Thyroid hormone receptor, v-ErbA, and chromatin

The thyroid hormone receptor and the highly related viral oncoprotein v-erbA are found exclusively in the nucleus as stable constituents of chromatin. Unlike most transcriptional regulators, the thyroid hormone receptor binds with comparable affinity to naked and nucleosomal DNA. In vitro reconstitution experiments and in vivo genomic footprinting have delineated the chromatin structural features that facilitate association with the receptor. Chromatin bound thyroid hormone receptor and v-erbA generate Dnase I hypersensitive sites independent of ligand. The unliganded thyroid hormone receptor and v-erbA associate with a corepressor complex containing NCoR, SIN3, and histone deacetylase. The enzymatic activity of the deacetylase and a chromatin environment are essential for the dominant repression of transcription by both the unliganded thyroid hormone receptor and v-erbA. In the presence of ligand, the thyroid hormone receptor undergoes a conformational change that weakens interactions with the corepressor complex while facilitating the recruitment of transcriptional coactivators such as p300 and PCAF possessing histone acetyltransferase activity. The ligand-bound thyroid hormone receptor directs chromatin disruption events in addition to histone acetylation. Thus, the thyroid hormone receptor and v-erbA make very effective use of their stable association with chromatin and their capacity to alter the chromatin environment as a major component of the transcription regulation process. This system provides an exceptionally useful paradigm for investigating the structural and functional consequences of targeted chromatin modification.

Differential regulation of three thyroid hormone-responsive matrix metalloproteinase genes implicates distinct functions during frog embryogenesis

Matrix metalloproteinases (MMPs) are a family of Zn(2+)-dependent extracellular proteases capable of degrading various proteinaceous components of the extracellular matrix (ECM). They are expressed in developmental and pathological processes such as postlactation mammary gland involution and tumor metastasis. Relatively few studies have been carried out to investigate the function of MMPs during embryogenesis and postembryonic organ development. Using Xenopus development as a model system, we and others have previously isolated three MMP genes as thyroid hormone response genes. They have distinct temporal and organ-specific regulations during thyroid hormone-dependent metamorphosis. We demonstrate here that three MMPs-stromelysin-3 (ST3), collagenases-3 (Col3), and collagenases-4 (Col4)-also have distinct spatial and temporal expression profiles during embryogenesis. Consistent with earlier suggestions that ST3 is a direct thyroid hormone response gene whereas Col3 and Col4 are not, we show that precocious overexpression of thyroid hormone receptors in the presence of thyroid hormone lead to increased expression of ST3, but not Col3. Furthermore, our whole-mount in situ hybridizations reveal a tight but distinct association of individual MMPs with tissue remodeling in different regions of the animal during embryogenesis. These results suggest that ST3 is likely to play a role in ECM remodeling that facilitate apoptotic tissue remodeling or resorption, whereas Col3 and Col4 appear to participate in connective tissue degradation during development.

p300 requires its histone acetyltransferase activity and SRC-1 interaction domain to facilitate thyroid hormone receptor activation in chromatin

We have characterized the mechanism by which coactivator p300 facilitates transcriptional activation mediated by the heterodimer of thyroid hormone (T3) receptor and 9-cis retinoid acid receptor (TR-RXR) in the context of chromatin. We demonstrate that, while p300 can enhance the transcriptional activation mediated by both liganded TR-RXR and GAL4-VP16, its histone acetyltransferase activity (HAT) is required for its ability to facilitate liganded TR-RXR- but not GAL4-VP16-mediated transcriptional activation. To understand how p300 is recruited by liganded TR-RXR, we have analyzed the interactions between TR-RXR and p300 as well as SRC-1 family coactivators. We show that, in contrast to a strong hormone-dependent interaction between TR-RXR and SRC-1 family coactivators, p300 displays minimal, if any, T3-dependent interaction with TR-RXR. However, p300 can be recruited by liganded TR-RXR through its interaction with SRC-1 family coactivators. Consistent with the protein-protein interaction profile described above, we demonstrate that the SRC-1 interaction domain of p300 is important for its ability to facilitate transcriptional activation mediated by TR-RXR, whereas its nuclear receptor interaction domain is dispensable. Collectively, these results reveal the functional significance of the HAT activity of p300 and define an indirect mode for the action of p300 in TR-RXR activation.

