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Disruption of integrin α5β1 signaling does not impair PDGF‐BB‐mediated stimulation of the extracellular signal‐regulated kinase pathway in smooth muscle cells

Smooth muscle cell (SMC) proliferation is dependent on both anchorage to the extracellular matrix by integrins and the presence of growth factors. Integrins and growth factor receptors transduce signals that seem to converge on the extracellular signal‐regulated (ERK) pathway, but the molecular basis for this interaction is not known. SMC proliferation has previously been shown to be supported by culture on fibronectin (FN), whereas cells cultured on laminin (LN) are growth inhibited. In the present study, we examined the mitogenic response to platelet‐derived growth factor BB (PDGF‐BB) in baboon SMCs cultured on FN vs. LN. Induction of DNA synthesis and the activity of ERK and the ERK activating kinase MKK‐1 were reduced only slightly after stimulation with PDGF‐BB in cells cultured on LN vs. those cultured on FN. We tested the possibility that endogenous FN secretion contributes to the ability of the cells to respond to PDGF stimulation during culture on LN. Inhibition of interactions between FN and integrin α5β1 by the competitive GRGDSP‐peptide or anti‐α5 integrin antibody restricted cell spreading, reduced cell‐surface staining for α5β1 and FN fibrils, and inhibited PDGF‐BB‐induced DNA synthesis. These results showed that SMC growth on LN required a provisional FN matrix. Although disruption of interactions between α5β1 and FN by the GRGDSP‐peptide prevented PDGF‐BB‐induced DNA synthesis, neither ERK activity nor translocation of ERKs into the nucleus was inhibited. These results show that integrins regulate SMC growth through pathways that function in parallel with, but distinct from, growth factor‐mediated ERK signaling. J. Cell. Physiol. 172:109–116, 1997. © 1997 Wiley‐Liss, Inc.