Lesson 5 Flashcards
Why is it important to study cardiovascular disesaes? How are lipids transported in the bloodflow? Speak about the different types of lipoproteins and their metabolic life
Pharmacology is particularly developed in the field of cardiovascular diseases, like atherosclerosis. This illness is associated with the accumulation of lipids in the intima, the inner part of the endothelium, this can happen both in veins and medium to large sized arteries. Studying these illnesses is fundamental because cardiovascular diseases are the first cause of death in western countries.
lipids are fundamental: they represent membrane main components, are used for our energy needs, can be precursors for important molecules like hormones and are also involved in the synthesis of different molecules. But, since lipids are not soluble, how do they travel in our body? how do they cross the bloodstream? The answer is that they form packages with proteins called lipoproteins formed both by lipids and proteins. In this way lipids can travel in the blood stream and reach different tissues. If lipids are too concentrated in our bloodstream they might cause atherosclerosis since lipid deposition can block the lumen of the vases. This pathology evolves in decades during which the symptoms are mostly invisible, the process of accumulation of lipid continues until we are far away from the homeostatic equilibrium, and just then we develop visible symptoms.
The development of cardiovascular diseases depends on many things: the concentration of lipid in the blood stream, hypertension, obesity, genetic predisposition, smoking, elevated blood concentrations of cholesterol-rich low-density lipoprotein (LDL), etc… all these factors can cause the cardiovascular diseases.
THE LIPOPROTEINS
a lipoprotein is a macroparticle, spheric in nature, composed by an external phospholipidic monolayer to which proteins are bound. Dispersed in the monolayer we also find free cholesterol, while lipids are also found in the core of the structure: here we find triglyceride (glycerine plus 3 fatty acids chains) and esterified cholesterol. This structure can move into the blood stream.
There are many different lipoproteins with different structures and different faiths. Lipoproteins can be differentiated on the basis of density, size, and protein content. Usually the larger the lipoprotein, the lower the density. The opposite is also true, that is because the density of the lipoproteins depends on the balance between proteins and lipids: the smaller lipoproteins have a higher protein composition and a lower lipid composition, so the higher the ratio between protein and lipid, the higher the density. Very large lipoproteins have a very low density and that is the case of the chylomicrons. with a higher density but a smaller size we have the VLDL (very low density lipoproteins) lipoproteins, then we have the IDL (intermediate density lipoproteins), LDL (low density lipoproteins) and HDL (high density lipoproteins) lipoproteins. So the nomenclature is based on the density the size of the lipoproteins. The lipoprotein particles can also be divided depending on their function: some are important for the delivery of trygliceride to the adipose tissue, others are involved in the transport of cholesterol.
The protein present on the lipoproteins of the monolayer are called apolipoproteins. Depending on the class of the lipoprotein there will be different apolipoproteins. We have apolipoproteins B, apolipoprotein C or E. The apolipoprotein composition of the lipoprotein determines its faith and its function. All the lipoproteins except one contain at least a type B apolipoprotein (ApoB) except for HDL lipoprotein.
ApoBs are important because they deliver triglycerides to muscles which it turns use them to produce ATP and adipose tissue which is useful as storage.
ApoB are contained in both chylomicrons and VLDL: Chylomicrons are formed in the intestine, that means they transport dietary triglycerides coming form the food we ate, so the exogenous triglycerides. The smaller VLDL are instead formed in the liver, meaning that they transport triglycerides which are synthetised endogenously.
The metabolic life of lipoproteins containing ApoB can be divided in three phases:
- assembly: the production of lipoproteins
- intravascular metabolism: after being produced they go into the blood in which they are active as transporters of lipids
- clearance: the end of the life for a lipoprotein which is mediated by specific receptors
All three steps can be modulated by drugs when necessary.
How are Apo-B-containing lipoprotein assembled?
For the assembly of ApoB lipoproteins, they all come form the same genes but have different editing. After the transcription of the DNA, we have that the mRNA can be edited either in the liver or in the small intestine. This happens thanks to the ApoB editing complex. In the liver all the mRNA is translated, so the ApoB is going to be full length, in this case the apolipoprotein is called B100, where 100 corresponds to the percentage of mRNA translated. While in the small intestine the complex induces the production of a stop codon, so only a part of mRNA is translated, so the final protein is going to be more ore less half of the full length and will be called ApoB48. This is why the two proteins have different functions:
the ApoB protein represents the starting point to create the lipoprotein: so when the ribosome is producing the ApoB protein there is a process called lipidation of the nascent protein during which while the protein is being synthetised, an enzyme called MTP immediately adds a triglyceride and a cholesteryl ester to the nascent ApoB protein and this leads to the production of the chylomicron when this process happens **in the intestine, and VLDL when it happens **in the liver.