Isometric force and endurance in soleus muscle of thyroid hormone receptor-alpha(1)- or -beta-deficient mice

The specific role of each subtype of thyroid hormone receptor (TR) on skeletal muscle function is unclear. We have therefore studied kinetics of isometric twitches and tetani as well as fatigue resistance in isolated soleus muscles of R-alpha(1)- or -beta-deficient mice. The results show 20-40% longer contraction and relaxation times of twitches and tetani in soleus muscles from TR-alpha(1)-deficient mice compared with their wild-type controls. TR-beta-deficient mice, which have high thyroid hormone levels, were less fatigue resistant than their wild-type controls, but contraction and relaxation times were not different. Western blot analyses showed a reduced concentration of the fast-type sarcoplasmic reticulum Ca(2+)-ATPase (SERCa1) in TR-alpha(1)-deficient mice, but no changes were observed in TR-beta-deficient mice compared with their respective controls. We conclude that in skeletal muscle, both TR-alpha(1) and TR-beta are required to get a normal thyroid hormone response.

An element in the region responsible for premature termination of transcription mediates repression of c-myc gene expression by thyroid hormone in neuroblastoma cells

The thyroid hormone (T3) blocks proliferation and induces differentiation of neuroblastoma N2a-beta cells that express the thyroid hormone receptor (TR) beta1 isoform. c-Myc is required for cell cycle progression, and this study shows that T3-induced neuronal differentiation is preceded by a rapid decrease of c-myc gene expression. A negative T3 responsive element (TRE), arranged as an inverted palindrome spaced by three nucleotides, has been identified within the first exon between nucleotides +237 and +268. The TRE is adjacent to the binding site for the transcriptional repressor CCCTC binding factor and maps precisely within the region of RNA polymerase II pausing and release, suggesting a direct implication of TR on premature termination of transcription. Furthermore, the TRE confers repression by T3 to an heterologous promoter only when inserted downstream of the transcription initiation site. Binding of CCCTC binding factor and TR to their cognate sites in the region of transcriptional attenuation, as well as direct interactions between both factors, could facilitate the formation of a repressor complex and the inhibition of c-myc gene expression. These studies provide insight into mechanisms by which TR mediate transcriptional repression and contribute to the understanding of the important effects of thyroid hormones on growth and differentiation of neuronal cells.

Thyroid hormone suppression of pituitary hormone gene expression

T3 suppression of TSH subunit gene transcription is an important step in maintaining thyroid hormone homeostasis, and recent investigations have increased our understanding of this process. Thyrotrope-specific proteins play a critical role in TSH subunit gene expression, and influence T3-mediated regulatory mechanisms. The structure and placement of the TSH gene TREs define suppressive regulation by T3, and this process is favored by the TR isoforms expressed in the pituitary. Elimination of TR beta function compromises the pituitary response to T3. TR beta 2, the isoform specifically expressed in pituitary and neural tissue, contains a transferable domain that both increases T3-independent gene transcription and enhances T3-suppressed transcription. The functional interaction of TR beta 2 with other regulatory proteins is distinct from that of other TR isoforms, and likely plays a critical role in pituitary physiology and in pituitary resistance to thyroid hormone. The development of novel thyrotrope cell lines will allow investigators to define new proteins and molecular mechanisms that distinguish negative from positive T3 transcriptional regulation.

Effects of T3R alpha 1 and T3R alpha 2 gene deletion on T and B lymphocyte development

