Cellular actions of thyroid hormone may be initiated within the cell nucleus, at the plasma membrane, in cytoplasm, and at the mitochondrion. Thyroid hormone nuclear receptors (TRs) mediate the biological activities of T(3) via transcriptional regulation. Two TR genes, alpha and beta, encode four T(3)-binding receptor isoforms (alpha1, beta1, beta2, and beta3). The transcriptional activity of TRs is regulated at multiple levels. Besides being regulated by T(3), transcriptional activity is regulated by the type of thyroid hormone response elements located on the promoters of T(3) target genes, by the developmental- and tissue-dependent expression of TR isoforms, and by a host of nuclear coregulatory proteins. These nuclear coregulatory proteins modulate the transcription activity of TRs in a T(3)-dependent manner. In the absence of T(3), corepressors act to repress the basal transcriptional activity, whereas in the presence of T(3), coactivators function to activate transcription. The critical role of TRs is evident in that mutations of the TRbeta gene cause resistance to thyroid hormones to exhibit an array of symptoms due to decreasing the sensitivity of target tissues to T(3). Genetically engineered knockin mouse models also reveal that mutations of the TRs could lead to other abnormalities beyond resistance to thyroid hormones, including thyroid cancer, pituitary tumors, dwarfism, and metabolic abnormalities. Thus, the deleterious effects of mutations of TRs are more severe than previously envisioned. These genetic-engineered mouse models provide valuable tools to ascertain further the molecular actions of unliganded TRs in vivo that could underlie the pathogenesis of hypothyroidism. Actions of thyroid hormone that are not initiated by liganding of the hormone to intranuclear TR are termed nongenomic. They may begin at the plasma membrane or in cytoplasm. Plasma membrane-initiated actions begin at a receptor on integrin alphavbeta3 that activates ERK1/2 and culminate in local membrane actions on ion transport systems, such as the Na(+)/H(+) exchanger, or complex cellular events such as cell proliferation. Concentration of the integrin on cells of the vasculature and on tumor cells explains recently described proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on certain cancer cells, including gliomas. Thus, hormonal events that begin nongenomically result in effects in DNA-dependent effects. l-T(4) is an agonist at the plasma membrane without conversion to T(3). Tetraiodothyroacetic acid is a T(4) analog that inhibits the actions of T(4) and T(3) at the integrin, including angiogenesis and tumor cell proliferation. T(3) can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin alphavbeta3. Downstream consequences of phosphatidylinositol 3-kinase activation by T(3) include specific gene transcription and insertion of Na, K-ATPase in the plasma membrane and modulation of the activity of the ATPase. Thyroid hormone, chief...

The nongenomic actions of thyroid hormone begin at receptors in the plasma membrane, mitochondria or cytoplasm. These receptors can share structural homologies with nuclear thyroid hormone receptors (TRs) that mediate transcriptional actions of T3, or have no homologies with TR, such as the plasma membrane receptor on integrin αvβ3. Nongenomic actions initiated at the plasma membrane by T4 via integrin αvβ3 can induce gene expression that affects angiogenesis and cell proliferation, therefore, both nongenomic and genomic effects can overlap in the nucleus. In the cytoplasm, a truncated TRα isoform mediates T4-dependent regulation of intracellular microfilament organization, contributing to cell and tissue structure. p30 TRα1 is another shortened TR isoform found at the plasma membrane that binds T3 and mediates nongenomic hormonal effects in bone cells. T3 and 3,5-diiodo-L-thyronine are important to the complex nongenomic regulation of cellular respiration in mitochondria. Thus, nongenomic actions expand the repertoire of cellular events controlled by thyroid hormone and can modulate TR-dependent nuclear events. Here, we review the experimental approaches required to define nongenomic actions of the hormone, enumerate the known nongenomic effects of the hormone and their molecular basis, and discuss the possible physiological or pathophysiological consequences of these actions.

