Lipids (Week 3 and 4--Edwards) Flashcards
Lipid disorders
Cardiovascular disease (hyperlipidemia, heart attack, stroke)
Obesity (linked to metabolic syndrome and diabetes)
Non alcoholic fatty liver disease (66% of diabetics have this)
Gall stones (can be fatal if block pancreatic duct and thus cause pancreatitis)
Minor diseases: Crohn’s disease, ileal resection, peroxisomal defects, respiratory distress syndrome, sphingolipidoses/defective degradation of substrates in lysosomes (Niemann-Pick C, Tay Sachs)
What exactly is a lipid?
There are hundreds/thousands of lipids and we don’t know the function of many of them
Includes: fatty acids, triglycerides, cholesterol, cholesterol esters, phospholipids, glycolipids, sphingolipids
Lipids are soluble in non-polar solvent but insoluble in water (except very short chain fatty acids)
What are lipids synthesized from?
Acetyl CoA
Acetyl CoA –> fatty acids –> triglycerides, phospholipids, sphingolipids, glycolipids, glycosphingolipids
Acetyl CoA –> cholesterol –> bile acids (in liver) or steroid hormones (in adrenals, testes, ovaries)
How do we absorb medium chain vs. long chain fatty acids?
Medium chain fatty acids/triglycerides: 6-12 carbons; do not require bile acids for hydrolysis in gut, so are absorbed directly into portal vein (ie milk)
Long chain fatty acids/triglycerides: 14-20 carbons; absorbed via chylomicrons
What are essential fatty acids?
FAs we don’t synthesize, so we need to eat them in our diet
Important for synthesizing arachidonic acid and other important fatty acids
What are saturated fatty acids?
Have no double bonds (saturated with as many hydrogens as they can)
Pack well
Found in meat
High dietary saturated fatty acids are risk factor for MI (whereas high dietary polyunsaturated FAs are protective)
Trans fatty acids
Produced by food industry (not natural) by partial hydrogenation to maintain better shelf life
Double bond is trans
Trans FAs increase risk of MI/heart attack
Triglyceride
Triglyceride (TG) AKA triacylglycerol (TAG)
3 fatty acids in ester linkage to 1 glycerol
Major dietary fat
Major storage fat
Phospholipids
Have glycerol, heterogeneous charged head group, 2 heterogeneous fatty acids
Lipases act on phospholipids to release fatty acids (could be arachidonic acid, which remember is precursor for thromboxanes, prostaglandins)
Cholesterol
Important for membrane fluidity
Precursor for bile acids/salt (thus important for lipid absorption), and for steroid hormones
Plasma lipoproteins
Have surface monolayer of specific proteins/apoproteins (may be structural, enzymes, enzyme activators, or recognition sites for cell surface receptors), phospholipids (polar, charged head group), cholesterol (which has hydroxyl group)
Core is tryglycerides and cholesterol esters (very hydrophobic)
Note: if cholesterol loses its hydroxyl group it will become a cholesterol ester and will go into core
What do plasma lipoproteins do?
Transport lipids through the blood
Lipids are transported within the plasma lipoproteins
Apoproteins on plasma lipoproteins (apoA-1, apoC-II, apoB) have specific functions involved in lipid metabolism, immunity, inflammation
What are the plasma lipoproteins we have?
HDL (good–correlates inversely with CHD)
VLDL (bad)
LDL (BAD–correlate with CHD)
Vary in size, density, composition, function
Are triglycerides correlated with heart disease?
Only very weakly
Digestion and absorption of lipid
1) Dietary lipids in intestinal lumen solubilized into micelles containing bile salts, then hydrolyzed to simpler lipids (FFA and monoglycerides)
2) Lipids enter enterocytes lining villi and complex lipids (triglycerides) re-synthesized and secreted into lymph as chylomicrons
3) Chylomicrons enter blood and deliver dietary lipid and fat soluble vitamins to other organs
Ezetimibe (aka Zetia)
Drug that inhibits cholesterol absorption (through NPC1L1)
Not used much any more because many risks, but is still on the market
How does pancreatic lipase work?
