Comparison of the mechanisms of nongenomic actions of thyroid hormone and steroid hormones

Direct Interaction of Mitochondrial Cytochrome c Oxidase with Thyroid Hormones: Evidence for Two Binding Sites

Thyroid hormones regulate tissue metabolism to establish an energy balance in the cell, in particular, by affecting oxidative phosphorylation. Their long-term impact is mainly associated with changes in gene expression, while the short-term effects may differ in their mechanisms. Our work was devoted to studying the short-term effects of hormones T2, T3 and T4 on mitochondrial cytochrome c oxidase (CcO) mediated by direct contact with the enzyme. The data obtained indicate the existence of two separate sites of CcO interaction with thyroid hormones, differing in their location, affinity and specificity to hormone binding. First, we show that T3 and T4 but not T2 inhibit the oxidase activity of CcO in solution and on membrane preparations with K_i ≈ 100–200 μM. In solution, T3 and T4 compete in a 1:1 ratio with the detergent dodecyl-maltoside to bind to the enzyme. The peroxidase and catalase partial activities of CcO are not sensitive to hormones, but electron transfer from heme a to the oxidized binuclear center is affected. We believe that T3 and T4 could be ligands of the bile acid-binding site found in the 3D structure of CcO by Ferguson-Miller’s group, and hormone-induced inhibition is associated with dysfunction of the K-proton channel. A possible role of this interaction in the physiological regulation of the enzyme is discussed. Second, we find that T2, T3, and T4 inhibit superoxide generation by oxidized CcO in the presence of excess H₂O₂. Inhibition is characterized by K_i values of 0.3–5 μM and apparently affects the formation of O₂^●− at the protein surface. The second binding site for thyroid hormones presumably coincides with the point of tight T2 binding on the Va subunit described in the literature.

CLICK-enabled analogues reveal pregnenolone interactomes in cancer and immune cells

Pregnenolone (P5) promotes prostate cancer cell growth, and de novo synthesis of intratumoural P5 is a potential cause of development of castration resistance. Immune cells can also synthesize P5 de novo. Despite its biological importance, little is known about P5's mode of actions, which appears to be context dependent and pleiotropic. A comprehensive proteome-wide spectrum of P5-binding proteins that are involved in its trafficking and functionality remains unknown. Here, we describe an approach that integrates chemical biology for probe synthesis with chemoproteomics to map P5-protein interactions in live prostate cancer cells and murine CD8⁺ T cells. We subsequently identified P5-binding proteins potentially involved in P5-trafficking and in P5's non-genomic action that may drive the promotion of castrate-resistance prostate cancer and regulate CD8⁺ T cell function. We envisage that this methodology could be employed for other steroids to map their interactomes directly in a broad range of living cells, tissues, and organisms.

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Developed four functional click-enabled analogues of pregnenolone (P5)

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Chemoproteomics prioritizes 62 P5 target proteins in live cancer and immune cells

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These include shared and distinct biochemical role of P5 in cancer and immune cells

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P5 activity in cancer and immune cells is mediated through non-genomic pathways

Environmental pollutant hexachlorobenzene induces hypertension in a rat model

Hexachlorobenzene (HCB) is a dioxin-like environmental pollutant, widely distributed in the environment. New research links exposure to high levels of persistent organic environmental toxicants to cardiovascular disease, however little is known about the effect of HCB on vascular function and on blood pressure. The purpose of the present study was to evaluate biochemical and cardiovascular changes resulting from subchronic HCB exposure. Adult female Sprague-Dawley rats were treated with vehicle or HCB (5 or 500 mg/kg b.w) for 45 days. Systolic blood pressure (BP), recorded by tail cuff plethysmography, was significantly increased at 35, 40 and 45 days of 500 mg/kg HCB-treatment. HCB (500 mg/kg) increased arterial thickness, while both 5 and 500 mg/kg HCB decreased proliferating cell nuclear antigen (PCNA) protein levels and cellular nuclei in abdominal aortas indicating a hypertrophic process. Also, aortas from both groups of HCB-treated rats presented higher sensitivity to noradrenalin (NA) and a significant decrease in maximum contractile response. Arteries from 500 mg/kg HCB-treated rats showed a significant increase in the levels of transforming growth factor-β1 (TGF-β1) mRNA and angiotensin II type1 receptor (AT₁), and a significant decrease in estrogen receptor alpha (ERα), endothelial nitric oxidide synthase (eNOS) protein expression and deiodinase II (DII) mRNA levels. In conclusion, we have demonstrated for the first time that subchronic HCB administration significantly increases BP and alters associated cardiovascular parameters in rats. In addition, HCB alters the expression of key vascular tissue molecules involved in BP regulation, such as TGF-β1, AT1, ERα, eNOS and DII.

Low-frequency stimulation induces a durable long-term depression in young adult hyperthyroid rats: the role of p38 mitogen-activated protein kinase and protein phosphatase 1

Long-term potentiation and long-term depression (LTD) are cellular mechanisms of learning and memory in the mammalian brain. We have previously shown that adult hyperthyroid rats showed a delay in the acquisition of a place learning task and attenuated long-term potentiation. However, changes in LTD in hyperthyroidism remain unclear. Rats were administered 0.2 mg/kg/day of L-thyroxine for 21 days starting at postnatal day 40 to induce hyperthyroidism. LTD was induced in the dentate gyrus using low-frequency stimulation (LFS) of the perforant pathway. The mRNA expressions of p38 mitogen-activated protein kinase (p38-MAPK) and protein phosphatase 1 (PP1) were evaluated using a quantitative reverse transcriptase PCR. In control rats, a standard LFS protocol induced a slight depression of the population spike (PS) amplitude during the induction phase of LTD (76±13% vs. baseline), but a small potentiation of the PS amplitude was observed in the early (107±18%) and late (111±20%) phases of LTD. Interestingly, in the hyperthyroid rats, the same LFS protocol induced a reliable LTD in the dentate gyrus of the hippocampus as evidenced by a marked depression in the PS amplitude during the induction (54±6% vs. baseline) and the early phases (56±8%) of LTD. Elevated mRNA levels of p38-MAPK and PP1 were observed in the hippocampus of the LFS-treated hyperthyroid rats compared with the hippocampus of the vehicle-treated hyperthyroid rats. No significant change in p38-MAPK or PP1 mRNA expression was observed in the euthyroid rats. The present study shows that a standard LFS protocol can induce a durable depression of synaptic strength and an upregulation of PP1 and p38-MAPK mRNA in hyperthyroid rats. We conclude that hyperthyroidism can induce molecular changes associated with degeneration of the hippocampus. The relationship between the levels of thyroid hormone and dementia requires further investigation.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Effect of non-genomic actions of thyroid hormones on the anaesthetic effect of propofol

