Molecular Mechanisms of Thyroid Hormone Action

Retinoic acid receptor β, a potential therapeutic target in the inhibition of adenovirus replication

Human adenoviruses (HAdVs) usually cause mild respiratory infections, but they can also lead to fatal outcomes for immunosuppressive patients. Unfortunately, there has been no specific anti-HAdV drug approved for medical use. A better understanding of the nature of virus-host interactions during infection is beneficial to the discovery of potential antiviral targets and new antiviral drugs. In this study, a time-course transcriptome analysis of HAdV-infected human lung epithelial cells (A549 cells) was performed to investigate virus-host interactions, and several key host molecules involved in the HAdV infection process were identified. The RARβ (retinoic acid receptor β) molecule, one of the upstream regulatory factors of differentially expressed genes (DEGs), played important roles in HAdV replication. The results of reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting showed that RARβ mRNA and protein were downregulated by HAdV infection in the A549 cells. The knockdown of RARβ by RARβ siRNA increased the HAdV production and the overexpression of RARβ decreased the HAdV production. Furthermore, FDA-approved Tazarotene, which is an RAR selective agonist with relatively more selectivity for RARβ, was found to inhibit HAdV replication in vitro. Taken together, our study presents a key host molecule in adenovirus infection, which could be developed as a potential host target to an anti-adenovirus drug. In addition, this study provides evidence for the re-exploitation of an FDA-approved small molecule for therapeutic applications in adenovirus replication.

The underlying mechanisms: how hypothyroidism affects the formation of common bile duct stones-a review

For decades, one well-known risk factor for the development of gallbladder stones has been hypothyroidism. Recent studies have interestingly reported that the risk in particular for common bile duct (CBD) stones increases in clinical and subclinical hypothyroidism. There are multiple factors that may contribute to the formation and/or accumulation of CBD stones in hypothyroid patients, including decreased liver cholesterol metabolism, diminished bile secretion, and reduced sphincter of Oddi relaxation. This paper focuses on the mechanisms possibly underlying the association between hypothyroidism and CBD stones. The authors conclude that when treating patients with CBD stones or microlithiasis, clinicians should be aware of the possible hypothyroid background.

Paradoxical triiodothyronine suppression of S14 transcription in permanent hepatic cell lines

Both in vivo and in primary rat hepatocyte culture, carbohydrate and triiodothyronine (T(3)) rapidly induce transcription of the rat S14 gene. To determine if regulation of this gene by T(3) is similar in human liver cells, we transfected the S14 upstream region into HepG2 cells. We chose this cell line because many others have used this cell line to study the effect of thyroid hormone on hepatic gene expression. We found that changing media glucose concentration did not affect S14 transcription. Furthermore, addition of T(3) to HepG2 cells caused a marked reduction of rat S14 transcription. This paradoxical reduction was dependent on cotransfection of the T(3) receptor. We obtained similar results in the other human hepatoma cell lines, HuH-7 and Hep3B. The paradoxical response was not limited to human cells. We found a similar response in the nonmalignant permanent mouse liver cell line, AML-12. This paradoxical response was specific to the S14 gene because transfection of all the cell lines with a CAT or luciferase reporter driven by a mouse mammary tumor virus promoter containing 1 or 4 copies of a palindromic thyroid hormone response element (TRE) showed marked induction by T(3). Our results show that T(3) abnormally regulates the S14 gene in proliferating liver cell lines of diverse origins. This paradoxical regulation by T(3) is caused by an interaction between T(3) and the thyroid hormone receptor. The factors that lead to this paradoxical response are not active in primary hepatocytes and normal intact liver.

Embryonic temperature affects metabolic compensation and thyroid hormones in hatchling snapping turtles

Temperature acclimation of adult vertebrates typically induces changes in metabolic physiology. During early development, such metabolic compensation might have profound consequences, yet acclimation of metabolism is little studied in early life stages. We measured the effect of egg incubation temperature on resting metabolic rate (RMR) and blood thyroid hormone levels of hatchling snapping turtles (Chelydra serpentina). Like many reptiles, snapping turtles have temperature-dependent sex determination (TSD), in which embryonic temperature determines sex. Therefore, we designed the experiments to separately measure effects of temperature and of sex on the response variables. We incubated eggs in the laboratory at 21. 5 degrees, 24.5 degrees, 27.5 degrees, and 30.5 degrees C, producing both sexes, all males, both sexes, and all females, respectively. Hatchling RMR, when measured at a common temperature (either 25 degrees or 31 degrees C), was negatively correlated with egg temperature in both males and females, such that RMR of turtles from 21.5 degrees C-incubated eggs averaged 160% that of turtles from 30.5 degrees C-incubated eggs. These results indicate that egg temperatures induced positive metabolic compensation in both sexes. Thyroid hormone levels of hatchlings showed similar correlations with egg temperature; thyroxine level of turtles from 21.5 degrees C-incubated eggs averaged 220% that of turtles from 30.5 degrees C-incubated eggs. To examine the possibility that thyroid hormones contribute to positive metabolic compensation, we added triiodothyronine to eggs during mid-incubation. RMR of hatchlings from these treated eggs averaged 131% that of controls, consistent with the previous possibility. Moreover, the effects of embryonic temperature on metabolic physiology, in combination with effects on sex, can result in differences in RMR and thyroid hormone levels between male and female hatchling turtles. Such differences may be important to the ecology and evolution of TSD.

