Thyroid hormone responsive genes in the murine hepatocyte cell line AML 12

Establishment of a human embryonic stem cell-based liver differentiation model for hepatotoxicity evaluations

The liver is one of the major targets of hormones, including thyroid hormones (THs), and many industrial chemicals, such as endocrine-disrupting chemicals. Those compounds may permeate the placenta barrier and pose a risk for embryonic development. Therefore, it is necessary to assess the toxic effects of those kind of industrial chemicals during liver development. In this study, to mimic liver specification in vitro, we differentiated human embryonic stem cells (ESCs) into functional hepatocyte-like cells. We performed this differentiation process in presence of two THs, triiodothyronine (T3) and thyroxine (T4), with the purpose of identifying biomarkers for toxicity screening. TH exposure (3, 30 and 300 nM) yielded to hepatocytes with impaired glycogen storage ability and abnormal lipid droplets' accumulation. Global gene expression analysis by RNA-seq identified a number of genes responsible for hepatic differentiation and function which were affected by 30 nM T3 and T4. Those differentially expressed genes were used to assess the potential developmental liver toxicity of two famous environmental pollutants, 2, 2, 4, 4-tetrabromodiphenyl ether (BDE-47) and decabromodiphenyl ether (BDE-209), at 10 nM to 1 μM treatments. Our findings demonstrate that BDE-47 and BDE-209, dysregulated pathways such as "chemical carcinogenesis", "steroid hormone biosynthesis" and "drug metabolism-cytochrome P450". Moreover, we were able to identify a set of 17 biomarkers, very useful to predict the potential developmental hepatotoxicity of industrial chemicals.

Thyroid Hormone at Near Physiologic Concentrations Acutely Increases Oxygen Consumption and Extracellular Acidification in LH86 Hepatoma Cells

Thyroid hormone (T3) has been known to regulate the basal metabolic rate for more than a century, but mechanistic understanding is lacking both at the level of the intact organism and in terms of how T3 alters energy expenditure in individual tissues. The current studies investigate the question of which metabolically relevant genes respond acutely as T3 concentrations increase through the physiologic range in liver cells. Because this has been technically unfeasible historically, we developed a modified protocol for extracellular flux analysis using a 96-well Extracellular Flux Analyzer (Seahorse Bioscience). Using a modified extracellular flux protocol and LH86 human hepatoma cells, we established an experimental system where small but significant changes in O2 consumption could be reproducibly quantified as hypothyroid cells were exposed to near-physiologic final concentrations of T3 approximately 2 orders of magnitude lower than most studies (0.04 nM free T3), in only 6-7 hours. Taking advantage of the nondestructive nature of 96-well Extracellular Flux Analyzer measurements, the acute, direct, transcriptional changes that occur were measured in the exact same cells demonstrating increased O2 consumption. An unbiased, genome-wide microarray analysis identified potential candidate genes related to fatty acid oxidation, angiogenesis, nucleotide metabolism, immune signaling, mitochondrial respiration, and cell proliferation. The identified transcriptome is likely enriched in the genes most important for mediating the energetic effects of T3 in hepatoma cells.

Thyroid hormone stimulates hepatic lipid catabolism via activation of autophagy

For more than a century, thyroid hormones (THs) have been known to exert powerful catabolic effects, leading to weight loss. Although much has been learned about the molecular mechanisms used by TH receptors (TRs) to regulate gene expression, little is known about the mechanisms by which THs increase oxidative metabolism. Here, we report that TH stimulation of fatty acid β-oxidation is coupled with induction of hepatic autophagy to deliver fatty acids to mitochondria in cell culture and in vivo. Furthermore, blockade of autophagy by autophagy-related 5 (ATG5) siRNA markedly decreased TH-mediated fatty acid β-oxidation in cell culture and in vivo. Consistent with this model, autophagy was altered in livers of mice expressing a mutant TR that causes resistance to the actions of TH as well as in mice with mutant nuclear receptor corepressor (NCoR). These results demonstrate that THs can regulate lipid homeostasis via autophagy and help to explain how THs increase oxidative metabolism.

