Phosphatidylinositide 3-kinase regulates angiotensin II-induced cytosolic phospholipase A2 activity and growth in vascular smooth muscle cells

Abstract

Angiotensin (Ang) II via the AT 1 receptor acts as a mitogen in vascular smooth muscle cells (VSMC) through stimulation of multiple signaling mechanisms, including tyrosine kinases and mitogen-activated protein kinase (MAPK). In addition, cytosolic phospholipase A 2 (cPLA 2)-dependent release of arachidonic acid (AA) is linked to VSMC growth and we have reported that Ang II stimulates cPLA 2 activity via the AT 1 receptor. The coupling of Ang II to the activation of cPLA 2 appears to involve mechanisms both upstream and downstream of MAPK such that AA stimulates MAPK activity which phosphorylates cPLA 2 to further enhance AA release. However, the upstream mechanisms responsible for activation of cPLA 2 are not well-defined. One possibility includes phosphatidylinositide 3-kinase (PI3K), since PI3K has been reported to participate in the upstream signaling events linked to activation of MAPK. However, it is not known whether PI3K is involved in the Ang II-induced activation of cPLA 2 or if this mechanism is associated with the Ang II-mediated growth of VSMC. Therefore, we used cultured rat VSMC to examine the role of PI3K in the Ang II-dependent phosphorylation of cPLA 2, release of AA, and growth induced by Ang II. Exposure of VSMC to Ang II (100 nM) increased [ 3H]thymidine incorporation, cell number, and the release of [ 3H]AA. Also, using Western analysis, Ang II increased the phosphorylation of MAPK and cPLA 2 which were blocked by the MAPK kinase inhibitor PD98059 (10 μM/L) . Similarly, the PI3K inhibitor LY294002 (10 μM/L) abolished the Ang II-mediated increase in MAPK phosphorylation, as well as phosphoserine-PLA 2. Further, inhibition of PI3K blocked the Ang II-induced release of AA and VSMC mitogenesis. However, exogenous AA was able to restore VSMC growth in the presence of LY294002, as well as reverse the inhibition of MAPK and cPLA 2 phosphorylation by LY294002. Thus, it appears from these data that Ang II stimulates the PI3K-sensitive release of AA which stimulates MAPK to phosphorylate cPLA 2 and enhance AA release. This mechanism may play an important role in the Ang II-induced growth of VSMC.