The production of these two lipoprotein is very similar but the site in which it happens is different, their major structural difference is the ApoB protein: ApoB100 for VLDLs and B48 for chylomicrons.
Because the triglyceride component of chylomicrons originates primarily from the diet the plasma concentration of chylomicrons varies in proportion to the dietary fat intake. And since these fat come from outside the organism, the assembly, secretion, and metabolism of chylomicrons are collectively referred to as the exogenous pathway of lipoprotein metabolism. Once triglycerides and cholesteryl esters are packaged together with ApoB48, ApoA1 is added as an additional structural apolipoprotein and the chylomicron particle is exocytosed into the lymphatic system for transport to the circulation.
Very-low-density lipoproteins (VLDL) contain triglycerides that are assembled by the liver using plasma fatty acids derived from adipose tissue or synthesized de novo, that is the reason why their pathway is called endogenous pathway. Hepatocytes synthesize triglycerides in response to increased free fatty acid flux to the liver, this typically happens during fasting, however dietary saturated fats as well as carbohydrates also stimulate the synthesis of triglycerides. VLDL particles are secreted directly into the circulation and may also acquire other apolipoproteins such as apoE, apoCI, apoCII, and apoCIII within the hepatocyte prior to secretion.
It is important to consider that the synthesis of apoB48 in the intestine and apoB100 in the liver is constitutive, but in the absence of triglycerides, for example during fasting, ApoB is degraded by a variety of cellular mechanisms. So we degrade ApoB proteins when we can not use them.
What happens to ApoB-containing lipoproteins one they enter the bloodstream? how are they metabolised?
Within the circulation, chylomicrons and VLDL particles must be activated in order to target triglyceride delivery to the muscle and adipose tissue. This activation requires the addition
of apoCII molecules. These apolipoproteins are specifically transported and transferred from HDL particles to VLDL and chylomicrons. HDL is a high density lipoprotein because of the huge amount of these proteins that need to be delivered to the other particles.
When VLDL and chylomicrons have apoCII attached, and are into the capillaries of the muscle or the adipose tissue, apoCII promotes the binding of the particle (VLDL or chylomicrons) to an enzyme called lipoprotein lipase LPL, which is bound to the surface of endothelial cells and mediates the hydrolysis of triglycerides. Without the apoCII the particles are not able to recognize said enzyme. The expression level and intrinsic activity of LPL in muscle and adipose tissue are regulated according to the fasting state, in fact the delivery of fatty acids happens preferentially to the muscle during fasting, since we need energy for the movements, and to the adipose tissue after a meal. So, depending on weather we are far or close to the meal we can decide the tissues where most triglycerides are delivered.
As LPL continues to hydrolyse triglycerides from chylomicrons and VLDL, the particles become progressively depleted of triglycerides and relatively enriched in cholesterol compared to the remaining amount of triglycerides. At this point the particles are called remnants, in particular VLDL remnants are also called IDL. The loss of around 50% of the triglycerides reduces the affinity of these particles for the LPL. At this point The ApoCII is transferred back to HDL in exchange for apoE which serves as a high-affinity ligand for receptor-mediated clearance of the remnants. Remnants of chylomicrons and VLDL are taken up by the liver in a three-step process.
- the remnants travel in the bloodstream and arrive in the liver. Here we find the fenestrae of the endothelium through which the remnants can enter into the hepatocytes. To do this they pass through a space called space of Disse, a location in the liver between a hepatocyte and a vessel. Here they interact with large heparan sulphate proteoglycans HSPG which sequestrate them into the hepatocytes.
- The second step is the particle remodelling which happens by hepatic lipase. This enzyme found in the hepatocytes promotes lipolysis of some residual triglycerides in the core of the remnants and also the release of fatty acids
- After all this, there is a receptor-mediated particle uptake this is the real clarence event and it is based **on four different pathways. There are that many pathways because these redundant mechanisms are useful for an efficient particle clearance. It is important to try and clean the bloodstream from these particles, infact normally only half of the remnants are actually cleared; this happens because the presence of the ApoB48 of chylomicrons allows a better clearance compared to the one of VLDL, meaning that the chylomicrons are immediately cleaned while a good 50% of VLDL remnants remain in the bloodstream. When this happens the survival VLDLs stay in the space of Disse, and here HDL comes to help by converting them into LDL particles, so this is where most of LDL comes form. As anticipated there are four receptor for the remnants on the hepatocytes. The uptake is mediated either by LDL-R, LRP (LDL related protein), LRP together with HSPG which, in this and in the next case, remains bound to the particle, or just by the HSPG.
What are the possible ways to reuptake and eliminate LDL, what happens when there is too much LDL in the bloodstream?
