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Abstract

Transforming growth factor (TGF)-β1, modulates the expression of extracellular matrix (ECM) glycoproteins, fibronectin and laminin and the adhesion of Moser colon cancer cells to these glycoproteins. Since adhesion can be altered through expression of cell-surface receptors, binding affinities of adhesion molecules for receptors, or both, we investigated the effect of TGF-β1, on the binding properties of fibronectin and laminin to their cell-surface receptors by saturation binding and Scatchard analyses using radiolabeled fibronectin and laminin. Fibronectin bound to its cell-surface receptor with high affinity (Kd = 1.25 × 10−9 M), Moser cells had approximately 7.1 × 104 fibronectin-binding sites per cell. TGF-β1 treatment rapidly up-modulated the number of cell-surface fibronectin-binding sites by 1.9-fold. The binding affinity of fibronectin for the receptor, however, was not altered. Laminin was found to bind to a higher-affinity and a lower-affinity receptor. Moser cells expressed approximately 1.1 × 103 higher-affinity laminin-binding sites and approximately 3.1 × 10 lower-affinity-binding sites per cell. TGF-β1 rapidly increased the expression of the higher-affinity sites 3-fold and the lower-affinity sites 5-fold. The binding affinity of both the higher-affinity and lower-affinity laminin receptors increased 3-fold after 2 and 6 hr of TGF-β1 treatment respectively. Concurrent with receptor modulation, TGF-β1 induced the secretion of fibronectin and laminin from Moser cells. Northern hybridization analyses showed a concurrent stimulation of the expression of the mRNAs for ligands (fibronectin and laminin) and the mRNAs for the integrin species of the fibronectin and laminin receptors (α5 and α6 subunits). Thus the production of fibronectin and laminin and the expression of their receptors were tightly co-regulated by TGF-β1.