类固醇激素非基因组作用的机制及意义

现已证明各种类型的类固醇激素均能通过非基因组作用快速改变生理过程。不同的激素,或同一激素对不同的细胞,其非基因组作用的机制各不相同,多种多样,并且不时有新的机制诞生。本文将迄今为止类固醇激素快速非基因组作用的可能机制作一综述,并初步阐述其可能的意义。     

Non-genomic effects of thyroid hormone in adult cardiac myocytes: relevance to gene expression and cell growth

Besides the well-characterized genomic action of thyroid hormone (TH), mediated by thyroid hormone receptors (TRs), accumulating data support the so-called non-genomic action of TH, which is often related to activation of signalling pathways. In this study, we sought to determine whether TH activates intracellular signalling pathways in the adult cardiac myocytes and whether such activation modulates cell growth and the expression of target proteins important in cardiac function. We demonstrate that TH promotes a rapid increase in the phosphorylation of several kinases, ERK1/2, PKCδ, p38-MAPK and Akt. This activation is inhibited by triiodothyroacetic acid (triac), which is a TH analogue known to displace the hormone from membrane bound receptors, indicating that this TH effect is mediated through a cell membrane-initiated mechanism. Furthermore, using specific inhibitors of the TH-activated kinases, we show that the long-term effects of TH on the expression of sarcoplasmic reticulum Ca²⁺-ATPase (SERCA), α- and β-myosin heavy chain (MHC) and cell growth are reverted, implying that what is initiated as a non-genomic action of the hormone interfaces with genomic effects. These data provide further insights into the underlying mechanisms of TH action in the heart with potentially important implications in the management of cardiac pathology.     

Non-genomic immunosuppressive actions of progesterone inhibits PHA-induced alkalinization and activation in T cells

Progesterone is an endogenous immunomodulator, and can suppress T-cell activation during pregnancy. When analyzed under a genome time scale, the classic steroid receptor pathway does not have any effect on ion fluxes. Therefore, the aim of this study was to investigate whether the non-genomic effects on ion fluxes by progesterone could immunosuppress phytohemagglutinin (PHA)-induced human peripheral T-cell activation. The new findings indicated that, first, only progesterone stimulated both [Ca2+]i elevation and pHi decrease; in contrast, estradiol or testosterone stimulated [Ca2+]i elevation and hydrocortisone or dexamethasone stimulated pHi decrease. Secondly, the [Ca2+]i increase by progesterone was dependent on Ca2+ influx, and the acidification was blocked by Na+/H+ exchange (NHE) inhibitor, 3-methylsulphonyl-4-piperidinobenzoyl, guanidine hydrochloride (HOE-694) but not by 5-(N,N-dimethyl)-amiloride (DMA). Thirdly, progesterone blocked phorbol 12-myristate 13-acetate (PMA) or PHA-induced alkalinization, but PHA did not prevent progesterone-induced acidification. Fourthly, progesterone did not induce T-cell proliferation; however, co-stimulation progesterone with PHA was able to suppress PHA-induced IL-2 or IL-4 secretion and proliferation. When progesterone was applied 72 h after PHA stimulation, progesterone could suppress PHA-induced T-cell proliferation. Finally, immobilization of progesterone by conjugation to a large carrier molecule (BSA) also stimulated a rapid [Ca2+]i elevation, pHi decrease, and suppressed PHA-induced proliferation. These results suggested that the non-genomic effects of progesterone, especially acidification, are exerted via plasma membrane sites and suppress the genomic responses to PHA. Progesterone might act directly through membrane specific nonclassical steroid receptors to cause immunomodulation and suppression of T-cell activation during pregnancy.     

Changes in thyroid hormone levels during zebrafish development

The zebrafish (Danio rerio) has been used as a model for the study of endocrine disrupting chemicals. This study set out to determine the profiles of whole-body thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels during the development of zebrafish from embryo to adult. Enzyme-linked immunoassay was used to analyze whole-body T4 and T3 contents. The results showed that whole-body T4 and T3 levels remained stable during the pre-hatching period (0-3 d) and increased significantly during early development after hatching. The T3 level peaked at 0.28 ± 0.01 ng g(-1) body weight at 10 days post-fertilization (dpf), and T4 peaked at 0.58 ± 0.09 ng g(-1) body weight at 21 dpf. Both thyroid hormones subsequently declined during later development. This study establishes a baseline for thyroid hormones in zebrafish, which will be vital for the understanding of thyroid hormone functions and in future studies of thyroid toxicants in this species.     

