Lack of Endothelial α1AMPK Reverses the Vascular Protective Effects of Exercise by Causing eNOS Uncoupling

Voluntary exercise training is an effective way to prevent cardiovascular disease, since it results in increased NO bioavailability and decreased reactive oxygen species (ROS) production. AMP-activated protein kinase (AMPK), especially its α1AMPK subunit, modulates ROS-dependent vascular homeostasis. Since endothelial cells play an important role in exercise-induced changes of vascular signaling, we examined the consequences of endothelial-specific α1AMPK deletion during voluntary exercise training. We generated a mouse strain with specific deletion of α1AMPK in endothelial cells (α1AMPKflox/flox x TekCre+). While voluntary exercise training improved endothelial function in wild-type mice, it had deleterious effects in mice lacking endothelial α1AMPK indicated by elevated reactive oxygen species production (measured by dihydroethidum fluorescence and 3-nitrotyrosine staining), eNOS uncoupling and endothelial dysfunction. Importantly, the expression of the phagocytic NADPH oxidase isoform (NOX-2) was down-regulated by exercise in control mice, whereas it was up-regulated in exercising α1AMPKflox/flox x TekCre+ animals. In addition, nitric oxide bioavailability was decreased and the antioxidant/protective nuclear factor erythroid 2-related factor 2 (Nrf-2) response via heme oxygenase 1 and uncoupling protein-2 (UCP-2) was impaired in exercising α1AMPKflox/flox x TekCre+ mice. Our results demonstrate that endothelial α1AMPK is a critical component of the signaling events that enable vascular protection in response to exercise. Moreover, they identify endothelial α1AMPK as a master switch that determines whether the effects of exercise on the vasculature are protective or detrimental.

Metastatic Melanoma Progression Is Associated with Endothelial Nitric Oxide Synthase Uncoupling Induced by Loss of eNOS:BH4 Stoichiometry

Melanoma is the most aggressive type of skin cancer due to its high capability of developing metastasis and acquiring chemoresistance. Altered redox homeostasis induced by increased reactive oxygen species is associated with melanomagenesis through modulation of redox signaling pathways. Dysfunctional endothelial nitric oxide synthase (eNOS) produces superoxide anion (O2−•) and contributes to the establishment of a pro-oxidant environment in melanoma. Although decreased tetrahydrobiopterin (BH4) bioavailability is associated with eNOS uncoupling in endothelial and human melanoma cells, in the present work we show that eNOS uncoupling in metastatic melanoma cells expressing the genes from de novo biopterin synthesis pathway Gch1, Pts, and Spr, and high BH4 concentration and BH4:BH2 ratio. Western blot analysis showed increased expression of Nos3, altering the stoichiometry balance between eNOS and BH4, contributing to NOS uncoupling. Both treatment with L-sepiapterin and eNOS downregulation induced increased nitric oxide (NO) and decreased O2• levels, triggering NOS coupling and reducing cell growth and resistance to anoikis and dacarbazine chemotherapy. Moreover, restoration of eNOS activity impaired tumor growth in vivo. Finally, NOS3 expression was found to be increased in human metastatic melanoma samples compared with the primary site. eNOS dysfunction may be an important mechanism supporting metastatic melanoma growth and hence a potential target for therapy.

Unripe Carica papaya Protects Methylglyoxal-Invoked Endothelial Cell Inflammation and Apoptosis via the Suppression of Oxidative Stress and Akt/MAPK/NF-κB Signals

Methylglyoxal (MGO), a highly reactive dicarbonyl compound, causes endothelial oxidative stress and vascular complications in diabetes. Excessive MGO-induced ROS production triggers eNOS uncoupling, inflammatory responses, and cell death signaling cascades. Our previous study reported that unripe Carica papaya (UCP) had antioxidant activities that prevented H2O2-induced endothelial cell death. Therefore, this study investigated the preventive effect of UCP on MGO-induced endothelial cell damage, inflammation, and apoptosis. The human endothelial cell line (EA.hy926) was pretreated with UCP for 24 h, followed by MGO-induced dicarbonyl stress. Treated cells were evaluated for intracellular ROS/O2•− formation, cell viability, apoptosis, NO releases, and cell signaling through eNOS, iNOS, COX-2, NF-κB, Akt, MAPK (JNK and p38), and AMPK/SIRT1 autophagy pathways. UCP reduced oxidative stress and diminished phosphorylation of Akt, stress-activated MAPK, leading to the decreases in NF-kB-activated iNOS and COX-2 expression. However, UCP had no impact on the autophagy pathway (AMPK and SIRT1). Although UCP pretreatment decreased eNOS phosphorylation, the amount of NO production was not altered. The signaling of eNOS and NO production were decreased after MGO incubation, but these effects were unaffected by UCP pretreatment. In summary, UCP protected endothelial cells against carbonyl stress by the mechanisms related to ROS/O2•− scavenging activities, suppression of inflammatory signaling, and inhibition of JNK/p38/apoptosis pathway. Thus, UCP shows considerable promise for developing novel functional food and nutraceutical products to reduce risks of endothelial inflammation and vascular complications in diabetes.

