Akt Regulates the Survival of Vascular Smooth Muscle Cells via Inhibition of FoxO3a and GSK3

Abstract

Apoptosis of vascular smooth muscle cells (VSMCs) may lead to atherosclerotic plaque instability and rupture, resulting in myocardial infarction, stroke, and sudden death. However, the molecular mechanisms mediating survival of VSMCs in atherosclerotic plaques remain unknown. Although plaque VSMCs exhibit increased susceptibility to apoptosis and reduced expression of the IGF1 receptor (IGF1R) when compared with normal VSMCs, a causative effect has not been established. Here we show that increased expression of the IGF1R can rescue plaque VSMCs from oxidative stress-induced apoptosis, demonstrating that IGF-1 signaling is a critical regulator of VSMC survival. Akt mediates the majority of the IGF1R survival signaling, and ectopic activation of Akt was sufficient to protect VSMCs in vitro. Both IGF1R and phospho-Akt expression were reduced in human plaque (intimal) VSMCs when compared with medial VSMCs, suggesting that Akt mediates survival signaling in atherosclerosis. Importantly, downstream targets of Akt were identified that mediate its protective effect as inhibition of FoxO3a or GSK3 by Akt-dependent phosphorylation protected VSMCs in vitro. We conclude that Akt and its downstream targets FoxO3a and GSK3 regulate a survival pathway in VSMCs and that their deregulation due to a reduction of IGF1R signaling may promote apoptosis in atherosclerosis.Insulin-like growth factor 1 (IGF1) 2 is a ubiquitous factor exhibiting pleiotropic effects on different cell types. Stimulation of the IGF1 receptor (IGF1R) initiates signaling pathways involved in cell proliferation, differentiation, transformation, and survival. IGF1R-dependent signaling is crucial for the survival of many cell types including vascular smooth muscle cells (VSMCs). VSMCs are the principle source of collagen and extracellular matrix that maintain the tensile strength of atherosclerotic plaques, and VSMC loss induces multiple features of plaque instability (1). In humans, rupture or erosion of the atherosclerotic plaque underlie the majority of myocardial infarctions, stroke, and sudden death (2). We have previously shown that VSMCs derived from atherosclerotic plaques (pVSMCs) are more sensitive to apoptosis than cells derived from non-diseased vessels (3) and exhibit a defect in IGF1-dependent survival signaling (4). Oxidative stress is increasingly implicated in the development of atherosclerosis (5) and increased oxidative damage, and elevated levels of DNA strand breaks occur in human atherosclerotic plaques (6). Oxidative stress reduces IGF1R expression and induces VSMC apoptosis in culture (7-10). Reduced IGF1R expression is also seen within plaques, suggesting that IGF1R-dependent survival regulates apoptosis in vivo (11,12). However, plaque VSMCs also show increased sensitivity to multiple proapoptotic stimuli, including the tumor suppressor gene P53 and death receptor ligation (13,14). It is therefore not known whether defective IGF1R expression alone is an important cause of reduced survival of pVSMCs.A major downstream effector of IGF1R ...

Vascular Smooth Muscle Cell Sirtuin 1 Protects Against DNA Damage and Inhibits Atherosclerosis

“…39 Human plaque VSMCs show increased apoptosis in culture and in vivo, in part, because of reduced expression of the insulin-like growth factor 1 receptor, leading to reduced signaling through the serine threonine kinase Akt. 31,40 FOXO3a is inhibited by Akt, and this inhibition mediates much of the protective effect of both IGF-1 and Akt in VSMCs. 40 Thus, reduced SIRT1 activity would augment the potent proapoptotic effect of FOXO3a in VSMCs.…”

“…31,40 FOXO3a is inhibited by Akt, and this inhibition mediates much of the protective effect of both IGF-1 and Akt in VSMCs. 40 Thus, reduced SIRT1 activity would augment the potent proapoptotic effect of FOXO3a in VSMCs.…”

Platelet-derived Growth Factor-DD Targeting Arrests Pathological Angiogenesis by Modulating Glycogen Synthase Kinase-3β Phosphorylation

“…PDGF-DD induced GSK3␤ Ser 9 phosphorylation and GSK3␤ Tyr 216 dephosphorylation in choroidal fibroblasts in vitro and in the retina in vivo. It is known that inhibition of GSK3␤ activity by Akt-dependant Ser 9 phosphorylation or through GSK3␤ Tyr 216 dephosphorylation promotes vascular cell survival (52,53). Moreover, the involvement of GSK3␤ in PDGF-DD-induced angiogenesis was confirmed by the aortic ring assay in vitro, and by the laser-induced CNV model in vivo.…”

“…7E). GSK3␤ induces vascular cell apoptosis (52). Inhibition of GSK3␤ by either Akt-dependent GSK3␤ Ser 9 phosphorylation or down-regulating its expression increases vascular cell survival (52,53).…”

Oxygen- and glucose-deprived culture promotes cell proliferation and invasion of vascular smooth muscle cells

“…VSMCs are differentiated from embryonic mesenchyme, and possess the functions of contraction, and synthesis and secretion of extracellular matrix. The structure and function of VSMCs may change significantly at different developmental stages and under different physiological or pathological states (11)(12)(13)(14). Recent studies have found that various growth factors, vasoactive substances, and cytokines released from damaged endothelial vessel wall can activate VSMC surface receptors and signal transduction, which result in VSMC cell proliferation and invasion (15)(16)(17)(18).…”