This review provides a summary of recent research on the role of high-density lipoprotein (HDL)/apolipoprotein A-I cholesterol efflux and immune cell function. Plasma concentrations of HDL have been known to inversely correlate with risk for coronary vascular disease. Bulk transport of HDL cholesterol from the peripheral tissues to the liver is a major pathway, termed reverse cholesterol transport, responsible for maintaining whole body cholesterol homeostasis. In addition to participating in this pathway, HDL and apolipoprotein A-I exert anti-inflammatory effects through different pathways. One pathway that seems to be important in atherosclerosis and autoimmunity is its role in modulation of T cell activation. HDL/apolipoprotein A-I helps regulate cell signaling by accepting membrane cholesterol from ATP binding cassette transporter A1 on immune cells and, thereby, fine tuning the amount of cholesterol present in plasma membrane lipid rafts.
apolipoprotein A-I; atherosclerosis; autoimmunity; cholesterol efflux; lipid rafts; nascent high density lipoprotein; T cells; T regulatory cells (CD4+CD25+FoxP3)
To examine placental growth factor’s (PlGF) predictive value in relation to coronary heart disease (CHD) risk in healthy women.
Methods and results
Among 32,826 women from the Nurses’ Health Study who provided blood samples at baseline, 453 CHD events were documented during 14 years of follow-up. Controls were matched to cases (2:1) for age, smoking, fasting status, and date of blood sampling. PlGF was inversely correlated with HDL-cholesterol (HDL-C), and positively correlated with several coronary risk factors. In multivariate models, women in the highest versus lowest quintile of PlGF had a greater risk of CHD (RR:1.58;95%CI:1.03-2.41). Additional adjustment for many coronary risk factors did not substantively alter this relationship, but HDL-C attenuated the association (RR:1.25;95%CI:0.81-1.94). In exploratory time to event analysis, higher PlGF levels, measured > 10 years prior to CHD event, but not < 10 years pre-clinical event, were associated with increased risk of CHD, even after adjustment for co-morbid conditions and HDL-C levels (RR:2.79;95%CI:1.19-6.56).
Elevated prediagnostic PlGF levels were modestly associated with subsequent risk of CHD events and results were attenuated after controlling for HDL-C. PlGF may be most strongly associated with long term prediction of CHD, consistent with a potential role in early plaque formation and growth.
PlGF; CHD; women
To evaluate direct versus indirect MCP-1/CCR2 signaling and identify the cellular producers and effectors for MCP-1 during neointimal hyperplasia (NIH) development in vein grafts (VG).
Methods and Results
Genomic analysis revealed an over-representation of 13 inflammatory pathways in WT VGs compared to CCR2KO VGs. Further investigation with various VG-host combinations of MCP-1 and CCR2 deficient mice were used to modify the genotype of cells both inside (graft intrinsic group) and outside of the vein wall (graft extrinsic group). CCR2 deficiency inhibited NIH only when present in cells extrinsic to the graft wall, MCP-1 deficiency required its effectiveness in cells both intrinsic and extrinsic to the graft wall to suppress NIH. Deletion of either MCP-1 or CCR2 was equally effective in inhibiting NIH. CCR2 deficiency in the predominant neointimal cell population had no impact on NIH. Direct MCP-1 stimulation of primary neointimal SMCs had minimal influence on cell proliferation and matrix turnover, confirming an indirect mechanism of action.
MCP-1/CCR2 axis accelerates NIH via its signaling in graft extrinsic cells, particularly circulating inflammatory cells, with cells both intrinsic and extrinsic to the graft wall being critical MCP-1 producers. These findings underscore the importance of systemic treatment for anti MCP-1/CCR2 therapies.
Leukocytes; Chemokines; Vein Graft; Neointimal Hyperplasia
To characterize the relationship between expression of EGF-like ligands and vascular NADPH oxidase expression and activity in a primate model of atherosclerosis.
Methods and Results
Adult male Cynomolgus monkeys were fed a normal or atherogenic (AS) diet for 45 months, after which animals from the AS group were placed on a normal diet for 8 months (regression). Expression of membrane-associated EGF-like ligands was increased in arteries from animals on the AS diet and normalized in the regression group. EGF-like ligands were distributed throughout atherosclerotic vessels but predominantly co-localized with macrophages. Consistent with ligand shedding, circulating heparin bound-EGF was elevated in plasma of AS monkeys but not in those on regression diet. Atherosclerosis was associated with activation of EGF receptor (EGFR) signaling. Expression of NADPH oxidase subunits Nox1 and Nox2 but not Nox4 or Nox5 was increased in arteries from monkeys on the AS diet and returned to normal with regression. Levels of Nox1 and Nox2 positively correlated with EGF-like ligands. In cultured monkey smooth muscle cells, treatment with EGF-like ligands increased Nox1 expression and activity.
