The Ang II-Induced Growth of Vascular Smooth Muscle Cells Involves a Phospholipase D-Mediated Signaling Mechanism

Angiotensin (Ang) II acts as a mitogen in vascular smooth muscle cells (VSMC) via the activation of multiple signaling cascades, including phospholipase C, tyrosine kinase, and mitogen-activated protein kinase pathways. However, increasing evidence supports signal-activated phospholipases A2 and D (PLD) as additional mechanisms. Stimulation of PLD results in phosphatidic acid (PA) formation, and PA has been linked to cell growth. However, the direct involvement of PA or its metabolite diacylglycerol (DAG) in Ang II-induced growth is unclear. PLD activity was measured in cultured rat VSMC prelabeled with [3H]oleic acid, while the incorporation of [3H]thymidine was used to monitor growth. We have previously reported the Ang II-dependent, AT1-coupled stimulation of PLD and growth in VSMC. Here, we show that Ang II (100 nM) and exogenous PLD (0.1–100 units/mL; Streptomyces chromofuscus) stimulated thymidine incorporation (43–208% above control). PA (100 nM–1 μM) also increased thymidine incorporation to 135% of control. Propranolol (100 nM–10 μM), which inhibits PA phosphohydrolase, blocked the growth stimulated by Ang II, PLD, or PA by as much as 95%, an effect not shared by other β-adrenergic antagonists. Propranolol also increased the production of PA in the presence of Ang II by 320% and reduced DAG and arachidonic acid (AA) accumulation. The DAG lipase inhibitor RHC-80267 (1–10 μM) increased Ang II-induced DAG production, while attenuating thymidine incorporation and release of AA. Thus, it appears that activation of PLD, formation of PA, conversion of PA to DAG, and metabolism of DAG comprise an important signaling cascade in Ang II-induced growth of VSMC.