Pancreatic lipase cleaves 2 ester bonds to create 2 fatty acids and 1 monoacylglycerol
In the lumen of the intestine
How does cholesterol esterase work?
Cholesterol esterase cleaves an ester bond to release cholesterol and 1 fatty acid
In the lumen of the intestine
What does blood look like after a fatty meal?
Milky white-ish appearance because chylomicrons in there reflect light
TGA levels will be very high and provide no useful clinical information
How are chylomicrons removed from the blood?
1) Chylomicrons in bloodstream, and liproprotein lipase (in endothelial cells that line blood vessels of muscle and adipose tissue) with ApoCII cofactor (on surface of chylomicron) hydrolyzes triglycerides in core of chylomicron
2) FFAs enter muscle (for beta oxidation for energy) or adipose tissue for storage (after re-synthesis of TGs)
3) Chylomicron remnant is smaller and has less TGs, and is taken up by the liver by receptor-mediated endocytosis
What would happen if patient had inactive/mutant LPL (or mutation in ApoCII)?
Hyperchylomicronemia: too many chylomicrons in the blood (blood white because of chylomicrons even after 12 hour fast)
Can lead to pancreatitis (huge lipoprotein particles stuck in vessels of pancreas); can lead to Xanthomata (bumps on skin that are TG deposits)
Treatment: low fat diet (still need some fat for essential fatty acids)
Obvi would have high TG in blood
What happens to a diabetic with low insulin (Type 1 Diabetes) in terms of LPL activity?
LPL requires insulin, so LPL activity decreases if no insulin (essentially like having inactive/mutant LPL)
If uncontrolled Type 1 Diabetes, can get hypertriglyceridemia and hyperchylomicronemia –> pancreatitis, xanthomata
A-beta-lipoproteinemia
Defect in gene (MTP: microsomal triglyceride transfer protein; causes decreased ApoB-100 and apoB-48) so cannot package TGs into chylomicrons to secrete chylomicrons from enterocytes into liver
No VLDL in plasma, and enterocytes loaded with lipid because can take up lipid but can’t get rid of it
Leads to steatorrhea
What does the liver do regarding VLDL, LDL, HDL?
Liver secretes VLDL –> LPL can act on VLDL to make TGs to give to cells (well, must break down into FFAs first) –> then VLDL turns into LDL and LDL can go to cells to contribute its cholesterol or can go back into liver
Liver also creates HDL, which can go to cells and be anti-inflammatory and take cholesterol from peripheral tissue back to liver to be secreted in bile
Where are LDL receptors located?
1) On peripheral tissues so LDL can go into tissue to give cells cholesterol for cell division
2) On liver (lots) so liver can take up LDL and get rid of the cholesterol in it by secreting it as bile or bile acids
What are similarities and differences between chylomicrons and VLDL?
Chylomicrons secreted by enterocytes, and release FFAs to go into adipose tissue/muscle while leaving behind a chylomicron remnant
VLDL is secreted by the liver and also releases FFAs to go into adipose tissue/muscle but leave behind an LDL (which goes to liver or peripheral tissue)
Both are acted upon by LPL and ApoCII
What do LDL and VLDL have in them?
LDL has cholesterol ester (because gives bad cholesterol to cells!)
VLDL has TGs (because just giving TGs/FFAs to cells)
What controls LDL levels in the plasma?
Partially by synthesis of VLDL (and thus LDL) and partially by hepatic expression levels of LDL receptor (LDLR) that clears LDL from blood
How do cells get their cholesterol?
1) LDL receptor on cell surface endocytoses LDL, turns into lysosome that secretes cholesterol into cytoplasm
2) Cell can synthesize cholesterol from acetate, and enzyme to regulate this is HMG-CoA reductase
Note: these two things go up and down together–if the cell needs more cholesterol, it uses both these pathways to get it
What happens to cholesterol once it’s in the cell?
Depends on what cell it’s in!
Liver can turn cholesterol to bile acids to secrete it
Adrenal cells can turn cholesterol to steroid hormones
etc etc
How important is the LDL receptor in controlling plasma LDL levels?
Very important!
