Regulation of proteins in the VLA cell substrate adhesion family: influence of cell growth conditions on VLA-1, VLA-2, and VLA-3 expression.

Cell quiescence resulting from culture of normal human fibroblasts in low serum (0.5%) was associated with a subsequent gradual increase in the expression of the cell-surface glycoprotein VLA-1, and a corresponding decrease in the expression of extracellular matrix adhesion receptors VLA-2 and VLA-3. Quantitation using either flow cytometry or immunoprecipitation showed that both the VLA-1/VLA-2 and VLA-1/VLA-3 ratios increased 10- to 28-fold and were still rising when cells remained quiescent for 20-30 days. Although induced by cell quiescence, changes in the levels of VLA-1, VLA-2, and VLA-3 continued to occur well after cell proliferation had stopped and thus do not directly correlate with cell cycle transition events. Despite prolonged serum deprivation resulting in elevated VLA-1/VLA-2 and VLA-1/VLA-3 ratios, growth-arrested cells remained viable and were fully capable of proliferating when restimulated. The increases in VLA-1/VLA-2 and VLA-1/VLA-3 ratios observed on quiescent cells were readily reversible, since after restimulation with 10% serum, these ratios quickly returned within 1-2 days to a level near that found on normal exponentially grown cells. Elevation of VLA-1/VLA-2 and VLA-1/VLA-3 ratios is generally associated with quiescence and is not due just to serum deprivation since density arrest of cells at confluence had similar effects on these ratios.