Transformation of Chicken Embryo Fibroblasts by v-src Uncouples β1 Integrin-Mediated Outside-in but Not Inside-out Signaling

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Abstract
Adhesion of cells to extracellular matrix is mediated by integrin family receptors. The process of receptor-ligand binding is dependent on metabolic energy and is regulated by intracellular signals, termed inside-out signals. The strength of the initial α5β1-mediated adhesion of v-src-transformed chicken embryo fibroblasts (v-srcCEF) was similar to that of normal CEF. A chemically cross-linked fibronectin substrate was able to restore cell spreading and the ability of v-srcCEF to assemble a fibronectin matrix. Over time, v-srcCEF showed decreased adhesion due to the reduction of α5β1-fibronectin bonds consequent on the reduction of substrate-bound fibronectin due to the secretion of proteases by v-srcCEF. Excess synthesis of hyaluronic acid by v-srcCEF also reduced the α5β1-fibronectin bonds and contributed to cell detachment at later times in culture. Thus, the adhesion defects were not due to a failure of α5β1 function and adhesion of the v-srcCEF was α5β1 dependent. Integrin-mediated adhesion also produces signals that affect cell proliferation and cell differentiation. An early consequence of these “outside-in” signals was the phosphorylation of FAK Y397 in direct proportion to the number of α5β1-fibronectin bonds formed. In contrast, v-srcCEF had an increased level of phosphorylation on five different tyrosines in FAK, and none of these phosphorylation levels were sensitive to the number of α5β1-fibronectin bonds. In the absence of serum, CEF proliferation was sensitive to changes in α5β1-mediated adhesion levels. Transformation by v-src increased the serum-free proliferation rate and made it insensitive to α5β1-mediated adhesion. Thus, the v-srcCEF were insensitive to the normal outside-in signals from α5β1 integrin.