These data identify EGF-like ligands as potential modulators of atherogenesis due in part to increased vascular NADPH oxidase activity.
NADPH oxidases; atherosclerosis; cell signaling; smooth muscle cells; EGFR
Endothelium dysfunction is an initiating factor in atherosclerosis. ADAM15 is a multi-domain metalloprotease recently identified as a regulator of endothelial permeability. However, whether and how ADAM15 contributes to atherosclerosis remains unknown.
Methods and Results
Genetic ablation of ADAM15 in apolipoprotein E-deficient mice lead to a significant reduction in aortic atherosclerotic lesion size (by 52%), plaque macrophage infiltration (by 69%), and smooth muscle cell deposition (by 82%). In vitro studies implicated endothelial derived ADAM15 in barrier dysfunction and monocyte transmigration across mouse aortic and human umbilical vein endothelial cell monolayers. This role of ADAM15 depended on intact functioning of the cytoplasmic domain, as evidenced in experiments with site-directed mutagenesis targeting the metalloprotease active site (E349A), the disintegrin domain (RDG→TDD) or the cytoplasmic tail. Further investigations revealed that ADAM15-induced barrier dysfunction was concomitant with dissociation of endothelial adherens junctions (VE-cadherin/γ-catenin), an effect that was sensitive to Src family kinase (SFK) inhibition. Through siRNA-mediated knockdown of distinct SFK members, c-Src and c-Yes were identified as important mediators of these junctional effects of ADAM15.
These results suggest that endothelial cell-derived ADAM15, signaling through c-Src and c-Yes, contributes to atherosclerotic lesion development by disrupting adherence junction integrity and promoting monocyte transmigration.
Vascular permeability; Metalloproteinase; Endothelial dysfunction; Intercellular junctions; Inflammation
We observed differences in atherosclerosis susceptibility in mouse inbred strains over the years as the health status of our animal rooms increased. Therefore, we investigated the effect of animal room health status on atherosclerosis susceptibility in different strains. As this data can also be used for genome-wide association mapping, we performed a mapping study and compared our results with previously found quantitative trait loci for atherosclerosis in mouse and human.
Methods and Results
Males and females from 48 inbred strains were housed in two animal rooms with different health status and given an atherogenic diet. We compared atherosclerosis susceptibility between animal rooms and between sexes and found that susceptibility is dependent on both health status and sex. Subsequently, the data were used for associations with loci on the mouse genome using 63,222 SNPs. Three loci in males and four loci in females were identified using the data from the low health status room. No significant associations were identified using the data from the high health status room.
Health status influences susceptibility to atherosclerosis and suggests that microbiological pressure plays an important role in the development of atherosclerosis in many strains. As we were only able to map susceptibility loci using the data from the lower health status room, we argue that susceptibility under these conditions is determined by a few key loci, while in the higher health status room different mechanisms might play a role in the differences in atherosclerosis susceptibility between strains and we did not have enough power to map the loci that are involved.
atherosclerosis; inbred strains; QTL
Mitochondrial dysfunction plays a key pathophysiological role in type 2 diabetes (T2DM) related endothelial dysfunction. Data delineating relationships between mitochondrial and endothelial dysfunction in humans with T2DM are lacking.
Methods and Results
In 122 human subjects, (60 with T2DM, 62 without T2DM) we measured endothelial function by brachial artery ultrasound (FMD%) and digital pulse amplitude tonometery (PAT). Endothelial function in arterioles isolated from gluteal subcutaneous adipose was measured by videomicroscopy. In arterioles and mononuclear cells, we measured inner mitochondrial membrane potential (Δψm), mitochondrial mass, and mitochondrial superoxide production using fluorophores. Endothelial function was impaired in T2DM versus non-diabetics. Δψm magnitude was larger and mitochondrial mass was lower in arterioles and mononuclear cells in T2DM. Mononuclear mitochondrial mass correlated with FMD% and PAT (r=0.38 and 0.33, P=0.001 and 0.02, respectively) and mononuclear mitochondrial superoxide production inversely correlated with FMD% (ρ=-0.58, P=0.03). Endothelial function was impaired in T2DM. Low doses of two different mitochondrial uncoupling agents (CCCP and DNP) that reduce Δψm magnitude and a mitochondrial-targeted antioxidant (MitoTEMPOL) improved endothelial function and reduced mitochondrial superoxide levels.
