PAX8-Peroxisome Proliferator-Activated Receptor γ (PPARγ) Disrupts Normal PAX8 or PPARγ Transcriptional Function and Stimulates Follicular Thyroid Cell Growth

Follicular thyroid carcinomas are associated with a chromosomal translocation that fuses the thyroid-specific transcription factor paired box gene 8 (PAX8) with the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ). This study investigated the transcriptional mechanisms by which PAX8-PPARγ regulates follicular thyroid cells. In HeLa cells, rat follicular thyroid (FRTL-5) cells, or immortalized human thyroid cells, PAX8-PPARγ stimulated transcription from PAX8-responsive thyroperoxidase and sodium-iodide symporter promoters in a manner at least comparable with wild-type PAX8. In contrast, PAX8-PPARγ failed to stimulate transcription from the thyroglobulin promoter and blocked the synergistic stimulation of this promoter by wild-type PAX8 and thyroid transcription factor-1. Unexpectedly, PAX8-PPARγ transcriptional function on a PPARγ-responsive promoter was cell-type dependent; in HeLa cells, PAX8-PPARγ dominantly inhibited expression of the PPARγ-responsive promoter, whereas in FRTL-5 and immortalized human thyroid cells PAX8-PPARγ stimulated this promoter. In gel shift analyses, PAX8-PPARγ bound a PPARγ-response element suggesting that its transcriptional function is mediated via direct DNA contact. A biological model of PAX8-PPARγ function in follicular thyroid cells was generated via constitutive expression of the fusion protein in FRTL-5 cells. In this model, PAX8-PPARγ expression was associated with enhanced growth as assessed by soft agar assays and thymidine uptake. Therefore, PAX8-PPARγ disrupts normal transcriptional regulation by stimulating some genes and inhibiting others, the net effect of which may mediate follicular thyroid cell growth and loss of differentiation that ultimately leads to carcinogenesis.