Expression of CFTR in human and bovine thyroid epithelium

The expression of cystic fibrosis transmembrane conductance regulator (CFTR) in the thyroid has not been documented to date, although a role for CFTR in the thyroid follicular epithelium is suggested both clinically, by the occurrence of subclinical hypothyroidism in patients with cystic fibrosis (CF), and physiologically, by the presence of low-conductance, adenosine 3',5'-cyclic monophosphate-activated Cl channels in the follicular cells. Using reverse transcriptase-polymerase chain reaction with nested primers derived from exons 13 and 14 of the human CF gene, we have now documented the presence of CFTR mRNA in the human thyroid. Western blot analyses using six antibodies directed against different domains of human CFTR showed that a 165-kDa band was present in membrane extracts from bovine and human thyroid. This protein has the predicted size of mature CFTR and was not detected with preimmune serum or preadsorbed antiserum. By immunofluorescence and immunoperoxidase, CFTR was located in the follicular cells, with a diffuse, intracellular labeling pattern. Quantitative analysis revealed that 64% of the follicles were CFTR positive, but only 16% of the follicular cells were stained per follicle. The number of CFTR-positive cells was inversely proportional to the size of the follicle. These results 1) demonstrate the expression of CFTR at the mRNA and protein levels in human and bovine thyroid follicular cells and 2) suggest that CFTR expression could be instrumental in follicular enlargement.