A novel mechanism of thyroid hormone-dependent negative regulation by thyroid hormone receptor, nuclear receptor corepressor (NCoR), and GAGA-binding factor on the rat cD44 promoter.

CD44 is an adhesion molecule in the extracellular matrix that shows various functions, including tumor genesis and metastasis. A recent study showed that CD44 expression level was strongly correlated with the generation of papillary thyroid carcinomas, the most prevalent malignancy of the thyroid gland. We report here that CD44 is negatively regulated by thyroid hormone (T(3)) through a novel mechanism. We demonstrate that nuclear receptor corepressor (NCoR) enhances thyroid hormone receptor (TR)-mediated basal transactivation by a weak TR.DNA interaction in the absence of T(3), which is repressed by T(3) through a transient TR .DNA interaction. Initially, we identified that CD44 was negatively directly transcriptionally T(3) -responsive. Deletion and mutation analysis indicated that both a weak TR and a GAGA-binding factor (GAF) binding sites on the CD44 promoter were required for negative regulation by T(3). The weak TR.DNA interaction was further confirmed by electrophoretic gel mobility shift assay, chromatin immunoprecipitation, and transfection assays using a non-DNA-binding TRalpha1 mutant. More interestingly, NCoR acted as a co-activator to enhance TR-mediated basal transactivation in the absence of T(3). This effect was eliminated by removal of TR or NCoR binding. Most strikingly, T(3) induced a remarkable increase in TR.DNA binding at 40-60 min after T(3) exposure that rapidly returned to basal levels, suggesting a T(3)-induced remodeling of chromatin structure at the early stage of T(3) stimulation resulting in repression. Therefore, we propose a mechanism by which NCoR, GAF, and TR interact with the CD44 negative T(3)-responsive element to enhance basal transactivation, whereas T(3) induces the remodeling of chromatin structure for repression.