Sodium butyrate selectively alters thyroid hormone receptor gene expression in GH3 cells

Intra-laboratory validation of yeast-based reporter gene assays for human thyroid hormone receptors

Thyroid hormones (THs) function by activating TH receptors (THRα and THRβ) on the target cells. Several chemicals adversely affect human health and ecosystems by disrupting TH signaling. Multiple assays for thyroid disruption have been reviewed for validation, especially with respect to assay reliability, sensitivity, efficiency, and technical criteria in the OECD Test Guidelines framework. The reporter gene assay is a sensitive method used to observe cellular events associated with signal transduction and gene expression. Some mammalian cell-based THR reporter gene assays have been developed optimized, and verified. In contrast, yeast-based reporter gene assays offer a cost-effective and rapid approach compared to mammalian-based assays and are considered advantageous for detecting direct or indirect interactions between test chemicals and the receptor of interest. In this study, the previously developed yeast-based reporter gene assays for human-derived THRα and THRβ were validated using several test substances, including THs and TH-disrupting chemicals, to establish scientific confidence. Our results showed that the yeast-based THRs reporter gene assays had high repeatability and the use of a fluorescent substrate improved the detection of TH disruption caused by several chemicals. Although inter-laboratory studies are needed to verify the acceptance and data interpretation criteria, our assays can provide information on the potential of chemicals to interfere with THR transactivation as animal-free in vitro assays.

Gut Microbiome Alterations in Patients With Thyroid Nodules

Thyroid nodules are found in nearly half of the adult population. Accumulating evidence suggests that the gut microbiota plays an important role in thyroid metabolism, yet the association between gut microbiota capacity, thyroid nodules, and thyroid function has not been studied comprehensively. We performed a gut microbiome genome-wide association study in 196 patients with thyroid nodules and 283 controls by using whole-genome shotgun sequencing. We found that participants with high-grade thyroid nodules have decreased number of gut microbial species and gene families compared with those with lower grade nodules and controls. There are also significant alterations in the overall microbial composition in participants with high-grade thyroid nodules. The gut microbiome in participants with high-grade thyroid nodules is characterized by greater amino acid degradation and lower butyrate production. The relative abundances of multiple butyrate producing microbes are reduced in patients with high-grade thyroid nodules and the relative abundances of L-histidine metabolism pathways are associated with thyrotropin-releasing hormone. Our study describes the gut microbiome characteristics in thyroid nodules and a gut-thyroid link and highlight specific gut microbiota as a potential therapeutic target to regulate thyroid metabolism.

Metabolomics for Investigating Physiological and Pathophysiological Processes

Metabolomics uses advanced analytical chemistry techniques to enable the high-throughput characterization of metabolites from cells, organs, tissues, or biofluids. The rapid growth in metabolomics is leading to a renewed interest in metabolism and the role that small molecule metabolites play in many biological processes. As a result, traditional views of metabolites as being simply the "bricks and mortar" of cells or just the fuel for cellular energetics are being upended. Indeed, metabolites appear to have much more varied and far more important roles as signaling molecules, immune modulators, endogenous toxins, and environmental sensors. This review explores how metabolomics is yielding important new insights into a number of important biological and physiological processes. In particular, a major focus is on illustrating how metabolomics and discoveries made through metabolomics are improving our understanding of both normal physiology and the pathophysiology of many diseases. These discoveries are yielding new insights into how metabolites influence organ function, immune function, nutrient sensing, and gut physiology. Collectively, this work is leading to a much more unified and system-wide perspective of biology wherein metabolites, proteins, and genes are understood to interact synergistically to modify the actions and functions of organelles, organs, and organisms.

