Intermittent High Glucose Enhances the Proliferation of Rat Aortic Vascular Smooth Muscle Cells More Than Constant High Glucose via the Mitogen-Activated Protein Kinase Pathway

Abstract

Background: The acute glucose fluctuations in diabetes can be a risk factor of cardiovascular events. Nonetheless, the mechanism of action of intermittent hyperglycemia on the proliferation of vascular smooth muscle cells (VSMCs) is poorly understood. We compared the effects of exposure to constant and intermittent hyperglycemia on in vitro proliferation of aortic VSMCs from obese diabetic rats. Methods: VSMCs from Otsuka Long-Evans Tokushima Fatty (OLETF) rats were incubated for 72 hours with different glucose concentrations: a constant normal-glucose medium (4.8 mM), constant high-glucose medium (24.5 mM), normal- and high-glucose media alternating every 12 hours, and mannose (12.0 mM; an osmotic control for high glucose levels). The proliferation of VSMCs was analyzed by a 3-[4,5-dimethylthiazol-2-yl]-diphenyltetrazolium bromide (MTT) assay. The molecular mechanism (including phospho-p44/42 mitogen-activated protein kinase [MAPK], mitogen-activated protein kinase kinase-1/2 (MEK1/2), Protein kinase B (Akt), and the apoptosis pathway) was then studied. Results: We detected enhanced proliferation of VSMCs incubated with the constant or intermittent high-glucose medium using the MTT assay (p<0.05). The proliferation of VSMCs was more pronounced in the intermittent high-glucose environment than at a constant high glucose concentration. In western blot analysis, treatment with the constant high-glucose medium for 72 hours increased phospho-p44/42 MAPK and phospho-MEK1/2 expression as compared to treatment with the normal-glucose medium. These effects were further enhanced by the intermittent high glucose concentration. Akt and phospho-Akt were not affected by hypergly cemia. In the apoptotic pathway, Bcl-x(L), phospho-Bad, and caspase-3 were not affected by hyperglycemia for 72 hours. Conclusion: Hyperglycemia increased the proliferation of VSMCs from OLETF rats via the MAPK pathway. These effects were further enhanced by intermittent exposure to the high glucose concentration.