Thyroid hormones bind to several nuclear receptors encoded by T3R alpha and T3R beta genes. There is now accumulating evidence that thyroid hormones act on the immune system. Indeed, mice deficient for thyroid hormones show a reduction in lymphocyte production. However, the mechanisms involved and, in particular, the role of the different thyroid hormone receptors in lymphocyte development have not been investigated. To address that question, we have studied lymphocyte development in mice deficient for the T3R alpha 1 and T3R alpha 2 gene products. A strong decrease in spleen cell numbers was found compared with wild-type littermates, B lymphocytes being more severely affected than T lymphocytes. A significant decrease in splenic macrophage and granulocyte numbers was also found. In bone marrow, a reduction in CD45+/IgM- pro/pre-B cell numbers was found in these mice compared with wild-type littermates. This decrease seems to result from a proliferation defect, as CD45+/IgM- cells incorporate less 5-bromo-2'-deoxyuridine in vivo. To define the origin of the bone marrow development defect, chimeric animals between T3R alpha-/- and Rag1-/- mice were generated. Results indicate that for B cells the control of the population size by T3R alpha 1 and T3R alpha 2 is intrinsic. Altogether, these results show that T3R alpha 1 or T3R alpha 2 gene products are implicated in the control of the B cell pool size.

Tissue-specific actions of thyroid hormone: insights from animal models

The major developmental targets for thyroid hormone are the brain, small intestine, and bone. Clear defects in gene regulation and tissue function as a consequence of TR gene inactivation can additionally be shown in the pituitary, hypothalamus, heart, and liver. TR gene knockout models show a clear distinction between thyroid hormone requirements for development and those that are required for functions in the adult animal. T3-mediated gene repression appears especially important in a number of tissues including brain, pituitary, and the heart. Preliminary evaluation of the combined TR knockout models suggests that hypothyroidism is associated with more significant abnormalities than receptor deficiency, indicating that the repressive action of the unliganded receptor may have physiological relevance. These various animal models should be very useful to design and test thyroid hormone analogues to selectively stimulate desired thyroid hormone actions.

Developmental exposure to polychlorinated biphenyls exerts thyroid hormone-like effects on the expression of RC3/neurogranin and myelin basic protein messenger ribonucleic acids in the developing rat brain

Polychlorinated biphenyls (PCBs) are a class of industrial compounds consisting of paired phenyl rings with various degrees of chlorination. They are now ubiquitous, persistent environmental contaminants that are routinely found in samples of human and animal tissues and are known to affect brain development. The effects of PCBs on brain development may be attributable, at least in part, to their ability to reduce circulating levels of thyroid hormone. However, the developmental effects of PCB exposure are not fully consistent with hypothyroidism. Because some individual PCB congeners interact strongly with various thyroid hormone binding proteins, several investigators have speculated that these congeners may be producing thyroid hormone-like effects on brain development. Therefore, we tested whether a mixture of PCBs, Aroclor 1254 (A1254), would produce an antithyroid or thyromimetic effect on the expression of known thyroid hormone-responsive genes in the developing brain. Pregnant female rats were fed various doses of A1254 (0, 1, 4, and 8 mg/kg) from gestational day 6 to weaning on postnatal day (P) 21. Pups derived from these dams were sampled on P5, P15, and P30. Total T4 was reduced by A1254 in a dose-dependent manner, but body weight of the pups or dams was not affected. The expression of RC3/Neurogranin and myelin basic protein was not affected by A1254 on P5 or P30. However, on P15, RC3/Neurogranin was elevated by A1254 in a dose-dependent manner, and myelin basic protein expression followed this general pattern. These data clearly demonstrate that the developmental effects of PCB exposure are not simply a function of PCB-induced hypothyroidism.

Transcriptional anti-repression. Thyroid hormone receptor beta-2 recruits SMRT corepressor but interferes with subsequent assembly of a functional corepressor complex

Thyroid hormone receptors (T3Rs) are hormone-regulated transcription factors. Different T3R isoforms are expressed in a tissue-specific and developmentally regulated manner. The T3Ralpha-1, beta-0, and beta-1 isoforms typically repress target gene expression in the absence of hormone and activate transcription in the presence of hormone. Intriguingly, however, the T3Rbeta-2 isoform fails to repress, and instead is able to activate transcription in both the absence and presence of hormone. We investigated the molecular mechanism behind this absence of repression by T3Rbeta-2. Repression by T3Ralpha-1, beta-0, and beta-1 is mediated by the ability of these isoforms to physically recruit a SMRT/N-CoR corepressor complex. We determined that the unliganded T3Rbeta-2 also recruits the SMRT corepressor; in contrast to the alpha-1, beta-0, and beta-1 isoforms, however, the T3Rbeta-2 protein interacts not only with the C-terminal "receptor-interaction domain" of SMRT, but also makes additional contacts with the N-terminal "silencing domain" of the SMRT corepressor. These additional, T3Rbeta-2-specific contacts interfere with the subsequent association of SMRT with mSin3, a crucial second subunit of the corepressor holo-complex. Our results suggest that T3Rbeta-2 regulates transcription through a novel anti-repression mechanism, recruiting SMRT, but preventing the subsequent formation of a functional corepressor complex.

