A. Datta et al., Transformation of chicken embryo fibroblasts by v-src uncouples beta 1 integrin-mediated outside-in but not inside-out signaling, MOL CELL B, 21(21), 2001, pp. 7295-7306
Adhesion of cells to extracellular matrix is mediated by integrin family re
ceptors. The process of receptor-ligand binding is dependent on metabolic e
nergy and is regulated by intracellular signals, termed inside-out signals.
The strength of the initial alpha5 beta1-mediated adhesion of v-src-transf
ormed chicken embryo fibroblasts (v-srcCEF) was similar to that of normal C
EF. A chemically cross-linked fibronectin substrate was able to restore cel
l spreading and the ability of v-srcCEF to assemble a fibronectin matrix. O
ver time, v-srcCEF showed decreased adhesion due to the reduction of alpha5
beta1-fibronectin bonds consequent on the reduction of substrate-bound fib
ronectin due to the secretion of proteases by v-srcCEF. Excess synthesis of
hyaluronic acid by v-srcCEF also reduced the alpha5 beta1-fibronectin bond
s and contributed to cell detachment at later times in culture. Thus, the a
dhesion defects were not due to a failure of alpha5 beta1 function and adhe
sion of the v-srcCEF was alpha5 beta1 dependent. Integrin-mediated adhesion
also produces signals that affect cell proliferation and cell differentiat
ion. An early consequence of these "outside-in" signals was the phosphoryla
tion of FAK Y397 in direct proportion to the number of alpha5 beta1-fibrone
ctin bonds formed. In contrast, v-srcCEF had an increased level of phosphor
ylation on five different tyrosines in FAK, and none of these phosphorylati
on levels were sensitive to the number of alpha5 beta1-fibronectin bonds. I
n the absence of serum, CEF proliferation was sensitive to changes in alpha
5 beta1-mediated adhesion levels. Transformation by v-src increased the ser
um-free proliferation rate and made it insensitive to alpha5 beta1-mediated
adhesion. Thus, the v-srcCEF were insensitive to the normal outside-in sig
nals from alpha5 beta1 integrin.