Mitochondrial dysfunction may play a central role in the origin and maintenance of endothelial dysfunction in T2DM.
mitochondria; endothelium; diabetes mellitus type 2; nitric oxide; oxidative stress
Vitamin D deficiency is common and associated with dyslipidemia. However, it is unclear if oral vitamin D supplementation improves the lipid profile. Therefore, we conducted a randomized, placebo-controlled trial to determine the short-term effects of vitamin D repletion on the lipid profile.
151 vitamin D deficient (25-hydroxyvitamin D [25(OH)D] <20ng/ml) adults with elevated risk for cardiovascular disease were randomized to receive either 50,000 IU of vitamin D3 weekly for 8 weeks or placebo. The primary outcome was the change in small LDL particle number. Secondary outcomes included changes in other NMR-based and chemical lipid fractions.
Vitamin D failed to improve the lipid profile. Compared to placebo, vitamin D repletion did not change small LDL particle number (mean change +18 nmol/l, 95% confidence interval (CI) [−80 to +116 nmol/L], P = 0.63). There were also no changes in the chemical lipid profile: total cholesterol (+5.8 mg/dl, 95%CI [−1.4 to +13.0 mg/dl], P =0.14); LDL cholesterol (+3.8 mg/dl, 95% CI [−2.5 to +10.2 mg/dl], P = 0.13); HDL cholesterol (+0.4 mg/dl 95% CI [−1.6 to +2.6 mg/dl], P = 0.71); triglycerides (+7.9 mg/dl 95%CI [−6.5 to +22.3 mg/dl]). In the vitamin D repletion group, exploratory multivariate regression analysis demonstrates that changes in LDL cholesterol were positively correlated with changes in serum calcium (P < .001) and inversely with changes in serum PTH (P = .02).
In contrast to the association between low 25(OH)D levels and dyslipidemia, correcting vitamin D deficiency in the short-term does not improve the lipid profile. Repletion of 25(OH)D levels raised serum calcium levels and decreased serum PTH levels. These expected physiologic responses to vitamin D therapy were correlated with a significant increase in LDL cholesterol.
Vitamin D; cholesterol; NMR lipids; cardiovascular disease
The proinflammtory cytokine tumor necrosis factor (TNF), primarily via TNF receptor 1 (TNFR1), induces NF-κB-dependent cell survival, and JNK and caspase-dependent cell death, regulating vascular endothelial cell (EC) activation and apoptosis. However, signaling by the second receptor, TNFR2, is poorly understood. The goal of this study is to dissect how TNFR2 mediates NF-κB and JNK signaling in vascular endothelial cells (EC), and its relevance with in vivo EC function.
Methods and Results
We show that TNFR2 contributes to TNF-induced NF-κB and JNK signaling in EC as TNFR2 deletion or knockdown reduces the TNF responses. To dissect out the critical domains of TNFR2 that mediate the TNF responses, we examine the activity of TNFR2 mutant with a specific deletion of the TNFR2 intracellular region, which contains conserved domain I, domain II, domain III, and two TRAF2-binding sites. Deletion analyses indicate that different sequences on TNFR2 have distinct roles in NF-κB and JNK activation. Specifically, deletion of the TRAF2-binding sites (TNFR2-59) diminishes the TNFR2-mediated NF-κB, but not JNK activation; whereas, deletion of domain II or domain III blunts TNFR2-mediated JNK but not NF-κB activation. Interestingly, we find that the TRAF2-binding sites ensure TNFR2 on the plasma membrane, but the di-leucine LL motif within the domain II and aa338-355 within the domain III are required for TNFR2 internalization as well as TNFR2-dependent JNK signaling. Moreover, domain III of TNFR2 is responsible for association with AIP1, a signaling adaptor critical for TNF-induced JNK signaling. While TNFR2 containing the TRAF2-binding sites prevents EC cell death, a specific activation of JNK without NF-κB activation by TNFR2-59 strongly induces caspase activation and EC apoptosis.
