Flipped Classroom: Lipoproteins I Flashcards
Lipoproteins are rich in
Cholesterol and triglycerides
Can vary over a greater range than glucose
Lipid levels
Hypercholesterolemia is usually a result of
High LDL/HDL ratios
Leads to narrowing of artery walls, decreased blood and oxygen supply to the heart, and possibly heart attack and death
Hypercholesterolemia
In population studies, there is a very strong correlation between plasma LDL cholesterol and the incidence of
Coronary Artery Disease (CAD)
What percentage of adult Americans have atherosclerotic narrowing of their coronary arteries?
70%
Contain cholesterol-laden foam cells, and their location depends on the cause of the dyslipidemia
Xanthomas
Patients with hypercholesterolemia but no hypertrygliceridemia have
Tendon xanthomas (achilles and hands/fingers)
Patients with combined hypercholesterolemia and hypertriglyceridemia have
-form over joints
Tuberous xanthomas
Yellowish cholesterol deposits under the skin of the eyelids are known as
Xanthelasmas
Xanthelasmas are most commonly seen in patients with high
LDL cholesterol
The type of LDL that leads to plaque formation
Oxidized LDL
The initiation of an atherosclerotic plaque involves which three stages of development?
- ) Fatty streak formation
- ) Conversion to fibrous plaque
- ) Alteration to complex lesion
Lined by a monolayer of endothelial cells that is in contact with blood overlying a basement membrane
Tunica intima
The initial steps of atherosclerosis includes the recruitment of monocyte-macrophages to the subendothelial space and the infiltration of
Oxidized plasma LDLs
These oxidized LDLs are taken up by macrophages, leading to the formation of
Foam cells
A these cholesterol laden foam cells accumulate, we see further recruitment of
Monocyte-macrophages
Release cytokines an growth factors that lead to smooth muscle cell proliferation and synthesis of extracellular matrix molecules such as collagen, elastin, and proteoglycan
Monocyte-macrophages
These changes form the earliest lesion of atherosclerosis, known as the
Fatty streak
Fatty streaks form in most people by the age of 20 in the
Aorta and coronary arteries
Plaque macrophages and smooth muscle cells can die in advancing lesions by
Necrosis and apoptosis
A mixture of lipid and subintimal smooth muscle that grows into the lumen
Atheroma
In a complicated lesion, the endothelial cell layer covering the lesion is lost. As a result, the surface of the lesion becomes
Thrombogenic (induces clot formation)
This results in an increase in cellular debris and the appearance of calcification and
Cholesterol crystals
Lastly, the complicated lesion emerges with calcification and further damage to the arterial wall and alterations in the structure of the
Fibrous cap
This leads to complete occlusion of the artery, called
Infarction
Also we can see disruption of the plaque and thrombosis, leading to
Stroke
A metabolic disorder caused by a mutation in the DHCR7 gene on chromosome 11
Smith-Lemli-Opitz Syndrome (SLOS)
This DHCR7 gene codes for an enzyme that is involved in the production of
Choleterol
Are unable to make enough cholesterol to support normal growth and development
SLOS patients
Shows the facial features of microcephaly, ptosis, broad nasal ridge, upturned nose, and micrognathia (small jaw)
SLOS
The most frequently reported clinical finding in the limbs of SLOS patients is
Syndactyly of the 2nd and 3rd toes
Cleft palate, short thumbs, and polydactyly are also signs of
SLOS
In a steady state individual, the normal levels of cholesterol can be twice the normal levels of
Glucose
Consists of four fused hydrocarbon rings (A-D) called the “steroid nucleus”
Cholesterol
Cholesterol has an 8 carbon branched hydrocarbon chain attached to
Carbon 17 of the D ring
Ring A of cholesterol has a hydroxyl group at
Carbon 3
Ring B of cholesterol has a double bond between carbons
5 and 6
Most plasma cholesterol is in an esterified form. I.e it has a
Fatty acid attached to carbon 3
A very hydrophobic compound
Cholesterol
Not found in membranes, and are normally only present in small amounts in most cells
Esterified cholesterol
Because of their hydrophobicity, cholesterol and its esters must be transported in association with a protein component (lipoprotein) or be solubilized by
Phospholipids and bile salts
The body’s supply of cholesterol, derived from the diet and de novo synthesis is always transported in the blood packaged into
Lipoprotein particles
The lipoprotein particle derived form the intestine is the
Chylomicron
The lipoprotein particle derived from the liver is the
VLDL
Part of the exogenous pathway of lipoprotein metabolism
Chylomicron
Part of the endogenous pathway of lipoprotein metabolism
-seen before in the context of de novo synthesized TAGs
VLDLs
Takes place in the cytosol of virtually all cell types
Cholesterol synthesis
The two major organs that contribute de novo cholesterol to the body’s pool
