Atherosclerosis in nonhuman primates: sequences and possible mechanisms of change in phospholipid composition and metabolism

Further evaluation of plasma sphingomyelin levels as a risk factor for coronary artery disease

BACKGROUND

Sphingomyelin (SM) is the major phospholipid in cell membranes and in lipoproteins. In human plasma, SM is mainly found in atherogenic lipoproteins; thus, high levels of SM may promote atherogenesis.

METHODS

We investigated in a median follow up of 6.0 years the association of SM with the incidence of a combined endpoint (myocardial infarction and cardiovascular death) in stable and unstable patients, and its relation to other marker of atherosclerosis in 1,102 patients with angiographically documented CAD and 444 healthy controls.

RESULTS AND DISCUSSION

Logistic regression analysis showed that SM categorized by median was associated with an elevated risk for CAD (HR 3.2, 95%CI 2.5-4.0, p < 0.05). SM levels were correlated with apoB (r = 0.34) and triglyceride levels (r = 0.31). In patients with stable angina (n = 614), SM categorized by median was not related to incidence of a combined endpoint (cardiovascular death and myocardial infarction) (p = 0.844 by Log-rank test). However, in patients with acute coronary syndrome (n = 488), elevated SM was related to the combined endpoint (p < 0.05 by Log-rank test), also in a multivariate Cox regression analysis including potential confounders (HR 1.8, 95%CI 1.0-3.3, p < 0.05).

CONCLUSION

The results of our study reveal that 1) human plasma SM levels are a risk factor for CAD; 2) the pro-atherogenic property of plasma SM might be related to metabolism of apoB-containing or triglyceride-rich lipoproteins; and 3) plasma SM levels are a predictor for outcome of patients with acute coronary syndrome.

Effects of chronic aortic coarctation on atherosclerosis and arterial lipid accumulation in the Watanabe hereditary hyperlipidemic (WHHL) rabbit

The effects of high blood pressure on atherosclerosis were examined in the Watanabe heritable hyperlipidemic (WHHL) rabbit. For this purpose, the subdiaphragmatic aorta of rabbits was partially ligated (coarctation) to increase blood pressure. Atherosclerosis was assessed 4 months later by morphometric analyses and quantitation of arterial lipids. Results were compared to control WHHL rabbits with matched plasma triglycerides and cholesterol levels. A marked increase in atherosclerotic lesions was observed in the thoracic aorta of the hypertensive rabbits without qualitative changes in its morphometric features. The cross sectional area of the atherosclerotic plaques of the ascending and descending aorta in the hypertensive rabbits was two- and six-times larger than in normotensive rabbits, respectively. Lesions represented 12.0% +/- 3.5% of the total medial cross sectional area of the descending aorta of normotensive rabbits, versus 45.0% +/- 5.7% in hypertensive rabbits. No lesions were observed downstream of the coarctation in hypertensive rabbits, nor in the normotensive rabbits. Accumulation of cholesterol and choline-containing phospholipids in the descending aorta of hypertensive rabbits was increased 3.2- and 1.5-fold, respectively, when compared to normotensive rabbits. Hypertension did not change the unesterified cholesterol/total cholesterol and sphingomyelin/lecithin + lysolecithin molar ratios. In conclusion, chronic coarctation enhances the atherosclerotic response in WHHL rabbits in the high blood pressure compartment, and reduces the variability of this response.

Arterial lipid biochemistry in the spontaneously hyperlipidemic Zucker rat and its similarity to early atherogenesis

In the present studies, arterial lipid metabolism was evaluated in the spontaneously hyperlipidemic obese Zucker rat (fa/fa), the lean Zucker rat (Fa/-), and the Sprague-Dawley (SD) rat. Mean serum cholesterol levels in the obese Zucker, lean Zucker and SD rats were 216 +/- 18 mg/dl, 145 +/- 14 mg/dl and 84 +/- 5 mg/dl, respectively. Arterial cholesterol content was in the same rank order as plasma cholesterol and ranged from a mean of 2.23 +/- 0.10 mg/gm wet wt. in the obese rats to 1.36 +/- 0.04 mg/gm wet wt. in the SD rats. The increased arterial sterol in the obese rats was associated with increased lipid metabolism activity. The in vitro incorporation of [14C]oleate into arterial cholesteryl esters was increased 3-4-fold (P less than 0.01) and incorporation into phospholipids and triglycerides was also elevated (P less than 0.001 and P less than 0.01, respectively). The arterial sterol content and arterial lipid metabolism pattern observed in obese Zucker rat aortas are similar to those found in vessels of other species undergoing atherogenic change.

Sphingomyelin and derivatives as cellular signals

This comprehensive review was necessitated by recent observations suggesting that sphingomyelin and derivatives may serve second messenger functions. It has attempted to remain true to the theme of cellular signalling. Hence, it has focussed on the lipids involved primarily with respect to their metabolism and properties in mammalian systems. The enzymology involved has been emphasized. An attempt was made to define directions in which signals may be flowing. However, the evidence presented to date is insufficient to conclusively designate the mechanisms of stimulated lipid metabolism. Hence, the proposed pathways must be viewed as preliminary. Further, the biologic functions of these lipids is for the most part uncertain. Thus, it is difficult to presently integrate this sphingomyelin pathway into the greater realm of cell biology. Nevertheless, the present evidence appears to suggest that a sphingomyelin pathway is likely to possess important bioregulatory functions. Hopefully, interest in this novel pathway will grow and allow a more complete understanding of the roles of these sphingolipids in physiology and pathology.