Is it possible to eliminate LDL? We know that in the Hepatocytes there’s the LDL-R receptor which can reuptake LDL and degrade it. Alas this is the only way to eliminate it. the mechanism is less efficient compared to the clearance pathway of the remnants, that is because of lower affinity of this receptor with the lipoprotein since they lack the ApoE. Because of this lower affinity the half-life of LDL is very long, around 2-4 days, and that is why 70% of our cholesterol is made of LDL.
A part for the liver which has 70% of the total LDL-R, these receptors can also be found in the macrophages, lymphocytes, smooth muscle cells, adrenal medulla and gonadal tissue where they are used for hormone production. These receptors can also be recycled on the surface of the membrane of the various cells. PCSK9 is a protein present in the plasma which has the role of degrading the LDL. So, the higher the amount of this protein, the lower the ability of the cell to reuptake LDL. There are mutations that can lead to an increased function of this protein, meaning that LDL in not reauptaken, there is also the opposite mutation, in this case people have an increased ability of reuptaking LDL.
When the lipoproteins are reuptaken the cholesterol is hydrolysed and taken to the ER where it is disassembled and its components can be reused. The amount of cholesterol present in the cell is very well regulated. the enzyme responsible for the entrance of cholesterol in the cell is reduced when there is a high amount of cholesterol, and there is also an increase for the enzyme responsible for the esterification of cholesterol.
Most of LDL receptors are in the liver, so that organ has the role of clearing LDL but, LDL can also travel into the blood and arrive in other degradation sites, like macrophages. The problem is that the LDL travelling in the bloodstream might remain into the endothelium of the vessels, in particular in intima where they bind to proteoglycans and could get oxidated. During this process there is a partial degradation of the ApoB100 protein and during these transformations the oxidated LDL become toxic. Being toxic they recall macrophages to the area, and the macrophages will uptake and phagocytise them. This happens thanks to the scavenger receptors SR of these cells that let them reuptake the oxidated LDL. the macrophages get full of LDL acquiring the shape of foam cells. Since they do not have a regulation mechanism they continue to accumulate LDL and become toxic too. Also, oxidated LDL can attract cytokines and thus cause an increase of the inflammation leading to endothelial injury and endothelial dysfunction. This modifies the intima and can cause the formation of the atherosclerotic plaque which in turn causes the blockage of the lumen, this happens also for medium and large sized arteries. Foam cells are the major component of the atherosclerotic plaques. when these cells die they release a lot of toxic compounds that destabilizes the plaque. when a plaque is broken we might have a heart attack. that is why high plasma level of LDL are a major risk factor for cardiovascular diseases.
Speak about the synthesis and metabolism of HDL. Why is it called “good cholesterol”?
HDL do not have ApoBs and have double function, they act as a reservoir for exchangeable Apos infact they are bound to ApoE, AI and AII, so they have all this proteins to bring to the other lipoproteins. and play a key role in the cholesterol homeostasis by removing excess cholesterol from cells and transporting through the plasma to the liver this process is called reverse cholesterol transport. This is the reason why HDL is called “good cholesterol”, because they are able to remove the excessive cholesterol from the cells and take it to the liver where it is degraded. All the cells in our body are able to synthetize cholesterol but only the liver can eliminate it.
HDL is formed in the liver. Its synthesis starts with ApoAI which gets lipidated thanks to the enzyme ABCA1 which adds a small amount of membrane phospholipid and unesterified cholesterol into ApoAI, so basically it acquires cholesterol from the the liver. In this way we have the production of pre-beta-HDL which contains cholesterol phospholipids and ApoAI. At this stage the pre-beta-HDL has the shape of a disk, but it becomes a sphere thanks the enzymes LCAT (lecithin cholesterol acyltransferase) and PLTP (phospholipid transfer protein). When it becomes a sphere it can better acquire cholesterol from different organs. In the non hepatic cells SR-BI is a scavenger receptor which mediates the efflux of excess cholesterol from the membranes. This transport can also be mediated by ABCG1 and ABCA1. At first the cholesterol is accumulated on its membrane but then it starts accumulating in its core too after getting esterificated. HDL basically gathers all the excess of cholesterol from the different organs and form the macrophages too.
Once HDL is full of cholesterol, it can go to the liver and interact with the remnants and exchange cholesterol with triglycerides. This is obtained with the interaction with another receptor called CETP (Cholesterol ester transfer protein) which helps the transport of cholesterol.
In the liver, HDL delivers the cholesterol. Here, once again, we find the scavenger receptor SR-BI, While in the non hepatic tissues this receptor works for the efflux of cholesterol, in the liver it works for the influx of cholesterol.
