Modulation of α₇-integrin-mediated adhesion and expression by platelet-derived growth factor in vascular smooth muscle cells

We showed previously that the expression of α₇-integrin in aortic vascular smooth muscle cells (VSMC) is enhanced in a rat model of atherosclerosis. In the present study, we investigated the effects of platelet-derived growth factor (PDGF) on α₇-integrin expression and VSMC adhesion and migration. Expression of the α₇-integrin gene was determined by real-time RT-PCR, whereas protein levels were determined by fluorescence-activated cell sorting analysis. PDGF increased α₇ cell surface protein expression (12 and 24 h: 3.3 ± 0.8- and 3.6 ± 0.4-fold, P < 0.05 vs. control) and mRNA levels (24 h: 3.1-fold, P < 0.05 vs. control) in a time-dependent manner. Actinomycin D and cycloheximide attenuated PDGF-induced increases in α₇- integrin, indicating the involvement of de novo mRNA and protein synthesis. Treatment with the MAPK inhibitors PD-98059, SP-600125, and SB-203580 attenuated PDGF-induced increases in mRNA. In contrast, PD-98059 and SP-600125, but not SB-203580, attenuated PDGF-induced increases in cell surface protein levels. PDGF-treated VSMC adhered to laminin more efficiently (42 ± 6% increase, P < 0.01), and this increase was partially inhibited by anti- α₇-integrin function-blocking antibody. However, PDGF did not alter migration on laminin, and there was no effect of the anti- α₇-integrin function-blocking antibody on basal or PDGF-stimulated migration. Immuno-fluorescence imaging revealed an increase in α₇-integrin distribution along the stress fibers. Together, these observations indicate that PDGF enhances α⁷-integrin expression in VSMC and promotes α₇-integrin-mediated adhesion to laminin.