Mutation in LDL receptor gene (Familial Hypercholesterolemia) thus less LDL receptor on liver, means increased plasma LDL (and higher risk for heart attack)
More LDL receptor on liver means decreased plasma LDL (and lower risk for heart attack)
What do statins do?
Statins add LDL receptors to the liver to remove more LDL from blood
Reduce plasma LDL 20-50%, and thus reduce risk of heart attack
Can you be homozygous for a mutation in the LDL receptor (Familial Hypercholesterolemia)?
Yes, but you have REALLY high LDL and total cholesterol, and can get heart attack REALLY young (younger than 13)
How many LDL receptors would a heterozygote for Familial Hypercholesterolemia have?
Half as many receptors as normal
How does cholesterol in the cell control LDLR and HMG-CoA reductase?
High cholesterol in the cell means low LDLR and low HMGCR (you don’t want any more cholesterol in the cell because you don’t need any more!) –> high cholesterol in the blood because can’t let any more into cells
Low cholesterol in the cell means high LDLR and high HMGCR (you can take more cholesterol into the cell)
How is the co-ordinate regulation of LDLR and HMG-CoA reductase the target of statins (and indirectly, bile acid sequestrants)?
Statins are competitive inhibitor of HMG-CoA reductase
If you inhibit HMG-CoA reductase, you keep the pool of cholesterol in the liver low, and that causes increased expression of LDLR which takes up cholesterol from blood!
Is it bad to keep taking up LDL into cells (if you are on a Statin, this is what you’re doing)?
In liver cells, this is fine! Because you take up CE from LDL, hydrolyze and release cholesterol, then convert cholesterol to bile acids and send that to gall bladder where 5% is flushed away every time it’s recycled (a few times per day), and bile is secreted into intestine for secretion
Liver does not become loaded with fat (steatotic), since it excretes lipid into bile
Other cells have LDLR, but most are on the liver
Statin
Competitive inhibitor of HMG-CoA reductase
Lower plasma LDL 20-55%
Very few side effects, only 0.2% have side effects (muscle weakness?)
HMG-CoA reductase binds statins better than it binds its real substrate (HMG-CoA)
Must go on statins for life, because if go off statins, cholesterol goes back up
Statins don’t help homozygotes because they don’t have any functional LDLR!
What is the treatment for someone who is hymozygous for Familial Hypercholesterolemia?
Liver transplant
Because no functional LDL receptor gene
Tangier disease
Low HDL
Results from mutation in ABCA1 gene (transmembrane protein that transports phospholipid and cholesterol out of liver to generate HDL)
Only 40 ABCA1-/- in the world, but ABCA1+/- also have low HDL
Can treat with niacin (nicotinic acid) to raise HDL ~25%
Familial hypoalphalipoproteinemia
Low HDL
Can occur as a result of mutations in various genes (apoAI)
What are normal cholesterol levels?
Total cholesterol <<200
LDL <100
HDL >60
What are major risk factors for MI/atherosclerosis?
High LDL (>160)
Low HDL (<40 in male; <50 in female)
Smoking
Hypertension (>/-140/90)
Family history of CHD (male first deg relative <55 and female <65)
Diabetes
Age (men >45; women >55)
Metabolic syndrome
Pre-diabetic state associated with high fasting TGs (>150)
What do LDL and HDL do in terms of forming atherosclerotic plaque?
LDL (bad) in blood enters endothelium of blood vessel (in intima) –> LDL is oxidized to cholesterol ester (CE) –> CE taken into macrophages via scavenger receptor, and macrophage turns into foam cell –> foam cells eventually die and release lipid into necrotic core of atherosclerotic plaque lesion on artery wall
HDL (good) in blood enters intima and prevents oxidation of LDL (is antioxidant) and unloads cholesterol out of macrophages (via ABCA1 receptor)
What are the initial clinical approaches to improving plasma lipids (and reducing MI/atherosclerosis)?
Tell patient to alter diet (lower saturated fat, cholesterol and carbs; increase polyunsaturated fat and plant sterols), alter lifestyle (no smoking, increase exercise, lose weight if obese)
However, this doesn’t usually help much–diet and exercise only change LDL 5-10%
If diet/exercise don’t help much, what are next steps to aimprove plasma lipids (and reduce MI/atherosclerosis)?