Hyperthyroidism is a common disease of the endocrine system and it is known that additional propofol anaesthesia is required during surgery for patients with hyperthyroidism compared with those with normal thyroid function. The aim of the present study was to determine the mechanism through which thyroid hormones (THs) inhibit the effect of propofol anaesthesia. Immunofluorescence techniques were used to verify the difference between the expression quantities of γ-aminobutyric acid type A (GABA_A) receptor subunits α2 and β2 in the dorsal root ganglions (DRGs) of rats with hyperthyroidism and those in normal rats. Perforated patch clamp recordings in the whole-cell mode were performed to detect the GABA-activated current in acutely isolated rat DRG neurons from rats with hyperthyroidism and normal rats. This method was also used to evaluate the change in the GABA-activated currents following the pre-perfusion of propofol with and without 3,3',5-L-triiodothyronine (T₃). Compared with normal rats, rats with hyperthyroidism expressed same quantities of GABA_A receptor α2 and β2 subunits in DRGs. In addition, no difference in GABA-activated currents in the acutely isolated DRG neurons from the two types of rat was observed (P>0.05). T₃ inhibits or minimises the augmentation effect of propofol on the GABA-activated currents (P<0.05). The inhibitory effect of T₃ on propofol was minimised by increasing the propofol concentration (P<0.05). The inhibitory effect of T₃ on the anaesthetic effect of propofol is achieved through the inhibition of the function of GABA_A receptors through the non-genomic actions of the THs, rather than by changing the number of GABA_A receptors. This inhibitory effect can be mitigated by increasing the propofol concentration. In conclusion, rats with hyperthyroidism require a larger dose of propofol to induce anaesthesia since the non-genomic actions of THs suppress GABA receptors, which in turn inhibits the anaesthetic action of propofol.

The steroid hormone 20-hydroxyecdysone via nongenomic pathway activates Ca2+/calmodulin-dependent protein kinase II to regulate gene expression

The steroid hormone 20-hydroxyecdysone (20E) triggers calcium signaling pathway to regulate 20E response gene expression, but the mechanism underlying this process remains unclear. We propose that the 20E-induced phosphorylation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) serves an important function in 20E response gene transcription in the lepidopteran insect Helicoverpa armigera. CaMKII showed increased expression and phosphorylation during metamorphosis. 20E elevated CaMKII phosphorylation. However, the G protein-coupled receptor (GPCR) and ryanodine receptor inhibitor suramin, the phospholipase C inhibitor U73122, and the inositol 1,4,5-triphosphate receptor inhibitor xestospongin C suppressed 20E-induced CaMKII phosphorylation. Two ecdysone-responsible GPCRs and Gαq protein were involved in 20E-induced CaMKII phosphorylation by RNA interference analysis. 20E regulated CaMKII threonine phosphorylation at amino acid 290, thereby inducing CaMKII nuclear translocation. CaMKII knockdown by dsCaMKII injection into the larvae prevented the occurrence of larval-pupal transition and suppressed 20E response gene expression. CaMKII phosphorylation and nuclear translocation maintained USP1 lysine acetylation at amino acid 303 by inducing histone deacetylase 3 phosphorylation and nuclear export. The lysine acetylation of USP1 was necessary for the interaction of USP1 with EcRB1 and their binding to the ecdysone response element. Results suggest that 20E (via GPCR activation and calcium signaling) activates CaMKII phosphorylation and nuclear translocation, which regulate USP1 lysine acetylation to form an EcRB1-USP1 complex for 20E response gene transcription.

ERK2-mediated phosphorylation of transcriptional coactivator binding protein PIMT/NCoA6IP at Ser298 augments hepatic gluconeogenesis

PRIP-Interacting protein with methyl transferase domain (PIMT) serves as a molecular bridge between CREB-binding protein (CBP)/ E1A binding protein p300 (Ep300) -anchored histone acetyl transferase and the Mediator complex sub-unit1 (Med1) and modulates nuclear receptor transcription. Here, we report that ERK2 phosphorylates PIMT at Ser(298) and enhances its ability to activate PEPCK promoter. We observed that PIMT is recruited to PEPCK promoter and adenoviral-mediated over-expression of PIMT in rat primary hepatocytes up-regulated expression of gluconeogenic genes including PEPCK. Reporter experiments with phosphomimetic PIMT mutant (PIMT(S298D)) suggested that conformational change may play an important role in PIMT-dependent PEPCK promoter activity. Overexpression of PIMT and Med1 together augmented hepatic glucose output in an additive manner. Importantly, expression of gluconeogenic genes and hepatic glucose output were suppressed in isolated liver specific PIMT knockout mouse hepatocytes. Furthermore, consistent with reporter experiments, PIMT(S298D) but not PIMT(S298A) augmented hepatic glucose output via up-regulating the expression of gluconeogenic genes. Pharmacological blockade of MAPK/ERK pathway using U0126, abolished PIMT/Med1-dependent gluconeogenic program leading to reduced hepatic glucose output. Further, systemic administration of T4 hormone to rats activated ERK1/2 resulting in enhanced PIMT ser(298) phosphorylation. Phosphorylation of PIMT led to its increased binding to the PEPCK promoter, increased PEPCK expression and induction of gluconeogenesis in liver. Thus, ERK2-mediated phosphorylation of PIMT at Ser(298) is essential in hepatic gluconeogenesis, demonstrating an important role of PIMT in the pathogenesis of hyperglycemia.

Thyroid hormone's role in regulating brain glucose metabolism and potentially modulating hippocampal cognitive processes

Cognitive performance is dependent on adequate glucose supply to the brain. Insulin, which regulates systemic glucose metabolism, has been recently shown both to regulate hippocampal metabolism and to be a mandatory component of hippocampally-mediated cognitive performance. Thyroid hormones (TH) regulate systemic glucose metabolism and may also be involved in regulation of brain glucose metabolism. Here we review potential mechanisms for such regulation. Importantly, TH imbalance is often encountered in combination with metabolic disorders such as diabetes, and may cause additional metabolic dysregulation and hence worsening of disease states. TH's potential as a regulator of brain glucose metabolism is heightened by interactions with insulin signaling, but there have been relatively few studies on this topic or on the actions of TH in a mature brain. This review discusses evidence for mechanistic links between TH, insulin, cognitive function, and brain glucose metabolism, and reaches the conclusion that TH may modulate memory processes, likely at least in part by modulation of central insulin signaling and glucose metabolism.

Overlapping nongenomic and genomic actions of thyroid hormone and steroids

Nuclear receptors for thyroid hormone and steroids are members of a receptor superfamily with similar molecular organization, but discrete transcriptional functions that define genomic actions of these nonpeptide hormones. Nongenomic actions of thyroid hormone and estrogens and androgens are initiated outside the nucleus, at receptors in the plasma membrane or in cytoplasm; these actions are largely regarded to be unique to the respective hormones. However, there is an increasing number of descriptions of overlapping nongenomic and genomic effects of thyroid hormone and estrogens and testosterone. These effects are concentrated in tumor cells, where, for example, estrogens and thyroid hormone have similar mitogen-activate protein kinase (MAPK)-dependent proliferative actions on ERα-positive human breast cancer cells, and where dihydrotestosterone also can stimulate proliferation. Steroids and thyroid hormone have similar anti-apoptotic effects in certain tumors. But thyroid hormone and steroids also have overlapping or interacting nongenomic and genomic actions in heart and brain cells. These various effects of thyroid hormone and estrogens and androgens are reviewed here and their possible clinical consequences are enumerated.

