Intermittent high glucose enhances proliferation of vascular smooth muscle cells by upregulating osteopontin.

OBJECTIVE: Hyperglycemia induces vascular smooth muscle cells (VSMCs) proliferation and may thus contribute to the formation of atherosclerotic lesions. Glucose fluctuations are strong predictor of diabetic vascular complications. We investigate the effects of exposure to constant and intermittent high glucose concentrations on the proliferation and matrix metalloproteinase (MMP)-2 activity of rat aortic VSMCs in culture, as well as the expression of osteopontin (OPN).
METHODS: Rat aortic VSMCs were grown to confluence and then exposed to 5 mmol/L glucose, 25 mmol/L glucose, or 5 mmol/L alternating with 25 mmol/L glucose in the absence or presence of neutralizing antibodies to OPN, beta3 integrin receptor and beta5 integrin receptor. The cell proliferation, MMP-2 activity and the expression of OPN were assessed.
RESULTS: In cultured VSMCs, treatment with constant or intermittent high glucose significantly increased [(3)H]thymidine incorporation in a time-dependent manner. A modest increase was observed at 12h, and further deteriorated afterwards, and reached the maximum expression at 48h. However, [(3)H]thymidine incorporation was more pronounced in intermittent high glucose than in constant high glucose. Treatment with constant high glucose for 48 h significantly increase cell number, MMP-2 activation, OPN protein and mRNA expression compared with VSMCs treated with the cells normal glucose, and these effects were further enhanced when VSMCs were treated with intermittent high glucose. In addition, neutralizing antibodies to either OPN or its receptor beta3 integrin but not neutralizing antibodies to beta5 integrin significantly suppressed increase in [(3)H]thymidine incorporation and MMP-2 activity induced by constant or intermittent high glucose.
CONCLUSIONS: In cultured VSMCs, constant high glucose concentrations enhanced MMP-2 activity, cell proliferation and OPN expression. These effects are enhanced following intermittent exposure to high glucose, indicating that short lived excursions in glycaemic control have important pathological effects on the development of diabetic atherosclerosis, which is mediated by the stimulation of OPN expression and synthesis.