly larger and polygonal with strong staining throughout the cytoplasm. These differences between the PT and PD were retained throughout foetal and neonatal rat development. The TSH cells in the PD of the adult or gestational rat were observed to contain Pit-1 factor by double immunostaining. However, TSH cells in the PT lacked Pit-1 factor. RT-PCR confirmed the absence of Pit-1 and TRbeta2 mRNA in the PT of the adult and late gestation rat pituitary gland. These results suggest that apparently distinct types of TSH cells in the PT develop independently from TSH cells in the PD.

Thyroid hormone decreases the number of adrenergic cells that develop in neural crest cultures and can inhibit the stimulatory action of retinoic acid

Environmental cues are known to be important in the migration, survival, and differentiation of neural crest cells and their derivatives. Retinoic acid (RA) can increase the number of adrenergic cells that develop in neural crest cultures in a dose dependent manner. These results with RA prompted us to investigate the effects of other retinoids and other related compounds on neural crest cultures. We have investigated the role of thyroid hormone (T3) in the development of adrenergic cells in quail neural crest cultures. T3 produced a significant decrease in the number of catecholamine-positive cells that developed in neural crest cultures after 7 days in vitro, as compared to untreated controls. The decrease in adrenergic cells produced by T3 was paralleled by a decrease in the number of tyrosine hydroxylase-positive cells, but T3 did not reduce either total or melanocyte cell number. Cultures were sensitive to T3 during the first 5 days in culture and T3 was not cytotoxic to adrenergic cells. The decrease in adrenergic cells seen with T3 was partially reversed by RA suggesting that these two compounds may be working through a common pathway.

Regulation of corticosteroid-binding globulin synthesis by 1alpha,25-dihyroxy-vitamin D3 (calcitriol), 9-cis-retinoic acid and triiodothyronine in cultured rat fetal hepatocytes

Evidence regarding the nature of the regulatory factors which directly act upon liver cells and extra-hepatic tissues to alter CBG synthesis is scarce. The present study used cultured rat fetal hepatocytes to investigate the involvement and possible interplay in this process of several members of the nuclear receptors superfamily: vitamin D (VDR), retinoic acids (RAR/RXR) and thyroid hormones (TR). Treatment of cells with 1alpha,25-(OH)2D3 (1,25-D) elicited a dose-dependent inhibition of basal CBG concentration in culture medium. Maximum inhibition to about 15% of control level was achieved with 0.1-1.0 nM, with an IC50 of 3.8 x 10(-12) M and with no significant change in binding affinity. Differential activation of RAR and RXR with either 9-cis-retinoic acid (9-cis-RA) or the RAR-selective synthetic retinoid TTNPB revealed that high doses of both drugs diminished CBG expression, though the former proved about 10-times more potent than the latter in this regard. Amplification by triiodothyronine (T3) of CBG synthesis failed to block the inhibitory effects of either 1,25-D or retinoids, as revealed by both binding capacity and mRNA measurements. Relative to CBG, 1,25-D similarly depressed the synthesis of alpha-fetoprotein (AFP), while on the contrary, retinoids and T3 were shown to cause opposite effects, as 9-cis-RA and TTNPB elevated and T3 decreased AFP expression. The present findings identify for the first time ligands of VDR and RAR/RXR as powerful negative regulators of both basal and T3-stimulated CBG biosynthesis in fetal hepatocytes and suggest lack of a functional interplay between TR and VR or RAR/RXR in these processes.

Tissue- and cell-specific distribution of proteins that interact with the human thyroid hormone receptor-beta

Variable tissue effects of thyroid hormones depend on many factors that could include tissue-specific proteins that interact with nuclear thyroid hormone receptors. To study this possibility, direct interactions between the thyroid hormone receptor beta and other nuclear proteins were examined by far western blotting assays. Purified hTR beta bound to human retinoid X receptor alpha and human general transcription factor IIB (TFIIB) immobilized on a membrane and it also bound to several different hTR beta binding proteins (TRBPs) in nuclear extracts from rat brain, kidney, liver, GH3 cells, and HeLa cells. Each tissue or cell line showed unique distributions of TRBPs and some TRBPs were regulated by thyroid hormone in vivo. These studies show that multiple proteins interact with the hTR beta and they could be important in regulating TR function.

