Vascular effects of non-esterified fatty acids: implications for the cardiovascular risk factor cluster

Oleic acid induces endothelin-1 expression through activation of protein kinase C and NF-kappa B

This study investigated the effect of oleic acid on the expression levels of endothelin-1 (ET-1) and on the signaling pathways mediating it in human aortic endothelial cells (HAECs). ET-1 mRNA expression was significantly increased by oleic acid in a dose- and time-dependent manner. Elevation of ET-1 expression in response to oleic acid was inhibited by the protein kinase C (PKC) inhibitor, GF109203X, or the NF-kappa B inhibitor, pyrrolidine dithiocarbamate. In addition, both PKC and NF-kappa B activities were significantly increased by oleic acid. Immunoblot analysis revealed that conventional PKCs (PKC-alpha and -beta II isoforms) were significantly increased in the membranous fractions of HAECs treated with oleic acid. PKC inhibitor completely abolished oleic acid-induced NF-kappa B activation, suggesting that PKC activation is upstream of NF-kappa B activation in oleic acid-induced ET-1 expression. These data suggest that elevated plasma oleic acid levels observed in obese, insulin-resistant subjects result in endothelial dysfunction, at least in part, through an increase in ET-1 expression.

Incorporation of beta cell redifferentiation therapy into a lipoprivic strategy for reversing type 2 diabetes

Type 2 diabetes (non-insulin-dependent diabetes mellitus, NIDDM), at least in the majority of patients characterized by insulin resistance and increased visceral fat, appears to be precipitated by the exposure of tissues to excessive levels of free fatty acids; this can contribute to the muscle insulin resistance, excessive hepatic gluconeogenesis, and beta cell dysfunction that collaborate to impair glycemic control. The resultant hyperglycemia, in turn, exacerbates the insulin resistance and beta cells dysfunction. The failure of glucose-stimulated insulin secretion (GSIS) in beta cells helps to sustain the elevations of serum glucose and free fatty acids, which in turn reinforce the failure of GSIS, possibly by inhibiting expression of the transcription factor IDX-1; NIDDM thus represents a vicious cycle that is not easily broken. A new strategy for achieving rapid loss of body fat - hepatothermic therapy (HT), an integrated approach involving exercise training, low-fat, low-glycemic-index food choices, and a supplementation program that promotes hepatic fatty acid oxidation - shows promise for alleviating the excessive fat exposure at the root of the diabetic syndrome, as well as the underlying insulin resistance syndrome responsible for increased macrovascular risk. However, when HT proves incapable of breaking the vicious cycle sustaining beta cell dysfunction, a supplementary strategy, beta cell redifferentiation therapy (BRT), may be required. BRT consists of a protocol in which near-normoglycemia is maintained for several weeks through use of intensive insulin therapy (e.g. artificial pancreas) or other effective measures, during which time beta cell GSIS can be expected to substantially recover owing to relief from glucolipotoxicity. Clinical experience demonstrates that this improved beta cell function, in certain cases, can persist for months or years after temporary BRT. A portion of the improved glycemic control achieved with very low calorie diets in NIDDM is reflective of improved GSIS, presumably consequent to a sustained reduction in diurnal glycemia. Long-lived analogs of glucagon-like peptide-1 (GLP-1) may find a key role in BRT; this incretin hormone not only potentiates GSIS, but also appears to increase the expression and activity of IDX-1 in beta cells, thus promoting beta cell redifferentiation. If HT is instituted prior to and following BRT to alleviate the FFA overexposure that initially precipitated the diabetic syndrome, it seems likely that the benefits of BRT will be conserved in the long term, thus enabling a reversal of NIDDM - in other words, maintenance of normoglycemia without medication. Since NIDDM is inherently preventable, its reversal should be the fundamental goal of diabetes therapy.

Tissue-specific expression of human lipoprotein lipase in the vascular system affects vascular reactivity in transgenic mice

1. The role of smooth muscle-derived lipoprotein lipase (LPL) that translocates to the endothelium surface on vascular dysfunction during atherogenesis is unclear. Thus, the role of vascular LPL on blood vessel reactivity was assessed in transgenic mice that specifically express human LPL in the circulatory system. 2. Aortic free fatty acids (FFAs) were increased by 69% in the transgenic mice expressing human LPL in aortic smooth muscle cells (L2LPL) compared with their non-transgenic littermates (L2). 3. Contractility to KCl was increased by 33% in aortae of L2LPL mice. Maximal contraction to phenylephrine (PE) was comparable in L2 and L2LPL animals, while the frequency of tonus oscillation to PE increased by 104% in L2LPL mice. 4. In L2LPL animals, *NO mediated relaxation to acetylcholine (ACh) and ATP was reduced by 47 and 32%, respectively. In contrast, endothelium-independent relaxation to sodium nitroprusside (SNP) was not different in both groups tested. 5. ATP-initiated Ca(2+) elevation that triggers *NO formation was increased by 41% in single aortic endothelial cells freshly isolated from L2LPL animals. 6. In aortae from L2LPL mice an increased *O(2)(-) release occurred that was normalized by removing the endothelium and by the NAD(P)H oxidase inhibitor DPI and the PKC inhibitor GF109203X. 7. The reduced ACh-induced relaxation in L2LPL animals was normalized in the presence of SOD, indicating that the reduced relaxation is due, at least in part, to enhanced *NO scavenging by *O(2)(-). 8. These data suggest that despite normal lipoprotein levels increased LPL-mediated FFAs loading initiates vascular dysfunction via PKC-mediated activation of endothelial NAD(P)H oxidase. Thus, vascular LPL activity might represent a primary risk factor for atherosclerosis independently from cholesterol/LDL levels.

