Insulin represses transcription of the thyroid stimulating hormone beta-subunit gene through increased recruitment of nuclear factor I

The insulin resistance is reversed by exogenous 3,5,3'triiodothyronine in type 2 diabetic Goto-Kakizaki rats by an inflammatory-independent pathway

PURPOSE

Diabetes mellitus (DM) has a multifactorial etiology that imparts a particular challenge to effective pharmacotherapy. Thyroid hormone actions have demonstrated beneficial effects in diabetic as well as obese rats. In both conditions, inflammation status plays a crucial role in the development of insulin resistance. Taking this into consideration, the present study aimed to demonstrate another possible pathway of thyroid hormone action on insulin sensitivity in a spontaneous type 2 diabetic rat model: the Goto-Kakizaki (GK) rats. GK animals present all typical hallmarks of type 2 DM (T2DM), except the usual peripheric inflammatory condition, observed in the other T2DM animal models.

METHODS

GK rats were treated or not with 3,5,3'triiodothyronine (T3). Insulin sensitivity, glucose tolerance, and proteins related to glucose uptake and utilization were evaluated in the skeletal muscle, white adipose tissue, and liver.

RESULTS

GK rats T3-treated presented enhanced insulin sensitivity, increased GLUT-4 content in the white adipose tissue and skeletal muscle, and increased hexokinase and citrate synthase content in skeletal muscle. Both non-treated and T3-treated GK rats did not present alterations in cytokine content in white adipose tissue, skeletal muscle, liver, and serum.

CONCLUSIONS

These results indicate that T3 improves insulin sensitivity in diabetic rats by a novel inflammatory-independent mechanism.

Thyroid hormone reduces inflammatory cytokines improving glycaemia control in alloxan-induced diabetic wistar rats

AIM

This study aimed at evaluating whether thyroid hormone treatment could improve glycaemia and insulin response in alloxan-induced diabetic rats by altering cytokine expression in the skeletal muscle and epididymal white adipose tissue (eWAT) as well as altering inflammatory cell infiltration in eWAT.

METHODS

Diabetes mellitus (DM) was induced in male Wistar rats by alloxan injection, and a subset of the diabetic rats was treated with T3 (1.5 μg per 100 g body weight) for a 28-day period (DT3 ). Cytokines were measured in serum (MILIplex assay kit) as well as in soleus and EDL skeletal muscles and eWAT by Western blotting. Thyroid function was evaluated by morphological, molecular and biochemical parameters. Cardiac function was assessed by measuring heart rate, blood pressure, maximal rate of pressure development (dp/dtmax ) and decline (dp/dtmin ) as well as the contractility index (CI). Sixty rats were used in the study.

RESULTS

Diabetic rats exhibited decreased thyroid function and increased inflammatory cytokines in serum, soleus muscle and eWAT. T3 treatment decreased glycaemia and improved insulin sensitivity in diabetic animals. These alterations were accompanied by decreased TNF-alpha and IL-6 content in soleus muscle and eWAT, and inflammatory cell infiltration in eWAT. T3 treatment did not affect cardiac function of diabetic rats.

CONCLUSIONS

The present data provide evidence that T3 treatment reduces glycaemia and improves insulin sensitivity in diabetic rats, and that at least part of this effect could result from its negative modulation of inflammatory cytokine expression.

Msx1 homeodomain transcription factor and TATA-binding protein interact to repress the expression of the glycoprotein hormone α subunit gene

Studying the regulatory mechanism of the glycoprotein hormone α subunit (αGSU) gene in thyrotropes is essential for understanding the synthesis of functional thyroid-stimulating hormone (TSH). Here, we investigated the influence of a homeodomain transcription factor Msx1 (Msh homeobox 1) on αGSU expression in thyrotropes. The transient expression of Msx1 inhibited the activity of an αGSU reporter gene, as well as its endogenous mRNA level in thyrotrope-derived αTSH cells. Luciferase reporter assays with serial deletion constructs and a close examination of the sequences revealed that the putative Msx1 binding site (PMS) in the αGSU promoter is not responsible for Msx1-mediated transcriptional repression. We also identified the TATA-box binding protein (TBP) as an interacting protein in thyrotropes. Interaction of TBP with Msx1 attenuates the inhibitory effect of Msx1 on αGSU gene expression in a DNA binding-independent manner. Furthermore, transient transfection studies with mutant Msx1 revealed that the interaction of TBP and Msx1 is critical for Msx1-mediated transcriptional repression of the αGSU. These results suggest that Msx1 functions as a transcriptional repressor of αGSU and that its interaction with TBP is an integral part of the mechanism by which Msx1 regulates the inhibition of αGSU gene expression.

A genome-wide association study of seasonal pattern mania identifies NF1A as a possible susceptibility gene for bipolar disorder

OBJECTIVE

The use of subphenotypes may be an effective approach for genetic studies of complex diseases. Manic episodes with a seasonal pattern may distinguish phenotypic subgroups of bipolar subjects that may also differ genetically.

METHOD

We have performed a genome-wide association study using GAIN genotype data from the Bipolar Genome Study (BiGS) and bipolar subjects that were categorized as having either seasonal or non-seasonal patterned manic episodes.

RESULTS

A bipolar case-only analysis identified three genomic regions that differed between seasonal and non-seasonal patterned manic episodes of bipolar subjects. The most significant association was for rs41350144, which lies within an intron of NF1A gene on 1p31 (P=3.08×10(-7), OR=2.27). Haplotype construction using flanking three SNPs (rs41453448, rs1125777, and rs12568010) spanning 7549bp showed a more significant association (P=2.12×10(-7), OR=0.4).

CONCLUSIONS

These data suggest that genetic variants in the NF1A gene region may predispose to seasonal patterned of mania in bipolar disorder.