The influence of lipoproteins (LDL and HDL) on PGI2-formation by isolated aortic preparations of rabbits

Cyclooxygenase-2-dependent prostacyclin formation is regulated by low density lipoprotein cholesterol in vitro

Reduction of plasma low density lipoprotein (LDL) levels is associated with a reduced risk of myocardial infarction, stroke, and death. Some of this clinical benefit may be derived from an improvement in endothelium-dependent vasodilation. In the present study, we examined the effects of LDL reduction on cyclooxygenase (COX) activity and prostacyclin (PGI2) production. Human umbilical vein endothelial cells exposed to reduced concentrations of LDL demonstrated increased PGI2 production in a dose-dependent manner (from 0.75+/-0.2 to 2.6+/-0.2 ng/mL, P<0.0001). This alteration in PGI2 production did not result from LDL-induced changes in PGI2 synthase expression. However, selective inhibition of COX-2, but not COX-1, blocked PGI2 production under low cholesterol conditions. Addition of exogenous cholesterol induces dose-dependent reductions in endothelial COX-2 expression as measured by reverse transcription-polymerase chain reaction and by Western blotting. Pretreatment of cells with actinomycin D, a transcription inhibitor, reduced COX-2-derived PGI2 production by 45.9% (from 0.55+/-0.09 to 0.25+/-0.08 ng/mL). Taken together, these observations indicate that endothelial PGI2 production is regulated by cholesterol at the transcriptional level and that cholesterol-sensitive transcriptional pathways that regulate COX-2 expression are present in vascular tissue.

Regulatory effects of HDL on smooth muscle cell prostacyclin release

One mechanism by which high density lipoproteins (HDLs) exert their protective effect against coronary artery disease could be related to the induction of prostacyclin (PGI(2)) release in the vessel wall. We have recently shown that HDL increases PGI(2) production in rabbit smooth muscle cells (RSMCs) and that this increase is dependent on cyclooxygenase-2 (Cox-2). Here we analyze the mechanism by which rabbit HDL induces PGI(2) release in RSMCs. Our results show that although HDL(2) and HDL(3) share a similar capacity to induce Cox-2 protein levels, HDL(3) stimulates a higher PGI(2) release than does HDL(2), probably because of their relative arachidonate contents. Acetylsalicylic acid pretreatment (300 micromol/L, 30 minutes) significantly reduced the HDL-induced PGI(2) release, suggesting that both preexisting and induced Cox-2 activities were involved in the HDL effect. Ca(2+)-dependent cytosolic phospholipase A(2) (cPLA(2)) and Cox-1 protein levels were not altered by HDL. Dexamethasone (2 micromol/L), which also inhibited the HDL-induced PGI(2) release, reduced significantly both Cox-2 mRNA and protein levels without affecting cPLA(2) and Cox-1 protein levels. In addition, methylarachidonyl fluorophosphonate, a potent inhibitor of cPLA(2), did not produce any effect on HDL-induced PGI(2) release. In the presence of cycloheximide, Cox-2 mRNA levels were induced by HDL and inhibited by dexamethasone, suggesting that HDL and dexamethasone work in the absence of de novo protein synthesis. These results indicate an early effect of HDL on PGI(2) biosynthesis, specifically increasing Cox-2. PD98059, an inhibitor of mitogen-activated protein kinase kinase, completely inhibited HDL-induced PGI(2) release, whereas GF109203X, a protein kinase C inhibitor, had no effect. Thus, HDL induces PGI(2) synthesis by a mechanism dependent on the mitogen-activated protein kinase pathway but independent of protein kinase C.