Isoform-specific transcriptional activity of overlapping target genes that respond to thyroid hormone receptors alpha1 and beta1

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that control multiple aspects of physiology and development. TRs are expressed in vertebrates as a series of distinct isoforms that exert distinct biological roles. We wished to determine whether the two most widely expressed isoforms, TR alpha 1 and TR beta 1, exert their different biological effects by regulating different sets of target genes. Using stably transformed HepG2 cells and a microarray analysis, we were able to demonstrate that TR alpha 1 and TR beta 1 regulate a largely overlapping repertoire of target genes in response to T(3) hormone. However, these two isoforms display very different transcriptional properties on each individual target gene, ranging from a much greater T(3)-mediated regulation by TR alpha 1 than by TR beta 1, to near equal regulation by both isoforms. We also identified TR alpha 1 and TR beta 1 target genes that were regulated by these receptors in a hormone-independent fashion. We suggest that it is this gene-specific, isoform-specific amplitude of transcriptional regulation that is the likely basis for the appearance and maintenance of TR alpha 1 and TR beta 1 over evolutionary time. In essence, TR alpha 1 and TR beta 1 adjust the magnitude of the transcriptional response at different target genes to different levels; by altering the ratio of these isoforms in different tissues or at different developmental times, the intensity of T(3) response can be individually tailored to different physiological and developmental requirements.

Mode of Action: Developmental Thyroid Hormone Insufficiency—Neurological Abnormalities Resulting From Exposure to Propylthiouracil

Because thyroid hormone is essential for normal brain development before and after birth, environmental chemicals that interfere with thyroid hormone signaling can adversely affect brain development. Adverse consequences of thyroid hormone insufficiency depend both on severity and developmental timing, indicating that environmental antithyroid factors may produce different effects at different developmental windows of exposure. Mechanistic studies can provide important insight into the potential impact of chemicals on human thyroid function, but relevance to humans must be systematically evaluated. This kind of analysis depends on data sets that include information about animals and humans. The drug 6-n-propyl-2-thiouracil (PTU) is used in animals to experimentally manipulate serum thyroid hormone levels, and in humans to treat patients, including pregnant women, with Graves' disease. A systematic analysis of the mode of action (MOA) of PTU in rats and in humans discloses similar modes of action. While the analysis predicts that PTU doses that produce thyroid hormone insufficiency in humans would adversely affect the developing brain, careful monitoring of PTU administration in pregnant and lactating humans keeps infant serum thyroid hormone levels within the normal range.

Hepatic regulation of fatty acid synthase by insulin and T3: evidence for T3 genomic and nongenomic actions

Fatty acid synthase (FAS) is a key enzyme of hepatic lipogenesis responsible for the synthesis of long-chain saturated fatty acids. This enzyme is mainly regulated at the transcriptional level by nutrients and hormones. In particular, glucose, insulin, and T(3) increase FAS activity, whereas glucagon and saturated and polyunsaturated fatty acids decrease it. In the present study we show that, in liver, T(3) and insulin were able to activate FAS enzymatic activity, mRNA expression, and gene transcription. We localized the T(3) response element (TRE) that mediates the T(3) genomic effect, on the FAS promoter between -741 and -696 bp that mediates the T(3) genomic effect. We show that both T(3) and insulin regulate FAS transcription via this sequence. The TRE binds a TR/RXR heterodimer even in the absence of hormone, and this binding is increased in response to T(3) and/or insulin treatment. The use of H7, a serine/threonine kinase inhibitor, reveals that a phosphorylation mechanism is implicated in the transcriptional regulation of FAS in response to both hormones. Specifically, we show that T(3) is able to modulate FAS transcription via a nongenomic action targeting the TRE through the activation of a PI 3-kinase-ERK1/2-MAPK-dependent pathway. Insulin also targets the TRE sequence, probably via the activation of two parallel pathways: Ras/ERK1/2 MAPK and PI 3-kinase/Akt. Finally, our data suggest that the nongenomic actions of T(3) and insulin are probably common to several TREs, as we observed similar effects on a classical DR4 consensus sequence.

Effects of 17beta-estradiol on neuronal cell excitability and neurotransmission in the suprachiasmatic nucleus of rat

17beta-Estradiol receptors have been found in several brain nuclei including the suprachiasmatic nucleus (SCN) of mammalian species. The SCN is believed to act as brain clock regulating circadian and circannual biological rhythms, such as body temperature, sleep, and mood. Here, we examined whether 17beta-estradiol (E2) could affect cell excitability and synaptic transmission in the SCN. Bath application of E2 (0.03-3 microM) increased the spontaneous firing frequency and depolarized cell membrane of the SCN neurons significantly. Furthermore, E2 (0.03-3 microM) increased (by about 25-150% of control) frequency of the miniature excitatory postsynaptic currents. Amplitude of the evoked excitatory postsynaptic currents was enhanced (by about 32% of control) after exposure to 1 microM E2. The paired-pulse ratio was reduced by E2. These effects were prevented by the estrogen receptor antagonist, ICI 182780. Exposure to the biologically inactive 17alpha-estradiol did not cause any significant changes in the parameters mentioned above. These findings are in favor of an implication of estrogen in modulation of neuronal activity in SCN and possibly regulating circadian rhythms.

Thyroid hormone effects on mitochondrial energetics

Thyroid hormones are the major endocrine regulators of metabolic rate, and their hypermetabolic effects are widely recognized. The cellular mechanisms underlying these metabolic effects have been the subject of much research. Thyroid hormone status has a profound impact on mitochondria, the organelles responsible for the majority of cellular adenosine triphosphate (ATP) production. However, mechanisms are not well understood. We review the effects of thyroid hormones on mitochondrial energetics and principally oxidative phosphorylation. Genomic and nongenomic mechanisms have been studied. Through the former, thyroid hormones stimulate mitochondriogenesis and thereby augment cellular oxidative capacity. Thyroid hormones induce substantial modifications in mitochondrial inner membrane protein and lipid compositions. Results are consistent with the idea that thyroid hormones activate the uncoupling of oxidative phosphorylation through various mechanisms involving inner membrane proteins and lipids. Increased uncoupling appears to be responsible for some of the hypermetabolic effects of thyroid hormones. ATP synthesis and turnover reactions are also affected. There appear to be complex relationships between mitochondrial proton leak mechanisms, reactive oxygen species production, and thyroid status. As the majority of studies have focused on the effects of thyroid status on rat liver preparations, there is still a need to address fundamental questions regarding thyroid hormone effects in other tissues and species.

Involvement of Kupffer cell-dependent signaling in T3-induced hepatocyte proliferation in vivo

Thyroid hormone-induced calorigenesis triggers liver oxidative stress with concomitant TNF-alpha production by Kupffer cells and up-regulation of gene expression. Considering that cyclin-dependent kinase-2 (CDK-2) performs essential functions for cellular proliferation, our aim was to test the hypothesis that l-3,3',5-triiodothyronine (T(3)) stimulates liver cell proliferation by upstream mechanisms involving CDK-2 expression dependent on Kupffer cell signaling. T(3) administration induced a calorigenic response at 60-70 h after treatment, with increased TNF-alpha generation and hepatic oxidative stress status, as shown by enhanced protein carbonyls and decreased glutathione content compared to controls. In this time interval, liver c-jun N-terminal kinase (JNK) phosphorylation, activator protein-1 (AP-1) DNA binding, and CDK-2 expression were enhanced, with concomitantly higher levels of the proliferation markers Ki-67 and proliferating cell nuclear antigen. These changes are abolished by administration of the Kupffer cell inactivator gadolinium chloride prior to T(3) treatment. We conclude that T(3) administration triggers liver CDK-2 expression and cellular proliferation through a cascade associated with Kupffer cell-dependent TNF-alpha generation, JNK phosphorylation, and AP-1 activation. Since CDK-2 promotes phase S progression within the cell cycle, this response may constitute a major mechanism involved in T(3)-induced liver preconditioning to ischemia/reperfusion injury.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Thyroid hormone homeostasis and action in the type 2 deiodinase-deficient rodent brain during development

