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

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 < 0.01), which was blocked by the cPLA 2 inhibitor MAFP (10 μM; 15 min). Similarly, the PI3K inhibitor LY294002 (10 μM; 15 min) and ERK1/2 inhibitor PD98059 (20 μM; 15 min) abolished the INS-mediated increase in cPLA 2 phosphorylation by 59% (p < 0.001), and by 75% ( p < 0.001), respectively. Further, inhibition of cPLA 2 with cPLA 2 inhibitor MAFP abolished the INS-stimulated ERK1/2 phosphorylation by 65% ( p < 0.01). Incubation of rat VSMCs with INS resulted in an increase of VSMCs proliferation by 85% ( p < 0.001). The effect of INS on VSMCs proliferation was significantly ( p < 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.