SQUALENE IN HUMAN AND RAT BLOOD PLASMA

Serum non-cholesterol sterols and cholesterol metabolism in childhood and adolescence

BACKGROUND AND AIMS

The profile of cholesterol metabolism, i.e., high absorption vs. high synthesis, may have a role in the development of atherosclerosis, the early lesions of which can be present already in childhood. Since there is no information on cholesterol metabolism in children from birth to adolescence, we evaluated cholesterol metabolism in 0-15 year-old children and adolescents without dyslipidemia.

METHODS

The study population consisted of 96 children (39 girls, 57 boys) divided into age groups <1 (n = 14), 1-5 (n = 37), 6-10 (n = 24), and 11-15 (n = 21) years. Cholesterol metabolism was assessed by analysing serum non-cholesterol sterols, biomarkers of cholesterol synthesis and absorption, with gas-liquid chromatography.

RESULTS

Serum non-cholesterol sterol ratios to cholesterol did not differ between gender. Cholesterol precursors squalene, cholestenol, and desmosterol were higher in the <1 year than in the older age groups, whereas lathosterol was highest in the 11-15 year old. Plant sterols were low in the age group <1 year, after which they did not differ between the groups. Cholestanol was not age-dependent. From the age of 1 year, cholesterol homeostasis was intact. Cholesterol absorption prevailed cholesterol synthesis from 1 to 10 years of age (e.g., lathosterol/cholestanol ratio 0.35 ± 0.03 and 0.45 ± 0.05 in 1-5 and 6-10 vs. 0.66 ± 0.08 in 11-15 year-old (mean ± SE, p < 0.001).

CONCLUSIONS

Serum non-cholesterol sterols had different individual profiles by age in childhood and adolescence. From 1 to 10 years of age, cholesterol absorption prevailed cholesterol synthesis. This novel finding emphasizes the importance of dietary aspects related to cardiovascular risk even from early childhood.

Final Report on the Safety Assessment of Squalane and Squalene

Squalane and Squalene have been identified as natural components of human sebum. Both ingredients are used in a variety of cosmetics at concentrations ranging from ≤0.1 to >50%. Animal studies indicate Squalene is slowly absorbed through the skin, while both compounds are poorly absorbed from the gastrointestinal tract. The acute animal toxicity of these ingredients by all routes is low. Both compounds are nonirritants to rabbit skin and eye at 100% concentration. Formulations containing Squalene indicate it is not a significant human skin irritant or sensitizer. Limited contact sensitization tests indicate Squalene is not a significant contact allergen or irritant. It is concluded that both Squalane and Squalene are safe as cosmetic ingredients in the present practices of use and concentration.

The validity of serum squalene and non-cholesterol sterols as surrogate markers of cholesterol synthesis and absorption in type 2 diabetes

OBJECTIVE

Serum non-cholesterol sterol ratios to cholesterol reflect cholesterol metabolism in non-diabetes populations. In type 2 diabetes (T2D) cholesterol metabolism is perturbed, and the role of squalene and non-cholesterol sterols has not been related to absolute cholesterol metabolism in detail.

AIM AND METHODS

We analyzed absolute cholesterol synthesis, absorption % of dietary cholesterol, and serum squalene and non-cholesterol sterol ratios (measured with gas-liquid chromatography) in 64 T2D subjects (age 41-74 years, BMI 21-40 kg/m(2)).

RESULTS

Serum precursors of cholesterol were related to cholesterol synthesis (e.g. serum squalene to cholesterol ratio vs absolute synthesis r=0.493, p<0.001), and serum cholestanol and plant sterol ratios were related to absorption % (e.g. cholestanol vs absorption %, r=0.455, p<0.001). Furthermore, the proportions of serum synthesis/absorption markers were correlated with variables of absolute cholesterol metabolism (e.g. squalene/sitosterol vs absolute synthesis/absorption %, r=0.569, p<0.001). Serum synthesis and absorption markers (lathosterol vs cholestanol, r=-0.545, r<0.001) and absolute synthesis and absorption (r=-0.540, p<0.001) were interrelated suggesting intact regulation of cholesterol metabolism in the whole study population. Absolute synthesis/absorption ratio indicated that a change of dietary cholesterol absorption by 1% changed the mean cholesterol synthesis by 27 mg/d to the opposite direction.

