Contribution of Insulin and Akt1 Signaling to Endothelial Nitric Oxide Synthase in the Regulation of Endothelial Function and Blood Pressure

Abstract

Abstract-Impaired insulin signaling via phosphatidylinositol 3-kinase/Akt to endothelial nitric oxide synthase (eNOS) in the vasculature has been postulated to lead to arterial dysfunction and hypertension in obesity and other insulin resistant states. To investigate this, we compared insulin signaling in the vasculature, endothelial function, and systemic blood pressure in mice fed a high-fat (HF) diet to mice with genetic ablation of insulin receptors in all vascular tissues (TTr-IR Ϫ/Ϫ ) or mice with genetic ablation of Akt1 (Akt1Ϫ/Ϫ). HF mice developed obesity, impaired glucose tolerance, and elevated free fatty acids that was associated with endothelial dysfunction and hypertension. Basal and insulin-mediated phosphorylation of extracellular signal-regulated kinase 1/2 and Akt in the vasculature was preserved, but basal and insulin-stimulated eNOS phosphorylation was abolished in vessels from HF versus lean mice. In contrast, basal vascular eNOS phosphorylation, endothelial function, and blood pressure were normal despite absent insulinmediated eNOS phosphorylation in TTr-IR Ϫ/Ϫ mice and absent insulin-mediated eNOS phosphorylation via Akt1 in Akt1Ϫ/Ϫ mice. In cultured endothelial cells, 6 hours of incubation with palmitate attenuated basal and insulinstimulated eNOS phosphorylation and NO production despite normal activation of extracellular signal-regulated kinase 1/2 and Akt. Moreover, incubation of isolated arteries with palmitate impaired endothelium-dependent but not vascular smooth muscle function. Collectively, these results indicate that lower arterial eNOS phosphorylation, hypertension, and vascular dysfunction following HF feeding do not result from defective upstream signaling via Akt, but from free fatty acid-mediated impairment of eNOS phosphorylation. Key Words: arterial insulin signaling Ⅲ hypertension Ⅲ endothelial dysfunction Ⅲ mice Ⅲ diabetes S timulation of insulin receptors in the vasculature leads to increased activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway and the mitogen-activated protein (MAP) kinase (eg, extracellular signaling-regulated kinase [ERK]1/2) pathway. [1][2][3][4] Insulin receptor (IR)-mediated stimulation of PI3K/Akt leads to endothelial nitric oxide (NO) synthase (eNOS) phosphorylation, NO production, and vasorelaxation. 4 -7 Insulin-mediated activation of ERK1/2 leads to endothelin (ET)-1 production, inhibition of eNOS phosphorylation, and subsequent vasocontraction. 1,4,8 Evidence from several experimental models of insulin resistance reveals impaired insulin-mediated PI3K/Aktdependent signaling in the vasculature, whereas ERK1/2 pathways are preserved or even augmented. 2,8,9 Collectively, these observations have led to the hypothesis that an imbalance in vascular insulin-mediated signaling can precipitate cardiovascular complications including endothelial dysfunction and hypertension. 1,4 Recently, it was shown that insulinmediated Akt phosphorylation was preserved, but NOmediated vasorelaxation was blunted, in arteries from obese, glucose ...

Mouse models of the metabolic syndrome

“…diet versus monogenic), and the method of measuring hypertension. For example, in C57BL/6J mice that are fed the same high-fat diet, diet-induced obesity can have differential effects on systolic blood pressure when measured by radiotelemetry and the tail cuff method (Gupte et al, 2008;Police et al, 2009;Symons et al, 2009). It is possible that the magnitude of the blood pressure increases (10-15 mmHg systolic pressure) in studies using radiotelemetry in obese mice may not be within the range of sensitivity of tail cuff methods.…”

Rodent models of diabetic cardiomyopathy

“…Western diets (high fat and high sucrose) lead to obesity, insulin resistance and diabetes, particularly when applied to C57BL/6 mice (Symons et al, 2009). However, the degree of hyperglycemia and insulin resistance is not as severe as that observed in leptin or leptin receptor mutant mice.…”

Ceramide synthase 5 mediates lipid-induced autophagy and hypertrophy in cardiomyocytes

“…While these transgenic models robustly induce lipid overload in cardiomyocytes, the very disruptions that produce lipotoxic DbCM also drastically alter patterns of lipid uptake and handling in a nonphysiologic way. In contrast, wild-type mice fed standard lard-based high-fat diets (LBD) failed to develop a DbCM-like phenotype until very late time points, if at all, and insulin resistance and other aspects of the diabetic phenotype were less pronounced in this model system than in transgenic animals (4)(5)(6)(7)(8)(9). This may result from the high levels of protective unsaturated fatty acids (UFAs) contained in these diets (10)(11)(12).…”