Stromal cells in normal colon and colon cancers: importance of thyroid hormone signaling.

Thyroid hormones (THs, namely T3 and T4) regulate intestinal development and homeostasis via thyroid hormone nuclear receptors (TRs), which are T3-modulated transcription factors. Previous work has highlighted the importance of THs and the TRα1 receptor in intestinal stem cell biology and tumor formation, through actions on WNT, NOTCH, and BMP signaling pathways, which mediate epithelial-stromal cell interactions. Recent findings underscore the critical role of stromal cells in maintaining homeostasis and interacting with colonic stem cells. Stromal cells, especially cancer-associated fibroblasts (CAFs), are also essential in colorectal cancer (CRC). While the TH/TR signaling on gut epithelia-stromal interactions is well characterized in amphibians during the TH-dependent metamorphosis process, its function in the normal mammalian colon is still poorly defined, and in CRCs, it remains underexplored. In addition, it is worth underlining that TRα1 mutations in patients are responsible for Resistance to Thyroid Hormone-α (RTH-α) syndrome. This syndrome is a complex pathology that recapitulates typical traits of hypothyroidism, including gut malfunction. Up to now, very little is known about the cellular alterations in the gut of RTH-α patients. This review summarizes recent studies on the roles of T3 and TRα1 in colon physiopathology, with an emphasis on epithelial/stromal or tumor/stromal interactions via cell-cell signaling.