Toll-like Receptor 2 (TLR2) Deficiency Abrogates Diabetic and Obese Phenotypes while Restoring Endothelial Function via Inhibition of NOX1

We have previously demonstrated a novel role of bone morphogenic protein-4 (BMP4) in inducing NOX1-dependent eNOS uncoupling, endothelial dysfunction, and inflammatory activation in type 2 diabetes mellitus (T2DM). However, it has remained unclear as to how BMP4 activates NOX1 and whether targeting the new mechanistic pathway revealed is effective in preserving endothelial function in T2DM. Here we observed that BMP4 induced marked, time-dependent increase in physiological binding between TLR2 and NOX1 in aortic endothelial cells, as well as increased binding of TLR2 to NOXO1. In high-fat diet fed <i>Tlr2^-/-</i>(TLR2 knockout) mice, the body weight gain was significantly less compared to WT (wild-type) mice both in males and females. The high-fat diet induced increases in fasting blood glucose levels, as well as in circulating insulin and leptin levels, were absent in <i>Tlr2^-/-</i>mice. High-fat feeding induced increases in overall fat mass, and fat mass of different pockets were abrogated in <i>Tlr2^-/-</i>mice. Whereas energy intake was similar in high-fat fed WT and <i>Tlr2^-/-</i>mice, TLR2 deficiency resulted in higher energy expenditure attributed to improved physical activity, which was accompanied by restored skeletal muscle mitochondrial function. In addition, TLR2 deficiency recoupled eNOS, reduced total superoxide production, improved H₄B and NO bioavailabilities in aortas and restored endothelium-dependent vasorelaxation. Collectively, our data strongly indicate that TLR2 plays important roles in the development of metabolic features of T2DM, and its related endothelial/vascular dysfunction. Therefore, targeting TLR2 may represent a novel therapeutic strategy for T2DM, obesity and cardiovascular complications via specific inhibition of NOX1.

Endothelial Dysfunction Driven by Hypoxia—The Influence of Oxygen Deficiency on NO Bioavailability

Cardiovascular diseases (CVDs) are the leading cause of death worldwide. The initial stage of CVDs is characterized by endothelial dysfunction, defined as the limited bioavailability of nitric oxide (NO). Thus, any factors that interfere with the synthesis or metabolism of NO in endothelial cells are involved in CVD pathogenesis. It is well established that hypoxia is both the triggering factor as well as the accompanying factor in cardiovascular disease, and diminished tissue oxygen levels have been reported to influence endothelial NO bioavailability. In endothelial cells, NO is produced by endothelial nitric oxide synthase (eNOS) from L-Arg, with tetrahydrobiopterin (BH4) as an essential cofactor. Here, we discuss the mechanisms by which hypoxia affects NO bioavailability, including regulation of eNOS expression and activity. What is particularly important is the fact that hypoxia contributes to the depletion of cofactor BH4 and deficiency of substrate L-Arg, and thus elicits eNOS uncoupling—a state in which the enzyme produces superoxide instead of NO. eNOS uncoupling and the resulting oxidative stress is the major driver of endothelial dysfunction and atherogenesis. Moreover, hypoxia induces impairment in mitochondrial respiration and endothelial cell activation; thus, oxidative stress and inflammation, along with the hypoxic response, contribute to the development of endothelial dysfunction.