Considerable indirect evidence suggests that the type 2 deiodinase (D2) generates T3 from T4 for local use in specific tissues such as pituitary, brown fat, and brain, and studies with a D2-deficent mouse, the D2 knockout (D2KO) mouse, have shown this to be the case in pituitary and brown fat. The present study employs the D2KO mouse to determine the role of D2 in the developing brain. As expected, the T3 content in the neonatal D2KO brain was markedly reduced to a level comparable with that seen in the hypothyroid neonatal wild-type mouse. However, the mRNA levels of several T3-responsive genes were either unaffected or much less affected in the brain of the D2KO mouse than in that of the hypothyroid mouse, and compared with the hypothyroid mouse, the D2KO mouse exhibited a very mild neurological phenotype. The current view of thyroid hormone homeostasis in the brain dictates that the T3 present in neurons is generated mostly, if not exclusively, from T4 by the D2 in glial cells. This view is inadequate to explain the findings presented herein, and it is suggested that important compensatory mechanisms must be in play in the brain to minimize functional abnormalities in the absence of the D2.

Developmental acquisition of T3-sensitive Na-K-ATPase stimulation by rat alveolar epithelial cells

Late in gestation, the developing air space epithelium switches from chloride and fluid secretion to sodium and fluid absorption. Absorption requires Na-K-ATPase acting in combination with apical sodium entry mechanisms. Hypothyroidism inhibits perinatal fluid resorption, and thyroid hormone [triiodothyronine (T3)] stimulates adult alveolar epithelial cell (AEC) Na-K-ATPase. This study explored the developmental regulation of Na-K-ATPase by T3 in fetal rat distal lung epithelial (FDLE) cells. T3 increased Na-K-ATPase activity in primary FDLE cells from gestational day 19 [both primary FDLE cells at embryonic day 19 (E19) and the cell line FD19 derived from FDLE cells at E19]. However, T3 did not increase the Na-K-ATPase activity in less mature FDLE cells, including primary E17 and E18 FDLE cells and the cell line FD18 (derived from FDLE cells at E18). Subsequent experiments assessed the T3 signal pathway to define whether it was similar in the late FDLE and adult AEC and to determine the site of the switch in responsiveness to T3. As in adult AEC, in the FD19 cell line, the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin blocked the T3-induced increase in Na-K-ATPase activity and plasma membrane quantity. T3 caused a parallel increase in phosphorylation of Akt at Ser473 in FDLE cells from E19, but not from E17 or E18. In the FD18 cell line, transient expression of a constitutively active mutant of the PI3K catalytic p110 subunit significantly augmented the Na-K-ATPase activity and the cell surface expression of Na-K-ATPase alpha(1) protein. In conclusion, FDLE cells from E17 and E18 lacked T3-sensitive Na-K-ATPase activity but acquired this response at E19. The developmental stimulation of Na-K-ATPase by T3 in rat FDLE cells requires activation of PI3K, and the acquisition of T3 responsiveness may be at PI3K or upstream in the signaling pathway.

Genomic-independent action of thyroid hormones on NTPDase activities in Sertoli cell cultures from congenital hypothyroid rats

The Sertoli cells play an essential role in the maintenance and control of spermatogenesis. The ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) and 5'-nucleotidase activities can modulate the extracellular adenine nucleotide levels, controlling nucleotide-mediated signaling events in Sertoli cells. Since thyroid hormones (TH) and adenine nucleotides and nucleosides play important modulatory roles in Sertoli cell proliferation and differentiation, the aim of our study was to investigate the effect of hypothyroidism upon the NTPDase and 5'-nucleotidase activities in Sertoli cell cultures, as well as to verify whether these effects may be reversed by short and long-term supplementation with TH. Congenital hypothyroidism was induced by adding 0.02% methimazole in the drinking water from day 9 of gestation and continually until 18 days of age. Hypothyroidism significantly decreased the extracellular ATP and ADP hydrolysis and this effect was significantly reversed when cell cultures were supplemented with 1 microM T3 or 0.1 microM T4 for 30 min. In contrast, AMP hydrolysis was not altered by hypothyroidism, but was increased by T4 supplementation for 24 h. The presence of the enzymes NTPDase 1, 2 and 3 was detected by RT-PCR in Sertoli cell cultures, however, hypothyroidism was not able to alter the expression of these enzymes. These findings demonstrate that TH modify NTPDase activities in hypothyroid Sertoli cells, probably via nongenomic mechanisms and, consequently, may influence the reproductive function throughout development.

Thyroid hormone mediated changes in gene expression can be initiated by cytosolic action of the thyroid hormone receptor beta through the phosphatidylinositol 3-kinase pathway

Thyroid hormone (TH) action is mediated principally through binding of the hormone ligand, 3,3,5-triiodothyronine (T₃), to TH receptors (TRs). This hormone-receptor interaction recruits other proteins to form complexes that regulate gene expression by binding to DNA sequences in the promoter of target genes. We recently described an extranuclear mechanism of TH action that consists of the association of TH-liganded TRβ with p85α [regulatory subunit of phosphatidylinositol 3-kinase (PI3K)] in the cytosol and subsequent activation of the PI3K, generating phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3]. This initiates the activation of a signaling cascade by phosphorylation of Akt, mammalian target of rapamycin (mTOR) and its substrate p70^S6K, leading to the stimulation of ZAKI-4α synthesis, a calcineurin inhibitor. Furthermore, we found that this same mechanism leads to induction of the transcription factor hypoxia-inducible factor (HIF-1α), and its target genes, glucose transporter (GLUT)1, platelet-type phosphofructokinase (PFKP), and monocarboxylate transporter (MCT) 4. These genes are of special interest, because their products have important roles in cellular glucose metabolism, from glucose uptake (GLUT1) to glycolysis (PFKP) and lactate export (MCT4). These results demonstrate that the TH-TRβ complex can exert a non-genomic action in the cytosol leading to changes in gene expression by direct (HIF-1α) and indirect (ZAKI-4α, GLUT1, PFKP) means.

Rapid modulation of TRH-like peptides in rat brain by thyroid hormones

Recent identification of membrane receptors for T4, T3, 3,5-T2, and 3-iodothyronamine that mediate rapid physiologic effects of thyroid hormones suggested that such receptors may supplement the regulation of TRH and TRH-like peptides by nuclear T3 receptors. For this reason 200 g male Sprague-Dawley rats received daily i.p. injections of PTU or T4. Levels of TRH and TRH-like peptides were measured 0, 2 h or 1, 2, 3, or 4 days later. Rapid increases or decreases in TRH and TRH-like peptide levels were observed in response to PTU and T4 treatments in various brain regions involved in mood regulation. Significant effects were measured within 2 h of T4 injection. Nuclear T3 receptor-mediated changes in gene expression altering translation, post-translational processing and constitutive release of peptides require more than 2 h. We conclude that non-genomic mechanisms may contribute to the psychiatric effects of thyroid disease and thyroid hormone adjuvant treatment for major depression.