Statins: increase hepatic LDL receptor so lower plasma LDL
Ezetimibe/Zetia: inhibit NPC1L1 in intestinal enterocytes to impair cholesterol absorption (recent concerns with this drug though, not used much)
Bile acid sequestrants: resin that binds bile acids and flushes them out through intestines; somehow results in increased hepatic LDL receptor (give you gas, diarrhea, taste bad so compliance issue)
Niacin: (nicotinic acid) raises HDL, reduces VLDL and LDL (but has side effects like flushing for males)
Fibrates: (gemfibrozil) lower VLDL and chylomicrons to lower TGs (these are plasma TG-rich lipoproteins)
Aspirin: anti thrombolitic reduces blood clotting
Nutritional aspects of developing coronary heart disease and stroke
Diet can raise LDL, raise BP, promote thrombogenesis, cause insulin resistance and Type 2 Diabetes
Three things in diet that are bad:
1) High intake of cholesterol
2) High intake of saturated or trans fatty acids
3) Excessive calorie intake
SREBP2
Transcription factor that regulates transcription of LDL receptor (good for getting low cholesterol) and HMG-CoA reductase (bad for getting low cholesterol) genes
SREBP2 processing/nuclear targeting is regulated by cholesterol
SREBP2 in ER, but proteases cleave and release end fragment which goes into the nucleus and drives expression of LDL receptor and HMGCR
Low cholesterol increases cleavage (activity) of SREBP2; high cholesterol decreases cleavage of SREBP2
What are the surgical treatments for atherosclerosis/MI?
Bypass surgery
Angioplasty (balloon) with or without stent
After surgery, patients put on statins, aspirin, beta blockers, ACE inhibitors (and maybe niacin if HDL is too low)
How do high carb diets lead to hypertriglyceridemia?
Carb can be converted to fatty acid by liver (via increase of pyruvate and acetyl CoA) and adipose tissue and leads to increased VLDL synthesis/secretion from the liver
What does the liver do in terms of lipids in the fed state?
1) Synthesizes fatty acids (glucose –> acetyl CoA –> FAs)
2) Takes up TGs from diet in cylomicron remnants
3) Synthesizes TGs from FAs and secretes VLDL
Overall: dietary lipids come in, go to liver as chylomicron remnant then liver spits it out as VLDL and that goes to give FAs to adipose tissue?
What does adipose tissue do in terms of lipids in the fed state?
1) Synthesizes FAs from glucose
2) Takes up FAs after hydrolysis of chylomicron TGs by LPL/apoCII
3) Synthesizes TGs from FAs and stores TGs as liquid droplets
Acetyl CoA Carboxylase (ACC)
Converts acetyl CoA to malonyl CoA (first step in liver in doing fatty acid synthesis)
Note: liver needs to synthesize FAs/TGs in order to make VLDLs to secrete
How is ACC regulated?
1) Allosteric: Inactive dimer is activated by citrate to make active ACC polymer; long chain acyl CoA in liver (from high fat diet) inhibits ACC polymer
2) Hormone: Insulin dephosphorylates inactive ACC to active ACC (glucagon phosphorylates active ACC to inactive ACC)
Remember, ACC increases synthesis of fatty acid, TGs, VLDLs –> LPL releases FAs from VLDL and gives FAs to adipose tissue (in adipose tissue they’re re-formed into TGs)
Malonyl CoA
Important in synthesizing fatty acids from acetyl CoA (add 2 carbon units until palmatate (C16) to create TGs and then VLDLs)
Requires fatty acid synthase (FAS)
Is activated by ACC (ACC turns acetyl CoA –> malonyl CoA)
(in liver)
High carb diet vs. high fat diet effects on fatty acid synthesis in the LIVER
High carb diet –> higher insulin levels –> increases expresion of ACC and FAS –> increases synthesis of FAs, TGs, VLDL secretion –> LPL releases FAs to adipose tissue
High fat diet –> lots of long chain acyl CoA in liver –> inhibits polymerization (activation) of ACC so you don’t synthesize fatty acids (makes sense because you ate a lot of them so don’t need to synthesize them)
How does TG accumulate in adipose tissue during the absorptive state?