Our data reveal that both internalization and AIP1 association are required for TNFR2-dependent JNK and apoptotic signaling. Controlling TNFR2-mediated JNK and apoptotic signaling in EC may provide a novel strategy for the treatment of vascular diseases.
TNF; TNFR2; JNK; endothelial cell; apoptosis
Experimental work has elucidated molecular and cellular pathways of inflammation that promote atherosclerosis. Unraveling the roles of cytokines as inflammatory messengers provided a mechanism whereby risk factors for atherosclerosis can alter arterial biology, and produce a systemic milieu that favors atherothrombotic events. The discovery of the immune basis of allograft arteriosclerosis demonstrated that inflammation per se can drive arterial hyperplasia, even in the absence of traditional risk factors. Inflammation regulates aspects of plaque biology that trigger the thrombotic complications of atherosclerosis. Translation of these discoveries to humans has enabled both novel mechanistic insights and practical clinical advances.
Fish oil (FO), containing n-3 fatty acids (FAs), attenuates atherosclerosis. We hypothesized that n-3 FA-enriched oils are atheroprotective through alteration of monocyte subsets and their trafficking into atherosclerotic lesions.
Methods and Results
Low density lipoprotein receptor knockout (LDLr−/−) and apolipoprotein E−/− (apoE) mice were fed diets containing 10% (calories) as palm oil (PO) and 0.2% cholesterol, supplemented with an additional 10% PO, echium oil (EO; containing 18:4 n-3) or FO. Compared to PO-fed LDLr−/− mice, EO and FO significantly reduced plasma cholesterol, splenic Ly6Chi monocytosis by ~50%, atherosclerosis by 40–70%, monocyte trafficking into the aortic root by ~50%, and atherosclerotic lesion macrophage content by 30–44%. In contrast, atherosclerosis and monocyte trafficking into the artery wall was not altered by n-3 FAs in apoE−/− mice; however, Ly6Chi splenic monocytes positively correlated with aortic root intimal area across all diet groups. In apoE−/− mice, FO reduced the percentage of blood Ly6Chi monocytes, despite an average two-fold higher plasma cholesterol relative to PO.
The presence of splenic Ly6Chi monocytes parallels the appearance of atherosclerotic disease in both LDLr−/− and apoE−/− mice. Furthermore, n-3 FAs favorably alter monocyte subsets independently from effects on plasma cholesterol, and reduce monocyte recruitment into atherosclerotic lesions.
monocytosis; n-3 fatty acids; fish oil; Echium oil; inflammation; Ly6C
Although the connection of oxidative stress and inflammation has been long recognized in diabetes, the underlying mechanisms are not fully elucidated. This study defined the role of 26S proteasomes in promoting vascular inflammatory response in early diabetes.
Methods and Results
The 26S proteasome functionality, markers of autophagy, and unfolded protein response (UPR) were assessed in: (a) cultured 26S proteasome reporter cells and endothelial cells challenged with high glucose, (b) transgenic reporter (UbG76V-GFP) and wild type (C57BL/6J) mice rendered diabetic, and (c) genetically diabetic (Akita and OVE26) mice. In glucose-challenged cells, and also in aortic, renal, and retinal tissues from diabetic mice, enhanced 26S proteasome functionality was observed, evidenced by augmentation of proteasome (chymotrypsin-like) activities and reduction in 26S proteasome reporter proteins, accompanied by increased nitrotyrosine-containing proteins. Also, while IκBα proteins were decreased, an increase was found in NF-κB nucleus translocation, which enhanced the NF-κB-mediated pro-inflammatory response, without affecting markers of autophagy or UPR. Importantly, the alterations were abolished by MG132 administration, siRNA knockdown of PA700 (proteasome activator protein complex), or superoxide scavenging in vivo.
Early hyperglycemia enhances 26S proteasome functionality, not autophagy or UPR, through peroxynitrite/superoxide-mediated PA700-dependent proteasomal activation, which elevates NF-κB-mediated endothelial inflammatory response in early diabetes.
diabetes; 26S proteasomes; oxidative stress; inflammatory response; NF-κB
Metabolic Syndrome (MetS), a clustering of risk factors for type 2 diabetes mellitus and cardiovascular disease, has been associated with cognitive dysfunction and brain abnormalities. This review describes the literature on the impact of MetS on brain and cognition and suggests directions for future research.
