Osteopontin (OPN) is an extracellular matrix protein that has been implicated in vascular smooth muscle cell (VSMC) adhesion. We have previously described the generation of OPN-deficient VSMC that displayed altered adhesion to collagen. We have examined further the causes and consequences of this altered adhesion. OPN-deficiency was associated with a significant reduction in surface expression of a1 and b1 integrins (mean fluorescence intensity a1: OPN-deficient 0.135 +/- 0.04 vs. control 0.313 +/- 0.05, p < 0.0001; b1: OPN-deficient 0.398 +/- 0.09 vs. control 0.570 +/- 0.05, p < 0.004). Treatment of normal VSMC with antibody to a1 recapitulated the adhesion defect. OPN-deficient cells without collagen exposure had an apoptotic fraction of 1.9%, which increased to 95.7% after 24 hours exposure to collagen. Exogenous OPN added to cultures within 15 minutes of plating restored normal cell adhesion, but did not prevent cells from undergoing apoptosis. Normal VSMC had no detectable apoptosis after 24 hours incubation in suspension, whereas OPN-deficient cells had an apoptotic fraction of 37.5% when incubated in suspension under the same conditions. The data suggest that OPN-deficient VSMC have two distinct abnormalities: an a1b1-mediated inability to adhere normally to collagen and an increased propensity for apoptosis.