What Lipid components are in Atherosclerotic Plaques
Lim. Tawatchai, MD., MPH., Japanese Medical Service Department, Bangkok, Thailand
Abstract The 1ststudy had classified lesions into 4 groups: fatty streaks, intermediate lesions, fibrous plaques and gruel plaques. They found cholesterol ester was the most predominant and free cholesterol was the 2nd rank of the lipid components in all groups. Fatty streaks had more cholesterol ester, less free cholesterol, a higher cholesterol oleate/ linoleate ratio, rare or no cholesterol crystals. Fibrous plaques and intermediate lesions had less free cholesterol and more cholesterol ester than gruel plaques, the other components were similar. Gruel plaques had a cholesterol ester oily phase, a free cholesterol crystal phase and a phospholipid liquid crystalline phase. Oleic and linoleic fatty acid were predominant fatty acid of cholesterol ester in all groups.
The 2nd study had analyzed specimens in 19 carotid and 7 femoral plaques. They compared with the values from each subject's plasma and xanthomas removed from 8 patients. They found advanced plaques demonstrated an increase of free cholesterol and cholesteryl ester,especially cholesteryl linoleate. Free cholesterol was higher when the plaque progressed. Oleic and linoleic fatty acid played a leading role in cholesterol ester.
The 3rd study had analysed the fatty acid composition of plaques compared with serum and adipose tissue from 9 white men (age 44-73). Fatty acid percentage content was averaged for individual and correlated with serum or adipose tissue. Positive associations were found between serum and plaque *n6 (r = 0.75) and *n3 (r = 0.93 ) polyunsaturated fatty acids(18:2n6, 20:4n6, 22:6n3,18:3n3)and monounsaturated fatty acids (r = 0.70),and also between adipose tissue and plaque n6 (r = 0.89) polyunsaturated fatty acids.
*n6 = omega 6 , *n3 = omega 3
Introduction
Although cardiovascular diseases are caused by various factors, lipid diet control may be the most popular and easy way to cope with dyslipidemia for people. While we have been concerning about how to prevent cardiovascular disease in the aspect of what kind of fat foods that can hinder the progression of atherosclerotic plaques and we are still consuming the predominant unsaturated fatty acid lipid follow the recommendation of the accredited international organizations[1] for nearly a half century,whereas the incident rate of the atherosclerotic diseases has not really decreased significantly.[Fig.1][Fig.2][2] Why many studies have showed the benefit of the coconut oil which has high content of saturated fatty acid in various ideas of healthy food recently even though their evidences are not strong enough to be determined the conclusion.[3] We root the concept that the bad cholesterol "LDL-c" level is the significant marker to indicate which one is good or bad lipid. If what kind of fat affects the LDL-c in blood to be high ,it will be considered to be related with the risk of cardiovascular disease(CVD) and should be avoided without elaborating the details of lipid components between the guilty lipid and plaques whether they are related or not. Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease during5-23 years of follow-up of 347,747 subjects, 11,006 developed cardiac heart diseases(CHD),stroke or CVD showed that there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD.*[4 ] The studies of fat components in atherosclerotic plaques may be the key to find out the testimony for the real causes of CHD or CVD.
