β-Catenin Regulates Deiodinase Levels and Thyroid Hormone Signaling in Colon Cancer Cells

BACKGROUND & AIMS: Activation of the beta-catenin/T-cell factor (TCF) complex occurs in most colon tumors, and its actions correlate with the neoplastic phenotype of intestinal epithelial cells. Type 3 deiodinase (D3), the selenoenzyme that inactivates thyroid hormone (3,5,3 ' triiodothyronine [T3]), is frequently expressed by tumor cells, but little is known about its role in the regulation of T3 signaling in cancer cells. METHODS: We measured D3 expression in 6 colon cancer cell lines and human tumors and correlated it with the activity of the beta-catenin/TCF complex. We also determined the effects of D3 loss on local thyroid hormone signaling and colon tumorigenesis. RESULTS: We show that D3 is a direct transcriptional target of the beta-catenin/TCF complex; its expression was higher in human intestinal adenomas and carcinomas than in healthy intestinal tissue. Experimental attenuation of beta-catenin reduced D3 levels and induced type 2 deiodinase (the D3 antagonist that converts 3,5,3 ',5 ' tetraiodothyronine into active T3) thereby increasing T3-dependent transcription. In the absence of D3, excess T3 reduced cell proliferation and promoted differentiation in cultured cells and in xenograft mouse models. This occurred via induction of Ecadherin, which sequestered beta-catenin at the plasma membrane and promoted cell differentiation. CONCLUSIONS: Deiodinases are at the interface between the beta-catenin and the thyroid hormone pathways. Their synchronized regulation of intracellular T3 concentration is a hitherto unrecognized route by which the multiple effects of beta-catenin are generated and may be targeted to reduce the oncogenic effects of beta-catenin in intestinal cells.