Comparative Analyses of the 12 Most Abundant PCB Congeners Detected in Human Maternal Serum for Activity at the Thyroid Hormone Receptor and Ryanodine Receptor

Polychlorinated biphenyls (PCBs) pose significant risk to the developing human brain; however, mechanisms of PCB developmental neurotoxicity (DNT) remain controversial. Two widely posited mechanisms are tested here using PCBs identified in pregnant women in the MARBLES cohort who are at increased risk for having a child with a neurodevelopmental disorder (NDD). As determined by gas chromatography-triple quadruple mass spectrometry, the mean PCB level in maternal serum was 2.22 ng/mL. The 12 most abundant PCBs were tested singly and as a mixture mimicking the congener profile in maternal serum for activity at the thyroid hormone receptor (THR) and ryanodine receptor (RyR). Neither the mixture nor the individual congeners (2 fM to 2 μM) exhibited agonistic or antagonistic activity in a THR reporter cell line. However, as determined by equilibrium binding of [3H]ryanodine to RyR1-enriched microsomes, the mixture and the individual congeners (50 nM to 50 μM) increased RyR activity by 2.4-19.2-fold. 4-Hydroxy (OH) and 4-sulfate metabolites of PCBs 11 and 52 had no TH activity; but 4-OH PCB 52 had higher potency than the parent congener toward RyR. These data support evidence implicating RyRs as targets in environmentally triggered NDDs and suggest that PCB effects on the THR are not a predominant mechanism driving PCB DNT. These findings provide scientific rationale regarding a point of departure for quantitative risk assessment of PCB DNT, and identify in vitro assays for screening other environmental pollutants for DNT potential.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Triiodothyronine induces UCP-1 expression and mitochondrial biogenesis in human adipocytes

Uncoupling protein (UCP)-1 expressed in brown adipose tissue plays an important role in thermogenesis. Recent data suggest that brown-like adipocytes in white adipose tissue (WAT) and skeletal muscle play a crucial role in the regulation of body weight. Understanding of the mechanism underlying the increase in UCP-1 expression level in these organs should, therefore, provide an approach to managing obesity. The thyroid hormone (TH) has profound effects on mitochondrial biogenesis and promotes the mRNA expression of UCP in skeletal muscle and brown adipose tissue. However, the action of TH on the induction of brown-like adipocytes in WAT has not been elucidated. Thus we investigate whether TH could regulate UCP-1 expression in WAT using multipotent cells isolated from human adipose tissue. In this study, triiodothyronine (T3) treatment induced UCP-1 expression and mitochondrial biogenesis, accompanied by the induction of the CCAAT/enhancer binding protein, peroxisome proliferator-activated receptor-γ coactivator-1α, and nuclear respiratory factor-1 in differentiated human multipotent adipose-derived stem cells. The effects of T3 on UCP-1 induction were dependent on TH receptor-β. Moreover, T3 treatment increased oxygen consumption rate. These findings indicate that T3 is an active modulator, which induces energy utilization in white adipocytes through the regulation of UCP-1 expression and mitochondrial biogenesis. Our findings provide evidence that T3 serves as a bipotential mediator of mitochondrial biogenesis.

Butyrate regulates leptin expression through different signaling pathways in adipocytes

Leptin is an adipocytokine that regulates body weight, and maintains energy homeostasis by promoting reduced food intake and increasing energy expenditure. Leptin expression and secretion is regulated by various factors including hormones and fatty acids. Butyrate is a short-chain fatty acid that acts as source of energy in humans. We determined whether this fatty acid can play a role in leptin expression in fully differentiated human adipocytes. Mature differentiated adipocytes were incubated with or without increasing concentrations of butyrate. RNA was extracted and leptin mRNA expression was examined by Northern blot analysis. Moreover, the cells were incubated with regulators that may affect signals which may alter leptin expression and analyzed with Northern blotting. Butyrate stimulated leptin expression, and stimulated mitogen activated protein kinase (MAPK) and phospho-CREB signaling in a time-dependent manner. Prior treatment of the cells with signal transduction inhibitors as pertusis toxin, G(i) protein antagonist, PD98059 (a MAPK inhibitor), and wortmannin (a PI3K inhibitor) abolished leptin mRNA expression. These results suggest that butyrate can regulate leptin expression in humans at the transcriptional level. This is accomplished by: 1) G(i) protein-coupled receptors specific for short-chain fatty acids, and 2) MAPK and phosphatidylinositol-3-kinase (PI3K) signaling pathways.