Thyroid hormone stimulates hepatic lipid catabolism via activation of autophagy

For more than a century, thyroid hormones (THs) have been known to exert powerful catabolic effects, leading to weight loss. Although much has been learned about the molecular mechanisms used by TH receptors (TRs) to regulate gene expression, little is known about the mechanisms by which THs increase oxidative metabolism. Here, we report that TH stimulation of fatty acid β-oxidation is coupled with induction of hepatic autophagy to deliver fatty acids to mitochondria in cell culture and in vivo. Furthermore, blockade of autophagy by autophagy-related 5 (ATG5) siRNA markedly decreased TH-mediated fatty acid β-oxidation in cell culture and in vivo. Consistent with this model, autophagy was altered in livers of mice expressing a mutant TR that causes resistance to the actions of TH as well as in mice with mutant nuclear receptor corepressor (NCoR). These results demonstrate that THs can regulate lipid homeostasis via autophagy and help to explain how THs increase oxidative metabolism.

Hepatic organic anion transporting polypeptide transporter and thyroid hormone receptor interplay determines cholesterol and glucose homeostasis

The role of organic anion transporting polypeptides (OATPs), particularly the members of OATP1B subfamily, in hepatocellular handling of endogenous and exogenous compounds is an important and emerging area of research. Using a mouse model lacking Slco1b2, the murine ortholog of the OATP1B subfamily, we have demonstrated previously that genetic ablation causes reduced hepatic clearance capacity for substrates. In this study, we focused on the physiological function of the hepatic OATP1B transporters. First, we studied the influence of the Oatp1b2 deletion on bile acid (BA) metabolism, showing that lack of the transporter results in a significantly reduced expression of Cyp7a1, the key enzyme of BA synthesis, resulting in elevated cholesterol levels after high dietary fat challenge. Furthermore, Slco1b2-/- mice exhibited delayed clearance after oral glucose challenge resulting from reduced hepatic glucose uptake. In addition to increased hepatic glycogen content, Slco1b2-/- mice exhibited reduced glucose output after pyruvate challenge. This is in accordance with reduced hepatic expression of phosphoenolpyruvate carboxykinase (PEPCK) in knockout mice. We show that this phenotype is due to the loss of liver-specific Oatp1b2-mediated hepatocellular thyroid hormone entry, which then leads to reduced transcriptional activation of target genes of hepatic thyroid hormone receptor (TR), including Cyp7a1 and Pepck but also Dio1 and Glut2. Importantly, we assessed human relevance using a cohort of archived human livers in which OATP1B1 expression was noted to be highly associated with TR target genes, especially for glucose facilitating transporter 2 (GLUT2). Furthermore, GLUT2 expression was significantly decreased in livers harboring a common genetic polymorphism in SLCO1B1.

CONCLUSION

Our findings reveal that OATP1B-mediated hepatic thyroid hormone entry is a key determinant of cholesterol and glucose homeostasis.

Nexus between epidermolysis bullosa and transcriptional regulation by thyroid hormone in epidermal keratinocytes

Abstract Thyroid hormone, T3, through the interaction of its receptor with the recognition sequences in the DNA, regulates gene expression. This regulation includes the promoter activity of keratin genes. The receptor shares coregulators with other members of the nuclear receptor family, including RXR. Intending to define the transcriptional effects of thyroid hormones in keratinocytes, we used Affymetrix microarrays to comprehensively compare the genes expressed in T3-treated and untreated human epidermal keratinocytes. The transcriptomes were compared at 1, 4, 24, 48, and 72 hours. Surprisingly, T3 induced only 9 and suppressed 28 genes, much fewer than expected. Significantly, genes associated with epidermolysis bullosa, a set of inherited blistering skin diseases, were found statistically highly overrepresented among the suppressed genes. These genes include Integrin beta4, Plectin, Collagen XVII, MMP1, MMP3, and MMP14. The data imply that in keratinocytes T3 could suppresses the remodeling by, attachment to, and production of extracellular matrix. The results suggest that topical treatment with T3 may be effective for alleviation of symptoms in patients with epidermolysis bullosa.