The rat growth hormone proximal silencer contains a novel DNA-binding site for multiple nuclear proteins that represses basal promoter activity

Cell-type-specific expression of the rat growth hormone (rGH) gene is determined by the interaction of both positive as well as negative regulatory proteins with cis-acting elements located upstream of the rGH mRNA start site. We have recently shown that the rat liver transcription factor NF1-L binds to the proximal rGH silencer (called silencer-1) to repress its transcriptional activity. However, this single factor proved to be insufficient by itself to confer cell-specific gene repression. We therefore attempted to identify other regulatory proteins interacting with silencer 1, which might be needed to achieve full cell-specific repression of that gene. A common recognition site for three yet uncharacterized nuclear proteins (designated as SBP1, SBP2 and SBP3) which bind a DNA sequence adjacent to the NF1-L-binding site in the rGH silencer-1 element were identified. UV crosslinking of DNA/protein complexes and nuclear protein fractionation/renaturation from SDS/polyacrylamide gels further indicated that the molecular masses for SBP1-3 are 41, 26 and 17 kDa respectively, the major species being the 26-kDa protein (SBP2) which account for 83% of the shifted SBP double-stranded oligonucleotide in gel mobility-shift assays. For this reason, most of this study focussed on the characterization of SBP2. We demonstrated that binding of NF1-L and SBP2 to their respective recognition sequence is a mutually exclusive event. Although an SBP-binding activity has been found in every non-pituitary tissue or cell line tested, no such activity could be detected in either rat pituitaries or rat pituitary GH4C1 cells. Insertion of the SBP element upstream of the basal promoter of the mouse p12 heterologous gene resulted in a consistent decrease in chloramphenicol acetyl transferase reporter gene expression following transient transfections in non-pituitary cells only, suggesting that the related SBP1-3 proteins might be involved in generally repressing gene transcription in a cell-specific manner.

3,5,3'-Triiodothyronine stimulates retinoic acid-induced differentiation in HL-60 cells

The human acute promyelocytic leukemia (APL) cell line HL-60 differentiates to functionally mature granulocytes by incubation with all-trans-retinoic acid (RA). Since T3 and RA are important in cell differentiation and development, and since their receptors are highly homological, we investigated the T3 effects on RA-induced HL-60 cell differentiation. Although T3 alone did not induce cell differentiation, RA-mediated differentiation was significantly enhanced in the presence of 10(-7) M T3. This effect of T3 was considered to be mediated, at least in part, by increased intracellular cAMP, since the phosphodiesterase inhibitor enhanced, and the protein kinase A antagonist partially blocked, T3 potentiation. When HL-60 cells were pretreated with RA for 20 h, T3 alone stimulated the cell differentiation. The time-course study showed that incubation with RA for 12 h was necessary for HL-60 cells to be primed to respond to T3 for differentiation. The present finding that T3 potentiates RA-induced HL-60 cell differentiation may raise the possibility that T3 supplement increases clinical remission in APL patients who are treated with RA.

Induction of type III-deiodinase activity in astroglial cells by retinoids

Thyroid hormones and retinoic acid (RA) are important modulators of growth, development, and differentiation. Type III deiodinase (D-III), which catalyzes thyroid hormones degradation in the brain and in cultured astroglial cells, is induced in astroglial cells by multiple pathways, including cAMP, 12.0-tetradecanoylphorbol-13-acetate (TPA), fibroblast growth factors, and thyroid hormones themselves. In the present study, the effects of retinoids on D-III activity were examined in astroglial cells cultures in a chemically defined medium devoid of hormones and growth factors. Incubation of astroglial cells with 5 microM all-trans-RA caused up to 200-fold increase in D-III activity, which reached a plateau after 48 h. The retinoid-induced increase in D-III activity was concentration dependent (0.5 microM all-trans-RA and 9-cis-RA producing half-maximal effect). Retinol was effective at physiological concentrations (1 and 10 microM). The 48 h effects of 5 microM all-trans-RA and 10 nM thyroid hormones on D-III activity were at least additive. Addition of 2 nM acidic fibroblast growth factor or 1 mM 8-bromo-cAMP for the last 8 h of a 48 h incubation with 5 microM all-trans-RA did not alter the induction by all-trans-RA, whereas 0.1 microM TPA in the same conditions produced an additive effect with all-trans-RA. All-trans-RA (5 microM) had little or no effect on type II deiodinase, the enzyme which catalyzes the activation of thyroxine to 3,5,3'-triiodothyronine.(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonistic effects of retinoic acid and triiodothyronine in the expression of corticoid-binding globulin (CBG) by cultured fetal hepatocytes