Molecular aspects of thyroid hormone actions

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, chiefly T(3) and diiodothyronine, has important effects on mitochondrial energetics and on the cytoskeleton. Modulation by the hormone of the basal proton leak in mitochondria accounts for heat production caused by iodothyronines and a substantial component of cellular oxygen consumption. Thyroid hormone also acts on the mitochondrial genome via imported isoforms of nuclear TRs to affect several mitochondrial transcription factors. Regulation of actin polymerization by T(4) and rT(3), but not T(3), is critical to cell migration. This effect has been prominently demonstrated in neurons and glial cells and is important to brain development. The actin-related effects in neurons include fostering neurite outgrowth. A truncated TRalpha1 isoform that resides in the extranuclear compartment mediates the action of thyroid hormone on the cytoskeleton.

Fasting-induced changes in the hypothalamus-pituitary-thyroid axis

Fasting induces profound changes in the hypothalamus-pituitary-thyroid (HPT) axis. The alterations observed in humans and rodents are similar in many ways, although they may be more pronounced and more acute in rodents. The molecular mechanisms underlying the resetting of HPT axis regulation in the framework of caloric deprivation are still incompletely understood. Fascinating studies in rats and mice have shown a dramatic downregulation of thyrotropin-releasing hormone (TRH) gene expression in hypophysiotropic paraventricular nucleus (PVN) neurons during fasting. Direct and indirect effects of decreased serum leptin, as well as effects of increased local triiodothyronine (T3) concentrations, in the hypothalamus during food deprivation contribute to the decreased activity of TRH neurons in the PVN. However, the relative contributions of these complex determinants remain to be defined in more detail. Pituitary thyroid-stimulating hormone (TSH) beta mRNA expression decreases during fasting, and this may be relatively independent of leptin and/or TRH, since leptin administration in this setting does not fully restore pituitary TSH expression, while it does restore TRH expression in the PVN. There may be a role for pituitary peptides, such as neuromedin B, in altered TSH gene expression during fasting. The observed decrease in serum thyroid hormone concentrations results to some extent from diminished thyroidal secretion of thyroid hormones, especially in rodents. Decreased thyroxine (T4) and T3 contribute to the downregulation of T3-responsive genes such as liver D1. The overall result of these complex HPT axis changes in various tissues during fasting is downregulation of the HPT axis, which is assumed to represent an energy-saving mechanism, instrumental in times of food shortage.

Cardiac angiotensin II type I and type II receptors are increased in rats submitted to experimental hypothyroidism

This study assessed the behaviour of angiotensin II (Ang II) receptors in an experimental hypothyroidism model in male Wistar rats. Animals were subjected to thyroidectomy and resting for 14 days. The alteration of cardiac mass was evaluated by total heart weight (HW), right ventricle weight (RVW), left ventricle weight (LVW), ratio of HW, RVW and LVW to body weight (BW) and atrial natriuretic factor (ANF) expression. Cardiac and plasma Ang II levels and serum T3 and T4 were determined. The mRNA and protein levels of Ang II receptors were investigated by RT-PCR and Western blotting, respectively. Functional analyses were performed using binding assays. T3 and T4 levels and the haemodynamic parameters confirmed the hypothyroid state. HW/BW, RVW/BW and LVW/BW ratios and the ANF expression were lower than those of control animals. No change was observed in cardiac or plasma Ang II levels. Both AT1/AT2 mRNA and protein levels were increased in the heart of hypothyroid animals due to a significant increase of these receptors in the RV. Experiments performed in cardiomyocytes showed a direct effect promoted by low thyroid hormone levels upon AT1 and AT2 receptors, discarding possible influence of haemodynamic parameters. Functional assays showed that both receptors are able to bind Ang II. Herein, we have identified, for the first time, a close and direct relation of elevated Ang II receptor levels in hypothyroidism. Whether the increase in these receptors in hypothyroidism is an alternative mechanism to compensate the atrophic state of heart or whether it may represent a potential means to the progression of heart failure remains unknown.