Thyroid hormones and retinoids: a possible link between genes and environment in schizophrenia

Phenotypic discordance for schizophrenia in monozygotic twins clearly indicates involvement of environmental factors as key determinants in disease development. Positive findings from genome scans, linkage and association studies apply in only a minority of those affected, while post-mortem brain investigations reveal altered expression of genes and proteins involved in numerous neurodevelopmental, metabolic and neurotransmitter pathways. Such altered expressions could result, on the one hand, from mutations in coding regions or polymorphisms in the promoter and regulatory regions in genes within those areas identified by gene searches or, on the other hand, from inadequate amounts of modulators, transporters and synthesizers of transcription factors necessary for regulation of the putative genes. Hormones and vitamins are such modulators. They could serve as bridges between genes and environment in schizophrenia. Multiple evidence supports the suggestion of retinoids and thyroid hormones as plausible actors in these roles. Both are not only essential for normal development of the central nervous system but also regulate the expression of many neurotransmitters, their synthesizing enzymes and receptors, and other genes in broader signaling transduction cascades affecting pathways that are altered in response to treatment. Functional and positional candidate genes include retinoic acid and thyroid hormone receptors, retinaldehyde dehydrogenases and deiodinases, which synthesize the powerful morphogens, retinoic acid and triiodothyronine, and the enzymes involved in their inactivation. This review highlights selective evidence supporting the retinoid and thyroid hormone hypotheses of schizophrenia.

Short-Term Effects of Thyroid Hormones on Cytoskeletal Proteins Are Mediated by GABAergic Mechanisms in Slices of Cerebral Cortex from Young Rats

: Thyroid hormones play important roles in brain function. However, few information is available about the effect of 3,5,3'-triiodo-L-thyronine (T(3)) or thyroxine (T(4)) on the in vitro phosphorylation of intermediate filament (IF) proteins from cerebral cortex of rats. In this study we investigated the involvement of GABAergic mechanisms mediating the effects of T(3) and T(4) on the in vitro incorporation of (32)P into IF proteins from cerebral cortex of 10-day-old male rats. Tissue slices were incubated with or without T(3), T(4), gamma-aminobutiric acid (GABA), kinase inhibitors or specific GABA antagonists and (32)P-orthophosphate for 30 min. The IF-enriched cytoskeletal fraction was extracted in a high salt Triton-containing buffer and the in vitro (32)P incorporation into IF proteins was measured. We first observed that 1 microM T(3) and 0.1 microM T(4) significantly increased the in vitro incorporation of (32)P into the IF proteins studied through the PKA and PKCaMII activities. A similar effect on IF phosphorylation was achieved by incubating cortical slices with GABA. Furthermore, by using specific GABA antagonists, we verified that T(3) induced a stimulatory effect on IF phosphorylation through noncompetitive mechanisms involving GABA(A), beyond GABA(B) receptors. In contrast, T(4) effects were mediated mainly by GABA(B) mechanisms. In conclusion, our results demonstrate a rapid nongenomic action of T(3) and T(4) on the phosphorylating system associated to the IF proteins in slices of cerebral cortex of 10 day-old male rats and point to GABAergic mechanisms mediating such effects.

The acute-phase response of the liver in relation to thyroid hormone-induced redox signaling

Recently, we reported that 3,3',5-triiodothyronine (T3) induces the expression of redox-sensitive genes as a nongenomic mechanism of T3 action. In this study, we show that T3 administration to rats (daily doses of 0.1 mg/kg ip for 3 consecutive days) induced a calorigenic response and liver glutathione depletion as an indication of oxidative stress, with higher levels of interleukin (IL)-6 in serum (ELISA) and hepatic STAT3 DNA binding (EMSA), which were maximal at 48-72 h after treatment. Under these conditions, the protein expression of the acute-phase proteins haptoglobin and beta-fibrinogen is significantly augmented, a change that is suppressed by pretreatment with alpha-tocopherol (100 mg/kg ip) or gadolinium chloride (10 mg/kg iv) before T3. It is concluded that T3 administration induces the acute-phase response in rat liver by a redox mechanism triggered at the Kupffer cell level, in association with IL-6 release and activation of the STAT3 cascade, a response that may contribute to reestablishing homeostasis in the liver and extrahepatic tissues exhibiting oxidative stress.

Short-term stimulation of lipogenesis by 3,5-L-diiodothyronine in cultured rat hepatocytes

Short-term effects of 3,5-l-diiodothyronine (T2) on lipid biosynthesis were studied in cultured hepatocytes from hypothyroid rats. A comparison with the effects of T3 was routinely carried out. After T2 addition to cell cultures, a distinct stimulation of fatty acid and cholesterol syntheses, measured as incorporation of [1-14C]acetate into these lipid fractions, was observed. The T2 dose-dependent effect on both metabolic pathways, already detectable at 10(-8)-10(-9) M, reached a 2-fold stimulation at 10(-5) M T2. At this concentration, the stimulatory effect was evident within 1 h of T2 addition to the hepatocytes and increased with time up to the length of the experimental period of 4 h. T2 stimulation of lipogenesis was also confirmed by incubating hepatocytes with [3H]H2O, used as an independent index of lipogenic activity. The effects of T2 are rather specific as 3,3',5,5'-tetraiodo-D-thyronine and 3,5-diiodo-L-tyrosine were practically ineffective on both fatty acid and cholesterol synthesis. Analysis of various lipid fractions showed that T2 addition to the cells produced a significant stimulation of the incorporation of newly synthesized fatty acids into both neutral and polar lipids. By comparing the effects induced by T2 with those seen in the presence of T3, it appeared that T2 was able to mimic T3 effects. Experiments conducted in the presence of cycloheximide, a protein synthesis inhibitor, indicated that the T2 stimulatory effect on fatty acid and cholesterol synthesis was essentially independent of protein synthesis.

Biological effects of 3,5-diiodothyronine (T(2))

This article is principally intended to describe the facts concerning the actions of 3,5-diiodothyronine (T(2)). Until recent years, T(2), because of its very low affinity for thyroid hormone receptors (THR), was considered an inactive metabolite of thyroid hormones (TH) (thyroxine (T(4)) and triiodo-L-thyronine (T(3))). Several observations, however, led to a reconsideration of this idea. Early studies dealing with the biological activities of this iodothyronine revealed its ability to stimulate cellular/mitochondrial respiration by a nuclear-independent pathway. Mitochondria and bioenergetic mechanisms seem to be major targets of T(2), although outside the mitochondria T(2) also has effects on carriers, ion-exchangers, and enzymes. Recent studies suggest that T(2) may also affect the transcription of some genes, but again the underlying mechanisms seem to be different from those actuated by T(3). The accumulated evidence permits the conclusion that the actions of T(2) do not simply mimic those of T(3) but instead are specific actions exerted through mechanisms that are independent of those actuated by T(3) and do not involve THR.

The roles of the iodothyronine deiodinases in mammalian development

The thyroid hormones (TH) are essential for normal development in vertebrate species. This review considers the roles that the three deiodinases, types 1, 2 and 3 (D1, D2, and D3), play in regulating intracellular levels of TH during this critical period. The focus is on rodents and humans with emphasis on brain development. There is little evidence to suggest that the D1 plays a significant role in development and this is substantiated by the absence of any obvious developmental impairment in a D1-deficient mouse model. There is, however, compelling indirect evidence pertaining to the importance of the D2 in development, particularly with respect to that of the brain. However, surprisingly, a D2-deficient mouse model exhibits a very mild phenotype. This, together with the fact that D2 activity is increased in hypothyroidism, suggests that this deiodinase may be of greater importance in development when supplies of thyroxine are limited. The D3 is clearly essential for development in the euthyroid mammal. Information, both indirect and that obtained from a D3-deficient mouse model, strongly suggests that its presence in placenta, uterus and some fetal tissues are critical for limiting exposure of fetal tissues to inappropriate levels of TH.