A variant form of the nuclear triiodothyronine receptor c-ErbAalpha1 plays a direct role in regulation of mitochondrial RNA synthesis

In earlier research, we identified a 43-kDa c-ErbAalpha1 protein (p43) in the mitochondrial matrix of rat liver. In the present work, binding experiments indicate that p43 displays an affinity for triiodothyronine (T3) similar to that of the T3 nuclear receptor. Using in organello import experiments, we found that p43 is targeted to the organelle by an unusual process similar to that previously reported for MTF1, a yeast mitochondrial transcription factor. DNA-binding experiments demonstrated that p43 specifically binds to four mitochondrial DNA sequences with a high similarity to nuclear T3 response elements (mt-T3REs). Using in organello transcription experiments, we observed that p43 increases the levels of both precursor and mature mitochondrial transcripts and the ratio of mRNA to rRNA in a T3-dependent manner. These events lead to stimulation of mitochondrial protein synthesis. In transient-transfection assays with reporter genes driven by the mitochondrial D loop or two mt-T3REs located in the D loop, p43 stimulated reporter gene activity only in the presence of T3. All these effects were abolished by deletion of the DNA-binding domain of p43. Finally, p43 overexpression in QM7 cells increased the levels of mitochondrial mRNAs, thus indicating that the in organello influence of p43 was physiologically relevant. These data reveal a novel hormonal pathway functioning within the mitochondrion, involving a truncated form of a nuclear receptor acting as a potent mitochondrial T3-dependent transcription factor.

Regulation of the mouse preprothyrotropin-releasing hormone gene by retinoic acid receptor

Retinoic acid (RA) has been reported to inhibit the secretion and synthesis of the pituitary TSH in vivo and in vitro. However, little is known about the influence of RA on the expression of the prepro-TRH gene. We therefore investigated whether the promoter activity of the mouse TRH gene is directly regulated by RA using a transient transfection assay into CV-1 cells. In the absence of cotransfected RA receptor (RAR), all-trans-RA did not affect the promoter activity. In contrast, the cotransfected RARalpha significantly stimulated promoter activity in the absence of ligand, and all-trans-RA reversed basal promoter activation. The cotransfected thyroid hormone receptor-beta (TRbeta), but not 9-cis-RA receptor (RXR), had an additive effect on the RAR-dependent stimulation. TR and RAR can similarly interact with the corepressor proteins, and the cotransfected nuclear receptor corepressor (N-CoR) has been demonstrated to augment the transcriptional stimulation of the TRH gene by unliganded TR. As observed with TR, the coexpression of a N-CoR variant significantly enhanced the ligand-independent stimulation by RAR. A mutant RAR (RAR403) lacking the C-terminal activation function-2 (AF-2) activation domain that was essential for ligand-induced corepressor release constitutively stimulated the promoter activity. The constitutive stimulation by RAR403 was augmented by the cotransfected N-CoR variant. A deletion analysis of the 5'-flanking region of the TRH gene revealed that the minimal promoter region for the regulation by RAR was -83 to +53, with a consensus half-site motif for the thyroid hormone response element at -57. In contrast to the strong binding of TR to the thyroid hormone response element half-site in gel retardation assays, no binding of RAR homodimer, RAR/ RXR heterodimer, or RAR/TR heterodimer was observed to the minimal promoter region. These results collectively suggest that RAR without heterodimerization with RXR and TR regulates transcription of the mouse TRH gene in cooperation with the corepressor, and that the DNA binding of RAR appeared to be unnecessary for regulation of the TRH gene promoter.