A literature search for reports of MetS and cognition and brain imaging was conducted for both non-elderly adults and adolescents. No studies were found describing MetS and brain or cognition among adolescents; therefore we also included studies investigating individual components of MetS in this age group. Most studies found associations between MetS and cognitive dysfunction. Multiple cognitive domains were affected by MetS in adults. In adolescents, the majority of findings were in executive functioning. Brain imaging literature in adults implicated MetS in ischemic stroke, white matter alterations and altered brain metabolism. For adolescents, individual MetS factors were linked to volume losses in the hippocampus and frontal lobes.
MetS negatively impacts cognitive performance and brain structure. Potential explanatory models include impaired vascular reactivity, neuroinflammation, oxidative stress, and abnormal brain lipid metabolism. We posit that insulin resistance-associated impairment in cerebrovascular reactivity is an important mechanism underlying brain deficits seen in MetS.
Metabolic Syndrome; Cognitive Performance; Adults; Adolescents; Brain Imaging
Smad4 is a central mediator of transforming growth factor-β/bone morphogenetic protein signaling that controls numerous developmental processes as well as homeostasis in the adult. The present studies sought to understand the function of Smad4 expressed in vascular smooth muscle cells (VSMC) in vascular development and the underlying mechanisms.
Methods and results
Breeding of Smad4flox/flox mice with SM22α-Cre mice resulted in no viable offspring with SM22α-Cre;Smad4flox/flox genotype in a total of 165 newborns. Subsequent characterization of 301 embryos between embryonic day 9.5 (E9.5) and E14.5 demonstrated that mice with SM22α-Cre;Smad4flox/flox genotype died between E12.5 and E14.5, due to decreased cell proliferation and increased apoptosis in the embryonic heart and arteries. Additionally, deletion of Smad4 more specifically in SMC with the inducible SMMHC-Cre mice, in which decreased cell proliferation was observed only in the artery but not the heart, also caused lethality of the knockout embryos at E12.5 and E14.5. The Smad4 deficient VSMC lacked smooth muscle α-actin filaments, decreased expression of SMC-specific gene markers, and markedly reduced cell proliferation, migration and attachment. Using specific pharmacological inhibitors and small-interfering RNAs, we demonstrated that inhibition of TGF-β signaling and its regulatory Smad 2/3 decreased VSMC proliferation, migration and expression of SMC-specific gene markers, while inhibition of BMP signaling only affected VSMC migration.
SMC-specific deletion of Smad4 results in vascular defects that lead to embryonic lethality in mice, which may be attributed to decreased VSMC differentiation, proliferation, migration, as well as cell attachment and spreading. The TGF-β signaling pathway contributes to VSMC differentiation and function; while the BMP signaling pathway regulates VSMC migration. These studies provide important insight into the role of Smad4 and its upstream Smads in regulating smooth muscle cell function and vascular development of mice.
Smad4; VSMC; proliferation; migration; vasculature development
Integrins mediate platelet adhesion and transmit “outside in” signals, leading to platelet spreading. Phosphoinositide 3-Kinases (PI3Ks) play a critical role in outside-in signaling and platelet spreading; however, the mechanisms of PI3K activation and function in outside-in signaling are unclear. We sought to determine the role of the Akt family of serine/threonine kinases and activation mechanisms of the PI3K/Akt pathway in outside-in signaling.
Methods and Results
Akt inhibitors and Akt3 knockout inhibited platelet spreading on fibrinogen, indicating that Akt is important in integrin outside-in signaling. Akt inhibitors and Akt3 knockout also diminished integrin-dependent phosphorylation of glycogen synthase kinase-3β (GSK-3β). Inhibition of GSK-3β reversed the inhibitory effects of Akt3 knockout and inhibitors of Akt or PI3K on platelet spreading, indicating that GSK-3β is a downstream target of Akt in outside-in signaling. Integrin-dependent activation of the PI3K-Akt pathway requires Src Family Kinase (SFK). Akt phosphorylation is also significantly inhibited in ADP receptor P2Y12 knockout platelets and further inhibited in P2Y12 knockout platelets treated with a P2Y1 antagonist. Consistently, P2Y12 knockout and P2Y1 inhibition together reduced platelet spreading.
These results demonstrate that integrin outside-in signaling and platelet spreading requires SFK-dependent and ADP receptor-amplified activation of the PI3K-Akt-GSK-3β pathway.
platelet; integrin; Akt; outside-in signaling; ADP
Epidemiologic data regarding the association between ABO blood groups and risk of coronary heart disease (CHD) have been inconsistent. We sought to investigate the associations between ABO blood group and CHD risk in prospective cohort studies.