3,5-diiodo-L-thyronine powerfully reduces adiposity in rats by increasing the burning of fats

The effect of thyroid hormones on metabolism has long supported their potential as drugs to stimulate fat reduction, but the concomitant induction of a thyrotoxic state has greatly limited their use. Recent evidence suggests that 3,5-diiodo-L-thyronine (T2), a naturally occurring iodothyronine, stimulates metabolic rate via mechanisms involving the mitochondrial apparatus. We examined whether this effect would result in reduced energy storage. Here, we show that T2 administration to rats receiving a high-fat diet (HFD) reduces both adiposity and body weight gain without inducing thyrotoxicity. Rats receiving HFD + T2 showed (when compared with rats receiving HFD alone) a 13% lower body weight, a 42% higher liver fatty acid oxidation rate, appoximately 50% less fat mass, a complete disappearance of fat from the liver, and significant reductions in the serum triglyceride and cholesterol levels (-52% and -18%, respectively). Thyroid hormones and thyroid-stimulating hormone (TSH) serum levels were not influenced by T2 administration. The biochemical mechanism underlying the effects of T2 on liver metabolism involves the carnitine palmitoyl-transferase system and mitochondrial uncoupling. If the results hold true for humans, pharmacological administration of T2 might serve to counteract the problems associated with overweight, such as accumulation of lipids in liver and serum, without inducing thyrotoxicity. However, the results reported here do not exclude deleterious effects of T2 on a longer time scale as well as do not show that T2 acts in the same way in humans.

Involvement of calcium-dependent mechanisms in T3-induced phosphorylation of vimentin of immature rat testis

Thyroid hormones have been shown to act at extra nuclear sites, inducing target cell responses by several mechanisms, frequently involving intracellular calcium concentration. It has also been reported that cytoskeletal proteins are a target for thyroid and steroid hormones and cytoskeletal rearrangements are observed during hormone-induced differentiation and development of rat testes. However, little is known about the effect of 3,5,3'-triiodo-L-thyronine (T3) on the intermediate filament (IF) vimentin in rat testes. In this study we investigated the immunocontent and in vitro phosphorylation of vimentin in the cytoskeletal fraction of immature rat testes after a short-term in vitro treatment with T3. Gonads were incubated with or without T3 and 32P orthophosphate for 30 min and the intermediate filament-enriched cytoskeletal fraction was extracted in a high salt Triton-containing buffer. Vimentin immunoreactivity was analyzed by immunoblotting and the in vitro 32P incorporation into this protein was measured. Results showed that 1 microM T3 was able to increase the vimentin immunoreactivity and in vitro phosphorylation in the cytoskeletal fraction without altering total vimentin immunocontent in immature rat testes. Besides, these effects were independent of active protein synthesis. The involvement of Ca2+-mediated mechanisms in vimentin phosphorylation was evident when specific channel blockers (verapamil and nifedipine) or chelating agents (EGTA and BAPTA) were added during pre-incubation and incubation of the testes with T3. The effect of T3 was prevented when Ca2+ influx was blocked or intracellular Ca2+ was chelated. These results demonstrate a rapid nongenomic Ca2+-dependent action of T3 in phosphorylating vimentin in immature rat testes.

Thyroid hormone can increase estrogen-mediated transcription from a consensus estrogen response element in neuroblastoma cells

Thyroid hormones (T) and estrogens (E) are nuclear receptor ligands with at least two molecular mechanisms of action: (i) relatively slow genomic effects, such as the regulation of transcription by cognate T receptors (TR) and E receptors (ER); and (ii) relatively rapid nongenomic effects, such as kinase activation and calcium release initiated at the membrane by putative membrane receptors. Genomic and nongenomic effects were thought to be disparate and independent. However, in a previous study using a two-pulse paradigm in neuroblastoma cells, we showed that E acting at the membrane could potentiate transcription from an E-driven reporter gene in the nucleus. Because both T and E can have important effects on mood and cognition, it is possible that the two hormones can act synergistically. In this study, we demonstrate that early actions of T via TRalpha1 and TRbeta1 can potentiate E-mediated transcription (genomic effects) from a consensus E response element (ERE)-driven reporter gene in transiently transfected neuroblastoma cells. Such potentiation was reduced by inhibition of mitogen-activated protein kinase. Using phosphomutants of ERalpha, we also show that probable mitogen-activated protein kinase phosphorylation sites on the ERalpha, the serines at position 167 and 118, are important in TRbeta1-mediated potentiation of ERalpha-induced transactivation. We suggest that crosstalk between T and E includes potential interactions through both nuclear and membrane-initiated molecular mechanisms of hormone signaling.

Impairment in short-term but enhanced long-term synaptic potentiation and ERK activation in adult hippocampal area CA1 following developmental thyroid hormone insufficiency

Thyroid hormones are critical for the development and maturation of the central nervous system. Insufficiency of thyroid hormones during development impairs performance on tasks of learning and memory that rely upon the hippocampus and impairs synaptic function in young hypothyroid animals. The present study was designed to determine if perturbations in synaptic function persist in adult euthyroid animals exposed developmentally to insufficient levels of hormone. Pre- and postnatal thyroid hormone insufficiency was induced by administration of 3 or 10 ppm propylthiouracil (PTU) to pregnant and lactating dams via the drinking water from gestation day (GD) 6 until postnatal day (PN) 30. This regimen produced a graded level of hormonal insufficiency in the dam and the offspring. Population spike and population excitatory postsynaptic potentials (EPSP) were recorded at the pyramidal cell layer and the stratum radiatum, respectively, in area CA1 of hippocampal slices from adult male offspring. PTU exposure increased baseline synaptic transmission, reduced paired-pulse facilitation, and increased the magnitude of the population spike long-term potentiation (LTP). Phosphorylation of the extracellular signal-regulated kinases (ERK1 and ERK2) was increased as a function of LTP stimulation in slices from PTU-exposed adult animals. On the other hand, no differences in the basal levels of synaptic proteins implicated in synaptic plasticity (total ERK, synapsin, growth-associated protein-43, and neurogranin) were detected. These results reinforce previous findings of persistent changes in synaptic function and, importantly extend these observations to moderate levels of thyroid hormone insufficiency that do not induce significant toxicity to the dams or the offspring. Such alterations in hippocampal synaptic function may contribute to persistent behavioral deficits associated with developmental hypothyroidism.