Liver and intestine
The source for all carbon atoms in cholesterol
Acetyl-CoA
The major co-factor required for cholesterol synthesis is
NADPH
Also consumed during cholesterol synthesis
ATP
The rate limiting step in cholesterol synthesis is catalyzed by
HMG CoA reductase (HMGR)
The rate limiting step of cholesterol synthesis, catalyzed by HMG CoA reductase produces
Mevalonic acid (releases free CoA)
The rate limiting step of cholesterol synthesis, catalyzed by HMG CoA reductase uses up
2 NADPH
The formation of mevalonic acid is the step of cholesterol synthesis that is
Regulated
Mevalonic acid gives rise to isoprene squaline, which ‘folds up’ and through a series of intramolecular reactions gives rise to
Lanosterol
Mevalonic acid gives rise not only to cholesterol, but also to
Terpenes (isoprenoids/isoprenes)
Farnesyl pyrophosphate, dolichol, ubiquinones, and geranyl pyrophosphate are the terpenes we get from
Mevalonic acid
Can be conjugated with proteins and then serve as lipid anchors
Farnesyl pyrophasphate and geranyl pyrophosphate
Can be reduced to ubiquinols which can donate their electrons to the electron transport chain
Ubiquionones
Excess cholesterol activates
-makes cholesterol esters
ACAT
Stimulates the proteolysis of HMG CoA reductase and influences RNA polymerase II’s rate of HMG-CoA reductase mRNA synthesis
Cholesterol
Stimulates the activity of HMG-CoA reductase
Insulin
Inhibits the activity of HMG-CoA reductase
Glucagon and cholesterol
Inhibits the uptake of cholesterol into liver cells
Excess cholesterol
Reduces the levels of HMG-CoA reductase through stimulation of its degredation
Cholesterol
Makes the cholesterol ore hydrophobic, enabling it to be packaged and stored and transported more easily
Esterification
In epithelial cells of the intestine, some cholesterol is converted to cholesterol ester by
ACAT
Hepatocytes and steroid producing cells store cholesterol as cholesterol esters inside lipid droplets in the
Cytosol
Uses a fatty acid from the phospholipid lecithin (phosphatidylcholine) on a peripheral cell to esterify cholesterol to cholesterol esters
Lecithin cholesterol acyltransferase (LCAT)
In the blood, LCAT is bound to
HDL
Regulation of HMGR through covalent modification occurs as a result of
Phosphorylation and dephosphorylation
HMGR is most active in the
Unphosphorylated form
Phorphorylated by AMP-activated protein kinase (AMPK) which decreases its activity
HMGR
Negatively affect cholesterol biosynthesis by increasing the activity of the inhibitor of phosphoprotein phasphatase inhibitor-1 (PPI-1)
Glucagon and epinephrine
Stimulates the removal of phasphates and thereby activates HMGR
Insulin
Cholesterol synthesis is trancriptionally controlled by
SREBP2
A cholesterol sensor that is the main regulator of HMGR activity
Sterol Regulatory Element Binding Protein (SREBP2)
An integral membrane protein of the ER that senses the concentration of cholesterol in the ER membrane
SREBP2
When the concentration of cholesterol in the ER is abnormally low, SREBP-containing visicles form and move to the
Golgi apparatus
There, proteases cleave
SREBP2
The N-terminal segment of SREBP2 then moves into the nucleus and enhances transcription of several genes, including the genes for
HMGR and also LDL receptors
When cholesterol levels are high, there is no stimulation of
SREBP2
HMGR is the target of a very widely used class of drugs called
Statins
Structural homologues of HMGR and compete for HMG-CoA binding on the reductase, reducing its ability to promote cholesterol synthesis
Statins
An exampleof a statin is
Lovastatin
Lower blood cholesterol levels while also restoring the liver cholesterol pool
Statins
The liver accomplishes this by upregulating the quantity of
LDL receptor molecules on hepatocytes
In a small subset of the population, statins can result in profound
Myopathy
Prescribed both as a preventative and in respone to CAD and MI
Statins
Most lipids are transported in the blood as part of soluble complexes called
Lipoproteins (LPs)
Plasma LPs are spherical particles composed of a hydrophobic lipid core surrounded by a
Hydrophilic layer
The lipid core contains primarily
Triglycerides (TG) and Cholesterol Esters (CE)
Composed of amphipathic lipids such as phospholipids, unesterified cholesterol, and special proteins called apolipoproteins or apoproteins
External layer of lipoproteins
These proteins facilitate lipid solubilization and help to maintain the structural integrity of
LPs
Serve as ligands for LP receptors and regulate the activity of LP metabolic enzymes
Apolipoproteins and apoproteins
What are the 4 major categories of lipoproteins?
- ) Lipoproteins
- ) VLDL
- ) LDL
- ) HDL
The largest lipoproteins but the least dense due to the large amount of triacylglycerides in these molecules
Chylomicrons
The smallest lipoproteins but the most dense due to low lipid and high apoprotein content
HDL