Lecithin: cholesterol acyltransferase and lysolecithin in coronary atherosclerosis

Of 100 arteriographically examined, hospitalized, male patients, those without myocardial infarctions were divided into the following categories: zero-, one-, two-, and three-vessel disease; patients diagnosed with myocardial infarction were classified separately. The fasting plasma samples from these patients were examined for concentrations of triglycerides and total cholesterol, lipoprotein profile, lecithin: cholesterol acyltransferase (LCAT) activity, and lysolecithin (LPC) concentration. Those parameters in this group which are commonly determined were consistent with the clinical classification of these patients. Of those remaining parameters, the LCAT activity was increased as the severity of coronary atherosclerosis increased and the changes in the activity of this enzyme were appropriately reflected by increases in LPC concentration and decreases in the proportion of the plasma cholesterol unesterified. The results of this study suggest that increased, rather than decreased, plasma LCAT activity and increased LPC concentrations are characteristic of coronary atherogenesis. The plasma concentrations of LPC observed in these atherosclerotic patients are more than sufficient to qualify this substance for its previously proposed roles of mediator of transmembrane diffusion of LDL and as an inhibitor of platelet aggregation.

Cultured heart cell reaggregates: a model for studying relationships between aging and lipid composition

Cultured heart cells serve as a common model for studying the electronphysiology and pharmacology of intact cells of the myocardium from which they are derived (Sperelakis, N. (1982) in Cardiovascular Toxicology (Van Stel, E.W., ed.), pp. 57-108, Raven Press, New York). In this study, heart cell reaggregates were used for investigating the relationship between lipid composition and aging of the heart cells. Spherical reaggregates were prepared from newborn, 3- and 18-month-old rats, respectively. They were grown for 6 days in culture and then analyzed for their lipid composition and creatine phosphokinase levels. There was an age-related increase in total phospholipids and cholesterol level per unit of cell protein. Due to a relatively greater increase in the cholesterol, the mole ratio of cholesterol to phospholipids increased with animal age. The phospholipid composition was also affected. Thus, sphingomyelin levels increased, while those of phosphatidylcholine decreased; these alterations became much more pronounced with increasing animal age. All these changes could be affected by adding small unilamellar vesicles composed of egg phosphatidylcholine to the growth medium on the 5th day after seeding. Such treatment resulted in a lesser ratio of cholesterol to phospholipid as well as sphingomyelin to phosphatidylcholine, without reducing the total phospholipid per unit protein; the level of creatine phosphokinase was also reduced. This study demonstrated that cultured heart reaggregates can serve as a model for studying aging of the whole animal. Its main advantage is the ability to employ cells from rats of any desired age. Currently this is not possible for cultured heart monolayers.

Alterations in the lipid and lipoprotein composition of prairie dogs fed a high cholesterol diet

Cholesterol feeding induced a marked hypercholesterolemia with a significant change in the chemical composition of different lipid and lipoprotein fractions. The change of lipoprotein composition in the prairie dog resembles that of human hyper-beta-lipoproteinemia. It is assumed that the mechanism of gallstone formation in humans is not identical to that of the prairie dog. The absolute increase in the hepatic free cholesterol in the prairie dog is due to the dietary cholesterol while the relative increase in free cholesterol in humans is due to a high FC/EC which is probably due to a defect in cholesterol esterification or cholesterol ester hydrolyzing activity.

A calorimetric study of the thermotropic behavior of aqueous dispersions of natural and synthetic sphingomyelins

A recently developed differential scanning calorimeter has been used to characterize the thermotropic behavior of aqueous dispersions of liposomes containing sphingomyelin. Liposomes derived from sheep brain sphingomyelin exhibit a broad gel-liquid crystalline phase transition in the temperature range of 20-45 degrees C. The transition is characterized by maxima in the heat capacity function at 31.2 and 37.1 degrees C and a total enthalpy change of 7.2 +/-0.4 kcal/mol. Beef brain sphingomyelin liposomes behave similarly but exhibit heat capacity maxima at 30, 32, and 38 degrees C and a total enthalpy change of 6.9 kcal/mol. The thermotropic behavior of four pure synthetic sphingomyelins is reminiscent of multilamellar lecithin liposomes in that a single, sharp, main transition is observed. Results obtained for liposomes containing mixtures of different sphingomyelins are complex. A colyophilized mixture of N-palmitoylsphingosinephosphorylcholine, N-stearoylsphingosinephosphorylcholine, and N-lignocerylsphingosinephosphorylcholine in a 1 : 1 : 1 mol ratio exhibits a single transition with a Tm below that observed for the individual components. On the other hand a 1 : 1 mixture of N-stearoylsphingosinephosphorylcholine and 1-palmitoyl-2-oleylphosphatidylcholine exhibits three maxima in the heat capacity function. It is clear from these results that the thermotropic behavior of sphingomyelin-containing liposomes is a complex function of the exact composition. Furthermore, it appears that the behavior of the liposomes derived from natural sphingomyelins cannot be explained in terms of phase separation of the individual components.

Sphingomyelin and other phospholipid metabolism in the rabbit atheromatous and normal aorta

Hypercholesterolemia induces atheroma formation, and the concentration of sphingomyelin is increased compared with that in the normal aortic intima-media. The aortic intima-media sphingomyelin concentration appears to increase exponentially with time on the cholesterol diet. Moreover, the entry rate of serum sphingomyelin into the aortic wall also appears to increase exponentially with time on the cholesterol diet and with the extent of aortic exposure to hypercholesterolemia. 32P-Phosphate incorporation into sphingomyelin and other phospholipids in the perfused rabbit aorta does not increase with atheromatosis, but the rate of sphingomyelin entry increases 27-fold during approximately the same period of atheroma formation and can account for all of the increase in sphingomyelin concentration in the intima-media.