Aortic stiffness is associated with visceral adiposity in older adults enrolled in the study of health, aging, and body composition

The central arteries stiffen with age, causing hemodynamic alterations that have been associated with cardiovascular events. Changes in body fat with age may be related to aortic stiffening. The association between vascular stiffness and body fat was evaluated in 2488 older adults (mean age, 74 years; 52% female; 40% black) enrolled in the Study of Health, Aging, and Body Composition (Health ABC), a prospective study of changes in weight and body composition. Clinical sites were located in Pittsburgh, Pa, and Memphis, Tenn. Aortic pulse wave velocity was used as an indirect measure of aortic stiffness. A faster pulse wave velocity indicates a stiffer aorta. Body fat measures were evaluated with dual energy x-ray absorptiometry and computed tomography. Independent of age and blood pressure, pulse wave velocity was positively associated with weight, abdominal circumference, abdominal subcutaneous fat, abdominal visceral fat, thigh fat area, and total fat (P<0.001 for all). The strongest association was with abdominal visceral fat. Elevated pulse wave velocity was also positively associated with history of diabetes and higher levels of glucose, insulin, and hemoglobin A1c (P<0.001 for all). In multivariate analysis, independent positive associations with pulse wave velocity were found for age, systolic blood pressure, heart rate, abdominal visceral fat, smoking, hemoglobin A1c, and history of hypertension. The association between pulse wave velocity and abdominal visceral fat was consistent across tertiles of body weight. Among older adults, higher levels of visceral fat are associated with greater aortic stiffness as measured by pulse wave velocity.

Hemodynamic effects of lipids in humans

Evidence suggests lipid abnormalities may contribute to elevated blood pressure, increased vascular resistance, and reduced arterial compliance among insulin-resistant subjects. In a study of 11 normal volunteers undergoing 4-h-long infusions of Intralipid and heparin to raise plasma nonesterified fatty acids (NEFAs), we observed increases of blood pressure. In contrast, blood pressure did not change in these same volunteers during a 4-h infusion of saline and heparin. To better characterize the hemodynamic responses to Intralipid and heparin, another group of 21 individuals, including both lean and obese volunteers, was studied after 3 wk on a controlled diet with 180 mmol sodium/day. Two and four hours after starting the infusions, plasma NEFAs increased by 134 and 111% in those receiving Intralipid and heparin, P < 0.01, whereas plasma NEFAs did not change in the first group of normal volunteers who received saline and heparin. The hemodynamic changes in lean and obese subjects in the second study were similar, and the results were combined. The infusion of Intralipid and heparin induced a significant increase in systolic (13.5 +/- 2.1 mmHg) and diastolic (8.0 +/- 1.5 mmHg) blood pressure as well as heart rate (9.4 +/- 1.4 beats/min). Small and large artery compliance decreased, and systemic vascular resistance rose. These data raise the possibility that lipid abnormalities associated with insulin resistance contribute to the elevated blood pressure and heart rate as well as the reduced vascular compliance observed in subjects with the cardiovascular risk factor cluster.

Acute regulation of fatty acid oxidation and amp-activated protein kinase in human umbilical vein endothelial cells

