The Chylomicron: Relationship to Atherosclerosis - Semantic Scholar
HDL measures, particle heterogeneity, proposed nomenclature, and relation to atherosclerotic cardiovascular events. Clin Chem ; Atherosclerosis is a cardiovascular disease in which lipids and inflammatory cells Two types of lipoproteins are triglyceride-rich: the chylomicrons and VLDL. The link between cholesterol and heart disease was recognized through the. In humans with the lipoprotein lipase deficiency disorder large amounts of chylomicrons and large very low-density lipoprotein (VLDL) accumulate in plasma.
The Chylomicron The chylomicron is assembled mainly in the ER and is then transported to the cis-golgi in prechylomicron transport vesicles PCVs [ 5 — 7 ]. The secretion of the particles from the intestine is regulated by MTP. Among the functions of MTP is the ability to initiate the incorporation of lipids into apo B preventing apo B degradation.
MTP acts as a chaperone to assist in apo B folding [ 8 ] For excellent review of intestinal lipid absorption see Iqbal and Hussain [ 9 ].
Inhibitors of MTP in the intestine cause steatorrhoea are associated with weight loss and a reduction in cholesterol and triglycerides in human studies [ 10 ]. The chylomicron may be thought of as a particle that has as its main function the transport of fat from the intestinal lumen to the liver. On the way to uptake by the liver, the chylomicron divests itself of lipid to the adipocyte for storage or the muscle and other cells for energy.
Chylomicron cholesterol is also available from the enterohepatic circulation and from de novo synthesis by the enterocyte. To study the chylomicron particle formation it is interesting to examine the effects of insulin resistance, a condition associated with an increase in chylomicrons. The fructose-fed hamster is a model of diet-induced insulin resistance.
Glucagon-like peptide-2 GLP-2a gastrointestinally derived intestinotropic hormone that links nutrient absorption to intestinal structure and function, was administered to hamsters and was found to increase secretion of apo B48, triglyceride-rich lipoprotein TRLand cholesterol mass. GLP 2 directly stimulated apo B48 secretion in jejunal fragments cultured ex vivo [ 12 ].
They further suggest that GLP2 represents a nutrient signal that regulates intestinal absorption of lipid and the assembly and secretion of chylomicrons from intestinal enterocytes. To examine how cholesterol might influence the chylomicron it is interesting to consider the impact of plant sterols which lower intestinal absorption of cholesterol.
Ezetimibe inhibits NPC1L1 by binding to the protein and preventing conformational changes necessary for translocation of cholesterol across the membrane [ 14 ].
International Journal of Vascular Medicine
They found that Ezetimibe added to a statin significantly decreased chylomicron cholesterol and triglyceride concentrations and postprandial apo B CD 36 deficient mice have enhanced synthesis of chylomicrons in the small intestine. They showed that triglyceride content and apo B48 mass were decreased and intestinal mucosal mRNA expression of fatty acid transfer protein 4 and apo B, along with fatty acid binding protein 2 FAB2diacylglycerol O-acyltransferase DGAT -1 and -2, and stearoyl-CoA desaturase SCD -1 which is involved in the synthesis and regulation of unsaturated fatty acids, were downregulated.
It seems therefore that Ezetimibe has more actions than just down regulating cholesterol absorption, and these studies may help in evaluating the role of cholesterol in chylomicron assembly. It is particularly difficult to understand how triglyceride absorption is altered since Ezetimibe has not been shown to cause weight loss or steatorrhoea. Turnover studies might help to understand the role of NPC1-L1 in fat metabolism.
It is possible that increased chylomicron particle clearance, due to the smaller load, plays a part in the above results. The search for an explanation as to how cholesterol in the body is so finely regulated has been intensive. Altman and Davis in their search for molecules that might inhibit cholesterol absorption discovered by chance a compound which is now known as Ezetimibe [ 19 ]. Elegant studies in mice demonstrated that knocking out this gene reduced cholesterol absorption by the same amount as happened when the wild mice were fed with Ezetimibe.
They showed that there was no further reduction in cholesterol absorption in the knockout mice when fed Ezetimibe. Statins, which inhibit HMGCoA reductase and cholesterol synthesis, have been shown to increase cholesterol absorption.
