Osteopontin deficiency in rat vascular smooth muscle cells is associated with an inability to adhere to collagen and increased apoptosis

Osteopontin (OPN) is an extracellular matrix protein that has been implicat ed in vascular smooth muscle cell (VSMC) adhesion. We have previously descr ibed the generation of OPN-deficient VSMC that displayed altered adhesion t o collagen. We have examined further the causes and consequences of this al tered adhesion. OPN-deficiency was associated with a significant reduction in surface expression of alpha1 and beta1 integrins (mean fluorescence inte nsity alpha1: OPN-deficient 0.135 +/- 0.04 vs. control 0.313 +/- 0.05, p < 0.0001; <beta>1: OPN-deficient 0.398 +/- 0.09 vs. control 0.570 +/- 0.05, p < 0.004). Treatment of normal VSMC with antibody to al recapitulated the a dhesion defect. OPN-deficient cells without collagen exposure had an apopto tic fraction of 1.9%, which increased to 95.7% after 24 hours exposure to c ollagen. Exogenous OPN added to cultures within 15 minutes of plating resto red normal cell adhesion, but did not prevent cells from undergoing apoptos is. Normal VSMC had no detectable apoptosis after 24 hours incubation in su spension, whereas OPN-deficient cells had an apoptotic fraction of 37.5% wh en incubated in suspension under the same conditions. The data suggest that OPN-deficient VSMC have two distinct abnormalities: an <alpha>1 beta1-medi ated inability to adhere normally to collagen and an increased propensity f or apoptosis.