In quest of thyroid hormone function in mature mammalian brain

Thyroid hormones (TH) have important functions in maturation, differentiation and metabolism during developmental periods in almost all types of tissues including brain of vertebrate animals. In humans' thyroid malfunction in early developmental stages cause severe neuropsychological abnormalities due to defective gene expression via nuclear receptor activation. However, role of TH in adult mammalian brain is lacking and unclear mainly because it was considered for a long time as a TH unresponsive tissue. Although adult brain contains a substantial number of TH nuclear receptors, no functional properties could be attributed. Recent findings suggest that T3 is distributed, concentrated, metabolized and binds to specific membrane sites within adult brain. In mature humans TH also reversibly regulates various neuropsychological symptoms produced in mature condition. This review discusses development of recent concepts and literature on role of TH and its importance in neuronal function in adult mammalian brain.     

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

The thyroid hormones L-thyroxine and triiodo-Lthyronine have profound effects on postenbryonic development of most vertebrates.Analysis of their 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 larvas of most anurans.This article will first describe the salient features of metamorphosis and its control by TH and other hormones.Emphasis will be laid on the key 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 other vertebrate species.     

Influence of thyroid hormone deficiency on the citrate synthase activity in rat brain regions during early postnatal development

The developmental pattern of citrate synthase activity has been studied in the liver and several brain areas of hypothyroid rats during the 4 first weeks of life. While citrate synthase activity in the liver showed a rise during the 2 first weeks of life, different patterns of enzyme activity were found in the brain regions of euthyroid animals. Citrate synthase activity increased in the cerebellum, decreased in the cerebral cortex and did not change significantly in the brain stem during the period studied. In the liver and brain areas, too, a decrease in citrate synthase activity was observed during hypothyroidism. From the 2nd week of birth, the citrate synthase activity in the brain but not in the liver was found to have recovered. The newly elevated citrate synthase activity coincided with a slight increase in thyroid hormone serum levels.     

Dual functions of thyroid hormone receptors during Xenopus development

Thyroid hormone (TH) plays a causative role in anuran metamorphosis. This effect is presumed to be manifested through the regulation of gene expression by TH receptors (TRs). TRs can act as both activators and repressors of a TH-inducible gene depending upon the presence and absence of TH, respectively. We have been investigating the roles of TRs during Xenopus laevis development, including premetamorphic and metamorphosing stages. In this review, we summarize some of the studies on the TRs by others and us. These studies reveal that TRs have dual functions in frog development as reflected in the following two aspects. First, TRs function initially as repressors of TH-inducible genes in premetamorphic tadpoles to prevent precocious metamorphosis, thus ensuring a proper period of tadpole growth, and later as activators of these genes to activate the metamorphic process. Second, TRs can promote both cell proliferation and apoptosis during metamorphosis, depending upon the cell type in which they are expressed.     

Stimulation of tubulin synthesis by thyroid hormone in the developing rat brain

The effect of triiodothyronine on the biogenesis of tubulin has been studied in the developing rat brain. In organ cultures of brains from newborn rats, the hormone stimulates the incorporation of [14C]leucine into tubulin by 60-80% within 2 h in the absence of any significant change in total protein synthesis. This stimulation is strictly age-dependent (only brains from late fetal or newborn rats are sensitive), dose-dependent (stimulation increases progressively and reaches a maximum level with physiological dose of the hormone) and displays tissue specificity. The temporal correspondence of the sensitivity of the rat brain to triiodothyronine with the period of normal rise in the level of tubulin and that of the maximal level of nuclear triiodothyronine receptors in the brain strongly suggests the involvement of the hormone in regulating the biogenesis of tubulin during the differentiation and maturation of neonatal rat brain.     

Effect of thyroid hormone on metabolic compartmentation in the developing rat brain