Effects of acetate and butyrate on the expression of leptin and short-form leptin receptor in bovine and rat anterior pituitary cells

The effects of acetate and butyrate on leptin and leptin receptor (OB-R) expression in bovine and rat anterior pituitary were examined. In bovine tissues, leptin gene expression using RT-PCR was observed in fat and anterior pituitary but not in liver. Isolated anterior pituitary cells cultured in Dulbecco's modified Eagle's medium (DMEM) for 3 days were further cultured for 48 h in DMEM containing 10 mM acetate or butyrate or without any fatty acids as control. Western blot analysis revealed that the abundance of leptin protein was greater in the presence of acetate and butyrate than that for the control culture. Leptin abundance was increased in a dose- and time-dependent manner in bovine anterior pituitary cells. However, leptin expression in rat cells, of which the basal level was much greater than that in ovine cells, was significantly decreased by the culture with butyrate. In addition, we studied the effects of both fatty acids on OB-R mRNA expression using semi-quantitative RT-PCR. The results showed that butyrate significantly decreased the expression in both bovine and rat cells. These findings indicate that acetate and butyrate enhance leptin expression in bovine, but not in rat anterior pituitary cells while butyrate suppresses OB-Ra expression in both rat and bovine pituitaries.

Sodium butyrate induces transcription from the G alpha(i2) gene promoter through multiple Sp1 sites in the promoter and by activating the MEK-ERK signal transduction pathway

Sodium butyrate, an erythroid differentiation inducer and a histone deacetylase inhibitor, increases G alpha(i2) levels in differentiating K562 cells. Here we show that sodium butyrate induces G alpha(i2) gene transcription via sequences at -50/-36 and -92/-85 in the G alpha(i2) gene promoter. Both sequences contain core sequence motif for Sp1 binding; electrophoretic mobility shift as well as supershift assays confirmed binding to Sp1. Transcription from the G alpha(i2) gene promoter was also activated by two other histone deacetylase inhibitors, trichostatin A and Helminthsporium carbonium toxin (HC toxin), which also induce erythroblastic differentiation in K562 cells. However, hydroxyurea, a potent erythroid differentiation inducer in these cells, did not activate transcription from this gene promoter, indicating that promoter activation is inducer-specific. Mutations within the Sp1 sites at -50/-36 and -92/-85 in the G alpha(i2) gene promoter substantially decreased transcriptional activation by sodium butyrate, trichostatin A, or HC toxin. Transfection with constitutively activated ERKs indicated that this promoter can be activated through the MEK-ERK signal transduction pathway. Inhibition of the MEK-ERK pathway with U0126 or reduction in the expression of endogenous ERK with an antisense oligonucleotide to ERK significantly inhibited sodium butyrate- and HC toxin-induced transcription but had no effect on trichostatin A-induced transcription. Inhibition of the JNK and p38 MAPKs, using selective inhibitors, had no effect on sodium butyrate-induced transcription. In cells in which sodium butyrate induction of promoter activation had been inhibited by various concentrations of U0126, constitutively activated ERK2 reversed this inhibition. These results show that the MEK-ERK signal transduction pathway is important in butyrate signaling, which eventually converges in the cell nucleus.

Physiological and molecular basis of thyroid hormone action

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

The effect of thyroid hormone and a long-acting somatostatin analogue on TtT-97 murine thyrotropic tumors