CONCLUSIONS

In T2D including varying body weight and altered cholesterol metabolism, serum non-cholesterol sterols and squalene reveal reliable information of cholesterol synthesis and absorption without complicated clinical and laboratory methods.

The distribution of squalene and non-cholesterol sterols in lipoproteins in type 2 diabetes

BACKGROUND

Lipoprotein distribution of non-cholesterol sterols was studied to evaluate in which lipoproteins they are carried in type 2 diabetes with body weight ranging from normal to overweight.

METHODS

Serum and lipoprotein squalene and non-cholesterol sterols were quantitated with gas-liquid chromatography in 33 diabetic subjects separated into normal (BMI < or = 25 kg/m2, n=10) and overweight (BMI > 25 kg/m2, n=23) groups.

RESULTS

Two-thirds of the non-cholesterol sterols were carried in LDL and one-fifth in HDL, whereas squalene was mainly in VLDL and LDL in both groups. In overweight versus normal weight subjects, the absorption marker concentrations and ratios to cholesterol in serum and lipoproteins were lower and those of synthesis higher. In both groups the synthesis and absorption markers were interrelated in all lipoproteins suggesting intact regulation of cholesterol metabolism. The absorption marker ratios to cholesterol were mostly carried in HDL (cholestanol) and IDL (campesterol and sitosterol), and synthesis markers in VLDL and IDL regardless of overweight. Synthesis marker ratios were underestimated in serum versus VLDL and IDL, and those of absorption markers in serum versus IDL and HDL (p<0.05 for all). Squalene was related to lathosterol in all lipoprotein fractions (e.g., in LDL r=+0.501, p<0.01) suggesting that in diabetes squalene, too, is an indicator of cholesterol synthesis.

CONCLUSIONS

The absorption sterols are carried in IDL and HDL, and the synthesis markers in VLDL and IDL regardless of weight. The lipoprotein squalene and non-cholesterol sterol ratios were under- or overestimated in serum, and whether their evaluation in lipoproteins versus in serum only gives better information on cholesterol metabolism should be investigated further also in normal population.

Cholesterol synthesis prevails over absorption in metabolic syndrome

The objective of this study was to investigate cholesterol metabolism and its association with glucose metabolism and genetic regulation in metabolic syndrome. Overall, 74 subjects with clinically defined metabolic syndrome and sex and age-matched controls (n=74) were recruited. Cholesterol metabolism was assayed with serum non-cholesterol sterols, surrogate markers of synthesis, and fractional absorption of cholesterol and was related to variables of glucose and insulin action and to the common polymorphisms of the ABCG5 and ABCG8 genes. Serum squalene and non-cholesterol sterols were analyzed with gas-liquid chromatography (GLC) and presented as ratios to cholesterol. Also, synthesis marker/absorption marker ratios were calculated. The subjects with metabolic syndrome had higher cholesterol synthesis marker ratios, including squalene, and lower absorption marker ratios than controls. When adjusted with body mass index (BMI) and waist circumference, differences in some of the absorption markers (plant sterols), but not in the synthesis markers, disappeared. Plasma glucose, serum triglycerides, and homeostasis model assessment (HOMA) index were positively associated with cholesterol synthesis/absorption marker ratios (r=0.264 to 0.353, P<0.05 for all). In multivariate analysis, the serum squalene ratio was the best variable of those of cholesterol metabolism explaining the presence of metabolic syndrome. The polymorphisms of ABCG5 and ABCG8 genes did not differ between the cases and controls. In conclusion, cholesterol synthesis prevails over absorption in metabolic syndrome. The high serum squalene ratio turned out to be associated with the prevalence of metabolic syndrome. The perturbations of cholesterol metabolism seem to be related to abdominal obesity, and weight reduction might normalize cholesterol metabolism.