Hormones, genes and the structure of sexual arousal

Despite the inherent difficulty of connecting individual genes with integrated mammalian behaviors, it has been determined that a series of genes are turned on by estrogenic hormones acting in forebrain. Their products are, in turn, facilitatory for female reproductive behaviors such as lordosis. The causal routes by which two genes contribute to the control of lordosis behavior, the classical estrogen receptor gene (ER-alpha) and a thyroid hormone (TH) receptor gene (TR-beta), have been delineated. Beyond the mechanisms underlying the expression of concrete, specific natural behaviors, lies the question of sexual motivation. Required as an intervening variable to explain fluctuations in natural behaviors in the face of constant stimuli, motivational states have both general and specific features. Most theoretical and experimental approaches toward the general aspects of motivation have depended heavily on concepts of 'arousal.' Sexual arousal is likely to depend both on very general, broadly distributed neuronal influences and on specific affiliative and sexual tendencies. Is 'general arousal' a monolithic, undifferentiated process? In no way can a review at this time settle such issues, but the reasons behind six new experimental approaches to these questions are described.

Peroxisome proliferator-activated receptors: a family of lipid-activated transcription factors

Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear transcription factors that belong to the steroid receptor superfamily. This family of PPARs includes PPARalpha, PPARdelta, PPARgamma1, and PPARgamma2. These PPARs are related to the T3 and vitamin D(3) receptors and bind to a hexameric direct repeat as a heterodimeric complex with retinoid receptor Xalpha. PPARs regulate the expression of a wide array of genes that encode proteins involved in lipid metabolism, energy balance, eicosanoid signaling, cell differentiation, and tumorigenesis. A unique feature of these steroid-like receptors is that the physiologic ligands for PPARs appear to be fatty acids from the n-6 and n-3 families of fatty acids and their respective eicosanoid products. This review describes the characteristics, regulation, and gene targets for PPARs and relates their effects on gene expression to physiologic outcomes that affect lipid and glucose metabolism, thermogenesis, atherosclerosis, and cell differentiation.

Oligodendrocyte development and thyroid hormone

Thyroid hormone plays an important role in brain development and is essential to ensure a normal myelination. The effects of thyroid hormone are mediated by nuclear thyroid hormone receptors, which act as ligand-regulated transcription factors. There are several isoforms encoded by two genes, alpha and beta. Developmental studies have shown that alpha isoforms are widely expressed in the fetal brain, while beta isoforms expression is more restricted with a dramatic increase that begins at birth in the rat. Remarkably, receptor number reaches maximal levels by postnatal day 10, when serum thyroid hormone levels also peak and myelination is the most prominent event in the developing rat brain. Likewise, oligodendrocyte precursor cells express alpha isoforms and expression of the beta isoforms is confined to the differentiated oligodendrocytes, suggesting that these isoforms might mediate different thyroid hormone effects in the oligodendrocyte lineage. Thyroid hormone acts at multiple steps in the development of oligodendrocytes: (a) Early in development, it can function as an instructive signal for the generation of oligodendrocytes and enhance the proliferation of the committed preprecursor oligodendrocyte cells. (b) Thyroid hormone regulates the number of oligodendrocyte generated by directly promoting their differentiation. Since oligodendrocytes are produced in vitro after the same period in culture regardless of whether thyroid hormone was added to the cultures, it has been suggested that thyroid hormone is required for neither the timing nor the generation of oligodendrocytes, but is necessary to achieve adequate oligodendrocyte numbers. (c) Finally, thyroid hormone increases morphological and functional maturation of postmitoitic oligodendrocytes by stimulation of the expression of various myelin genes.