Increased FFA/TG from:
1) Chylomicron breakdown by LDL
2) VLDL breakdown by LDL
3) Glucose uptake (stimulated by insulin) –> –> FA synthesis –> VLDL secretion
What does the liver do in terms of lipids in the fasting state?
In fasting state, liver decreases FA synthesis and increases FA oxidation
What does adipose tissue do in terms of lipids in the fasting state?
No dietary lipid or carbohydrate means insulin low, glucagon and epi high
High glucagon and epi activate lipases in adipocytes that hydrolyze TGs and release FFA into blood for use by muscle and liver (they need this because there’s no food coming in, gotta use stores!)
Why do you have so many FFAs in diabetes?
Diabetes is like a fasting state because low insulin and high glucagon and epi
Tons of TG hydrolysis into FFA and monoglyceride in adipose tissue, and FFAs released into blood
What is the molecular mechanism of glucagon and epi causing TG –> FFA in adipocytes?
Epi and glucagon bind cell surface receptors on fat cells –> adenylate cyclase –> increased cAMP –> 3 lipases hydrolyze TG to FFA:
1) Adipocyte triglyceride lipase (ATGL)
2) Hormone sensitive lipase (HSL)
3) Monoglyceride lipase (MGL)
What does a low calorie diet (or fasting) do to fatty acid synthesis in the liver?
Low insulin, high glucagon/epi
ACC is inactive (phosphorylated) so decreased FA synthesis
Decreased TG synthesis as a result of decreased FA synthesis
Increased hydrolysis of TGs in adipocytes, releasing FFA to blood/muscle/liver for energy!
You’re not SYNTHESIZING FAs to release them, you’re BREAKING DOWN TGs to create/release FFAs
What cellular organelles are involved in degradation of lipids?
Mitochondria: beta oxidation (degradation) of fatty acids/energy generation
Peroxisomes: oxidation of very long chain fatty acids; bile acid synthesis; metabolism of H2O2 by catalase
Lysosomes: degrade lipids, proteins, glycoproteins, sphingolipids
How do fatty acids travel through the blood?
Attached to albumin
What happens in someone with diabetes in terms of FAs and beta oxidation?
In diabetes, FFA levels are high (TG hydrolysis in adipose tissue) so lots of FA going to LIVER –> mitochondria in liver do beta oxidation of these FFAs, which creates a lot of ketone bodies (from fatty acyl CoA)
Remember, ketone bodies are ONLY produced in the liver!
What are the ketone bodies that are released from the liver?
Acetoacetate and 3-hydroxybutyrate (beta-hydroxybutyrate)
Note: acetoacetate turns into acetone in the blood and this is what we smell as fruity breath (?)
What happens to the ketone bodies that are released by the liver?
Brain can use ketone bodies (but not FFAs) for energy
Muscle also uses ketone bodies for energy
Why are ketone bodies produced in both diabetes and in starvation?
In starvation, glycogen stores depleted, causes high rate of beta oxidation of FAs in liver; also in starvation, no insulin and high glucagon/epi
In diabetes, have no insulin but high glucagon/epi which causes lots of TG hydrolysis, lots of FAs in blood, lots of FAs in liver to be beta oxidized
Lysosomal storage diseases
Approximately 40 diseases that result from defect in degradation of lipids, LDL, etc and resulting accumulation of substrates and disruption of cell function
All are recessive Mendelian inheritance
1 out of 5,000 live births
Disease may APPEAR tissue-sepcific becase substrate of enzyme very important to a certain tissue (Ex: Tay-Sachs patients can’t degrade GM2 which is very important to degrade in the brain bc its produced so rapidly)
Significance of the fact that lysosomal storage diseases inherited in recessive Mendelian fashion
Enzyme deficiency present in all cells and tissues (except mature RBCs), so can diagnose using fibroblasts, body fluids, leukocytes, etc
Most of these diseases are genetically heterogeneous, but some mutations may be more commin in some geographic areas or ethnic groups
Note: most have severe, intermediate, and mild forms of disease because it’s not always inactivation of the affected gene–could just decrease the activity
Wolman’s disease
Inactive cholesterol esterase
CE accumulates in lysosomes (cannot be hydrolyzed to cholesterol and FA like normal)
Niemann-Pick C
Mutated cholesterol exporter on lysosomes
Cholesterol accumulates in lysosomes
Usually affects children but can be any time from infancy to adulthood
Causes neurological disorders/death
Note: this single gene disorder causes a ton of diff symptoms (distonia, enlarged spleen and liver, tremors, etc etc)
How do lysosomal enzymes usually get to the lysosome?