Because of its ability to bring cholesterol to the liver to get it degradated HDL prevents the risk of cardiovascular diseases and is thus called “good cholesterol”.
All this cholesterol can be transformed in the liver by cholesterol-7-alpha-hydroxylase which transforms **cholesterol in **bile acid and is then eliminated by biliary secretion. Bile acid, unlike cholesterol, are highly soluble in water and stimulate the biliary secretion of phospholipids and cholesterol also promotes the formation of micelles.
Overall cholesterol balance depends on the disposition of both cholesterol and bile acids. Most bile acid molecules are not lost in the faeces but are taken up and recycled by high-affinity transport proteins. The process of recycling bile acids between the liver and intestine is referred to as enterohepatic circulation. this circulation is **highly efficient, in fact less then 5% of biliary acids get lost, so this is the only amount of biliary acids that we have to synthetise again.
what types of dislipidaemias do we have? when should they be treated?
he dyslipidaemias can be divided into:
- hypercholesterolemia: which normally is the situation in which we have high levels of total plasma cholesterol, normally high levels of mostly LDL cholesterol. But we have a normal concentration of triglycerides. This can be due to familial hypercholesterolemia, in which an autosomal dominant disease involves defects in the LDL receptor. Or it could be due to familial defective apoB100, an **autosomal dominant disorder, in which mutations in the apoB100 protein lead to decreased affinity of the LDL particle for the LDL receptors. Or due to a *gain-of-function mutations in PCSK9: in this case there is an **increased PCSK9 function and decreased LDL receptor expression on cell surfaces. At last hypercholesterolemia can also be caused by **familial combined hyperlipidaemia,* characterized by different combinations of hyperlipidaemia or by polygenic hypercholesterolemia which might be the result of complex gene-environment interactions since the majority of patients have no defined genetic cause for this disorder.
- hypertriglyceridemia: when we have a too high concentration of triglycerides in the plasma. This can be caused by familial hypertriglyceridemia, an **autosomal dominant disorder with normal LDL concentrations but often low HDL concentrations. It might also be caused by *familial lipoprotein lipase deficiency, an autosomal recessive disorder caused by the absence of active LPL, or it might be due to an *apoCII deficiency. More commonly, hypertriglyceridemia develops with age, weight gain, obesity, and diabetes.
- mixed hyperlipidemia: Patients with mixed hyperlipidaemia exhibit a complex lipid profile that may consist of elevated total cholesterol, LDL cholesterol, and triglyceride concentrations, while HDL cholesterol is often reduced. It might be caused by familial combined hyperlipidaemia, which is quite **common, and is normally associated with moderately elevated concentrations of fasting triglycerides and total cholesterol and reduced concentrations of HDL cholesterol. Or it might be due to *dysbetalipoproteinemia characterized by an increased cholesterol-rich chylomicrons and IDL-like particles. Finally it could be due to *lysosomal acid lipase deficiency, a ****rare lysosomal storage disorder caused by mutations in the gene coding for lysosomal acid lipase.
- Disorders of HDL metabolism such as decreased HDL cholesterol which is an independent risk factor for development of atherosclerosis and cardiovascular disease. Numerous rare genetic defects in HDL metabolism have been identified, including defects in apoAI, ABCA1, and LCAT. For this conditions there are no effective treatments currently available. Commonly, low HDL is associated with visceral obesity and insulin resistance. The patient might also have elevated concentrations of HDL occurring in the setting of aerobic activity, alcohol consumption, estrogen use and corticosteroid therapy. Recently, reductions in CETP activity have been characterized as a relatively common genetic cause of increased HDL levels.
As we saw, these specific situations might be secondary to other disorders such as: diabetes, chronic renal failure, excess of alcohol, glucocorticoid treatment, treatment with oral extrogens, anorexia nervosa and other situations. All of these problems are able to raise the levels of triglyceride and can cause the rise of cholesterol as well.
Elevated plasma lipid concentrations raise the risk of cardiovascular disease. The increased risk of cardiovascular mortality is most closely linked to elevated levels of LDL cholesterol and decreased levels of HDL cholesterol. Instead, hypertriglyceridemia represents an independent risk factor which can further increase the risk when it **is associated with low HDL-cholesterol concentrations, even when LDL-cholesterol concentrations are normal. So, the causes of hyperlipidemia are multifactorial:
- well-defined monogenic diseases and the contributions of genetic polymorphisms,
- less well-defined gene–environment interactions
The decision to treat elevated cholesterol is based on estimations of the risk of cardiovascular disease. Infact, the treatments for cholesterol should only be administered when there is an actual risk of cardiovascular disease, for instance, if the patient only has high levels of LDL, this does not mean that he should be treated unless there are other risk factors.