Methods and Results
Two large, prospective cohort studies (the Nurses’ Health Study [NHS] including 62,073 women and the Health Professionals Follow-up Study [HPFS] including 27, 428 men) were conducted with more than 20 years of follow-up (26 years in NHS and 24 years in HPFS). A meta-analysis was performed to summarize the associations from the present study and previous studies. In NHS, during 1,567,144 person-years of follow-up, 2,055 participants developed CHD; in HPFS, 2,015 participants developed CHD during 517,312 person-years of follow-up. ABO blood group was significantly associated with the risk of developing CHD in both women and men (log-rank test; P = 0.0048 and 0.0002 respectively). In the combined analysis adjusted for cardiovascular risk factors, compared with participants with blood group O, those with blood groups A, B or AB, were more likely to develop CHD (adjusted hazard ratios [95% CI] for incident CHD were 1.06 [ 0.99-1.15], 1.15 [ 1.04-1.26], and 1.23 [1.11-1.36]; respectively). Overall, 6.27% of the CHD cases were attributable to inheriting a non-O blood group. Meta-analysis indicated that non-O blood group had higher risk of CHD (RR= 1.11; 95% CI, 1.05 – 1.18; P = 0.001) compared with O blood group.
These data suggest that ABO blood group is significantly associated with CHD risk. Compared with other blood groups, those with the blood type O have moderately lower risk of developing CHD.
ABO; coronary heart disease; cohort study; meta-analysis
Bone morphogenetic proteins (Bmps) are important mediators of inflammation and atherosclerosis, though their mechanism of action is not fully understood. To better understand the contribution of the Bmp signaling pathway plays in vascular inflammation, we investigated the role of Bmper (Bmp-endothelial cell precursor-derived regulator), an extracellular Bmp modulator, in an induced in vivo model of inflammation and atherosclerosis.
Methods and Results
We crossed apolipoprotein E-deficient (ApoE−/−) mice with mice missing one allele of Bmper (Bmper+/−, used in place of Bmper−/− mice that die at birth) and measured the development of atherosclerosis in mice fed a high fat diet. Bmper haploinsufficiency in ApoE−/− mice (Bmper+/−;ApoE−/− mice) led to a more severe phenotype compared to Bmper+/+;ApoE−/− mice. Bmper+/−;ApoE−/− mice also exhibited increased Bmp activity in endothelial cells in both the greater and lesser curvatures of the aortic arch, suggesting a role for Bmper in regulating Bmp-mediated inflammation associated with laminar and oscillatory shear stress. siRNA knockdown of Bmper in human umbilical vein endothelial cells caused a dramatic increase in the inflammatory markers ICAM1 and VCAM1 at rest and following exposure to oscillatory and laminar shear stress.
We conclude that Bmper is a critical regulator of Bmp-mediated vascular inflammation and that the fine-tuning of Bmp and Bmper levels is essential in the maintenance of normal vascular homeostasis.
Bone morphogenetic protein; Bmp endothelial cell precursor-derived regulator; atherosclerosis; inflammation; fluid shear stress
Impaired insulin signaling is central to the development of the metabolic syndrome and can promote cardiovascular disease indirectly through development of abnormal glucose and lipid metabolism, hypertension and a proinflammatory state. However, insulin action directly on vascular endothelium, atherosclerotic plaque macrophages, and in the heart, kidney, and retina has now been described, and impaired insulin signaling in these locations can alter progression of cardiovascular disease in the metabolic syndrome and affect development of microvascular complications of diabetes. Recent advances in our understanding of the complex pathophysiology of insulin’s effects on vascular tissues offer new opportunities for preventing these cardiovascular disorders.
Insulin resistance is a characteristic feature of obesity and Type 2 diabetes and impacts the heart in various ways. Impaired insulin-mediated glucose uptake is a uniformly observed characteristic of the heart in these states, although changes in upstream kinase signaling are variable and dependent on the severity and duration of the associated obesity or diabetes. The understanding of the physiological and pathophysiological role of insulin resistance in the heart is evolving. To maintain its high energy demands, the heart is capable of utilizing many metabolic substrates. Although, insulin signaling may directly regulate cardiac metabolism, its main role is likely the regulation of substrate delivery from the periphery to the heart. In addition to promoting glucose uptake, insulin regulates long chain fatty acid uptake, protein synthesis, and vascular function in the normal cardiovascular system. Recent advances in understanding the role of metabolic, signaling, and inflammatory pathways in obesity have provided opportunities to better understand the pathophysiology of insulin resistance in the heart. This review will summarize our current understanding of metabolic mechanisms for and consequences of insulin resistance in the heart and discuss potential new areas for investigating novel mechanisms that contribute to insulin resistance in the heart.