It is generally accepted that endothelial cells generate most of their ATP by anaerobic glycolysis and that very little ATP is derived from the oxidation of fatty acids or glucose. Previously, we have reported that, in cultured human umbilical vein endothelial cells (HUVECs), activation of AMP-activated protein kinase (AMPK) by the cell-permeable activator 5-aminoimidazole-4-carboximide riboside (AICAR) is associated with an increase in the oxidation of (3)H-palmitate. In the present study, experiments carried out with cultured HUVECs revealed the following: (1) AICAR-induced increases in palmitate oxidation during a 2-hour incubation are associated with a decrease in the concentration of malonyl coenzyme A (CoA) (an inhibitor of carnitine palmitoyl transferase 1), which temporally parallels the increase in AMPK activity and a decrease in the activity of acetyl CoA carboxylase (ACC). (2) AICAR does not stimulate either palmitate oxidation when carnitine is omitted from the medium or oxidation of the medium-chain fatty acid octanoate. (3) When intracellular lipid pools are prelabeled with (3)H-palmitate, the measured rate of palmitate oxidation is 3-fold higher, and in the presence of AICAR, it accounts for nearly 40% of calculated ATP generation. (4) Incubation of HUVECs in a glucose-free medium for 2 hours causes the same changes in AMPK, ACC, malonyl CoA, and palmitate oxidation as does AICAR. (5) Under all conditions studied, the contribution of glucose oxidation to ATP production is minimal. The results indicate that the AMPK-ACC-malonyl CoA-carnitine palmitoyl transferase 1 mechanism plays a key role in the physiological regulation of fatty acid oxidation in HUVECs. They also indicate that HUVECs oxidize fatty acids from both intracellular and extracellular sources, and that when this is taken into account, fatty acids can be a major substrate for ATP generation. Finally, they suggest that AMPK is likely to be a major factor in modulating the response of the endothelium to stresses that alter its energy state.

Nonesterified fatty acids in blood pressure control and cardiovascular complications

The fact that cardiovascular risk factors cluster among individuals with the insulin resistance syndrome strongly suggests a common pathogenetic denominator. For many years, abnormalities of nonesterified fatty acid metabolism have been implicated in the disturbances of carbohydrate and lipid metabolism that characterize the cluster. However, until more recently, evidence implicating fatty acids in the hemodynamic and vascular abnormalities that affect patients with this syndrome was lacking. Observations from epidemiological, clinical, and basic science suggest that fatty acids can raise blood pressure and contribute to the development of hypertension. The effects of fatty acids on blood pressure may be mediated in part by inhibition of endothelial nitric oxide synthase activity and endothelium-dependent vasodilation. Fatty acids can also increase alpha1-adrenoceptor-mediated vascular reactivity and induce vascular smooth muscle migration and proliferation. The adverse effects of fatty acids appear to be mediated in part through induction of oxidative stress. Fatty acids interact with other components of the risk factor cluster, including increased angiotensin II, to synergistically augment oxidative stress in cultured vascular smooth muscle cells. Oxidative stress is implicated in the pathogenesis of insulin resistance, hypertension, vascular remodeling, and vascular complications. A clearer definition of the specific reactive oxygen signaling pathways involved and interventions aimed at altering these pathways could lead to more rationale antioxidant therapy and improved outcomes.

A sequence variation in the mitochondrial glycerol-3-phosphate dehydrogenase gene is associated with increased plasma glycerol and free fatty acid concentrations among French Canadians

FAD-dependent glycerol-3-phosphate dehydrogenase (mGPD) enzyme is located in the mitochondrial inner membrane where it catalyzes irreversible oxidation reactions. Type 2 diabetes mellitus (DM) is a multifactorial disorder associated with physiological abnormalities in the glycerol and free fatty acids (FFA) metabolic pathways. In the present study, we have evaluated the association among the mGPD H264R sequence variation and postabsorptive plasma FFA and glycerol concentrations in a sample of French Canadians with and without type 2 DM. A sample of 81 recently diagnosed type 2 DM and 318 nondiabetic, nonobese, normotriglyceridemic French Canadians were screened for the presence of the mGPD H264R genetic variant using a PCR-RFLP-based method. The 318 nondiabetic subjects were free of known type 2 DM covariates (fasting glucose <7.0 mmol/L, body mass index <29 kg/m(2), fasting glycerol <2.0 mmol/L and absence of the N288D sequence variation in the glycerol kinase gene, fasting triglyceride <2.5 mmol/L). The association of mGPD H264R sequence variation with plasma FFA and glycerol concentrations was assessed in different regression models. Among non-DM individuals, the R allele (HR and RR genotypes) was associated with increased plasma FFA and glycerol concentrations (P < 0.05). However, the mean plasma FFA and glycerol concentrations were not affected by the H264R genotype in the type 2 DM sample. Overall, mean plasma FFA concentrations in non-DM RR homozygotes reached values that were similar to those achieved in patients with type 2 diabetes (0.87 +/- 0.63 vs 0.90 +/- 0.48 mmol/L). After controlling for age, gender, body mass index, fasting glucose, and fasting triglyceride concentrations, the relative odds of having fasting plasma FFA levels above the 90th percentile (0.9 mmol/L) in the absence of DM was increased by twofold in H264R heterozygotes (P = 0.04) and fourfold among R264 homozygotes (P = 0.009) compared to noncarriers. In the absence of DM, the mGPD R allele was also associated with higher plasma glycerol concentrations (P < 0.05). Results in non-DM individuals suggest that the mGPD R allele is associated with DM intermediate phenotypes. The absence of a relation between mGPD genotype and DM is in accordance with the view that DM is a complex phenotype in which increased plasma FFA or glycerol concentrations result from metabolic alterations which might obscure the effect of the mGPD polymorphism.