Molecular and functional aspects of the hairless (hr) gene in laboratory rodents and humans

For many years, hairless and rhino mouse mutants have provided a useful and extensively exploited model for studying different aspects of skin physiology, including skin aging, pharmacokinetic evaluation of drug activity and cutaneous absorption, skin carcinogenesis, and skin toxicology. Interestingly, however, hairless and rhino mice have rarely been studied for their primary cellular defect - hairlessness - and thus, the hairless gene itself and its physiological functions have been largely overlooked for decades. The recent identification of the human homolog of the hairless gene on human Chromosome 8p12 confirmed the clinical significance of the phenomenon of "hairlessness" in humans, which was predicted on the basis of similarities between hairless mice and a congenital hair disorder characterized by atrichia with papules. Mutations in the hairless gene of mice provide instructive models for further studies of hr gene function, and may facilitate insights into the pathophysiology of different human disorders associated with the disruption of hr gene activity. We provide an overview of current data on the structure and expression patterns of the hr gene, and of mutations at the hairless locus in mice and humans, including the genetic basis of different alleles, the pathology of hairlessness, reproductive and immunological defects, and susceptibility to dioxin toxicity. On the basis of our current understanding of hairlessness, we speculate on the putative functions of the hr gene product in skin physiology, and particularly, in hair follicle biology.

Characterization of a low affinity thyroid hormone receptor binding site within the rat GLUT4 gene promoter

Previous studies have demonstrated that thyroid hormone (T3) stimulates insulin-responsive glucose transporter (GLUT4) transcription and protein expression in rat skeletal muscle. The aim of the present study was to define a putative thyroid hormone response element (TRE) within the rat GLUT4 promoter and thus perhaps determine whether T3 acts directly to augment skeletal muscle GLUT4 transcription. To this end, electrophoretic mobility shift analyses were performed to analyze thyroid hormone receptor (TR) binding to a previously characterized 281-bp T3-responsive region of the rat GLUT4 promoter. Indeed, within this region, a TR-binding site of the standard DR + 4 TRE variety was located between bases -457/ -426 and was shown to posses a specific affinity for in vitro translated TRs. Interestingly, however, the GLUT4 TR-binding site demonstrated a significantly lower affinity compared to a consensus DR + 4 TRE, and only bound TRs appreciatively in the form of high affinity heterodimers, in this case with the cis-retinoic acid receptor. In conclusion, these data demonstrated the presence of a specific TR-binding site within a T3-responsive region of the rat GLUT4 promoter and thus support the supposition that thyroid hormone acts directly to stimulate GLUT4 transcription in rat skeletal muscle. Moreover, characterization of a novel TR-binding site with low affinity suggests an additional mechanism by which the intrinsic activity and responsiveness of thyroid hormone regulated genes may be modulated.

Application of a novel method for the comparison of DNA binding parameters of the two human thyroid hormone receptor subtypes hTR alpha 1 and hTR beta 1

DNA-binding characteristics of the two human thyroid hormone receptors alpha 1 and beta 1 (hTR alpha 1 and hTR beta 1) were studied by applying the recently developed solid-phase scintillation technique. Biotinylated double stranded oligonucleotides containing thyroid hormone response elements (TRE) were immobilized to streptavidin coated scintillating microtiter plates. The TRE:s consisted of variants of the consensus core sequence AGGTCA as monomers or as dimers in direct repeats. Equilibrium binding of radioactive labelled hTR alpha 1 and hTR beta 1 were studied. Metabolically 35S-labelled hTR (in vitro translated cDNA) as well as hTR expressed in the baculovirus-system and labelled with 125I-triiodothyronine (125I-T3) were used. In binding saturation experiments, the affinity for the TRE:s investigated did not differ greatly between hTR alpha 1 and hTR beta 1. No significant effects of T3 on the amplitude of DNA binding of either hTR alpha 1 or hTR beta 1 to the single site response elements could be demonstrated. Receptor binding to direct repeats was stimulated by the hormone in the case of the hTR beta 1. The hTR alpha 1 binding to direct repeats was not significantly altered by T3. The single site octameric variant of a TRE, TAAGGTCA, was observed to bind tighter to the hTR:s as compared to the hexameric variant AGGTCA. In the binding competition format, with one response element immobilized and other (un-biotinylated) added to the reaction mixture, there was a larger dynamic range for the affinity constants (IC50) as compared to the affinity constants (Kd) obtained in the binding saturation experiments. The present quantitative results confirm previous reports obtained with qualitative methods like gel shift assays. The method described here is applicable in basic research concerning characterisation of DNA binding of nuclear receptors. It also lends itself to automatization in high capacity formats.