Decreased Expression and Uncoupling of Endothelial Nitric Oxide Synthase in the Cerebral Cortex of Rats with Thioacetamide-Induced Acute Liver Failure

Acute liver failure (ALF) is associated with deregulated nitric oxide (NO) signaling in the brain, which is one of the key molecular abnormalities leading to the neuropsychiatric disorder called hepatic encephalopathy (HE). This study focuses on the effect of ALF on the relatively unexplored endothelial NOS isoform (eNOS). The cerebral prefrontal cortices of rats with thioacetamide (TAA)-induced ALF showed decreased eNOS expression, which resulted in an overall reduction of NOS activity. ALF also decreased the content of the NOS cofactor, tetrahydro-L-biopterin (BH4), and evoked eNOS uncoupling (reduction of the eNOS dimer/monomer ratio). The addition of the NO precursor L-arginine in the absence of BH4 potentiated ROS accumulation, whereas nonspecific NOS inhibitor L-NAME or EDTA attenuated ROS increase. The ALF-induced decrease of eNOS content and its uncoupling concurred with, and was likely causally related to, both increased brain content of reactive oxidative species (ROS) and decreased cerebral cortical blood flow (CBF) in the same model.

Toll-like Receptor 2 (TLR2) Deficiency Abrogates Diabetic and Obese Phenotypes while Restoring Endothelial Function via Inhibition of NOX1

We have previously demonstrated a novel role of bone morphogenic protein-4 (BMP4) in inducing NOX1-dependent eNOS uncoupling, endothelial dysfunction, and inflammatory activation in type 2 diabetes mellitus (T2DM). However, it has remained unclear as to how BMP4 activates NOX1 and whether targeting the new mechanistic pathway revealed is effective in preserving endothelial function in T2DM. Here we observed that BMP4 induced marked, time-dependent increase in physiological binding between TLR2 and NOX1 in aortic endothelial cells, as well as increased binding of TLR2 to NOXO1. In high-fat diet fed <i>Tlr2^-/-</i>(TLR2 knockout) mice, the body weight gain was significantly less compared to WT (wild-type) mice both in males and females. The high-fat diet induced increases in fasting blood glucose levels, as well as in circulating insulin and leptin levels, were absent in <i>Tlr2^-/-</i>mice. High-fat feeding induced increases in overall fat mass, and fat mass of different pockets were abrogated in <i>Tlr2^-/-</i>mice. Whereas energy intake was similar in high-fat fed WT and <i>Tlr2^-/-</i>mice, TLR2 deficiency resulted in higher energy expenditure attributed to improved physical activity, which was accompanied by restored skeletal muscle mitochondrial function. In addition, TLR2 deficiency recoupled eNOS, reduced total superoxide production, improved H₄B and NO bioavailabilities in aortas and restored endothelium-dependent vasorelaxation. Collectively, our data strongly indicate that TLR2 plays important roles in the development of metabolic features of T2DM, and its related endothelial/vascular dysfunction. Therefore, targeting TLR2 may represent a novel therapeutic strategy for T2DM, obesity and cardiovascular complications via specific inhibition of NOX1.

Toll-like Receptor 2 (TLR2) Deficiency Abrogates Diabetic and Obese Phenotypes while Restoring Endothelial Function via Inhibition of NOX1

We have previously demonstrated a novel role of bone morphogenic protein-4 (BMP4) in inducing NOX1-dependent eNOS uncoupling, endothelial dysfunction, and inflammatory activation in type 2 diabetes mellitus (T2DM). However, it has remained unclear as to how BMP4 activates NOX1 and whether targeting the new mechanistic pathway revealed is effective in preserving endothelial function in T2DM. Here we observed that BMP4 induced marked, time-dependent increase in physiological binding between TLR2 and NOX1 in aortic endothelial cells, as well as increased binding of TLR2 to NOXO1. In high-fat diet fed <i>Tlr2^-/-</i>(TLR2 knockout) mice, the body weight gain was significantly less compared to WT (wild-type) mice both in males and females. The high-fat diet induced increases in fasting blood glucose levels, as well as in circulating insulin and leptin levels, were absent in <i>Tlr2^-/-</i>mice. High-fat feeding induced increases in overall fat mass, and fat mass of different pockets were abrogated in <i>Tlr2^-/-</i>mice. Whereas energy intake was similar in high-fat fed WT and <i>Tlr2^-/-</i>mice, TLR2 deficiency resulted in higher energy expenditure attributed to improved physical activity, which was accompanied by restored skeletal muscle mitochondrial function. In addition, TLR2 deficiency recoupled eNOS, reduced total superoxide production, improved H₄B and NO bioavailabilities in aortas and restored endothelium-dependent vasorelaxation. Collectively, our data strongly indicate that TLR2 plays important roles in the development of metabolic features of T2DM, and its related endothelial/vascular dysfunction. Therefore, targeting TLR2 may represent a novel therapeutic strategy for T2DM, obesity and cardiovascular complications via specific inhibition of NOX1.