Thyroid hormone inhibits thyrotropin (TSH) production and thyrotrope growth. Somatostatin has been implicated as a synergistic factor in the inhibition of thyrotrope function. We have previously shown that pharmacological doses of thyroid hormone (levothyroxine [LT4]) inhibit growth of murine TtT-97 thyrotropic tumors in association with upregulation of somatostatin receptor type 5 (sst5) mRNA and somatostatin receptor binding. In the current study, we examined the effect of physiological thyroid hormone replacement alone or in combination with the long-acting somatostatin analogue, Sandostatin LAR, on thyrotropic tumor growth, thyrotropin growth factor-beta (TSH-beta), and sst5 mRNA expression, as well as somatostatin receptor binding sites. Physiological LT4 replacement therapy resulted in tumor shrinkage in association with increased sst5 mRNA levels, reduced TSH-beta mRNA levels and enhanced somatostatin receptor binding. Sandostatin LAR alone had no effect on any parameter measured. However, Sandostatin LAR combined with LT4 synergistically inhibited TSH-beta mRNA production and reduced final tumor weights to a greater degree. In this paradigm, Sandostatin LAR required a euthyroid status to alter thyrotrope parameters. These data suggest an important interaction between the somatostatinergic system and thyroid hormone in the regulation of thyrotrope cell structure and function.

The localization of thyroid hormone receptor mRNAs in human bone

Thyroid hormones have well-documented effects on the skeleton although the mechanism of their action on bone is poorly understood. We have recently reported the presence of different thyroid hormone receptor isoforms in human bone. However, there is evidence to suggest that the expression of thyroid hormone receptor (TR) protein may not necessarily correlate with its mRNA. In this study, we used specific digoxigenin-labeled ribo probes to investigate the expression of TRalpha1, variant TRalpha2, TRbeta1, and in particular TRbeta2 mRNA in human osteophytic bone and osteoclastoma tissue in situ. The number of positive cells was expressed as the percentage of the total number of cells of the same phenotype. In osteophytes, at sites of endochondral ossification, TRalpha1, variant TRalpha2, TRbeta1, and TRbeta2 mRNA were widely distributed in undifferentiated, proliferating, mature and hypertrophic chondrocytes. At sites of bone remodeling, TRalpha1 mRNA was expressed in the majority (> 90%) of osteoblasts. TRbeta1 and the variant TR-alpha2 mRNA were moderately expressed in approximately 75% of cells with only a few osteoblasts (< 25%) expressing TRbeta2 mRNA. All the TR transcripts were highly expressed in multinucleated osteoclasts in osteoclastoma tissue. The distribution of TR mRNAs was similar to TR receptor protein expression (as we have previously reported) in both osteophytic bone and osteoclastoma tissue except TRalpha1 mRNA that was highly expressed in osteoclasts and in undifferentiated, proliferating, mature, and hypertrophic chondrocytes in contrast to its receptor protein expression. This study highlights the importance of studying both TR mRNA and receptor proteins in triiodothyronine (T3) responsive tissues. This is also the first demonstration of the presence of TRbeta2 mRNA in bone. The role of TRbeta2 in mediating the actions of thyroid hormones in bone is not known and requires further investigation.

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

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

Regulation of glyceraldehyde-3-phosphate dehydrogenase in differentiating HD3 cells

Red blood cells usually replenish their ATP pools by glycolysis, fueled by glucose imported via the cell membrane. Mature red cells of some species (e.g. pig, chicken) have, however, been reported to show very low glucose transport. The subject of this study was the possible dependency of the level of a key glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAD) on glucose transporter activity during the maturation of chicken red cells. The chicken pronormoblast cell line, HD3, was used as a model system. These cells contain higher levels of GAD and glucose transporter activities than normal chicken bone marrow cells, but reduce their levels during maturation. In an attempt to assess whether the decrease in GAD activity is regulated by the glucose transport, the chicken GLUT3 expressed under the control of viral promoter was introduced into HD3 cells by retroviral infection (pDOL-cGT3). Upon cell differentiation and maturation, both cells with and without the exogenous transporter decreased GAD activity. Butyric acid did not affect the regulation of GAD activity upon differentiation. These results show that the development of chicken red cells is manifested by reduction of their GAD activity and that this is not affected by their sugar transporter activity. The very low GAD activity in embryonic chicken red cells is thus due to a loss of this activity at an early stage in their development. Because of the very low glucose transport and GAD activities in mature chicken red cells, rates of glycolysis are likely to be low and suggesting an alternative pathway for ATP production in these cells.