The endoplasmic reticulum (ER) plays a critical role in ensuring proper folding of newly synthesized proteins. Aberrant ER stress is reported to play a causal role in cardiovascular diseases. However, the effects of ER stress on vascular smooth muscle contractility and blood pressure remain unknown. The aim of this study was to investigate whether aberrant ER stress causes abnormal vasoconstriction and consequent high blood pressure in mice.
Methods and Results
ER stress markers, vascular smooth muscle contractility, and blood pressure were monitored in mice. Incubation of isolated aortic rings with tunicamycin or MG132, 2 structurally unrelated ER stress inducers, significantly increased both phenylephrine-induced vasoconstriction and the phosphorylation of myosin light chain (Thr18/Ser19), both of which were abrogated by pretreatment with chemical chaperones or 5-Aminoimidazole-4-carboxamide ribonucleotide and metformin, 2 potent activators for the AMP-activated protein kinase. Consistently, administration of tauroursodeoxycholic acid or 4-phenyl butyric acid, 2 structurally unrelated chemical chaperones, in AMP-activated protein kinase-α2 knockout mice lowered blood pressure and abolished abnormal vasoconstrictor response of AMP-activated protein kinase-α2 knockout mice to phenylephrine. Consistently, tunicamycin (0.01 μ/g per day) infusion markedly increased both systolic and diastolic blood pressure, both of which were ablated by coadministration of 4-phenyl butyric acid. Furthermore, 4-phenyl butyric acid or tauroursodeoxycholic acid, which suppressed angiotensin II infusion–induced ER stress markers in vivo, markedly lowered blood pressure in angiotensin II–infused mice in vivo.
We conclude that ER stress increases vascular smooth muscle contractility resulting in high blood pressure, and AMP-activated protein kinase activation mitigates high blood pressure through the suppression of ER stress in vivo.
AMPK; ER stress; hypertension; vascular smooth muscle
Insulin resistant states are associated with increased fatty acid flux to liver and intestine, which stimulates the production of triglyceride-rich lipoproteins (TRL). ApoC-III production and plasma and TRL concentrations are increased in insulin resistance and may contribute to the hypertriglyceridemia of these conditions. The mechanism underlying that increase is not known, but because apoC-III and VLDL production are closely linked we hypothesized that FFAs may stimulate TRL apoC-III production.
Methods and Results
We used Intralipid/heparin (IH) to raise plasma FFA in 12 healthy men in the fed state, and stable isotopes to examine apoC-III metabolism. TRL apoC-III concentration was significantly higher in the IH study, and this increase was associated with higher production (PR) and fractional catabolic rate (FCR). The increase in production was greater than in FCR (90% versus 30%, respectively), accounting for the elevated concentration. Glycerol infusion had no effect on apoC-III concentration, PR, or FCR compared to saline, indicating that the effect was not attributable to glycerol released from intralipid.
These findings confirm that TRL apoC-III production is stimulated by an acute elevation of plasma FFAs, suggesting a novel regulatory pathway that may play a role in the overproduction of TRL apoC-III in insulin resistant states.
apolipoprotein C3; free fatty acids; insulin resistance; lipoprotein
Obesity is associated with insulin resistance, chronic low-grade inflammation and atherosclerosis. Toll-like receptor 4 (TLR4) participates in the cross-talk between inflammation and insulin resistance, being activated by both lipopolysaccharide and saturated fatty acids. This study was undertaken to determine whether TLR4 deficiency has a protective role in inflammation, insulin resistance and atherosclerosis induced by a diabetogenic diet.
Methods and Results
TLR4 and LDL receptor double knockout (Tlr4−/−Ldlr−/−) mice and Ldlr−/− mice were fed either a normal chow or a diabetogenic diet for 24 weeks. Tlr4−/−Ldlr−/− mice fed a diabetogenic diet showed improved plasma cholesterol and triglyceride levels but developed obesity, hyperinsulinemia and glucose intolerance equivalent to obese Ldlr−/− mice. Adipocyte hypertrophy, macrophage accumulation and local inflammation were not attenuated in intra-abdominal adipose tissue in Tlr4−/−Ldlr−/− mice. However, TLR4 deficiency led to markedly decreased atherosclerosis in obese Tlr4−/−Ldlr−/− mice. Compensatory up-regulation of TLR2 expression was observed both in obese TLR4 deficient mice and in palmitate-treated TLR4-silenced 3T3-L1 adipocytes.