Rapid nongenomic effects of 3,5,3'-triiodo-L-thyronine on the intracellular pH of L-6 myoblasts are mediated by intracellular calcium mobilization and kinase pathways

L-T3 and L-T4 activated the Na+/H+ exchanger of L-6 myoblasts, with a fast nongenomic mechanism, both in the steady state and when cells undergo acid loading with ammonium chloride. Monitored with the intracellular pH-sensitive fluorescent probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein, activation of the exchanger appeared to be initiated at the plasma membrane, because T3-agarose reproduced the effect of L-T3, and triiodothyroacetic acid, a hormone analog previously shown to inhibit membrane actions of thyroid hormone, blocked the action of L-T3 on the exchanger. We show here for the first time that transduction of the hormone signal in this nongenomic response requires tyrosine kinase-dependent phospholipase C activation and two different signaling pathways: 1) mobilization of intracellular calcium, assessed by the fluorescent probe fura-2, through activation of inositol trisphosphate receptors and without contributions from extracellular calcium or ryanodine receptors; and 2) protein phosphorylation involving protein kinase C and MAPK (ERK1/2), as shown by the use of kinase inhibitors and by immunoblotting for activated kinases.

Thyroid hormone treatment of hypothyroid rats restores the regenerative capacity and the mitochondrial membrane permeability properties of the liver after partial hepatectomy

We have investigated the effect of thyroid hormone on recovery of liver mass and on the mitochondrial membrane permeability properties during rat liver regeneration after 70% partial hepatectomy (PH). In the euthyroid state, liver weight starts to recover 24 h after PH and is completely restored 96 h after PH. Cyclosporin A (CsA)-sensitive mitochondrial permeability transition (MPT) occurs 24 h after PH, and it has been suggested to act in the signaling mechanism for hepatocyte proliferation. In this study we show that hypothyroidism delays recovery of the liver mass, being only 50% of the initial weight 96 h after PH, and alters the duration and mode of MPT occurrence, first inducing a CsA-insensitive swelling 24 h after PH, followed by a CsA-sensitive swelling 96 h after PH. The occurrence of both CsA-sensitive and -insensitive swelling is shown to be associated with an increase in mitochondrial calcium content. Concurrent with mitochondrial swelling, external release of matrix proteins from mitochondria, such as aspartate aminotransferase and malate dehydrogenase, is shown to be CsA insensitive 24 h after PH and CsA sensitive 96 h after PH. After thyroid hormone administration to hypothyroid rats, the liver regenerative capacity is restored, and the duration and mode of MPT occurrence as well as changes in mitochondrial calcium content become similar to those observed in the euthyroid condition. The results of the present study suggest the involvement of a mitochondria-mediated pathway in regulation of the liver regenerative process by thyroid hormone.

Thyroxine-thyroid hormone receptor interactions

Thyroid hormone (TH) actions are mediated by nuclear receptors (TRs alpha and beta) that bind triiodothyronine (T(3), 3,5,3'-triiodo-l-thyronine) with high affinity, and its precursor thyroxine (T(4), 3,5,3',5'-tetraiodo-l-thyronine) with lower affinity. T(4) contains a bulky 5' iodine group absent from T(3). Because T(3) is buried in the core of the ligand binding domain (LBD), we have predicted that TH analogues with 5' substituents should fit poorly into the ligand binding pocket and perhaps behave as antagonists. We therefore examined how T(4) affects TR activity and conformation. We obtained several lines of evidence (ligand dissociation kinetics, migration on hydrophobic interaction columns, and non-denaturing gels) that TR-T(4) complexes adopt a conformation that differs from TR-T(3) complexes in solution. Nonetheless, T(4) behaves as an agonist in vitro (in effects on coregulator and DNA binding) and in cells, when conversion to T(3) does not contribute to agonist activity. We determined x-ray crystal structures of the TRbeta LBD in complex with T(3) and T(4) at 2.5-A and 3.1-A resolution. Comparison of the structures reveals that TRbeta accommodates T(4) through subtle alterations in the loop connecting helices 11 and 12 and amino acid side chains in the pocket, which, together, enlarge a niche that permits helix 12 to pack over the 5' iodine and complete the coactivator binding surface. While T(3) is the major active TH, our results suggest that T(4) could activate nuclear TRs at appropriate concentrations. The ability of TR to adapt to the 5' extension should be considered in TR ligand design.

Selective activators of thyroid hormone receptors

An excess of thyroid hormone (TH) leads to a mix of deleterious (increased heart rate, muscle wasting and osteoporosis) and beneficial effects (reduced serum cholesterol and lipoprotein A and weight loss). All of these actions are mediated by nuclear thyroid hormone receptors (TRs), however, genetic evidence suggests that different TR isoforms do not contribute equally to individual TH effects. Thus, TR isoform selective activators could mimic the beneficial aspects of TH excess while avoiding the harmful effects. This article reviews new selective TR activators, their mechanism of action (they work by targeting the TR-beta isoform) and their actions in animal models. It is clear that these compounds represent a promising new avenue for the treatment of lipid disorders and obesity.

Thyroid hormone causes mitogen-activated protein kinase-dependent phosphorylation of the nuclear estrogen receptor

Activated by thyroid hormone, the MAPK (ERK1/2) signaling pathway causes serine phosphorylation by MAPK of several nucleoproteins, including the nuclear thyroid hormone receptor beta1. Because estrogen can activate MAPK and cause MAPK-dependent serine phosphorylation of nuclear estrogen receptor (ER)alpha, we studied whether thyroid hormone also promoted MAPK-mediated ERalpha phosphorylation. Human breast cancer (MCF-7) cells were incubated with physiological concentrations of l-T(4) or 17beta-estradiol (E(2)) for 15 min to 24 h, and nuclear ERalpha and serine-118-phosphorylated ERalpha were identified by Western blotting. Serine-118-phosphorylated ERalpha was recovered at 15 min in nuclei of MCF-7 cells exposed to either T(4) or E(2). The T(4) effect was apparent at 15 min and peaked at 2 h, whereas the E(2) effect was maximal at 4-6 h. T(4)-agarose was as effective as T(4) in causing phosphorylation of ERalpha. T(4) action on ERalpha was inhibited by PD 98059, an inhibitor of ERK1/2 phosphorylation, and by tetraiodothyroacetic acid, a T(4) analog that blocks cell surface-initiated actions of T(4) but is not itself an agonist. Electrophoretic mobility shift assay of nuclear extracts from T(4)-treated and E(2)-treated cells showed similar specific protein-DNA-binding. Indexed by [(3)H]thymidine incorporation and nuclear proliferating cell nuclear antigen, MCF-7 cell proliferation was stimulated by T(4) and T(4)-agarose to an extent comparable with the effect of E(2). This T(4) effect was blocked by either PD 98059 or ICI 182,780, an ER antagonist. Thus, T(4), like E(2), causes phosphorylation by MAPK of nuclear ERalpha at serine-118 in MCF-7 cells and promotes cell proliferation through the ER by a MAPK-dependent pathway.

Multi-tissue gene-expression analysis in a mouse model of thyroid hormone resistance

BACKGROUND

Resistance to thyroid hormone (RTH) is caused by mutations of the thyroid hormone receptor beta (TRbeta) gene. To understand the transcriptional program underlying TRbeta mutant-induced phenotypic expression of RTH, cDNA microarrays were used to profile the expression of 11,500 genes in a mouse model of human RTH.

