The C-terminal domain of the adenomatous polyposis coli (Apc) protein is involved in thyroid morphogenesis and function

Adenomatous polyposis coli (APC) is a multifunctional protein as well as a tumor suppressor. To determine the functions of the C-terminal domain of Apc, we have investigated Apc ^1638T/1638T mice, which express a truncated Apc that lacks the C-terminal domain. Apc ^1638T/1638T mice are tumor free and exhibit growth retardation. In the present study, we analyzed the morphology and functions of the thyroid gland in Apc ^1638T/1638T mice. There was no significant difference in the basal concentration of serum thyroid hormones between Apc ^1638T/1638T and Apc ^+/+ mice. Thyroid follicle size was significantly larger in Apc ^1638T/1638T mice than in Apc ^+/+ mice. The extent of serum T4 elevation following exogenous thyroid-stimulating hormone (TSH) injection was lower in Apc ^1638T/1638T mice than in Apc ^+/+ mice. TSH also induced a greater reduction in thyroid follicle size in Apc ^1638T/1638T mice than in Apc ^+/+ mice. Analyses using immunohistochemistry and electron microscopy indicated that follicular epithelial cells in Apc ^1638T/1638T mice had an enlarged rough endoplasmic reticulum of irregular shape. These results suggest that the C-terminal domain of Apc is involved in thyroid morphology and function.