Role of thyroid hormones and their receptors in peripheral nerve regeneration

After peripheral nerve injury in adult mammals, reestablishment of functional connections depends on several parameters including neurotrophic factors, the extracellular matrix, and hormones. However, little is known about the contribution of hormones to peripheral nerve regeneration. Thyroid hormones, which are required for the development and maturation of the central nervous system, are also important for the development of peripheral nerves. The action of triiodothyronine (T3) on responsive cells is mediated through nuclear thyroid hormone receptors (TRs) which modulate the expression of specific genes in target cells. Thus, to study the effect of T3, it is first necessary to know whether the target tissues possess TRs. The fact that sciatic nerve cells possess functional TRs suggests that these cells can respond to T3 and, as a consequence, that thyroid hormone may be involved in peripheral nerve regeneration. The silicone nerve guide model provides an excellent system to study the action of local administration of T3. Evidence from such studies demonstrate that animals treated locally with T3 at the level of transection have more complete regeneration of sciatic nerve and better functional recovery. Among the possible regulatory mechanisms by which T3 enhances peripheral nerve regeneration is rapid action on both axotomized neurons and Schwann cells which, in turn, produce a lasting and stimulatory effect on peripheral nerve regeneration. It is probable that T3 up- or down-regulates gene expression of one or more growth factors, extracellular matrix, or cell adhesion molecules, all of which stimulate peripheral nerve regeneration. This could explain the greater effect of T3 on nerve regeneration compared with the effect of any one growth factor or adhesion molecule.

Divergent roles for thyroid hormone receptor beta isoforms in the endocrine axis and auditory system

Thyroid hormone receptors (TRs) modulate various physiological functions in many organ systems. The TR alpha and TR beta isoforms are products of 2 distinct genes, and the beta 1 and beta 2 isoforms are splice variants of the same gene. Whereas TR alpha 1 and TR beta 1 are widely expressed, expression of the TR beta 2 isoform is mainly limited to the pituitary, triiodothyronine-responsive TRH neurons, the developing inner ear, and the retina. Mice with targeted disruption of the entire TR beta locus (TR beta-null) exhibit elevated thyroid hormone levels as a result of abnormal central regulation of thyrotropin, and also develop profound hearing loss. To clarify the contribution of the TR beta 2 isoform to the function of the endocrine and auditory systems in vivo, we have generated mice with targeted disruption of the TR beta 2 isoform. TR beta 2-null mice have preserved expression of the TR alpha and TR beta 1 isoforms. They develop a similar degree of central resistance to thyroid hormone as TR beta-null mice, indicating the important role of TR beta 2 in the regulation of the hypothalamic-pituitary-thyroid axis. Growth hormone gene expression is marginally reduced. In contrast, TR beta 2-null mice exhibit no evidence of hearing impairment, indicating that TR beta 1 and TR beta 2 subserve divergent roles in the regulation of auditory function.

Short-chain fatty acids and thyroid hormone interact in regulating enterocyte gene transcription

BACKGROUND

Enterocyte differentiation is known to be regulated by a variety of extracellular compounds, among which are triiodothyronine (T3) and the short-chain fatty acids (SCFAs). Because several SCFAs are known to induce histone hyperacetylation, and T3 action has been recently linked to chromatin structure, we sought to investigate the interplay between SCFAs and T3 in regard to the enterocyte differentiation marker, intestinal alkaline phosphatase (IAP).

METHODS

Caco-2 cells were transiently transfected with a reporter construct containing 2.4 kb of the human IAP gene 5' flanking region (IAP2.4CAT). Cotransfections were carried out with and without thyroid hormone receptor-1 (TR beta-1) or histone deacetylase-1 (HDAC-1) expression plasmids. Cells were treated with 5 mmol/L SCFAs (propionic, butyric, valeric, or caproic acids as propionate, butyrate, valerate, or caproate, respectively), with and without 10 nmol/L T3. Reporter gene activity was measured and the level of histone acetylation assessed by means of acid-urea-triton (AUT) gel assays.

RESULTS

TR beta-1 cotransfection caused a marked decrease in IAP reporter gene activity, which is consistent with the well-known phenomenon of ligand independent repression (LIR), whereas T3 treatment reversed the LIR and caused further reporter gene activation. Treatment with SCFAs similarly resulted in a complete blockage of LIR, and, in fact, turned the TR beta-1 into a transcriptional activator, even in the absence of T3. Concomitant treatment with T3 and butyric acid produced an additive effect on IAP transactivation. In contrast, cotransfection with HDAC-1 attenuated the effects of SCFAs on IAP gene activation. AUT gel studies demonstrated histone hyperacetylation in response to SCFA treatment.

