Smooth muscle cells isolated from the neointima after vascular injury exhibit altered responses to platelet-derived growth factor and other stimuli.

A variety of evidence suggests that vascular smooth muscle cells (SMC) exhibit a more immature phenotype when stimulated by injury to replicate in the adult. One growth characteristic common to immature (embryonic, fetal, and neonatal) SMC is a markedly reduced responsiveness to platelet-derived growth factor (PDGF) and other mitogenic stimuli. Here we demonstrate that SMC isolated from the 14-day neointima of experimentally injured carotid arteries exhibit a similar growth phenotype. The proliferative responses of neointimal cells to the BB homodimer of PDGF, which interacts with both forms of the PDGF receptor, were up to twenty-fold less (as assessed by BrdU immunocytochemistry) than that of adult control tunica media cells over a wide range of PDGF concentrations. Paradoxically, these cells expressed abundant mRNA for the alpha- and beta-subunits of the PDGF receptor (by RT-PCR) and expressed abundant PDGF receptor protein (by Western blotting). Addition of PDGF-BB to neointimal SMC induced significant autophosphorylation of the PDGF receptor, suggesting that the PDGF receptors were fully functional. The chemotactic responses of neointimal SMC to PDGF, in in vitro migration assays, were identical to that of control medial cells. The data further establish the existence of vascular SMC phenotypes characterized by a refractoriness to growth stimulation by specific mitogens, and provide further evidence for the reiteration of developmentally regulated programs following vascular injury in vivo.