Inhibition of platelet-derived growth factor BB-induced expression of glyceraldehyde-3-phosphate dehydrogenase by sodium butyrate in rat vascular smooth muscle cells

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a key regulatory enzyme of glycolysis, which exists in nuclei and functions as a DNA-binding protein as well as a nuclear protein, appears to be modulated by cellular activities. Exposure of quiescent rat smooth muscle cells (SMCs) to platelet-derived growth factor BB (PDGF-BB), which stimulates SMCs proliferation, caused a time-dependent increase in mRNA for GAPDH and its catalytic activity. Treatment of quiescent SMCs with sodium butyrate (SB), which is shown to inhibit PDGF-BB-induced SMC proliferation, caused a time- and concentration-dependent decrease in PDGF-BB-induced GAPDH mRNA expression and its catalytic activity. Nuclear run-on studies revealed that the PDGF-BB-induced rate of GAPDH gene transcription was reduced by about 50% in the presence of 5 mmol/L SB. The protein synthesis inhibitor, cycloheximide, failed to abolish the SB-inhibited PDGF-BB-induced rate of transcription of GAPDH, suggesting that SB is not dependent on ongoing protein synthesis to exert its effects on PDGF-BB-induced GAPDH transcription. Furthermore, measurement of GAPDH mRNA stability at various times after the inhibition of transcription with actinomycin D indicated that 5 mmol/L SB has no significant effect on the half-life of PDGF-BB-induced mRNA. The reduction in PDGF-BB-induced GAPDH expression by SB is probably caused by a cycloheximide-insensitive transcriptional mechanism. Thus, the inhibition of PDGF-BB-induced expression of GAPDH by SB suggests a link between SMC proliferation, energy consumption, and GAPDH gene upregulation.

Histone acetylation influences thyroid hormone and retinoic acid-mediated gene expression

Thyroid hormone (T3) and retinoic acid (RA) receptors regulate transcription of the rat growth hormone (GH) gene through binding to a common hormone response element (HRE) in the promoter. We have investigated the effect of histone acetylation on hormone-dependent expression of the rat GH gene. We examined the effect of butyrate, which induces histone hyperacetylation, and trichostatin A (TSA), a highly specific inhibitor of histone deacetylases. GH-mRNA levels were significantly increased in pituitary GH4C1 cells incubated with T3 and RA, and this response was further stimulated in the presence of 1 mM butyrate. The effect of butyrate was mimicked by TSA. Butyrate and TSA also enhanced the activity of recombinant constructs containing the GH promoter directing chloramphenicol acetyl transferase (CAT) reporter gene expression. CAT activity increased by 4- to 8-fold after incubation with 1 nM T3 and 1 microM RA, and this response was stimulated 2- to 4-fold further in the presence of 0.25 mM butyrate. This concentration of butyrate did not influence basal expression of CAT. TSA produced a dose-dependent increase of CAT activity in the absence of ligands, and between 5 and 200 nM potentiated the effect of T3 and RA. These compounds also increased the hormonal response of constructs in which the HRE was linked to heterologous [mouse mammary tumor virus (MMTV) and thymidine kinase (TK)] promoters. With butyrate >1 mM, basal activity of the GH promoter increased by more than 10-fold and the effect of T3 and RA was no longer observed. Overexpression of T3 receptors was able to counteract the stimulation of basal CAT levels caused by butyrate. Thus, in the absence of ligand, the T3 receptor acts as a constitutive repressor of gene expression. Upon binding of the hormone, the T3 receptor is converted into an activator. Our findings suggest that histone acetylation, which alters chromatin structure, may play an important role in hormone-mediated transcriptional regulation.

