Adiponectin expression and metabolic markers in obesity and Type 2 diabetes.

BACKGROUND: Adiponectin has emerged over the last decade as a key adipokine linking obesity, insulin resistance, and Type 2 diabetes. However, the molecular mechanisms controlling adiponectin expression in adipose tissue are not fully elucidated. Furthermore, increasing evidence indicates that peroxisome proliferator-activated receptor- γ (PPAR-γ) plays an important, and beneficial, role in modulating adiponectin expression. AIM: The aim of the present study was to assess the separate role of obesity and Type 2 diabetes in the relationship between endogenous PPAR-γ signaling and adiponectin expression in subcutaneous adipose tissue. SUBJECTS AND METHODS: Enzyme-linked immuno sor bent assay and real time quantitative PCR analysis were carried out in overweight, obese, and/or diabetic Tunisian patients who underwent an abdominal surgery. RESULTS: These results collectively indicate that circulating levels of adiponectin were decreased in all overweight, obese, and/or diabetic (p<0.001). However, the subcutaneous mRNA expression of adiponectin was reduced only in diabetics (p<0.01) but presents some discrepancies in obese individuals. Moreover, mRNA levels of adiponectin were positively correlated with levels of mRNA encoding PPARγ and its heterodimeric partner retinoid X receptor-α (RXR-α), in both obese and diabetic patients. CONCLUSION: Our study on Tunisian patients shows impaired regulation of circulating and mRNA adiponectin levels dependent of metabolic disorders in obesity and Type 2 diabetes. The data suggest that subcutaneous adipose tissue may play an important role in modulating adiponectin expression in diabetes and obesity. Moreover, adiponectin mRNA could be potentially regulated by endogenous PPARγ/RXRα-dependent pathways.

Nuclear corepressor and silencing mediator of retinoic and thyroid hormone receptors corepressor expression is incompatible with T(3)-dependent TRH regulation.

Ligand-independent repression by thyroid hormone (T(3)) receptors on positive T(3)-responsive genes requires corepressor proteins. However, the role of corepressors in regulating genes such as hypothalamic TRH, which are under negative control by T(3), is largely unknown. We examined the expression of mRNAs encoding the corepressors NCoR (nuclear corepressor) and SMRT (silencing mediator of retinoic and thyroid hormone receptors) in the TRH-producing paraventricular nucleus of the mouse hypothalamus. Further, we carried out in vivo functional studies by overexpression of both corepressors. Three lines of evidence show that NCoR and SMRT expression is incompatible with physiological regulation of TRH. First, Northern blotting revealed TRH and NCoR mRNA expressions to be inversely correlated during postnatal development and as a function of thyroid status. Second, in situ hybridization showed that NCoR and SMRT mRNA expression profiles in the paraventricular nucleus were distinct from that of TRH mRNA. Third, over-expression of full length NCoR and SMRT in the hypothalamus abolished T(3)-dependent repression of TRH-luciferase. However, over-expression of NCoR or SMRT did not affect either T(3)-independent activation of TRH-luciferase transcription, or transcription from a positively regulated T(3)-response element. We conclude that T(3) -dependent feedback on TRH expression is unlikely to involve the corepressors NCoR or SMRT.

Transcriptional repression of TRH promoter function by T3: analysis by in vivo gene transfer.

We consider how an integrated in vivo model can be used to study the specific transcriptional effects of specific receptors in neuroendocrine systems. Our example is the role of thyroid receptor (TR) isoforms in mediating negative feedback effects of T3 on TRH (thyrotropin releasing hormone) expression. The in vivo transfection method employed polyethylenimine (PEI) to introduce genes directly into specific regions of the brains of mice, rats, and Xenopus tadpoles. In the mouse model, the technique has served to examine TR effects on TRH transcription and on the pituitary-thyroid axis end point: thyroid hormone secretion. When a TRH-luciferase construct is introduced into the hypothalami of newborn mice TRH-luciferase transcription is regulated physiologically, being significantly increased in hypothyroidism and decreased in T3-treated animals. When various T3-binding forms of TRbeta or TRalpha are expressed in the hypothalamus, all TRbeta isoforms give T3-dependent regulation of TRH transcription, whereas TRalpha isoforms block T3-dependent transcription. Moreover, TR transcriptional effects are correlated with physiological consequences on circulating T4. Thus, somatic gene transfer shows TR subtypes to have distinct, physiologically relevant effects on TRH transcription. The approach is an appealing alternative to germinal transgenesis for studying specific neuroendocrine regulations at defined developmental stages in different species.

Physiological regulation of hypothalamic TRH transcription in vivo is T3 receptor isoform specific.

Thyroid hormone (tri-iodo-thyronine, T3) exerts transcriptional effects on target genes in responsive cells. These effects are determined by DNA/protein interactions governed by the type of T3 receptors (TRs) in the cell. As TRs show tissue and developmental variations, regulation is best addressed in an integrated in vivo model. We examined TR subtype effects on thyrotropin-releasing hormone (TRH) transcription and on the pituitary/thyroid axis end point: thyroid hormone secretion. Polyethylenimine served to transfect a TRH-luciferase construct containing 554 bp of the rat TRH promoter into the hypothalami of newborn mice. Transcription from the TRH promoter was regulated in a physiologically faithful manner, being significantly increased in hypothyroidism and decreased in T3-treated animals. Moreover, when various ligand binding forms of mouse or chicken TRbeta and TRalpha were expressed with TRH-luciferase, all forms of TRbeta gave T3-dependent regulation of TRH transcription, whereas transcription was T3 insensitive with each TRalpha tested. Moreover, chicken TRalpha increased TRH transcription sixfold, whereas mouse TRalpha decreased transcription. These transcriptional effects had correlated physiological consequences: expression of the chicken TRalpha in the hypothalamus of newborn mice raised circulating T4 levels by fourfold, whereas mouse TRalpha had opposite effects. Thus, TR subtypes have distinct, physiologically relevant effects on TRH transcription.