Proteins are synthesized in the ER, transported to golgi –> in golgi, have enzyme called PGlcNAc which adds a complex onto the protein –> cleavage of this added part leaves mannose 6-phosphate (M6P) on protein –> there are M6P receptors in the golgi that bind these proteins and take them to pre-lysosome then to lysosome
Some M6P-tagged proteins get out of golgi and into blood, but not many normally
How can we treat lysosomal storage diseases?
For most diseases, no treatment, but for $100,000 per year, can give exogenous recombinant enzyme with M6P artifically added –> enzyme attaches to M6P receptors on cell surface (all cells have some M6P receptor) and gets into affected cell
I-cell Disease
Defect in PGlcNAc transferase
Loss of M6P lysosomal targeting signal, so lysosomal proteins can’t get to lysosome and are instead secreted into blood (and urine); also get buildup of these enzymes’ substrates in the lysosome (nothing there to convert them!)
Get “inclusions” which are abnormal cells with all the substrates (junk) in all the lysosomes
However, note that these enzymes are NOT active in the blood because they need a very low pH to operate
Why can’t normal fibroblasts take up enzymes secreted by I-cell fibroblasts?
I-cell fibroblasts secrete enzymes with no M6P tag on them!
So even though all cells have M6P receptors, if the enzyme has no M6P, it can’t be picked up by cells
Note: Fibroblasts (CELLS) from I cell disease can take up enzymes secreted by normal fibroblasts (or, recombinant enzymes, as long as they have M6P tag on them)
Tay-Sachs
Lysosomal storage disease that used to be most prevalent in Ashkenazi Jews, but now they know to screen for disease so has decreased
Can be a number of mutations that lead to inability to degrade GM2 so can’t do PCR screen
Ganglioside GM2 (glycosphingolipid) accumulates in neurons because baby makes GM2 at really high rate in brain and can’t degrade it
Causes neurodegeneration, blindness, muscular weakness, seizures, cherry red macula, and early death
Note: considered specifically a mutation in hexA gene (hexB would cause Sandhoff’s disease–different, more broad, visceral symptoms)
Enzyme HexA
HexA is the enzyme needed to degrade GM2; enzyme defective in Tay-Sachs
Requires subunits from hexA gene (makes alpha subunit), hexB gene (makes beta subunit), and coactivator to be active
Get Tay-Sachs if you have mutation in hexA gene or hexB gene (but if mutation in hexB gene, would also get Sandhoff’s disease)
How could you get adult onset Tay-Sachs?
Partial inactivating mutation in hexA gene
Since we can’t do DNA testing (PCR) to find mutations, how do we screen for carriers of Tay-Sachs?
Remember, homozygotes die
Take blood or lymphocytes and check for Hex activity while heating
HexA is unstable (inactive) in heat, but HexB is stable (active)
So in normal person, would lose all activity of HexA in heat which would look like a huge decrease in Hex activity, carrier would only lose half of HexA activity and Tay-Sachs would lose no activity
Note: Tay-Sachs/carriers just don’t have much activity to begin with, so don’t have as much of a percent decrease
Possible treatments for lysosomal storage diseases
1) Bone marrow transplant (but doesn’t get to brain)
2) Enzyme replacement therapy (also doesn’t get to brain)
3) Gene therapy (doesn’t work that well yet)
How do you increase your HDL?
Exercise (12 miles per week)
Niacin
Mutation in cholesterol ester transfer protein (CETP) (facilitates transfer of CEs and TGs between lipoproteins, so can take a TG from LDL and give it to HDL while taking CE from HDL and giving it to LDL)