Opposite effects of sodium butyrate on CCK mRNA and CCK peptide levels in RIN cells

The effects of the differentiation-inducing agent sodium butyrate on cholecystokinin (CCK) expression was investigated in the pancreatic islet tumor cell line RIN 1056E, which contains high levels of CCK-like immunoreactivity (CCK-LI). Exposure to butyrate for 24 h dose-dependently inhibited cell proliferation and increased the cell content in CCK-LI over the concentration range 0.1-8 mM. With 2 mM butyrate, cell proliferation was decreased by 50% and CCK-LI content was increased by 300%, whereas the level of steady-state CCK mRNA was reduced by 75%. Cycloheximide (10 μg/mL) abolished the sodium butyrate-induced increase in CCK-LI content. This article reports the novel finding that butyrate exerts opposite effects on CCK mRNA and immunoreactivity. The butyrate-induced increase in cellular CCK-LI content is entirely dependent on continuing protein synthesis.

Sodium butyrate inhibits platelet-derived growth factor-induced proliferation of vascular smooth muscle cells

Sodium butyrate (SB), a naturally occurring short-chain fatty acid, was investigated for its therapeutic value as an antiproliferative agent for vascular smooth muscle cells (SMCs). At 5-mmol/L concentration, SB had no significant effect on rat SMC proliferation. However, at the same concentration, SB inhibited platelet-derived growth factor (PDGF)-AA-, -AB-, and -BB-induced proliferation of SMCs. Exposure of SMCs to PDGF-BB resulted in activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of beta-PDGF-receptor (beta-PDGFR). The activated beta-PDGFR physically associated and phosphorylated signaling molecules such as ras-GTPase activating protein (GAP) and phospholipase C gamma (PLC gamma). SB, in the absence of PDGF-BB, caused neither beta-PDGFR tyrosine phosphorylation nor phosphorylation and association of GAP and PLC gamma with beta-PDGFR. PDGF-BB-enhanced activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of tyrosine residues of beta-PDGFR were unaffected by SB irrespective of whether SMCs were preincubated with SB before exposure to PDGF-BB plus SB or incubated concomitantly with PDGF-BB plus SB. Likewise, phosphorylation and association of GAP and PLC gamma with PDGF-BB-activated beta-PDGFR were unaffected. In addition, SB did not block PDGF-BB-stimulated, PLC gamma-mediated production of inositol triphosphate. Similarly, PDGF-BB-induced beta-PDGFR degradation was unaffected when SMCs were exposed to PDGF-BB plus SB, and SB by itself had no influence on beta-PDGFR degradation. Unlike beta-PDGFR kinase activity, mitogen-activated protein kinase (MAP-kinase) activity was stimulated by SB by about 2.7-fold. Exposure of SMCs to PDGF-BB caused an approximately 11.4-fold increase in MAP-kinase activity and this increase in activity was not significantly affected when cells were coincubated with PDGF-BB and SB (10.3-fold). However, pretreatment of SMCs with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB abolished most of the PDGF-BB-induced MAP-kinase activity (4.6-fold). Transcription of growth response genes such as c-fos, c-jun, and c-myc were induced by PDGF-BB, and their induction was suppressed, particularly c-myc, by incubating SMCs with PDGF-BB plus SB. Similarly, preincubation of cells with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB diminished PDGF-BB-induced transcription of c-fos, c-jun, and c-myc. However, SB by itself had no significant effect on c-fos, c-jun, and c-myc transcription.(ABSTRACT TRUNCATED AT 400 WORDS)

The expression of human plasma cholesteryl-ester-transfer protein in HepG2 cells is induced by sodium butyrate. Quantification of low mRNA levels by polymerase chain reaction

Although human plasma cholesteryl-ester-transfer protein (CETP) is primarily synthesized in the liver, its expression in a number of transformed liver cell lines is very low. However the use of the human hepatoma cell line HepG2 as a model system for the regulation of CETP on mRNA level is facilitated by a quantitative reverse-transcribed polymerase chain reaction. We demonstrate a time-dependent and concentration-dependent 3-4-fold induction of CETP mRNA by sodium butyrate. CETP mass in the medium is also augmented; however, the effect on protein level is less pronounced.