It has also been shown that low absorbers of cholesterol respond better to statins than high absorbers [ 22 ]. The mechanism of action of NPC1L1 has recently been further elucidated. It has been shown that cholesterol promotes the formation and endocytosis of NPC1L1 which appears to be an early step in cholesterol uptake.
It is interesting that this domain does not bind to plant sterols; thus it now seems that plasma membrane-bound NPC21L1 binds exogenous cholesterol and this binding facilitates the formation of NPC1L1-flotiln-cholestrerol microdomains that are then internalized into cells through the clathrin AP2 pathway.
In animal studies we have demonstrated an increase in cholesterol absorption in diabetes [ 25 ]; we asked the question as to whether diabetes might be associated with an increase in cholesterol absorption through stimulation of NPC1-L1. We demonstrated in animal models of diabetes that NPC1L1 was upregulated [ 26 ] and in diabetic patients, and we demonstrated an increase in NPC1L1 mRNA [ 27 ] suggesting a mechanism for an increase in cholesterol absorption.
In the Psammomys Obesus, a model of type 2 diabetes, the animals exhibiting weight gain, hyperinsulinaemia, and hypercholesterolaemia, NPC1-L1 protein and gene expression were both significantly reduced in the intestine, and the authors found a lower capacity to absorb cholesterol compared to controls [ 28 ].
This may suggest interspecies variation but it is a surprising finding considering that this animal model of diabetes has been shown to have increased production of intestinal lipoprotein-containing apo B48 [ 29 ]. There is a sterol regulatory element in the promoter and a sterol-sensing domain of NPCILI which appears to regulate cholesterol absorption in response to cholesterol intake.
These findings were accompanied by an increase in the transcription factors, sterol regulatory binding protein SREBP 2 and hepatic nuclear factor HNF There may be other transporters of cholesterol, for example, scavenger receptor class B type 1 SR-B1 [ 33 ] which is located both in the apical and basolateral membranes of the enterocyte. They may also play a role in cholesterol absorption.
For review see Iqbal and Hussein [ 9 ]. ATP Binding Cassette Proteins G5 and G8 The mechanism whereby the body is almost completely unable to absorb plant sterols was a mystery until recently.
Study of the familial condition, sitosterolaemia, unlocked the mystery [ 34 ]. Sitosterolaemia is a rare condition associated with early and severe atherosclerosis. The condition is associated with normal or slightly elevated cholesterol whereas total sterols are markedly increased. Search for polymorphisms in putative genes, controlling plant sterol absorption or perhaps one should say blocking plant sterol absorption, identified ATP binding cassette proteins ABC G5 and G8 in the intestine [ 35 ].
Further work demonstrated that these two gene products work in tandem to reexcrete both plant sterols virtually completely and cholesterol to a lesser extent in a regulated way [ 36 ]. The genes were also found to be expressed in the liver where they are responsible for controlling cholesterol reexcretion into the bile [ 37 ].
It appears that these two genes are very important regulators of cholesterol and together with NPC1-L1 protein are responsible for cholesterol homeostasis in the body. The authors studied mildly hypercholesterolaemic noncoronary subjects using cholestanol to cholesterol ratio as a surrogate marker of cholesterol absorption efficiency.
Since diabetes is so frequently associated with dyslipidaemia and atherosclerosis, the ABCs became a target for research. They found that levels were partially normalised on insulin supplementation.
Cholesterol, lipoproteins and the liver
In another study of streptozotosin diabetic rats ABCG5 and G8 were both very significantly reduced in the intestine [ 26 ]. In the Psammomys Obesus, another model of diabetes, Levy et al. There was a significant negative correlation between chylomicron cholesterol and both ABCG5 and G8 [ 27 ]. These two genes appear to play an important role in the dysregulation of cholesterol metabolism in diabetes.
Calcium appears to regulate lipids, at least in post menopausal women where supplementation has been shown to favorably alter lipids. Intestinal Microsomal Triglyceride Transfer Protein Intestinal microsomal triglyceride transfer protein MTP plays a major role in the "assembly of the chylomicron particle and therefore of cholesterol and triglyceride metabolism.
MTP has become a hot topic since inhibitors of intestinal MTP have been shown to lower triglyceride without causing hepatic steatosis at least in animal studies [ 43 — 45 ]. Although many polymorphisms of MTP have been described, some of which have considerable impact on LDL cholesterol in both nondiabetic and diabetic subjects [ 4647 ], it is difficult to know whether the results mainly stemmed from the effect in the liver rather than the intestine.
