Cardiology Research and Practice Other mechanisms whereby Ang II may promote vascular remodeling and formation of vascular lesions are the modu- lation of vascular cell migration, decreased vascular smooth muscle apoptosis, and extracellular matrix deposition [50– 52]. These multiple actions of Ang II are mediated via complex intracellular signaling pathways including stimula- tion of the PLC-IP3-DAG cascade, tyrosine kinases, MAP kinases, and RhoA/Rho kinase [53]. Intracellular signaling pathways that are stimulated after binding of the peptide to its cell-surface receptors, of which two major subtypes have been characterized, AT1R and AT2R [54, 55]. Although Ang IV receptor was identiﬁed recently, as insulin-regulated aminopeptidase, however, its roles in angiogenesis still remain unknown. In humans, AT1R is widely expressed in blood vessels, kidney, heart, liver, and adrenal glands, whereas AT2R is present largely in foetal tissue, decreasing quickly after birth, with relatively low amounts normally expressed in adult tissue [53]. AT1R mediates proangio- genic eﬀect through enhancement of inﬂammation and leukocytes inﬁltration, while AT2R mediates antiangiogenic eﬀect through regulation of apoptosis. AT2R expression is increased in pathological circumstances associated with cardiac and vascular remodeling or inﬂammation. Although they diﬀer in their unique actions, both of the receptors play a crucial role in regulating VSMC function. 2.3. Reactive Oxygen Species and Inﬂammatory Cells in the Formation of Atherosclerosis. Experimental and clinical studies using Ang II, ACE inhibitors, and AT1 receptor blockers have provided indirect evidence supporting the role of oxidative stress in the pathogenesis of endothelial dysfunc- tion and atherogenesis, independent of the hemodynamic stress of blood pressure [56, 57]. Growing evidence suggests that vascular reactive oxygen species (ROS) play a key role in atherogenesis. Besides its vasoconstrictive properties, Ang II, via the AT1 receptor, generates O2-production in endothelial cells, adventitial ﬁbroblasts, vascular smooth muscle cells (VSMCs), and mesangial cells through activation of nicoti- namide adenine dinucleotide (reduced form)/NADH phos- phate (reduced form) (NADH/NAD(P)H) oxidase leading to endothelial dysfunction, growth, and inﬂammation [58, 59]. Among many ROS generator, nicotinamide dinucleotide phosphate (NAD(P)H) oxidase dependent pathway is an important one in vascular system [60]. Recent researches demonstrated that in endothelial cells as well as in VSMCs, NAD(P)H-dependent oxidase represents the most signiﬁcant O2-source. Interestingly, this oxidase is activated upon stimulation with Ang II, suggesting that under all conditions of an activated circulating and/or local RAS endothelial dysfunction secondary to increased vascular O2-production is expected. In a previous study by Barry-Lane et al. [61], it has been demonstrated that NAD(P)H oxidase is a crucial enzyme in the pathogenesis of atherosclerosis by analyzing the genetically altered mice that are lacking for both apolipoprotein E (ApoE) and p47phox, one subunit of NAD(P)H oxidase. In this interesting study, signiﬁcant reduction in atherosclerotic lesion was shown in the double knockout mice, compared with that of ApoE-deﬁcient mice. 3 ROS takes actions not only as a regulator of vascular tonus but also as a second messenger to modify the vascular cell phenotypes. ROS stimulates janus kinase (JAK)/STAT (signal transducers and activators of transcription), Akt, and mitogen-activated protein kinase pathways [62–65]. Increased oxidative stress contributes to endothelial dysfunc- tion and to vascular inﬂammation by stimulating the redox- sensitive transcription factors (NF-kappa B) and by upregu- lating adhesion molecules, cytokines, and chemokines. In the cardiovascular system, the major catalytic subunits of NAD(P)H oxidase are, nox 1, gp91phox (nox 2), and nox 4, and the regulatory subunits are p22phox, p47phox, p67phox, and rac [59]. The four phox subunits are knownt to be upregulated in endothelial cells and VSMCs from vessels exposed to Ang II [59]. Ang II induces ROS production, one of the most signiﬁcant mediators of the atherogenic actions of RAS. The ROS produced by Ang II contributes to the pathogenesis of vascular diseases by inactivating nitric oxide, impairing endothelial function, enhancing VSMC growth and proliferation, and stimulating proatherogenic, inﬂammatory, and adhesion molecule expression [66–68]. Importantly, the Ang II-induced elevation in O2-generation in the vessel wall does not seem to be related to the hemodynamic eﬀects of Ang II, because norepinephrine- induced hypertension did not have a similar eﬀect [57]. Ang II is a key mediator of oxidative stress and decreased activity of NO. Ang II causes the activation of NADH/NADPH oxidase that results in the production of superoxide anion and, subsequently, hydrogen peroxide [69]. Moreover, it has been shown that Ang II plays a cruical role in neointimal monocyte inﬁltration through NF- kappa B activation and monocyte chemoattractant protein- 1(MCP-1) expression an important eﬀect that is blocked by angiotensin converting enzyme (ACE) inhibitors [22]. Although Ang II activates NF-kappa B and upregulates expression of cytokines such as interleukin-6 and tumor necrosis factor-α, pharmacological blockade of AT1R with angiotensin receptor blockers would not be so eﬃcient to inhibit cytokine production entirely [38, 70, 71]. Ang II regulates not only adhesion molecule expres- sions like vascular cellular adhesion molecule-1 (VCAM- 1), intercellular adhesion molecule-1 (ICAM-1) and P- selectin but also cytokine, chemokine, and growth factor secretion within the arterial wall [72]. Alternatively, RAS can adjust the activation of complement system in both atherosclerosis and renal injury [73, 74]. This inﬂammatory cascade accelerate the vascular inﬂammatory response by elevating inﬂammatory cell recruitment to vessel walls. After migrating into the vessel wall, monocytes transform into macrophages and contribute to lipid deposition in the plaque [75, 76]. Chemokines and MMPs secreted from monocytes/macrophages cause acceleration of atheroscle- rotic lesions [77]. Furthermore, angiotensin II favors the intraplaque recruitment of monocytes and lymphocytes and directly enhances TNF-α, IL-6 and cyclooxygenase-2 expression in atherosclerotic arteries [1, 78–80]. Moreover recruited leukocytes themselves have NAD(P)H oxidase subunits and serve as a source of ROS [81]. In addition, Ang II triggered activation of transcription factor nuclear