Cultures of rat fetal hepatocytes were used to investigate the effects and interplay of triiodothyronine (T3) and retinoic acid (RA) in the regulation of gene expression of CBG, compared to that of alpha-fetoprotein (AFP). The cultured cells showed cytological features typical to hepatocytes and actually synthesized CBG and AFP, as evidenced from in situ hybridization with specific radioactive probes. Time course studies indicated that CBG (but not AFP) binding capacity in culture medium and cell mRNA levels disappeared with a half-life of about 2 days, thereby reflecting the decrease previously seen in hepatic CBG mRNA contents during embryonic life. The Kd values for CBG binding were unchanged under these conditions. Culturing of hepatocytes in the presence of T3 resulted in dose-dependent stimulations of both medium CBG and cell mRNA levels, with an EC50 concentration of about 10(-9) M. In sharp contrast, RA caused a reduction in CBG biosynthesis (IC50 = 1.7 x 10(-7) M) and, in addition, antagonized the stimulatory influence of T3. Under the same experimental conditions, AFP synthesis failed to be affected in a similar fashion. We conclude that thyroid hormones and RA directly act on hepatocytes to specifically regulate the expression of CBG in an antagonistic way.

Steroid-induced inhibition of adenosine transport in cultured chromaffin cells

1. Adenosine transport is subjected to regulation by hormones. Glucocorticoids, sexual steroids, and retinoic acid inhibit adenosine transport in chromaffin cells after a long-term incubation period (24 hr). No effects were observed after a short-term incubation period (10 min). 2. The kinetic parameters of transporters were studied. No significant changes were observed for the affinity constant (Km), whose value remains at 1 +/- 0.2 microM after 24-hr incubation in the presence of these compounds. The maximal velocity (Vmax) was significantly modified, with a decrease of about 20% in all cases. 3. NBTI binding was not modified in its affinity constant or maximal bound capacity (Bmax) by the presence of these compounds for a 24-hr incubation period. Thus the efficiency of transporters (quotient Vmax/Bmax) changed from 10.9 +/- 0.08 adenosine molecules transported per transporter per sec in the control cells to 9.1 +/- 0.07 in hormone-treated cultured cells. 4. The thyroid hormone (T3) significantly increased adenosine transport in a long-term incubation period in chromaffin cells (24 hr). This activatory effect is antagonized by steroid hormones and retinoic acid.

Regulation of thyroid hormone receptor beta-2 mRNA levels by retinoic acid

The thyroid hormone receptor, TR beta-2, whose expression is limited to the pituitary and parts of the central nervous system, is strongly negatively regulated at the pre-translational level by thyroid hormone (T3). We have investigated whether retinoic acid (RA), whose receptors (RARs) share a high degree of homology with the thyroid hormone receptors (TRs), can regulate this gene in a manner similar to T3, as has been shown for the growth hormone (GH) gene. GH3 cells were incubated with 10 nM T3, 1 microM RA or both for 48 h and then TR beta-2 mRNA levels determined by RNA blot hybridization analysis. We observed a 73% decrease in TR beta-2 mRNA levels after incubation with T3 and a two-fold increase in TR beta-2 mRNA levels after incubation with RA alone. In the presence of RA, the T3 effect on TR beta-2 mRNA levels was blunted with mRNA levels decreasing by only 20%. We investigated the mechanism by which retinoic acid increases and opposes the effects of T3 on levels of TR beta-2 mRNA. In transient transfection experiments using a reporter plasmid containing the TR beta-2 promoter and in nuclear run on assays, we found no effect of RA on TR beta-2 gene transcription. We then investigated whether the effects of RA were mediated at the post-transcriptional level. Determination of the apparent half-life of TR beta-2 mRNA using the transcriptional inhibitor, actinomycin D, showed that RA had no effect on TR beta-2 mRNA stability.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinoic acid decreases retinoic acid and triiodothyronine nuclear receptor expression in the liver of hyperthyroidic rats

Retinoic acid (RA) and triiodothyronine (T3) exert many of their actions by binding to specific nuclear receptors (respectively, RA receptor (RAR) and T3) receptor (TR) belonging to a 'superfamily' of receptors. Some heterologous regulation of these receptors has been shown, and in particular regulation of the maximum binding capacity of TR by either retinol or RA. Now, using hyperthyroidic rats as a model, the effect of RA on binding capacity and on the mRNA levels of TR and RAR was investigated. The results show that the benefit of vitamin A treatment for the hyperthyroidic state, which has been described for a long time, could be the result of a down-heteroregulation of TR by RA, the active metabolite of retinol.