Changes in gene expression of DOR and other thyroid hormone receptors in rat liver during acute-phase response

Non-thyroidal illness is characterized by low tri-iodothyronine (T3) serum level under acute-phase conditions. We studied hepatic gene expression of the newly identified thyroid hormone receptor (TR) cofactor DOR/TP53INP2 together with TRs in a rat model of aseptic abscesses induced by injecting intramuscular turpentine-oil into each hind limb. A fast (4-6 h) decrease in the serum level of free thyroxine and free T3 was observed. By immunohistology, abundant DOR protein expression was detected in the nuclei of hepatocytes and ED-1(+) (mononuclear phagocytes), CK-19(+) (biliary cells), and SMA(+) (mesenchymal cells of the portal tract) cells. DOR signal was reduced with a minimum at 6-12 h after the acute-phase reaction (APR). Immunohistology also showed a similar pattern of protein expression in TRα1 but without a significant change during APR. Transcripts specific for DOR, nuclear receptor co-repressor 1 (NCoR-1), and TRβ1 were down-regulated with a minimum at 6-12 h, whereas expression for TRα1 and TRα2 was slightly and significantly up-regulated, respectively, with a maximum at 24 h after APR was initiated. In cultured hepatocytes, acute-phase cytokines interleukin-1β (IL-1β) and IL-6 down-regulated DOR and TRβ1 at the mRNA level. Moreover, gene expression of DOR and TRs (TRα1, TRα2, and TRβ1) was up-regulated in hepatocytes by adding T3 to the culture medium; this up-regulation was almost completely blocked by treating the cells with IL-6. Thus, TRβ1, NCoR-1, and the recently identified DOR/TP53INP2 are abundantly expressed and down-regulated in liver cells during APR. Their down-regulation is attributable to the decreased serum level of thyroid hormones and most probably also to the direct action of the main acute-phase cytokines.

Studies of complex biological systems with applications to molecular medicine: the need to integrate transcriptomic and proteomic approaches

Omics approaches to the study of complex biological systems with potential applications to molecular medicine are attracting great interest in clinical as well as in basic biological research. Genomics, transcriptomics and proteomics are characterized by the lack of an a priori definition of scope, and this gives sufficient leeway for investigators (a) to discern all at once a globally altered pattern of gene/protein expression and (b) to examine the complex interactions that regulate entire biological processes. Two popular platforms in "omics" are DNA microarrays, which measure messenger RNA transcript levels, and proteomic analyses, which identify and quantify proteins. Because of their intrinsic strengths and weaknesses, no single approach can fully unravel the complexities of fundamental biological events. However, an appropriate combination of different tools could lead to integrative analyses that would furnish new insights not accessible through one-dimensional datasets. In this review, we will outline some of the challenges associated with integrative analyses relating to the changes in metabolic pathways that occur in complex pathophysiological conditions (viz. ageing and altered thyroid state) in relevant metabolically active tissues. In addition, we discuss several new applications of proteomic analysis to the investigation of mitochondrial activity.

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.

High-selenium yeast supplementation in free-living North American men: no effect on thyroid hormone metabolism or body composition

In a prior study, we observed decreased serum 3,3',5-triiodothyronine (T(3)), increased serum thyrotropin and increased body weight in five men fed 297 microg/d of selenium (Se) in foods naturally high in Se while confined in a metabolic research unit. In an attempt to replicate and confirm those observations, we conducted a randomized study of high-Se yeast supplements (300 microg/d) or placebo yeast administered to 42 healthy free-living men for 48 weeks. Serum thyroxine, T(3) and thyrotropin did not change in supplemented or control subjects. Body weight increased in both groups during the 48-week treatment period and remained elevated for the 48-week follow-up period. Body fat increased by 1.2 kg in both groups. Energy intake and voluntary activity levels were not different between the groups and remained unchanged during the treatment period. Dietary intakes of Se, macronutrients and micronutrients were not different between groups and remained unchanged during the treatment period. These results suggest that our previous observation of a hypothyroidal response to high-Se foods was confounded by some aspect of the particular foods used, or were merely chance observations. Because of the high dose and long administration period, the present study suggests that the effects of Se supplements on thyroid hormone metabolism and energy metabolism in healthy North American men with adequate Se status do not represent a significant risk for unhealthy weight gain.