The intestinal inhibitors of MTP which have no effect on the liver should answer this question in the future. In the rabbit increased intestinal MTP mRNA is associated with increase in chylomicron particle numbers [ 48 ], but in the rat it is associated with larger particles [ 49 ].
The fructose-fed insulin-resistant hamster model had an increase in MTP protein mass, and this was associated with an increase in the triglyceride-rich intestinally derived lipoproteins [ 50 ]. De novo triglyceride synthesis, apo B48 biogenesis, and triglyceride-rich lipoprotein assembly were all increased. MTP activity and protein expression, however, were not altered. The homozygous form of the polymorphism has been shown to be associated with a higher concentration of LDL 3 in diabetic patients of Chinese origin, but there was no effect in the heterozygous subjects [ 53 ].
It has been suggested that the T allele might interact with visceral obesity and hyperinsulinaemia in nondiabetic subjects [ 55 ]. These intriguing findings have been further investigated by Aminoff et al.
These studies, together with the knowledge that the heart secretes Apo B-containing lipoproteins, suggest that reduction in MTP in the heart results in lipid accumulation in the heart and is followed by IHD or the susceptibility to IHD perhaps through reduction in availability of free fatty acids for energy at times of acute stress. Of course another theory is that just like the liver the decrease in secretion of VLDL through reduction in MTP function that leads to hepatic steatosis in the heart leads to accumulation of fat that may be toxic to the myocardium.Atherosclerosis: LDL & Apo-B in Focus
Last year Bharadwaj et al. They showed that lypolysis is involved in the uptake of core lipids from triglyceride-rich lipoproteins. AMPK plays a central role in energy homeostasis. In the heart it increases during ischaemia and is thought to be implicated in the pathophysiology of cardiovascular and metabolic disease.
AMPK has been targeted as being of value in the production of new therapies for cardiac and metabolic disease. AMPK is insulin sensitive, and in diabetes a reduction in AMPK activity leads to a decrease in muscle glucose uptake thus shifting fuel from glucose to fat for cardiac myocyte function.
The Chylomicron: Relationship to Atherosclerosis
A further recent link to the possible importance of MTP activity in the cardiac muscle in diabetes is the finding that the redox-sensitive transcription factor NF-E2-related factor 2 NrF2 is suppressed by extracellular signaling-related kinase ERK leading to an increase in stress-induced insulin resistance in cardiac myocytes [ 62 ].
The authors also showed in the hearts of streptozotosin-induced diabetic mice downregulation of glucose utilization. This enzyme is also found in many other tissues including the intestine and white adipose tissue, tissues that are active in triglyceride syntheis. These enzymes catalyse the final step of the triglyceride pathway [ 63 ], their substrates being diacyl glycerol and fatty acyl CoA.
DGAT-1 deficient mice are resistant to diet-induced obesity and have increased sensitivity to insulin and leptin [ 6465 ], hence the excitment in discovering DGAT inhibitors [ 6667 ]. Some studies showed an effect in normolipidemic subjects [ 7071 ].
The role of fenofibrate in the prevention of atherosclerosis is still disputed with large trials such as the field study[ 73 ] failing to demonstrate benefit for primary endpoints although secondary endpoints suggested that benefit and the DAIS study [ 74 ], an angiographic study in diabetes, certainly demonstrated significant reduction in the progression of atherosclerotic lesions Figure 1.
Dietary triglyceride, phospholipid, and cholesterol, together with intestinally synthesized cholesterol for which 3-Hydroxymethylglutaryl coenzyme A HMG-CoA reductase is the rate-limiting enzyme and recycled biliary cholesterol together with intestinally derived ApoB48 are assembled under the influence of microsomal triglyceride transfer protein MTP to form the chylomicron. Prior to incorporation into the chylomicron, cholesterol is esterified by acylcoenzyme A: The particle is assembled under the regulation of microsomal triglyceride transfer protein and delivered to the lymphatics.
In newborn swine, intestinal epithelial cells that had overexpressed apo A-IV increased the lipid content of the chylomicron particle [ 7677 ]. Further studies showed that the mechanism was through upregulation of MTP at the pretranslational level [ 78 ]. A meal rich in fat increases Apo A IV synthesis. The function of apo AIV on HDL is not clear, but reduced levels are associated with cardiovascular disease, and transgenic overexpression protects mice fed a high-fat diet from atherosclerosis [ 79 ] suggesting that Apo A IV plays an important regulatory role in fat absorption and storage.