studies review
The 1st research was a research of the Lipids of Human Atherosclerotic lesions that demonstrated lesions from fatty streaks developed to advanced plaques.[5] It was a research from the Biophysics Division, Department of Medicine,Boston University School of Medicine, Massachusetts and was presented in the Annual Meeting of the American Heart Association17-20 Nov,1975. They collected 95 plaque lesions from 26 persons and classified in to 4 groups : fatty streaks,intermediate lesions, fibrous plaques, and gruel(atheromatous) plaques by the chemical and physical property identifying methods.[Fig.3][Fig.4] The data were summarized in table 1 and 2
Fatty streaks had relatively more cholesteryl oleate(C18:1),and less cholesteryl linoleate (C18:2) than advanced plaques.(table 2) The free cholesterol and cholesteryl linoleate increased higher amount in advanced plaques.(table1) The large amount of lipid component of all lesions were cholesterol ester and free cholesterol respectively. The unsaturated fatty acid C18:1 (oleic acid) and C18:2 (linoleic acid) were the first two mainly obvious fatty acid component of cholesterol ester and C16:0(stearic acid) was in the 3rd rank. It implied that the more plaques progress the more Iinoleic fatty acid and free cholesterol deposit. Cholesterol monohydrate crystals were precipitated predominantly in advanced plaques.[Fig.5]
The second research was Lipids of human atherosclerotic plaques and xanthomas : clues to the mechanism of plaque progression.[6] It was conducted by the Section of Clinical Nutrition and Lipid Metabolism, Department of Medicine,and the Division of Vascular Surgery, Department of Surgery,The Oregon Health Sciences University, Portland,OR. Journal of Lipid Research ,Volume 24,1983
The advanced atherosclerotic plaques were 19 carotid and 17 femoral obliterative plaques obtained at enarterectomy. Xanthomas were removed from 8 patients. These were compared with values from each subject's plasma. Results showed the composition of cholesterol ester fatty acid that summarized in table 3.
Oleic(18:1) and linoleic(18:2) acid were the most common fatty acid found in the cholesterol esters of all plasma and tissues (table3 and table 4).Together they amounted to more than 60% of the total fatty acids in plaque,xanthoma and plasma. In plasma linoleic acid predominated for 50.0%of the cholesterol ester fatty acids. In femoral and carotid plaques linoleic acid contributed 40.0% and 38.1%,respectively,while in xanthomas it was only 17.4%of CE fatty acids. The oleic acid was reverse in order. In xanthoma cholesterol ester, oleic acid was the highest 47.8% of total fatty acids while in carotid, femoral plaques and plasma was 25.5%,23.5% and 20.2%,respectively.
An alternative way to express these tissue-plasma differences in cholesterol ester fatty acids was to consider the ratio of linoleic acid to oleic acid (18:2 to 18:1) ( Table 5) In xanthomas the ratio is 0.36 while in carotid ,femoral plaques and plasma ,it was1.62,1.73 and 2.51, respectively. While the ratio in tissues and plasma differed several-fold, the sum of linoleic(18:2)and oleic(18:1) acid were similar in all samples.
The percentage of free cholesterol to total cholesterol in xanthomas, atherosclerotic plaques and plasma were shown in Table 7. The percentage of free cholesterol increased from the lower value in plasma (23)and xanthomas( 25.5) to higher value in femoral (50.4), and carotid plaques(56.6). The detail of each subjects in carotid plaques and femoral plaques grouping were shown in Table 6. This research showed data, similar to previous one, that oleic(18:1) and linoleic(18:2)acid were the predominant fatty acids component in plaques. The linoleic acid and free cholesterol values increased relatedly with the progression of advanced plaques too.
The 3rd research: Dietary polyunsaturated fatty acid and composition of human aortic plaques, was conducted by Wynn institute for Metabolic Research, 21 Wellington Rd., London UK. Lancet 1994[7]
They collected 135 aortic plaques sample from 9 white men(age 44-73). Fatty acids percentage contents were averaged for each individual and correlated with serum or adipose tissue percentage contents by Pearson's test. These analysis incorporated a Bonferroni correction and other statistical analytic system to assess the correlations between plaques, serum and adipose tissue's fatty acid contents.
Correlations were found among plaque's 18:2n6 (linoleic acid) content, serum and adipose tissue's 18:2n6(linoleic acid)content.[Table 8] It demonstrated that the dietary intake of this essential fatty acid directly influenced the plaque composition. The others were 20:4n6 might reflect vessel-wall catabolism of 18:2n6, 22:6n3(DHA) was the final polyunsaturated fatty acid derivative of the parent essential fatty acid 18:3n3 (alpha-linolenic acid), the dietary intake of 18:3n3 also appeared to influence plaque composition, 18:1n9 (oleic acid)content suggested that vessel wall accumulation was influenced by serum levels.
