Platelet-derived growth factor inhibits smooth muscle cell adhesion to fibronectin by ERK-dependent and ERK-independent pathways

The adhesion of cells to the extracellular matrix plays a major role in cell migration. Pretreatment with platelet-derived growth factor (PDGF) inhibited the adhesion of smooth muscle cells to fibronectin by 80%. This inhibition decreased as concentrations of fibronectin increased. In the presence of 200 muM GRGDS peptide, only 45% of PDGF-treated cells adhered to fibronectin compared with 80% of control cells. This indicates that a decrease in integrin avidity was induced by PDGF. Cell adhesion was partially restored when the activation of the extracellular signal-regulated kinase (ERK) was inhibited with PD98059. The remaining inhibition of adhesion (50%) was independent of the fibronectin concentration, suggesting that the ERK pathway is involved in the decrease in integrin avidity. This was confirmed by depleting ERK protein levels by treatment with ERK antisense oligonucleotide. The adhesion of ERK control oligonucleotide-treated cells decreased by 41% when the concentration of GRGDS peptide was increased from 50 to 200 muM but only decreased by 11% in ERK antisense oligonucleotide-treated cells. Treatment with PDGF also delayed focal complex assembly and inhibited stress fiber formation. Consistent with a delay in tyrosine phosphorylation of paxillin, PDGF treatment caused a lag in focal complex formation, although this was not associated with any change in Src family tyrosine kinase activity. Our results indicate that PDGF inhibits smooth muscle cells adhesion by two pathways. The first involves an ERK-dependent decrease in integrin avidity; the second involves the ERK-independent inhibition of focal complex assembly.