CONCLUSION

One or more DNA cis-elements in the human IAP gene mediate ligand independent repression by the TR beta-1, an effect that can be entirely reversed by those SCFAs that induce histone hyperacetylation. In addition T3 and SCFAs can act in concert to induce IAP gene transcription, demonstrating an important link between triiodothyronine and histone hyperacetylation in regard to enterocyte-specific gene expression.

Involvement of T3Ralpha- and beta-receptor subtypes in mediation of T3 functions during postnatal murine intestinal development

BACKGROUND & AIMS

Thyroid hormones are implicated in intestinal development. Their effects are mediated by nuclear receptors, which are transcriptional regulators activated upon binding of triiodothyronine. The aim of this study was to define the involvement of the receptor subtypes during intestinal development.

METHODS

We used strains of knockout mice lacking T3Ralpha, T3Rbeta, or both receptors, encoded by T3Ralpha and T3Rbeta genes.

RESULTS

Morphological features and expression of digestive enzymes and of two intestinal regulators, Cdx-1 and Cdx-2, were compared in wild-type and T3Ralpha, T3Rbeta, and T3Ralphabeta knockout animals. T3Ralpha-/- mice had abnormal intestinal morphology, assessed by a decrease in the number of epithelial cells along the crypt-villus axis and a decrease in proliferating crypt cells. Expression of Cdx-1 and Cdx-2, and of the digestive enzymes, was down-regulated. These parameters can be partially reversed by T3 injection. A similar (jejunum) or more severe (ileum) phenotype was found in T3Ralphabeta double mutants. In contrast, no changes occurred in T3Rbeta mice.

CONCLUSIONS

These data describe for the first time a direct effect of TH through the T3Ralpha-receptor subtypes on postnatal intestinal mucosa maturation. They also suggest that T3Rbeta receptors are dispensable but can partially substitute for T3Ralpha.

Local activation and inactivation of thyroid hormones: the deiodinase family

Tissue-specific activation and inactivation of ligands of nuclear receptors which belong to the steroid retinoid-thyroid hormone superfamily of transcription factors represents an important principle of development- and tissue-specific local modulation of hormone action. Recently, several enzyme families have been identified which act as 'guardians of the gate' of ligand-activated transcription modulation. Three monodeiodinase isoenzymes which are involved in activation the 'prohormone' L-thyroxine (T4), the main secretory product of the thyroid gland, have been identified, characterized, and cloned. Both, type I and type II 5'-deiodinase generate the thyromimetically active hormone 3,3',5-triiodothyronine (T3) by reductive deiodination of the phenolic ring of T4. Inactivation of T4 and its product T3 occurs by deiodination of iodothyronines at the tyrosyl ring. This reaction is catalyzed both the type III 5-deiodinase and also by the type I enzyme, which has a broader substrate specificity. The three deiodinases appear to constitute a newly discovered family of selenocysteine-containing proteins and the presence of selenocysteine in the protein is critical for enzyme activity. Whereas the selenoenzyme characteristics of the type I and type III deiodinases are definitively established some controversy still exists for the type II 5'-deiodinase in mammals. The mRNA probably encoding the type II 5'-deiodinase subunit is markedly longer than those of the two other deiodinases and its selenocysteine-insertion element is located more than 5 kB downstream of the UGA-codon in the 3'-untranslated region. The three deiodinase isoenzymes show a distinct development- and tissue-specific pattern of expression, operate at individual optimal substrate levels, are differently regulated and modulated by hormones, cytokines, signaling pathways, natural factors, and pharmaceuticals. Whereas circulating T3 mainly originates from hepatic production via the type I 5'-deiodinase, the local cellular thyroid hormone concentration in various tissues including the central nervous system is controlled by complex para-, auto-, and intracrine interactions of all three deiodinases. Local thyroid hormone availability is further modulated by conjugation reactions of the phenolic 4'-OH-group of iodothyronines, which also inactivate the thyroid hormones.

