CELL-ADHESION TO EXTRACELLULAR-MATRIX REGULATES THE LIFE-CYCLE OF INTEGRINS

The expression of alpha 5 beta 1 integrin on the surface of fibroblast s requires adhesion to substratum. We have examined the basis for this adhesion-dependent surface expression by comparing the life cycle of integrins in parallel cultures of adherent and nonadherent cells. Resu lts of biosynthetic labeling experiments in NRK fibroblasts showed tha t the synthesis and biosynthetic processing of the beta 1 integrin sub unit proceed in the absence of cell attachment; however, when examinin g the behavior of preexisting cell surface integrins, we observed that the alpha beta 1 integrins are internalized and degraded when adhesio n to substratum is blocked. A kinetic analysis of integrin internaliza tion in cycloheximide-treated NRK cells showed that each of the fibrob last integrins we examined (in both the beta 1 and beta 3 families) ar e lost from the cell surface after detachment from substratum. Thus, t he default integrin life cycle in fibroblasts involves continuous synt hesis, processing, transport to the cell surface, and internalization/ degradation. Interestingly, studies with NIH-3T3 cells expressing alph a 1 beta 1 integrin showed that the loss of cell-surface alpha 5 beta 1 integrin is blocked by adhesion of cells to dishes coated with type IV collagen (a ligand for alpha 1 beta 1 integrin) as well as fibronec tin. Similarly, adhesion of these cells to dishes coated with type IV collagen stabilizes the surface expression of alpha 5 beta 1 as well a s alpha 1 beta 1 integrin. We propose that the adhesion of fibroblasts to extracellular matrix protein alters the integrin life cycle and pe rmits retention of these proteins at the cell surface where they can p lay important roles in transmitting adhesion-dependent signals.