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

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

c-erb-A mRNA correlates with T3-receptor levels in liver and pituitary of tumor rats

In the rat tumor model of the sick euthyroid syndrome, differential regulation of T3-induced cellular responses have been demonstrated in liver and anterior pituitary. These effects occur with a concomitant decrease in nuclear thyroid hormone receptor (TR) number as measured by the binding of 125I-labeled T3. To explore the possibility that these altered responses to T3 in tumor rats resulted from changes in the expression of different TR forms, we correlated the relative abundance of mRNAs encoding each receptor form with the concentration of TR measured by specific T3 binding. In anterior pituitary of tumor rats, TR beta-1 and beta-2 mRNA levels decreased to 51 and 45%, respectively, compared to controls; rat c-erb A alpha-2 mRNA, which encodes a TR-related DNA alpha-binding protein that does not bind T3, decreased to 46% of control. These findings correlate with a decrease in nuclear T3 binding capacity that has been shown to be 63% of control. The level of TR beta-1 mRNA, the only quantifiable TR form in liver, was decreased to 61% of control in the same hepatic tissue that revealed a 50% decrease in TR as measured by specific T3 binding. The coordinate down-regulation of all TR mRNA forms to a degree that parallels the decrease in TR number as measured by specific T3 binding suggests that the differential regulation of T3-mediated effects in illness is by a mechanism other than changing concentrations of specific receptor forms.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of sodium butyrate on 1,25-dihydroxyvitamin D3 receptor activity in primary chick kidney cells

The genomic effects of the steroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are mediated by high affinity nuclear associated specific receptors that belong to the superfamily of ligand induced transcription factors. The carboxylic acid, sodium butyrate--a potent inhibitor of histone deacetylase--is known to modulate gene expression in a variety of systems. Specific binding of 1,25(OH)2D3 to its receptor was examined in primary chick kidney cells, the chick macrophage cell line HD-11, and other mammalian cell lines such as ROS 17/2.8, HT-29 and CV-1 cells, that were all cultured in the presence or absence of 1 mM sodium butyrate. Treatment with n-butyrate resulted in significant (4.0-4.5-fold) increases in 1,25(OH)2D3 receptor binding without changing binding affinity only in the primary cultures of chick renal epithelial cells and the chick macrophage cell line but not in the other heterologous receptor-positive cell lines. The maximum increase in receptor binding was evident at 1 mM butyrate concentration. This effect reached a maximum at 15 h treatment, beyond which there was slow attenuation in increased binding until 24 h. The butyrate induced increases in receptor activity was associated with increases in the 1,25(OH)2D3-mediated induction of calbindin-D28K protein only in primary chick kidney cultures but not in the macrophage cell line (HD-11). Similarly, calbindin-D28K promoter activity was enhanced only in butyrate-treated primary chick kidney cultures, transfected with chimeric plasmids containing the 5' flanking sequence of the calbindin-D28K promoter fused to the chloramphenicol acetyl transferase (CAT) reporter gene but not in HD-11 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone and apotransferrin regulation of growth hormone secretion by GH1 rat pituitary tumor cells in iron restricted serum-free defined medium

Growth hormone (GH) production by GH1 rat pituitary tumor cells in iron restricted serum-free defined medium requires apotransferrin (apoTf) and triiodothyronine (T3). As measured by radioimmunoassay, apoTf plus T3 induced GH levels 2 to 4-fold above controls. Deletion of either apoTf or T3 arrested GH secretion. ApoTf/T3 defined medium regulated GH production as effectively as whole serum. Because glucocorticoids enhance GH secretion in serum containing cultures, the effects of dexamethasone were evaluated in apoTf/T3 defined medium. The steroid hormone showed no enhancing effects unless the cells were exposed to serum prior to incubation in apoTf/T3 defined medium. Even under these conditions, the response to dexamethasone remained T3 dependent. These observations indicate that a yet to be characterized serum factor(s), other than apoTf, regulates the response to the steroid hormone. This is the first report of thyroid hormone regulation of GH secretion by rat pituitary tumor cells under completely serum-free chemically defined conditions.