Mice devoid of all known thyroid hormone receptors are viable but exhibit disorders of the pituitary-thyroid axis, growth, and bone maturation

Thyroid hormone (T3) has widespread functions in development and homeostasis, although the receptor pathways by which this diversity arises are unclear. Deletion of the T3 receptors TRalpha1 or TRbeta individually reveals only a small proportion of the phenotypes that arise in hypothyroidism, implying that additional pathways must exist. Here, we demonstrate that mice lacking both TRalpha1 and TRbeta (TRalpha1(-/-)beta-/-) display a novel array of phenotypes not found in single receptor-deficient mice, including an extremely hyperactive pituitary-thyroid axis, poor female fertility and retarded growth and bone maturation. These results establish that major T3 actions are mediated by common pathways in which TRalpha1 and TRbeta cooperate with or substitute for each other. Thus, varying the balance of use of TRalpha1 and TRbeta individually or in combination facilitates control of an extended spectrum of T3 actions. There was no evidence for any previously unidentified T3 receptors in TRalpha1(-/-)beta-/- mouse tissues. Compared to the debilitating symptoms of severe hypothyroidism, the milder overall phenotype of TRalpha1(-/-)beta-/- mice, lacking all known T3 receptors, indicates divergent consequences for hormone versus receptor deficiency. These distinctions suggest that T3-independent actions of T3 receptors, demonstrated previously in vitro, may be a significant function in vivo.

Amphibian metamorphosis as a model for studying the developmental actions of thyroid hormone

A salient feature of the thyroid hormones. L-thyroxine (T4) and triiodo-L-thyronine (T3), is the multiplicity of their physiological and biochemical actions in a wide variety of vertebrates. Important among these is their profound effects on postembryonic development. Analysis of their developmental action in mammals is vitiated by the exposure of the developing foetus to a number of maternal factors which do not allow one to specifically define the role of thyroid hormone (TH) or that of other hormones and factors that modulate its action. Amphibian metamorphosis is obligatorily dependent on TH which can initiate all the diverse physiological manifestations of this postembryonic developmental process (morphogenesis, cell death, re-structuring, etc.) in free-living embryos and larvae of most anurans. It is therefore an ideal model for studying the mechanisms underlying the hormonal regulation of postembryonic development. This article will first summarize the key features of metamorphosis and its control by TH and other hormones. Emphasis will be laid on the important role played by TH receptor (TR), in particular the phenomenon of TR gene autoinduction, in initiating the developmental action of TH. Finally, it will be argued that the findings on the control of amphibian metamorphosis enhance our understanding of the regulation of postembryonic development by TH in mammals and other vertebrate species.

Recent advances in understanding thyroid hormone receptor coregulators

Thyroid hormones (L-triiodothyronine, T3; L-tetraiodothyronine, T4) regulate normal cellular growth and development, and general metabolism as well. Their various actions are mediated by the thyroid hormone receptor, a ligand-dependent transcriptional factor belonging to the nuclear hormone receptor superfamily. The recent discovery of coregulators (coactivators, corepressors, and cointegrators) has greatly enhanced our understanding of thyroid hormone receptor functions. Hence we review and discuss, in brief, the potential role of thyroid hormone receptor coregulators involved in diverse cellular activities.

Different functions for the thyroid hormone receptors TRalpha and TRbeta in the control of thyroid hormone production and post-natal development

The biological activities of thyroid hormones are thought to be mediated by receptors generated by the TRalpha and TRbeta loci. The existence of several receptor isoforms suggests that different functions are mediated by specific isoforms and raises the possibility of functional redundancies. We have inactivated both TRalpha and TRbeta genes by homologous recombination in the mouse and compared the phenotypes of wild-type, and single and double mutant mice. We show by this method that the TRbeta receptors are the most potent regulators of the production of thyroid stimulating hormone (TSH). However, in the absence of TRbeta, the products of the TRalpha gene can fulfill this function as, in the absence of any receptors, TSH and thyroid hormone concentrations reach very high levels. We also show that TRbeta, in contrast to TRalpha, is dispensable for the normal development of bone and intestine. In bone, the disruption of both TRalpha and TRbeta genes does not modify the maturation delay observed in TRalpha -/- mice. In the ileum, the absence of any receptor results in a much more severe impairment than that observed in TRalpha -/- animals. We conclude that each of the two families of proteins mediate specific functions of triiodothyronin (T3), and that redundancy is only partial and concerns a limited number of functions.