Enhancement of an analytical method for the determination of squalene in anthrax vaccine adsorbed formulations

Specific lots of anthrax vaccine adsorbed administered to members of the U.S. Armed Forces have been alleged to contain squalene, a chemical purported to be associated with illnesses of Gulf War veterans. A method of enhanced sensitivity for determining squalene in anthrax vaccine adsorbed using high-performance liquid chromatography with photodiode array detection has been developed, validated, and applied to 44 bottles of 38 lots of anthrax vaccine. In 43 bottles of 37 lots, no squalene was detected within a detection limit of 1ng/0.5ml dose (2 parts-per-billion). One lot, FAV008, was found to contain trace amounts of squalene at 7, 9, and 1microgl(-1), levels considerably below normal human plasma levels (290microgl(-1)). The overall results of this investigation provide direct evidence for the absence of squalene in nearly all of anthrax vaccine preparations tested.

Dietary squalene increases cholesterol synthesis measured with serum non-cholesterol sterols after a single oral dose in humans

Studies considering long-term squalene consumption have revealed no consistent effects on serum cholesterol levels but the immediate effect of dietary squalene on cholesterol synthesis has not been studied. Thus, the effect of a single dose of dietary squalene on postprandial lipid metabolism was studied in 16 male volunteeers aged 22-79 years. Two oral fat meals a week apart were administered to every subject, one without (control) and the other with 500 mg of squalene. Lipids, retinyl palmitate, squalene and non-cholesterol sterols were measured at baseline and after 3, 4, 6, 9, 12 and 24 h postprandially in plasma, chylomicron, VLDL and VLDL bottom and, in six randomly chosen subjects, also in IDL, LDL and HDL. In the fasting samples, squalene was mainly transported in LDL and HDL, whereas in squalene-supplemented postprandium most of squalene was carried in the triglyceride-rich lipoproteins. Postprandial squalene and retinyl palmitate curves closely resembled each other. After the squalene-enriched dietary fat load, squalene was significantly increased compared to control fat loads in plasma, chylomicrons, VLDL and IDL. Squalene addition increased significantly lathosterol/campesterol ratio in chylomicrons and VLDL at 12 h and in VLDL bottom at 9-12 h, and increased significantly VLDL lanosterol/campesterol ratio at 12 h, indicating enhanced cholesterol synthesis caused by squalene. Plasma plant sterol levels remained unchanged. In conclusion, a single oral dose of squalene representing a potential daily dietary amount increases cholesterol synthesis within 9-12 h detected in chylomicrons, VLDL and VLDL bottom.

Serum squalene in postmenopausal women without and with coronary artery disease

Squalene, found in earlier studies in human atherosclerotic plaques, was measured in the serum of postmenopausal women with coronary artery disease (CAD) (n = 25) and randomly chosen age-matched healthy controls (n = 30). The squalene concentrations of the whole population ranged from 37.5 to 115.5 microg/dl, and were higher in serum of the CAD than healthy women (91.4+/-2.6 versus 65.2+/-2.6 microg/dl, P = 0.000), a finding observed also in relation to cholesterol (43.8+/-1.8 versus 32.9+/-1.1 10(2)x mmol/mol of cholesterol, P = 0.000). The squalene concentration was also increased in chylomicrons, VLDL and d>1.006 g/ml lipoproteins, and the proportions to cholesterol in VLDL and d>1.006 g/ml lipoproteins. The respective squalene and cholesterol concentrations were related to each other in serum, VLDL and d>1.006 g/ml lipoproteins (r = 0.52, 0.85 and 0.55, respectively), whereas the correlation with triglycerides was seen only in VLDL (r = 0.84) over the whole population. Besides enhanced intestinal secretion, it remains to be shown whether higher serum squalene in postmenopausal coronary women is due to increased cholesterol synthesis or a defect in squalene conversion to lanosterol.