There was another interesting research studied in mice : Fatty acid-induced mitochondrial uncoupling elicits inflammasome-independent IL-alpha and sterile vascular inflammation in atheosclerosis, Nature Immunology 14,2013. [8]
Mice were fed with the high cholesterol diet for 3 months or 6 months. Fatty acid profiles in atheromas were assessed by gas chromatography and mass spectrometry. The data of fatty acid components were different markedly between the end of each periods. After 3 months, palmitic acid(16:0) and stearic acid(18:0), both saturated fatty acids were preponderant( ~22%,20%), whereas after 6 months, oleic acid(18:1)and linoleic acid(18:2), both unsaturated fatty acids converted to be predominant (~45%,18%) in higher proportion than palmitic and stearic acid (~8%,12%). These might be the clues to explain the uncorrected conclusions why we assumed the effects of saturated fatty acid lipid related to cardiac risks in many previous studies. The length of time, a very important factor, was not appropriately enough to analyze the fact of data.
The Fatty acid compositions in the main lipids of human lipoprotein classes[Table 9] are interesting too.[9] The predominant fatty acid components of Triacylglycerols, Cholesterol Esters and Phospholipids in LDL are also linoleic acid(18:2) and oleic acid(18:1) which correlate with the plaques studies.
conclusion
According to data of these studies, unsaturated fatty acids were the main components of plaques, especially oleic(18:1) and linoleic(18:2) acid. The early stage of plaque(Fatty streak),cholesterol ester was preponderance and oleic acid was the leading role, but in the advanced stage(Fibrous &Gruel plaque), it converted to linoleic acid to be predominant. Free cholesterol, which was in the 2nd rank of quantity, also increaseed similarly to linoleic when advanced plaques progressed. The excess of free cholesterol deposition in cellular membrane disturbed the function of ACAT (A:cholesterol acyltransferase system) intracellular esterification [10][11], so free cholesterol accumulated and precipitated to be cholesterol crystals form which might have a role to induce inflammatory process and fibrous formation consequently. Since monoenoic cholesterol esters had higher melting points than polyunsaturated esters[12][13], predominant linoleic acids in advanced plaque tended to be more liquefied form (Gruvel plaque). The correlations between unsaturated fatty acids in plasma and plaque, adipose tissue were also significantly demonstrated, especially linoleic and oleic acid were mainly obvious fatty acid components in plaque.
Evidences from the studies showed the clues of plaques formation composing 2 main parts. One was cholesterol ester which had large amount of oleic(18:1) and linoleic(18:2)fatty acid components. The other was free cholesterol, the 2nd rank of lipid composition forming cholesterol crystals, which was relatively large proportion in gruel plaques. Even though in the aspect of high cholesterol diet causing dyslipidemia correlating with CVD is seem to be supported by data of these studies, but the factor of food plays a role less than the intrinsic factor of cholesterol synthesis in human body. The free cholesterol in bile which is the main source of cholesterol absorption in small bowel[14],only one quarter from food intake[15], nevertheless high cholesterol diet limitation should be concerned.
Refer to the data of American civilian consumption in statistical abstract of the United State:2012 [16] , We acknowledge that American people cannot control fat diet consumption good enough to follow the dietary guideline. The excess of food energy and cholesterol consumption per capita per day may affect the insignificantly decreased incident rate of CHD in American population.
Eventually the lipid foods that may prevent CHD should be some kind of lipid containing small quantity of oleic and linoleic acid. It may be an unfamiliar new concept of dietary recommendation, especially for essential fatty acid as linoleic acid. It is too difficult to change the long-time belief of the good or bad lipid in common sense of consumers. Only more studies or the meta-analysis supporting the new concepts by the strong evidences are proved and approved, so we can conclude what kind of lipid diet that related to CVD.
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