Procyanidin B2-induced LKB1-AMPK activation mitigates vascular smooth muscle cell proliferation through inhibition of mTOR signaling

Vascular smooth muscle cell (VSMC) proliferation contributes to intimal thickening in atherosclerosis and restenosis diseases. As a proanthocyanidin type B, procyanidin B2 (PB2) is abundantly found in cocoa, apples, and grapes and is reported to have vascular protective effects. However, the mechanisms by which PB2 inhibits proliferation of VSMCs are not clearly understood. Therefore, the purpose of this study was to investigate the underlying mechanism of PB2-induced inhibition of cell proliferation in VSMCs. We found that PB2 dose- and time-dependently increased phosphorylation of liver kinase B1 (LKB1) and AMP-activated protein kinase (AMPK) in VSMCs. AMPK is a serine-threonine kinase and serves as a key sensor of cellular energy. PB2 induced LKB1 translocation from nucleus to cytosol which led to AMPK activation. In addition, PB2-induced AMPK activation decreased cell proliferation and cell cycle progression by inhibiting mammalian target of rapamycin signaling pathway. Transfection with LKB1 or AMPK siRNA and transduction of dominant-negative isoforms of the α1 and α2 subunits of AMPK eliminated anti-proliferative effects of PB2. These results demonstrate that PB2 might be a preventive agent for cardiovascular disorders such as atherosclerosis and hypertension.