Treatment of cerebrotendinous xanthomatosis: effects of chenodeoxycholic acid, pravastatin, and combined use

Treatments by oral administration of chenodeoxycholic acid (CDCA) alone, 3-hydroxy-3-methylglutaryl (HMG) CoA reductase inhibitor (pravastatin) alone, and combination of the two drugs were attempted for 7 patients with cerebrotendinous xanthomatosis (CTX). CDCA treatment at a dose of 300 mg/day reduced serum cholestanol (67.3% reduction), lathosterol (50.8%), campesterol (61.7%) and sitosterol (12.7%). However, the sera of the patients changed to be "atherogenic"; total cholesterol, triglyceride and low-density lipoprotein (LDL)-cholesterol were increased, while high-density lipoprotein (HDL)-cholesterol was decreased. Contrarily, pravastatin at a dose of 10 mg/day improved the sera of the patients to be markedly "anti-atherogenic", but the reductions of cholestanol (30.4%), lathosterol (44.0%), campesterol (22.9%) and sitosterol (9.6%) were inadequate. Combined treatment with CDCA and pravastatin showed good overlapping of the effects of each drug alone. The sera of the patients were apparently more "anti-atherogenic" than those after CDCA treatment. Serum cholestanol concentration was still 2.7 times higher than in controls, but the serum lathosterol level was within the normal range, indicating that the enhancement of overall cholesterol synthesis in the patients was sufficiently suppressed. Plant sterol levels were also within the normal range. The combination of CDCA and pravastatin was a good treatment for CTX, based on the improvement of serum lipoprotein metabolism, the suppression of cholesterol synthesis, and reductions of cholestanol and plant sterol levels. In all of 7 patients, the progression of disease was arrested, but dramatic effects on clinical manifestations, xanthoma, and electrophysiological findings could not be found after the treatment of these drugs.

Postabsorptive metabolism of dietary squalene

Serum squalene, a non-steroid intermediate of cholesterol biosynthesis, originates mainly from endogenous cholesterol synthesis and partly from diet, especially in populations consuming a lot of olive oil rich in squalene. Its postabsorptive metabolism has not been studied in detail in humans. Its presence in chylomicrons and VLDL suggests that the removal of dietary squalene may reflect the metabolism of intestinal lipoproteins. Accordingly, we studied the postabsorptive metabolism of 1 g dietary squalene in 16 healthy subjects with apolipoprotein (apo) E 3/3 phenotype and in five type III hyperlipidemic apo E 2/2 homozygotes known to have a retarded chylomicron remnant removal, and compared the results with vitamin A fat load test. About 40% of the basal and 90% of the postabsorptive squalene was in lipoproteins < 1.019 g/ml. The peak concentrations of chylomicron squalene were at 6 h, and of triglyceride-rich nonchylo-fraction at 9-12 h in the controls. The peak values occurred later than those of vitamin A. At 24 h the levels still exceeded the basal ones. In type III dyslipoproteinemia, most of the basal and postabsorptive squalene was in lipoproteins of density less than 1.019 g/ml, the peak postabsorptive values occurred later than in the controls and the serum values remained above the control levels for up to 24 h. The squalene and vitamin A areas under the incremental response curves (AUC) were higher than in the control group. The AUCs of the two markers in chylomicron were correlated negatively and those in LDL+HDL were correlated positively with fasting HDL cholesterol levels, the respective correlations being opposite with fasting VLDL triglycerides. The postabsorptive profile of squalene levels resembled that of vitamin A in both groups, except that the squalene curves were shifted to a later time period. Thus, a delayed clearance of chylomicron remnants could be detected by analyzing serum squalene 6-24 h after the squalene-supplemented fat meal.

