A novel hypothesis regarding the possible involvement of cytosolic phospholipase 2 in insulin-stimulated proliferation of vascular smooth muscle cells

Abstract

Insulin (INS) via INS receptor acts as a mitogen in vascular smooth muscle cells (VSMCs) through stimulation of multiple signaling mechanisms, including p42/44 mitogen-activated protein kinase (ERK1/2) and phosphatidyl inositol-3 kinase (PI3K). In addition, cytosolic phospholipase 2 (cPLA(2)) is linked to VSMCs proliferation. However, the upstream mechanisms responsible for activation of cPLA(2) are not well defined. Therefore, this investigation used primary cultured rat VSMCs to examine the role of PI3K and ERK1/2 in the INS-dependent phosphorylation of cPLA(2) and proliferation induced by INS. Exposure of VSMCs to INS (100 nM) for 10 min increased the phosphorylation of cPLA(2) by 1.5-fold (p lt 0.01), which was blocked by the cPLA(2) inhibitor MAFP (10 mu M; 15 min). Similarly, the PI3K inhibitor LY294002 (10 mu M; 15 min) and ERK1/2 inhibitor PD98059 (20 mu M; 15 min) abolished the INS-mediated increase in cPLA(2) phosphorylation by 59% (p lt 0.001), and by 75% (p lt 0.001), respectively. Further, inhibition of cPLA2 with cPLA2 inhibitor MAFP abolished the INS-stimulated ERK1/2 phosphorylation by 65% (p lt 0.01). Incubation of rat VSMCs with INS resulted in an increase of VSMCs proliferation by 85% (p lt 0.001). The effect of INS on VSMCs proliferation was significantly (p lt 0.01) reduced by pretreatment with MAFP. Thus, we hypothesized that INS stimulates VSMCs proliferation via a mechanism involving the PI3K-dependent activation of cPLA(2) and release of arachidonic acid (AA), which activates ERK1/2 and further amplifies cPLA(2) activity. (C) 2009 International Federation for Cell Biology. Published by Elsevier Ltd. All rights reserved.