HDL-induced prostacyclin release in smooth muscle cells is dependent on cyclooxygenase-2 (Cox-2)

Cyclooxygenase-1 (Cox-1) and Cox-2 are key enzymes in the conversion of arachidonic acid to prostaglandins and other eicosanoids. We studied the effects of plasma HDL and LDL on the synthesis of prostacyclin, Cox-1/Cox-2 mRNA, and protein expression by rabbit aortic smooth muscle cells. Prostacyclin synthesis was measured by enzyme immunoassay (EIA) of the stable metabolite of prostacyclin (PGI2), 6-ketoprostaglandin F1 alpha. HDL (150 micrograms/mL) induced release of PGI2 to values 3.46 +/- 0.3-fold above control. Incubations with LDL did not induce release of PGI2. N-(2-cyclohexyloxy-4-nitrophenyl)methanesulfonamide (NS-398), a selective irreversible Cox-2 inhibitor, blocked the HDL-induced PGI2 synthesis. Cycloheximide, actinomycin D, and dexamethasone downregulated HDL-induced PGI2 synthesis; therefore, HDL induced de novo synthesis of protein and Cox-2 mRNA. In addition, Northern blot analyses did not reveal differences in Cox-1 mRNA levels between control and HDL-treated cells, whereas Cox-2 mRNA levels were significantly increased in treated cells. Western blot analysis also showed an increase in the levels of Cox-2 protein. Therefore, the effects of HDL on PGI2 synthesis are mediated via upregulation of Cox-2 expression.

The case of low-dose aspirin for the prevention of myocardial infarction: but how low is low?

Although there is firm consensus that aspirin reduces the incidences (and severity) of reinfarction if given to patients after a myocardial infarction or in patients with unstable angina, there is disagreement about the optimum dose that should be used. We make the case that it could be considerably lower than the "medium dose" (75-320 mg daily) in current usage and put forward the view that a dose of 30 mg daily is sufficient. The arguments are based on the presystemic, exclusively pharmacokinetic inhibition of thromboxane synthesis in platelets by very low-dose aspirin and the importance of maintaining prostacyclin production by endothelial cells. Little attention has been in the past to the endogenous myocardial protection that results from prostacyclin generation. Such low doses have the additional advantage of not inducing gastrointestinal ulceration. We conclude that more extensive clinical trials with such "low-dose" aspirin preparations are warranted.

Characteristic features of long-living patients with familial hypercholesterolemia in Japan

OBJECTIVE

To assess characteristics of long-living familial hypercholesterolemics (FHs) in comparison with younger patients.

DESIGN

Cross-sectional study.

SETTING

Lipid clinics of a university hospital and 14 related medical institutions.

PATIENTS

A total of 335 heterozygous FHs including 17 patients over 70 years old. The average ages of the aged (> or = 70 years old) and the younger groups were 73.5 +/- 3.7 and 46.5 +/- 15.0 years, respectively.

MEASUREMENTS

Medical history, serum lipids and apolipoproteins, and radiographic measurement of Achilles tendon xanthomas.

MAIN RESULTS

Age distribution of FHs suggests shorter life of FHs compared with the general population. The age distribution of FH females was shifted to older age compared with that of FH males (P < 0.01). No significant differences were found between the levels of serum lipids and apolipoproteins in the aged and the younger groups. The thickness of the Achilles tendon was positively correlated with the product of excess total serum cholesterol and age in the patients (P < 0.01). Progression of Achilles tendon thickening was less in females than in males. A few cases of longevity could not be explained by any of the anti-atherogenic factors including female gender, a relatively low concentration of serum total cholesterol, a high concentration of HLD-cholesterol, a non-smoking habit, and a familial predisposition for longevity.

CONCLUSIONS

The female gender was found to be one of the most important factors for long survival of FHs. The different progression of Achilles tendon thickening in females and males may be related to the slower development of atherosclerosis and higher survival rate of the female patients.