Serum noncholesterol sterols related to cholesterol metabolism in familial hypercholesterolemia

Serum cholesterol precursor levels and plant sterol were related to parameters of cholesterol metabolism in 22 patients with heterozygous familial hypercholesterolemia. The serum levels of cholesterol precursor sterols were inversely related to fractional absorption of dietary cholesterol and subsequently positively to overall cholesterol synthesis. The serum plant sterol levels, on the contrary, were significantly associated with fractional cholesterol absorption and negatively with overall cholesterol synthesis. These results were confirmed also with multivariate analyses. Fecal lanosterol, a cholesterol precursor, was related positively to cholesterol synthesis measured by sterol balance and serum precursors and negatively to serum plant sterols. The serum precursor and plant sterol levels were inversely significantly related to each other, indicating that the higher cholesterol absorption efficiency the higher are the serum plant sterol levels and the lower the precursor sterol contents and the overall cholesterol synthesis.

Absorption and metabolic fate of dietary 3H-squalene in the rat

The absorption and metabolic fate of dietary squalene was investigated on the rat by administering a single oral dose of 3H-squalene and 14C-cholesterol. Experiments on rats with cannulated thoracic duct revealed that 3H-squalene was, like 14C-cholesterol, absorbed through the lymphatic vessels and that ca. 20% of absorbed 3H-squalene was cyclized to sterols during the transit through the intestinal wall. Feces contained 3H-sterols, indicating that newly synthesized mucosal sterols had been secreted into the gut lumen. In intact animals, 3H-squalene appeared in the circulation more rapidly than 14C-cholesterol and did not persist to any significant extent in the squalene-rich adipose and muscle tissues. The increase in dietary squalene load (8-48 mg) decreased the absorption percentage of 3H-squalene (45-26%) but did not affect the absorption of 14C-cholesterol (47%). Determination of fecal steroids revealed that during the first days absorbed 3H-squalene was eliminated to a significantly higher extent than 14C-cholesterol as fecal bile acids (34% vs 11%). The experiments indicate that the rat intestine has a marked capacity for absorbing dietary squalene and that the absorbed squalene is preferentially converted to bile acids in the liver.

Fate of intravenously administered squalene in the rat

To investigate the metabolism of serum squalene rats were given intravenously serum or isolated lipoprotein containing [3H]squalene and [14C]cholesterol. Labeled squalene disappeared multiexponentially from serum and the rate of disappearance was consistently faster than for [14C]cholesterol. [3H]Squalene given by injection did not accumulate in tissues, but was rapidly cyclized to sterols, resulting in the labeling of serum methyl sterols and cholesterol as well as biliary and fecal sterols and bile acids. Independent of the form of administration, the fractional conversion of squalene to serum cholesterol was less than one. This was caused by the fact that [3H]squalene was eliminated initially more rapidly than serum [14C]cholesterol in the feces and was converted to a greater extent than serum cholesterol to bile acids, whereas both labels were eliminated in parallel as neutral sterols. The results support the role of newly synthesized hepatic cholesterol as the preferred substrate of bile acid synthesis.

Inhibition of rat hepatic sterol formation from squalene by plasma lipoproteins

The conversion of 3H-squalene to sterols by rat liver microsomes and cytosol was inhibited by individual rat and human plasma lipoproteins at various concentrations. This inhibition was also observed with added human high density apolipoprotein, but triglycerides, cholesterol, or cholesteryl esters had no inhibitory effects. Lipoproteins and apo high density lipoprotein (HDL) were demonstrated to bind 3H-squalene in vitro. The binding of 3H-squalene by apo HDL could be reversed by increasing concentration of liver cytosol containing sterol carrier protein.

