Crossing the hurdles of thyroid hormone receptor-α activation

Fundamentally distinct roles of thyroid hormone receptor isoforms in a thyrotroph cell line are due to differential DNA binding

Thyroid hormones have a profound influence on human development and disease. The hypothalamic-pituitary-thyroid axis involves finely tuned feedback mechanisms to maintain thyroid hormone (TH) levels. Despite the important role of TH-negative feedback in regulating this axis, the mechanism by which this occurs is not clearly defined. Previous in vivo studies suggest separate roles for the two thyroid hormone receptor isoforms, THRA and THRB, in this axis. We performed studies using a unique pituitary thyrotroph cell line (TαT1.1) to determine the relative roles of THRA and THRB in the regulation of Tshb. Using chromatin immunoprecipitation assays, we found that THRB, not THRA, bound to the Tshb promoter. By selectively depleting THRB, THRA, or both THRA and THRB in TαT1.1 cells, we found that simultaneous knockdown of both THRB and THRA abolished T(3)-mediated down-regulation of Tshb at concentrations as high as 100 nm T(3). In contrast, THRA knockdown alone had no effect on T(3)-negative regulation, whereas THRB knockdown alone abolished T(3)-mediated down-regulation of Tshb mRNA levels at 10 nm but not 100 nm T(3) concentrations. Interestingly, chromatin immunoprecipitation assays showed that THRA becomes enriched on the Tshb promoter after knockdown of THRB. Thus, a likely mechanism for the differential effects of THR isoforms on Tshb may be based on their differential DNA-binding affinity to the promoter.

New drug targets for the treatment of obesity

There is a huge void in the current pharmacological treatment options for obesity. This gap is surprising given the high prevalence and associated costs of obesity. Many factors have prevented active drug development, including the poor safety and efficacy of earlier antiobesity drugs. However, there are now several compelling targets on the horizon. The new generation of antiobesity drugs offers hope for the management of obesity, but no single agent is likely to be a panacea. Rather, obesity will need to be managed like many other chronic diseases, with combination therapies and long-term treatment in order to achieve sustained success. New targets have arisen as more research has been performed to understand the complex circuitry that controls energy homeostasis. The goal of this review is to discuss the latest pharmacological agents and strategies that are under development and that may eventually be used for the treatment of obesity.