RESULTS

We analyzed transcript levels in cerebellum, heart and white adipose tissue from a knock-in mouse (TRbetaPV/PV mouse) that harbors a human mutation (referred to as PV) and faithfully reproduces human RTH. Because TRbetaPV/PV mice have elevated thyroid hormone (T3), to define T3-responsive genes in the context of normal TRbeta, we also analyzed T3 effects in hyperthyroid wild-type gender-matched littermates. Microarray analysis revealed 163 genes responsive to T3 treatment and 187 genes differentially expressed between TRbetaPV/PV mice and wild-type littermates. Both the magnitude and gene make-up of the transcriptional response varied widely across tissues and conditions. We identified genes modulated in T3-dependent PV-independent, T3- and PV-dependent, and T3-independent PV-dependent pathways that illuminated the biological consequences of PV action in vivo. Most T3-responsive genes that were dysregulated in the heart and white adipose tissue of TRbetaPV/PV mice were repressed in T3-treated wild-type mice and upregulated in TRbetaPV/PV mice, suggesting the inappropriate activation of T3-suppressed genes in RTH.

CONCLUSIONS

Comprehensive multi-tissue gene-expression analysis uncovered complex multiple signaling pathways that mediate the molecular actions of TRbeta mutants in vivo. In particular, the T3-independent mutant-dependent genomic response unveiled the contribution of a novel 'change-of-function' of TRbeta mutants to the pathogenesis of RTH. Thus, the molecular actions of TRbeta mutants are more complex than previously envisioned.

Proteins of multiple classes may participate in nongenomic steroid actions

Responses to steroids initiated from non-nuclear receptors impinge on a wide variety of cellular responses and utilize nearly all known signal transduction webs. While the mechanisms by which steroid receptors localize in the membrane are still unclear, it is apparent that this alternative localization allows steroid receptors to participate in a wide range of complex functions influencing cell proliferation, death, and differentiation. The central debate still remains the identity of the protein class or classes that mediate membrane-initiated (nongenomic) responses. The data thus far have supported several possibilities, including: nuclear steroid receptor-like forms in non-nuclear locations; other known (nonsteroid) membrane receptors or channels with additional steroid-binding sites; enzymes; transporters; receptors for serum steroid-binding proteins; unique and previously undescribed proteins; or chimeras of typical steroid receptor domains with other unique or known protein domains. Categorizing membrane steroid receptor proteins based exclusively on the actions of antagonists and agonists, without considering cell context and protein partnering issues, may mislead us into predicting more receptor subtypes than really exist. However, the plethora of signaling and functional outcomes may indicate the participation of more than one kind of steroid-binding protein. Resolving such unanswered questions will require future investigative focus on this alternative arm of steroid action, which is likely to yield as many therapeutic opportunities as have nuclear steroid mechanisms.

Thyroid-stimulating hormone receptor and its role in Graves' disease

The thyroid-stimulating hormone (TSH, or thyrotropin) receptor (TSHR) mediates the activating action of TSH to the thyroid gland, resulting in the growth and proliferation of thyrocytes and thyroid hormone production. In Graves' disease, thyroid-stimulating autoantibodies can mimic TSH action and stimulate thyroid cells. This leads to hyperthyroidism and abnormal overproduction of thyroid hormone. TSHR-antibodies-binding epitopes on the receptor molecule are well studied. Mechanism of TSHR-autoantibodies production is more or less clear but a susceptibility gene, which is linked to their production, is still unknown. Genetic studies show no linkage between the TSHR gene and Graves' disease. Among three common polymorphisms in the TSHR gene, only the D727E germline polymorphism in the cytoplasmic tail of the receptor showed an association with the disease, and this association is weak. The absence of a strong genetic effect of the TSHR polymorphisms in such a common and complex disorder as Graves' disease may be explained by a high degree of evolutionary conservation in TSHR. This can be shown by naturally existing germline and somatic mutations in the TSHR gene that cause various types of nonautoimmune and hereditary thyroid disease.

Aldosterone potentiates 1,25-dihydroxyvitamin D3 action in renal thick ascending limb via a nongenomic, ERK-dependent pathway

Recently, we demonstrated that aldosterone inhibits HCO3- absorption in the rat medullary thick ascending limb (MTAL) via a nongenomic pathway blocked by inhibitors of extracellular signal-regulated kinase (ERK) activation. Here we examined the effects on the MTAL of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], which regulates cell functions through nongenomic mechanisms in nonrenal systems. Addition of 1 nM 1,25(OH)2D3 to the bath decreased HCO3- absorption by 24%, from 15.0 +/- 0.3 to 11.4 +/- 0.5 pmol. min-1. mm-1 (P < 0.001). This inhibition was maximal within 60 min and was eliminated by pretreatment with actinomycin D, cycloheximide, or inhibitors of protein kinase C. In MTAL bathed with 1 nM aldosterone [added 15-20 min before 1,25(OH)2D3], the absolute (5.6 +/- 0.3 vs. 3.6 +/- 0.3 pmol. min-1. mm-1) and fractional (49 +/- 2 vs. 24 +/- 2%) decreases in HCO3- absorption induced by 1,25(OH)2D3 were significantly greater than those in the absence of aldosterone (P < 0.05). The effect of aldosterone to potentiate inhibition by 1,25(OH)2D3 was not affected by spironolactone but was eliminated by the MAPK kinase/ERK inhibitor U-0126. U-0126 did not affect inhibition of HCO3- absorption by 1,25(OH)2D3 alone. Aldosterone induced rapid activation of ERK via a transcription-independent pathway. We conclude that 1) 1,25(OH)2D3 inhibits HCO3- absorption in the MTAL via a genomic pathway involving protein kinase C, which may contribute to 1,25(OH)2D3-induced regulation of urinary net acid and/or Ca2+ excretion and 2) aldosterone potentiates inhibition by 1,25(OH)2D3 through an ERK-dependent, nongenomic pathway. These results identify a novel regulatory interaction whereby aldosterone acts via nongenomic mechanisms to enhance the genomic response to 1,25(OH)2D3. Aldosterone may influence a broad range of biological processes, including epithelial transport, by modifying the response of target tissues to 1,25(OH)2D3 stimulation.

Short-term activation by low 17beta-estradiol concentrations of the Na+/H+ exchanger in rat aortic smooth muscle cells: physiopathological implications

Low physiological concentrations of 17beta-estradiol increased the intracellular pH of rat aortic smooth muscle cells by a rapid nongenomic mechanism. This effect was due to stimulation of the Na+/H+ exchanger activity, measured using the intracellular pH-sensitive fluorescent probe 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein. The 17beta-estradiol gave rise to a bell-shaped dose response, with a maximum at 10-12 m and no significant effect at 10-9 m. The specificity of the effect was verified by the use of the Na+/H+ exchanger inhibitor 5-(ethyl-N-isopropyl)amiloride and the lack of effect of the isomer 17alpha-estradiol. Inhibitors of the nuclear estrogen receptors, tamoxifen and ICI 182,780, completely prevented activation of the exchanger by 17beta-estradiol. The effect of low estrogen concentrations on the intracellular pH was mimicked by both norepinephrine and phenylephrine, suggesting a connection between the increase of intracellular pH and the muscle contraction process. The transduction mechanism for this nongenomic effect of estrogens did not involve modulation of the cAMP content, whereas inositol 1,4,5-trisphosphate, protein kinase C and MAPK pathways appear to play a role, as indicated by both pharmacological approaches and immunoblot experiments on protein kinase C translocation and ERK phosphorylation. These results for the first time provide evidence for a nongenomic effect of low physiological concentrations of 17beta-estradiol on intracellular pH that, together with other factors, may contribute to the development of hypertension and atherosclerosis in men and postmenopausal women and increase the risk of cardiovascular disease. Paradoxically, the lack of stimulation at high physiological estradiol levels could explain the protective effects found in premenopausal women.