Serum squalene levels in hepatobiliary diseases

Serum squalene levels did not change in patients with acute hepatitis, chronic active hepatitis and liver cirrhosis, but were significantly reduced in patients with intra- and extrahepatic cholestasis. The ratio of cholesterol to squalene remained normal in patients with acute hepatitis as well as chronic active hepatitis, while being slightly decreased in patients with liver cirrhosis. On the other hand, in patients with cholestasis the ratio was markedly raised. From these observations we confirmed abnormal sterol metabolism in hepatobiliary diseases, and clinical usefulness of the ratio of cholesterol to squalene to distinguish hepatocellular injury and cholestasis.

Effect of alimentation on human serum squalene levels

Five subjects consumed a meal containing ca. 1.5 mg squalene. Postprandial serum samples showed that in three of the subjects squalene and lipid levels reached maxima at 3 hr. Squalene levels in the low density lipoproteins of these subjects also increased. The results indicate that alimentation is a major factor in the variation in serum squalene levels.

Studies of the in vivo metabolism of mevalonic acid in the normal rat

Studies were performed to examine synthesis, tissue localization, and metabolism of mevalonic acid in normal rats. Circulating mevalonate was found to have a rapid turnover phase of 5 min and a slower phase of 40-50 min. Under in vitro conditions the synthesis of mevalonate is carried out most actively by the liver and only to a minor extent by the other tissues studied. The most unexpected finding of this study was that both in vivo and in vitro the kidneys rather than the liver are the primary site of the metabolism of circulating mevalonate. Whereas mevalonate in the liver is rapidly transformed to cholesterol, the major products of mevalonate metabolism in the renal tissues during the same time period are squalene and lanosterol. Exogenous in contrast to circulating mevalonate is metabolized primarily in the intestine.

Sterol synthesis in the human arterial intima

Intimal sterol synthesis was examined in isolated human arterial segments obtained at surgery or at postmortem examination. The tissues were incubated with acetate-1-(14)C and mevalonate-2-(14)C and the incorporation of these precursors into sterols was determined. Intimal sterols were isolated by multiple chromatographic techniques and purified by bromination and oxidation procedures. The results indicate that the arterial intima can incorporate acetate and mevalonate into cholesterol, cholestanol, and squalene. Cholestanol was the major sterol synthesized by the arterial wall, but cholesterol production was also consistently observed. The findings suggest that local synthesis is a potential source of sterol accumulation within the arterial wall. The conversion of cholesterol to other sterols was also studied in terminally ill patients receiving labeled cholesterol before death. Tissue analyses revealed the presence of labeled cholestanol as well as cholesterol in the tissue 5-104 days after labeled cholesterol administration. The results demonstrate the conversion of cholesterol to cholestanol in man and suggest that the exchange of cholestanol between the blood and tissues is similar to that of cholesterol.

Blood cholesterol and hydrocortisone production in man: quantitative aspects of the utilization of circulating cholesterol by the adrenals at rest and under adrenocorticotropin stimulation

A kinetic study of the conversion of blood cholesterol into hydrocortisone was carried out in two patients through prolonged infusions of cholesterol-4-(14)C. The following points appear to be established by our observations:1) The infused tracer behaved metabolically like endogenous cholesterol; it could therefore serve as a means of labeling plasma cholesterol for investigating its utilization by the adrenal cortex.2) At rest, about 80% of hydrocortisone derived from plasma cholesterol, the other 20% thus being synthesized in situ from acetate and other unlabeled precursors.3) Under ACTH stimulation the participation of plasma cholesterol in the synthesis of hydrocortisone was the same as at rest; the conversion of plasma cholesterol into hydrocortisone was thus proportional to the production of glucocorticosteroids by the adrenal glands.4) The specific activities of hydrocortisone allowed us to trace its adrenal precursors including adrenal cholesterol. The kinetics of the replacement of adrenal cholesterol by plasma cholesterol underlined the functional heterogeneity of the former. The experimental data were compatible with the following model: A fraction of plasma cholesterol entering the adrenal cell is immediately available for metabolism and conversion into steroid hormones, and another fraction turns over slowly, representing some form of storage.