TLR4 deficiency decreases atherosclerosis without affecting obesity-induced inflammation and insulin resistance in LDL receptor deficient mice. Alternative pathways may be responsible for adipose tissue macrophage infiltration and insulin resistance that occurs in obesity.
Toll-like receptor 4; insulin resistance; atherosclerosis; inflammation; diabetogenic diet
Factor VII activating protease (FSAP) activates FVII as well as pro-urokinase and inhibits platelet-derived growth factor-BB, thus regulating haemostasis- and remodeling-associated processes in the vasculature. A genetic variant of FSAP (Marburg I polymorphism) results in low enzymatic activity and is associated with an enhanced risk for carotid stenosis and stroke. We postulate that there are additional substrates for FSAP that will help to explain its role in vascular biology and have searched for such a substrate.
Results and Methods
Using screening procedures to determine the influence of FSAP on various haemostasis-related processes on endothelial cells we discovered that FSAP inhibited tissue factor pathway inhibitor (TFPI), a major anti-coagulant secreted by these cells. Proteolytic degradation of TFPI by FSAP could also be demonstrated by Western blotting and the exact cleavage sites were determined by N-terminal sequencing. The Marburg I variant of FSAP had a diminished ability to inhibit TFPI. A monoclonal antibody to FSAP, that specifically inhibited FSAP binding to TFPI, reversed the inhibitory effect of FSAP on TFPI.
The identification of TFPI as a sensitive substrate for FSAP increases our understanding of its role in regulating haemostasis and proliferative remodeling events in the vasculature.
HABP2; FSAP; Marburg I SNP; Atherosclerosis; thrombosis; TFPI
Diminished cholesterol efflux activity of apolipoprotein B (apoB)–depleted serum is associated with prevalent coronary artery disease, but its prognostic value for incident cardiovascular events is unclear. We investigated the relationship of cholesterol efflux activity with both prevalent coronary artery disease and incident development of major adverse cardiovascular events (death, myocardial infarction, or stroke).
Approach and Results
Cholesterol efflux activity from free cholesterol–enriched macrophages was measured in 2 case– control cohorts: (1) an angiographic cohort (n=1150) comprising stable subjects undergoing elective diagnostic coronary angiography and (2) an outpatient cohort (n=577). Analysis of media from cholesterol efflux assays revealed that the high-density lipoprotein fraction (1.063
Heightened cholesterol efflux to apoB-depleted serum was paradoxically associated with increased prospective risk for myocardial infarction, stroke, and death. The majority of released radiolabeled cholesterol from macrophages in cholesterol efflux activity assays does not reside within a high-density lipoprotein particle.
cholesterol efflux; high-density lipoprotein-cholesterol; macrophage; prognosis
Multiple biochemical, metabolic and signal transduction pathways contribute to insulin resistance. In this review, we present the evidence that the post-translational process of protein glycation may play role in insulin resistance. The post-translational modifications, the advanced glycation endproducts (AGEs), are formed and accumulate by endogenous and exogenous mechanisms.
Methods and Results
AGEs may contribute to insulin resistance by a variety of mechanisms, including generation of tumor necrosis factor-alpha, direct modification of the insulin molecule thereby leading to its impaired action, generation of oxidative stress, and impairment of mitochondrial function, as examples. AGEs may stimulate signal transduction via engagement of cellular receptors, such as RAGE, or receptor for AGE. AGE-RAGE interaction perpetuates AGE formation and cellular stress via induction of inflammation, oxidative stress and reduction in the expression and activity of the enzyme, glyoxalase I that detoxifies the AGE precursor, methylglyoxal, or MG.
Once set in motion, glycation-promoting mechanisms may stimulate ongoing AGE production and target tissue stresses that reduce insulin responsiveness. Strategies to limit AGE accumulation and action may contribute to prevention of insulin resistance and its consequences.
Advanced glycation endproducts; receptors; insulin resistance; type 2 diabetes
Results 1-25 (754)
This will clear all selections from your clipboard. Do you wish proceed?
Clipboard is full! Please remove an item and try again.