Sustained orbital shear stress stimulates smooth muscle cell proliferation via the extracellular signal-regulated protein kinase 1/2 pathway

Objective: Nonlaminar shear stress stimulates smooth muscle cell (SMC) proliferation and migration in vivo, especially after an endothelial-denuding injury. To determine whether sustained shear stress directly stimulates SMC proliferation in vitro, the effect of orbital shear stress on SMC proliferation, phenotype, and extracellular signal-regulated protein kinase 1/2 (ERK1/2) phosphorylation was examined. Methods: Bovine SMCs were exposed to orbital shear stress (210 rpm) for up to 10 days, with and without the ERK1/2 upstream pathway inhibitor PD98059 (10 mu M) or the p38 pathway inhibitor SB203580 (10 mu M). Proliferation was directly counted and assessed with proliferation cell nuclear antigen. Western blotting was used to assess activation of SMC ERK1/2 and SMC phenotype markers. Results: SMCs exposed to sustained orbital shear stress (10 days) had 75% increased proliferation after 10 days compared with static conditions. Expression of markers of the contractile phenotype (alpha-actin, calponin) was decreased, and markers of the synthetic phenotype (vimentin, beta-actin) were increased. ERK1/2 was phosphorylated in the presence of orbital shear stress, and orbital shear-stress-stimulated SMC proliferation was inhibited in the presence of PD98059 but sustained in the presence of SB203580. Orbital shear-stress-induced changes in SMC phenotype were also inhibited in the presence of PD98059. Conclusion: Orbital shear stress directly stimulates SMC proliferation in long-term culture in vitro and is mediated, at least partially, by the ERK1/2 pathway. The ERK1/2 pathway may also mediate the orbital shear-stress-stimulated switch from SMC contractile to synthetic phenotype. These results suggest that shear-stress-stimulated SMC proliferation after vascular injury is mediated by a pathway amenable to pharmacologic manipulation.