Integrin binding to immobilized collagen and fibronectin stimulates the proliferation of human thyroid cells in culture.

The expression of integrins of the beta1 family and their possible biological effects were investigated in normal human thyroid cells in monolayer culture. The expression of beta1 and alpha(1-6) integrin subunits was determined by flow cytofluorometry with specific antibodies. Follicular cells of subconfluent monolayer cultures expressed alpha2beta1 and alpha3beta1 at high levels, while alpha1beta1 was only slightly expressed, and alpha4beta1, alpha5beta1, and alpha6beta1 were never detected. Cell attachment assays were performed in fibronectin-, type I collagen-, and laminin-coated microtiter plates. Thyroid cells, while adherent to collagen and fibronectin, showed poor attachment to laminin despite the abundance of their putative receptors alpha2beta1 and alpha3beta1. In serum-free medium, collagen and fibronectin induced cytoskeletal organization, change of cell shape from round to flat, and cell spreading. [3H]Thymidine incorporation and proliferation assays were used to evaluate the effects of collagen and fibronectin on DNA synthesis and cell growth in the absence of a change in spreading or cell shape. Both substrates, in low serum-containing medium, induced a concentration-dependent increase in [3H]thymidine incorporation partially inhibited by RGD-containing peptides that blocked the cell attachment. Thyrocytes cultured in low serum-containing medium on immobilized fibronectin or collagen showed a dose-dependent stimulation of proliferation. These data indicate that fibronectin and collagen can regulate the cytoskeletal organization and cell shape and stimulate the proliferation of normal human thyroid cells in culture and that integrins mediate these effects of extracellular matrix proteins.