Inability of vascular smooth muscle cells to proceed beyond S phase of cell cycle, and increased apoptosis in symptomatic carotid artery disease.

OBJECTIVE: Microemboli passing through the cerebral artery downstream from high-grade carotid artery stenosis produce transient ischemic symptoms and may result in stroke. Rupture of carotid artery plaque is the main source of microemboli in high-grade internal carotid artery stenosis. However, the mechanisms underlying plaque rupture are unclear. We hypothesized that vascular smooth muscle cells (VSMC) from plaque in patients with symptoms of carotid artery stenosis undergo increased apoptosis and decreased proliferation, compared with VSMC in patients without symptoms.
METHODS: VSMC were isolated by means of enzymatic dissociation from plaque removed at carotid endarterectomy in patients with symptoms of carotid artery stenosis, eg, hemispheric transient ischemic attacks, amaurosis fugax, or stroke, and patients with high-grade stenosis without symptoms. VSMC were cultured and immunostained with smooth muscle alpha-actin and caldesmon antibodies to ensure purity. TUNEL assay and annexin V labeling were performed to identify VSMC undergoing apoptosis. Proliferation assay with [(3)H] thymidine incorporation was performed in VSMC stimulated with fetal bovine serum (FBS), and cell cycle profile was analyzed with DNA staining with Vindelov reagent.
RESULTS: We isolated VSMC from symptomatic plaque that showed gross ulceration, and asymptomatic plaque. Apoptosis, as measured with the TUNEL assay, in VSMC from symptomatic plaque was 5.45% +/- 0.8%, and in asymptomatic plaque was 1.20% +/- 0.2%. Annexin V labeling revealed that 26.8% +/- 3.8% cells were labeled for phosphatidylserine in VSMC in symptomatic plaque, compared with 4.8% +/- 0.3% cells in asymptomatic plaque. VSMC in asymptomatic plaque showed significantly increased uptake of [(3)H] thymidine at all concentrations of FBS, compared with symptomatic plaque. In the presence of 10% FBS, VSMC from asymptomatic plaque progressed through the S phase of the cell cycle, whereas significantly increased numbers of VSMC from symptomatic plaque were arrested in the S phase.
CONCLUSION: Increased numbers of VSMC from symptomatic plaque undergo apoptosis, compared with VSMC from asymptomatic plaque. This could be due to inability of VSMC from symptomatic plaque to progress beyond the S phase of the cell cycle. Decreased proliferation and increased loss of VSMC as a result of apoptosis in symptomatic plaque may result in plaque rupture, leading to development of symptoms.