Fibronectin is required to prevent thyroid cell apoptosis through an integrin-mediated adhesion mechanism

Apoptosis or programmed cell death occurs in a wide variety of cell types when adhesion to extracellular matrix (ECM) is denied. Invasion and metastasis by tumor cells involve the loss of normal cell-ECM contacts and require independence from such control mechanisms. We studied whether the immortalized thyroid cell line TAD-2 is a model suitable to investigate thyroid cell-ECM interaction, and we analyzed the role of integrin-fibronectin (FN) interaction in apoptosis. Adhesion, spreading, and cytoskeleton organization in TAD-2 cultured cells were dependent upon integrin-FN interaction. Cell spreading and cytoskeletal organization were coupled to deposition of insoluble FN induced by serum. Expression of integrin-FN receptors was demonstrated by flow cytofluorometry with specific antibodies, and strong integrin-dependent adhesion was demonstrated by attachment assays to immobilized FN. Apoptosis, occurring in different culture conditions, was determined by cell morphology and DNA electrophoretic analysis and quantitated by flow cytometry in propidium iodide-stained cells. Thyroid cells underwent apoptosis in the presence of serum when adhesion was prevented by specific peptides that inhibit integrin binding to FN (RGD-containing peptides) or by coating the culture plates with agar. In serum-free cultures, apoptosis was prevented by insoluble FN immobilized on the plates, but not by soluble FN. These results suggest that the TAD-2 cell line is a good model to study thyroid cell-ECM interaction, that FN, assembled into insoluble matrix, is required for cytoskeletal organization and to prevent thyroid cell apoptosis, and that integrin-mediated adhesion is involved in this process.