Effect of ethanol on low density lipoprotein and platelet composition

The present study was designed to investigate the effect of ethanol (EtOH) dose on low density lipoprotein (LDL) and platelet composition. Male squirrel monkeys were divided into three groups designated Control, Low, and High EtOH, and fed isocaloric liquid diets containing 0%, 12%, and 24% of calories as EtOH, respectively. After four months of treatment, monkeys fed the 12% alcohol dose had LDL and platelet cholesterol concentrations similar to Controls. By contrast, platelet membranes from High EtOH animals contained significantly more cholesterol which was associated with higher levels of plasma LDL cholesterol and apolipoprotein B. Blood platelet count, size, and mass were similar for all groups and circulating platelet aggregates were absent in the two alcohol cohorts. Despite elevations in platelet cholesterol mass and thromboxane A2 (TXA2) precursor, phospholipid arachidonate, platelet responsiveness, measured as thromboxane formed in response to a collagen challenge in vitro, and the cholesterol/phospholipid molar ratio, were not significantly altered by high dose alcohol. Normal platelet activity in High EtOH monkeys may have resulted from a significant increase in the platelet phospholipid polyunsaturated/saturated fatty acid ratio and a non-significant increase in platelet phospholipid mass, both of which would have a fluidizing effect on platelet membranes. Our data indicate that low EtOH intake has no effect on platelet composition and function while unfavorable platelet cholesterol enrichment following consumption of high dose ethanol may arise from elevations in plasma LDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Longitudinal and cross-sectional studies of the relationship between 6-keto PGF1 alpha and high density lipoproteins

Two studies were performed to examine the relationship between Prostacyclin (PGI2) and high density lipoprotein cholesterol (HDL-C). A longitudinal study examined the stable metabolite of PGI2, 6-keto PGF1 alpha, along with HDL-C, and total cholesterol (TC) before (Week 0), during (Week 4), and after (Week 9) an eight week aerobic conditioning program. 6-keto PGF1 alpha was measured by radioimmunoassay using 125I, and HDL-C and TC were spectrophotometrically analysed. Maximal oxygen uptake and resting heart rate data obtained at Week 0 confirmed that the training group (E) was not different from the sedentary group (C). Results obtained at Week 9 indicated that maximal oxygen uptake was higher (p less than 0.05) and resting heart rate lower (p less than 0.05) in the E group, while these variables remained unchanged in group C. Levels of 6-keto PGF1 alpha, HDL-C, and TC did not differ between groups E and C when Week 0 was compared with Week 9. However, within the E group, significantly lower concentrations of 6-keto PGF1 alpha (p less than 0.05) and HDL-C (p less than 0.05) were found at Week 4 compared with Week 0. A cross-sectional study involving 52 subjects examined the relationship between 6-keto PGF1 alpha and several cardiovascular disease risk factors which included HDL-C. The only significant correlation (r = 0.50 p less than 0.05) observed was between 6-keto PGF1 alpha and HDL-C.

Plasma factors influencing PGI2-like activity in patients with IgA nephropathy and Henoch-Schönlein purpura

Plasma factors influencing vascular PGI2-like activity (PSA) were studied in 45 patients with IgA nephropathy, 18 with Henoch-Schönlein purpura, including 8 children with nephrotic syndrome, and 41 controls. The results were compared with the levels of plasma high-density lipoprotein (HDL), low-density lipoprotein (LDL) and fatty acid components of plasma phospholipids. The plasma of 38 of 45 patients with IgA nephropathy and 14 with Henoch-Schönlein purpura showed a diminished ability or no ability to support PSA. Twenty-three patients with IgA nephropathy and 10 with Henoch-Schönlein purpura exhibited an inhibitory activity against PGI2 production. The plasma HDL level was lower, while the LDL level and the LDL/HDL ratio were significantly higher in IgA nephropathy and Henoch-Schönlein purpura cases than in the controls. A high LDL/HDL ratio was associated with a low plasma PSA. The levels of arachidonic acid and its precursor were not lower in the plasma of patients than in the controls. The decreased PGI2 synthesis may play an important role in the pathogenesis of IgA nephropathy and Henoch-Schönlein purpura, but it can not be explained by reduced PG precursors. LDL may have an inhibitory, and HDL a protective effect on PGI2 synthesis.