Fasting jejunal lipid content was examined in morbidly obese persons some of whom had diabetes [ 80 ]. The diabetic patients had higher chylomicron triglycerides and apo B In the fructose-fed hamster, a model of insulin resistance, it has been demonstrated that the intestine is not responsive to the insulin-induced downregulation of the apo B48 lipoprotein production found in the chowfed animals [ 81 ].
The mechanism appears to be through disturbance in the ERK pathway which involves both insulin signaling and lipoprotein overproduction.
These results are in keeping with the studies which have shown that insulin resistance and diabetes are associated with an increase in MTP in the liver and that MTP is negatively regulated by insulin [ 82 — 84 ]. In further studies reported by the Toronto group in [ 85 ] the prechylomicron transport vesicles PCVs were characterized, and proteomic profiles were developed.
The results the authors suggest have increased our understanding of the assembly and transport of nascent chylomicrons in insulin-resistant states. The intestinal enterocyte has a short half-life but yet manages to finely control fat absorption so that even the largest fat meal does not pass through unabsorbed. The mechanism involves increase in chylomicron number as shown by increased apo B and increased size as shown by the fat content of the chylomicron.
They found that MTP expression seemed to be a limiting factor for apo B lipoprotein secretion. NR2F1 and IRE1B were found more in the crypts than in the villi suggesting the mechanism whereby only the enterocyte has the facility to absorb fat and that MTP is the limiting factor. The function of LDL is to deliver cholesterol to cells, where it is used in membranes, or for the synthesis of steroid hormones blue pathway.
Cells take up cholesterol by receptor-mediated endocytosis. Receptors are recycled to the cell surface, while hydrolysis in an endolysosome releases cholesterol for use in the cell.
HDL is involved in reverse cholesterol transport. Excess cholesterol is eliminated from the body via the liver, which secretes cholesterol in bile or converts it to bile salts. The liver removes LDL and other lipoproteins from the circulation by receptor-mediated endocytosis. Additionally, excess cholesterol from cells is brought back to the liver by HDL in a process known as reverse cholesterol transport green pathway.
It travels in the circulation where it gathers cholesterol to form mature HDL, which then returns the cholesterol to the liver via various pathways. Disorders and Drug Treatments The link between cholesterol and heart disease was recognized through the study of individuals with familial hypercholesterolemia.
Individuals with this disorder have several-fold higher levels of circulating LDL due to a defect in the function of their LDL receptors. As well, because cholesterol cannot get into cells efficiently, there is no negative feedback suppression of cholesterol synthesis in the liver.
Individuals with familial hypercholesterolemia may have strokes and heart attacks starting in their 30's. More common in the general population is dyslipidemia, which is the term that is used if lipid levels are outside the normal range. In a typical lipid profile, the fasting levels of total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides are determined.
Low levels of HDL cholesterol the so-called "good cholesterol" are an independent risk factor, because reverse cholesterol transport works to prevent plaque formation, or may even cause regression of plaques once they have formed. HDL may also have anti-inflammatory properties that help reduce the risk of atherosclerosis. Fasting triglyceride levels are used to estimate the level of VLDL. High levels of triglycerides are also associated with an increased risk for atherosclerosis, although the mechanism is not entirely clear.
The most important drugs for the treatment of dyslipidemia are by far, the statins. Statins have been shown in multiple clinical trials to reduce cardiovascular events and mortality.
Inhibition of cholesterol synthesis further decreases circulating LDL because reduced levels of cholesterol in the hepatocyte cause it to upregulate expression of LDL receptors.
In the past, several different drugs have been used to treat dyslipidemia, however the most recent treatment guidelines recommend mainly statin therapy at different intensities according to the patient's risk for cardiovascular disease. However, statins may cause adverse effects in some patients, or in others, statins by themselves may not provide sufficient lowering of LDL cholesterol.
These patients may benefit from the use of the other two drugs listed below. Two PCSK9 inhibitor drugs were approved in Praluent and evolocumab tradename: Because they are monoclonal antibody drugs, they must be administered by injection. For instance, patients with familial hypercholesterolemia are good candidates for treatment with a PCSK9 inhibitor. In clinical trials, these drugs were able to substantially lower LDL cholesterol.