Glucose lowers the threshold for human aortic vascular smooth muscle cell migration: inhibition by protein phosphatase-2A

Atherosclerosis, which occurs prematurely in individuals with diabetes, incorporates vascular smooth muscle cell (VSMC) chemotaxis. Glucose, through protein kinase C-β_II signalling, increases chemotaxis to low concentrations of platelet-derived growth factor (PDGF)-BB. In VSMC, a biphasic response in PDGF-beta receptor (PDGF-βR) level occurs as PDGF-BB concentrations increase. The purpose of this study was to determine whether increased concentrations of PDGF-BB and raised glucose level had a modulatory effect on the mitogen-activated protein kinase/extracellular-regulated protein kinase pathway, control of PDGF-βR level and chemotaxis.

Cultured aortic VSMC, exposed to normal glucose (NG) (5 mmol/l) or high glucose (HG) (25 mmol/l) in the presence of PDGF-BB, were assessed for migration (chemotaxis chamber) or else extracted and immunoblotted.

At concentrations of PDGF-BB <540 pmol/l, HG caused an increase in the level of PDGF-βR in VSMC (immunoblotting) versus NG, an effect that was abrogated by inhibition of aldose reductase or protein kinase C-β_II. At higher concentrations of PDGF-BB (>540 pmol/l) in HG, receptor level was reduced but in the presence of aldose reductase or protein kinase C-β_II inhibitors the receptor levels increased. It is known that phosphatases may be activated at high concentrations of growth factors. At high concentrations of PDGF-BB, the protein phosphatase (PP)2A inhibitor, endothall, caused an increase in PDGF-βR levels and a loss of biphasicity in receptor levels in HG. At higher concentrations of PDGF-BB in HG, the chemoattractant effect of PDGF-BB was lost (chemotaxis chamber). Under these conditions inhibition of PP2A was associated with a restoration of chemotaxis to high concentrations of PDGF-BB.

The biphasic response in PDGF-βR level and in chemotaxis to PDGF-BB in HG is due to PP2A activation.