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Enzymatic 5'-deiodination of 3,3',5'-triiodothyronine (rT3) and 3,3',5,5'-tetraiodothyronine (thyroxine, T4) was studied in microsomal preparations of rat cerebral cortex. Evidence was obtained for the existence of two thiol-dependent 5'-deiodinase entities. One of these predominates in tissue from euthyroid and long-term hypothyroid rats, is specific for rT3, follows "ping-pong" kinetics with dithiothreitol as the cosubstrate, and is inhibited by propylthiouracil (PrSUra) and iodoacetate. Inhibition by PrSUra is uncompetitive with rT3 and competitive with dithiothreitol. These properties are shared with the 5'-deiodinase activity of liver and kidney. The activity of a second type of 5'-deiodinase is highest in cerebral cortex from short-term hypothyroid rats, prefers T4 to rT3 as the substrate, is insensitive to PrSUra and iodoacetate, and follows "sequential" reaction kinetics. A similar PrSUra-insensitive 5'-deiodinase activity is also found in pituitary but is not detectable in liver and kidney; it seems, therefore, characteristic oftissues in which local T4 to 3,3',5-triiodothyronine (T3) conversion supplies a major portion of the total intracellular T3.The main secretory product of the thyroid gland, 3,3',5,5'-tetraiodothyronine (thyroxine, T4) is converted to the biologically more potent 3,3',5-triiodothyronine (T3) in many tissues of the body by a process termed 5'-deiodination. There appears to be a differentiation between tissues such as the liver and the kidneys, where most of the T3 produced is returned to the circulation, and tissues such as the brain and the pituitary, where the T4 to T3 conversion seems to serve local purposes (1). Another distinction between the deiodinase activities at these loci is the effect ofchanges in thyroid status. Thus, hypothyroidism is associated with decreased T4 to T3 conversion in rat liver and kidney, whereas increased conversion rates are observed in the pituitary and the brain (2)(3)(4)(5). Not only are the changes opposite but also they occur more rapidly in the central organs (6,*).3,3',5'-Triiodothyronine (reverse T3, rT3), an inactive intermediary product in the metabolism of T4, is in fact a better substrate for the enzymes that convert T4 to T3 in the liver and the kidney (7)(8)(9). The enzymes are located in membrane-derived fractions ofthese tissues, depend on thiols for activity, and are effectively inhibited by derivatives of 2-thiouracil-e.g., 6-propyl-2-thiouracil (PrSUra) (7-15).A third difference with T4 5'-deiodinase activities ofthe liver and the kidney is that conversion of T4 to T3 in the pituitary and the brain is not affected by PrSUra in vivo or in vitro (3,4,16,17). However, brain rT3 5'-deiodination is sensitive to PrSUra (18, t pooled tissue from 10 rats (approximately 5 g wet weight) was homogenized in 5-6 vol (vol/wt) of ice-cold 0.32 M sucrose, 10 mM Hepes (pH 7.0), containing 10 mM dithiothreitol (DTT), using a hand-driven, all-glass homogenizer. The homogenate was centrifuged at 4°C for 15 min at 15,000 X g. The pellet was wa...

Based on these findings, we propose three possible routes for thyroid hormone feedback on TRH neurons in the human PVN: 1) local thyroid hormone uptake from the vascular compartment within the PVN, 2) thyroid hormone uptake from the cerebrospinal fluid in the third ventricle followed by transport to TRH neurons in the PVN or IFN neurons projecting to TRH neurons in the PVN, and 3) thyroid hormone sensing in the IFN of the mediobasal hypothalamus by neurons projecting to TRH neurons in the PVN.

A B S T R A C T We have studied 5'-deiodination of thyroxine (T4) and 3,3',5'-triiodothyronine (rT3) in rat pituitary tissue in vitro, with respect to substrate specificity, reaction kinetics, effects of 6-n-propyl-2-thiouracil (PTU), and the time course of effects of thyroid hormone depletion and repletion. Removal of one phenolic iodine or both tyrosyl iodines from the T4 molecule resulted in compounds that were not deiodinated, but alterations in the alanine side chain had little effect.5'-Deiodination of 2 nM rT3 by pituitary microsomes from euthyroid rats was inhibited >90% by 1 mM PTU, but was inhibited <10% by 100 nM T4. The apparent Michaelis constant (K.n) and maximum velocity (Vll~ax) for rT3 at 20 mM dithiothreitol (DTT) were 33 nM and 84 pmol/mg protein per h. This reaction followed ping-pong type reaction kinetics when concentrations of DTT sitive rT3 5'-deiodination (K, = 1.3 nM). T4 5'-deiodination by hypothyroid microsomes was not affected by PTU, was competitively inhibited by rT3 (Ki, 1.7 nM), and exhibited sequential type reaction kinetics with DTT as cosubstrate. When T4 5'-deiodination was measured in euthyroid and hypothyroid microsomes, respectively, the apparent Km and Vma. for T4 at 20 mM DTT, were 0.9 nM and 0.55 pmol/mg protein per h (euthyroid), and 0.8 nM and 6.9 pmol/mg protein per h (hypothyroid).The T4 5'-deiodination rate and the PTU-insensitive, but not total, rT3 5'-deiodination rate (i.e. measured in the presence and the absence of 1 mM PTU, respectively) in pituitary homogenates were significantly elevated 24 h after thyroidectomy. PTU-insensitive activity continued to increase until at .30 d after thyroidectomy it was 11 times the PTU-insensitive activity in controls. At the latter time, PTU-sensitive rT3 5'-deiodinase activity appeared to be decreased. The increase in PTU-insensitive T4 and rT3 5'-deiodination observed 48

In rats subjected to thyroidectomy there was a two- to fourfold increase in cerebral cortex iodothyronine 5'-deiodinase activity within 24 hours. This increase was prevented by thyroxine replacement. The increased cortical 5'-deiodinase in chronically hypothyroid rats was normalized within 4 hours by a single intravenous injection of triiodothyronine. These results indicate that the adult central nervous system can give a very rapid biochemical response to thyroid hormone.

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