1. The effects of treatment with thyroid hormone (tri-iodothyronine) and of neonatal thyroidectomy on the cerebral metabolism of [U-¹⁴C]leucine were investigated during the period of functional maturation of the rat brain extending from 9 to 25 days after birth. 2. Age-dependent changes in the labelling of brain constituents under normal conditions appear to depend on changes in the availability of blood-borne [¹⁴C]leucine resulting from differential rates of growth of body and brain; but developmental changes in the pool size of free leucine and in the rates of protein synthesis and oxidation of leucine are also involved. 3. Treatment with thyroid hormone had no significant effect on the conversion of leucine carbon into proteins and lipids; and the age-dependent changes in the concentration and specific radioactivity of leucine were similar to controls. On the other hand there was an acceleration in the conversion of leucine carbon into amino acids associated with the tricarboxylic acid cycle. These observations indicate that leucine oxidation was the process mainly affected. 4. The specific radioactivity of glutamine relative to that of glutamate was used as an index of metabolic compartmentation in brain tissue. Treatment with thyroid hormone advanced the development of metabolic compartmentation. 5. Neonatal thyroidectomy led to a marked decrease in the conversion of leucine carbon into proteins and lipids and to a significant increase in the amount of ¹⁴C combined in the amino acids associated with the tricarboxylic acid cycle. The age-dependent increase in the glutamate/glutamine specific-radioactivity ratio was strongly retarded. 6. The increased conversion of leucine carbon into cerebral amino acids applied to glutamate and aspartate, but not to glutamine and γ-aminobutyrate. This observation facilitated the understanding of the effects of thyroid deprivation on brain metabolism and provided new evidence for the allocation of morphological structures to the metabolic compartments in brain tissue. 7. In contrast with the marked effects of the thyroid state on metabolic compartmentation, it had relatively little effect on the developmental changes in the concentration of amino acids in the brain. 8. The rate of conversion of leucine carbon into the `cycle amino acids' both under normal conditions and in thyroid deficiency indicated a special metabolic relationship between glutamate and aspartate on the one hand, and glutamine and γ-aminobutyrate on the other.     

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

The thyroid hormones L-thyroxine and triiodo-L-thyronine have profound effects on postembryonic development of most vertebrates. Analysis of their 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. This article will first describe the salient features of metamorphosis and its control by TH and other hormones. Emphasis will be laid on the key 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 other vertebrate species.     

Short-term effects of thyroid hormone in prenatal development and cell differentiation

Extranuclear or nongenomic effects of thyroid hormones do not require interaction with the nuclear receptor, but are probably mediated by specific membrane receptors. This review will focus on the extranuclear effects of thyroid hormones on plasma membrane transport systems in non mammalian cells: chick embryo hepatocytes at two different stages of development, 14 and 19 days. At variance with mammals, the chick embryo develops in a closed compartment, beyond the influence of maternal endocrine factors. Thyroid hormones inhibit the Na+/K+-ATPase but stimulate the Na+/H+ exchanger and amino acid transport System A with different dose-responses: a bell-shaped curve in the case of the exchanger and a classic saturation curve in the case of System A. These effects are mimicked by the analog 3,5-diiodothyronine. Signal transduction is mediated by interplay among kinases, mainly protein kinase C and the MAPK pathway, initially primed by second messengers such as Ca2+, IP3, and DAG as in mammalian cells. Thyroid hormones and 3,5-diiodothyronine stimulate thymidine incorporation and DNA synthesis, associated with the increased levels and activity of cyclins and cyclin-dependent kinases involved in the G1/S transition, and also these effects have their starting point at the plasma membrane. Increasing evidence now demonstrates that thyroid hormones act as growth factors for chick embryo hepatocytes and their extranuclear effects are important for prenatal development and differentiation.     

Cooperative nongenomic and genomic actions on thyroid hormone mediated-modulation of T cell proliferation involve up-regulation of thyroid hormone receptor and inducible nitric oxide synthase expression

Thyroid hormones (THs) exert a broad range of actions on development, growth, and cell differentiation by both genomic and nongenomic mechanisms. THs regulate lymphocyte function, but the participation of nongenomic actions is still unknown. Here the contribution of both genomic and nongenomic effects on TH-induced division of T cells was studied by using free and noncell permeable THs coupled to agarose (TH-ag). THs-ag led to cell division, but to a lesser extent than free hormones. THs induced nongenomically the rapid translocation of protein kinase C (PKC) ζ isoform to cell membranes, extracellular-signal-regulated kinases (ERK1/2) phosphorylation and nuclear factor-κB (NF-κB) activation. The signaling cascade include sphingomyelinases acting up-stream the activation of PKCζ isoform, while ERK and NF-κB are activated downstream this PKC isoenzyme. Both free and THs-ag increased the protein and mRNA levels of TH nuclear receptor TRα1, while only free hormones incremented the inducible NOS gene and protein levels as well as a calcium independent NOS activity. Both effects were blunted by PKCζ inhibition. These results indicate that THs, by triggering a nongenomic signaling cascade that involves Smases-mediated activation of PKCζ, lead to ERK 1/2 and NF-κB activation and to the genomic increase of TRs and the inducible nitric oxide synthase protein and mRNA levels, improving T lymphocyte proliferation. These finding not only contribute to the understanding of the mechanisms involved in TH modulation of lymphocyte physiology, but would also point out for the first time the interplay between genomic and nongenomic TH actions in T cells.