RESUMO
Thyroid hormone receptor ß (TRß) suppresses tumor growth through regulation of gene expression, yet the associated TRß-mediated changes in chromatin assembly are not known. The chromatin ATPase brahma-related gene 1 (BRG1; SMARCA4), a key component of chromatin-remodeling complexes, is altered in many cancers, but its role in thyroid tumorigenesis and TRß-mediated gene expression is unknown. We previously identified the oncogene runt-related transcription factor 2 (RUNX2) as a repressive target of TRß. Here, we report differential expression of BRG1 in nonmalignant and malignant thyroid cells concordant with TRß. BRG1 and TRß have similar nuclear distribution patterns and significant colocalization. BRG1 interacts with TRß, and together, they are part of the regulatory complex at the RUNX2 promoter. Loss of BRG1 increases RUNX2 levels, whereas reintroduction of TRß and BRG1 synergistically decreases RUNX2 expression. RUNX2 promoter accessibility corresponded to RUNX2 expression levels. Inhibition of BRG1 activity increased accessibility of the RUNX2 promoter and corresponding expression. Our results reveal a mechanism of TRß repression of oncogenic gene expression: TRß recruitment of BRG1 induces chromatin compaction and diminishes RUNX2 expression. Therefore, BRG1-mediated chromatin remodeling may be obligatory for TRß transcriptional repression and tumor suppressor function in thyroid tumorigenesis.
Assuntos
Montagem e Desmontagem da Cromatina/genética , Subunidade alfa 1 de Fator de Ligação ao Core/genética , DNA Helicases/fisiologia , Proteínas Nucleares/fisiologia , Receptores beta dos Hormônios Tireóideos/fisiologia , Fatores de Transcrição/fisiologia , Carcinogênese/genética , Células Cultivadas , Regulação para Baixo/genética , Regulação da Expressão Gênica , Células HEK293 , Humanos , Neoplasias da Glândula Tireoide/genética , Neoplasias da Glândula Tireoide/patologiaRESUMO
Dysregulation of the thyroid hormone receptor (TR)ß is common in human cancers. Restoration of functional TRß delays tumor progression in models of thyroid and breast cancers implicating TRß as a tumor suppressor. Conversely, aberrant expression of the runt-related transcription factor 2 (Runx2) is established in the progression and metastasis of thyroid, breast, and other cancers. Silencing of Runx2 diminishes tumor invasive characteristics. With TRß as a tumor suppressor and Runx2 as a tumor promoter, a compelling question is whether there is a functional relationship between these regulatory factors in thyroid tumorigenesis. Here, we demonstrated that these proteins are reciprocally expressed in normal and malignant thyroid cells; TRß is high in normal cells, and Runx2 is high in malignant cells. T3 induced a time- and concentration-dependent decrease in Runx2 expression. Silencing of TRß by small interfering RNA knockdown resulted in a corresponding increase in Runx2 and Runx2-regulated genes, indicating that TRß levels directly impact Runx2 expression and associated epithelial to mesenchymal transition molecules. TRß specifically bound to 3 putative thyroid hormone-response element motifs within the Runx2-P1 promoter ((-)105/(+)133) as detected by EMSA and chromatin immunoprecipitation. TRß suppressed Runx2 transcriptional activities, thus confirming TRß regulation of Runx2 at functional thyroid hormone-response elements. Significantly, these findings indicate that a ratio of the tumor-suppressor TRß and tumor-promoting Runx2 may reflect tumor aggression and serve as biomarkers in biopsy tissues. The discovery of this TRß-Runx2 signaling supports the emerging role of TRß as a tumor suppressor and reveals a novel pathway for intervention.