Lipids Flashcards
how is cholesterol eliminated from the body?
humans cannot degrade cholesterol
it is eliminated through bile as
1. unmodified cholesterol
2. bile acids/salts (converted cholesterol)
bile = bile acids/slats + phospholipids + cholesterol + bilirubin
what are the major sources of liver cholesterol?
- hepatic de novo synthesis
- diet (chylomicron remnants)
- extra-hepatic tissue (HDL, LDL)
liver exports cholesterol via VLDL, bile, bile salts
what is the building block for cholesterol synthesis? where does cholesterol synthesis occur in the cell?
2 acetyl CoA (2C each) are the building blocks of cholesterol
synthesis takes place in cytosol
requires a lot of NADPH (reducing power) and ATP! - only made when energy state is high (well-fed)
what is the rate-limiting/committed step of cholesterol synthesis? what is the clinical significance of this?
HMG CoA Reductase: converts HMG CoA to mevalonic acid
statins competitively inhibits HMG CoA Reductase!
what is the mechanism of statins?
competitively inhibit HMG CoA Reductase, rate-limiting step of cholesterol synthesis
why are patients on statins sometimes told to take supplements of coenzyme Q?
statins competitively inhibit HMG CoA reductase, rate-limiting step of cholesterol synthesis
downstream in the pathway, other important products are produced, including dolichol (glycoprotein synthesis) and ubiquinone ( = CoQ, for electron transport)
therefore, statins decrease production of these as well!
how is cholesterol synthesis regulated by gene expression?
HMG CoA Reductase (rate-limiting enzyme) is regulated by SREBP2 (transcription factor - sterol responsive element binding protein)
SREBP2 is bound by SCAP (SREBP2 cleavage activating protein) in ER membrane
high cholesterol levels: SREBP2-SCAP is bound by INSIG and ubiquitated for proteolytic degradation
low cholesterol levels: SREBP2-SCAP binds COP-II and is taken to nuclear SRE (sterol response element) —> HMG CoA reductase and LDL receptor gene are transcribed
where is transcription factor SREBP2 (regulates HMG CoA reductase transcription) found in times of high and low cholesterol, respectively?
SREBP2 = sterol responsive element binding protein
high cholesterol: bound to ER membrane
low cholesterol: transported to golgi, proteolytically released from membrane, enters nucleus to bind SRE (sterol response element)
how is cholesterol synthesis regulated by HMG CoA reductase degradation?
when there is high cholesterol, presence of sterols induce binding of INSIG (inhibitory protein binding SREBP2) to HMG CoA reductase itself
—> HMG CoA reductase is ubiquitinated and proteasomaly degraded
therefore INSIG causes both HMGR degradation AND suppresses HMGR transcription
how is cholesterol synthesis regulated by phosphorylation/dephosphorylation?
HMG CoA reductase (rate-limiting enzyme of cholesterol synthesis) is phosphorylated by AMP-activated kinase (AMPK) —> enzyme inactivated
AMP is allosteric regulator for AMPK
this makes sense because cholesterol synthesis requires a lot of NADPH and ATP, so you don’t want to make it when energy is low
how is cholesterol synthesis regulated by hormones?
insulin (well-fed/high energy) promotes DEphosphorylation of HMG CoA Reductase (rate-limiting) —> active enzyme
glucagon (starved/low energy) promotes phosphorylation of HMGR —> INactive enzyme
what are the 4 levels of regulation of de novo cholesterol synthesis in the body? bonus if you can name a pharmacological method
- regulated gene expression of HMG CoA reductase (HMGR)
- degradation of HMGR
- HMGR phosphorylation (OFF) / dephosphorylation (ON)
- hormonal (insulin vs glucagon)
- statins: competitively inhibit HMGR
what is found within a lipoprotein particle (the core)?
polar monolayer surface (amphipathic phospholipids, cholesterol)
non-polar core of triacylgylcerols (TAG) and cholesteryl esters (CE)
put these is order from smallest to largest: VLDL, LDL, chylomicron, HDL
HDL (high density) - mostly protein
LDL (low density) - cholesterol
VLDL (very low density) - TAG
chylomicron (simply massive) - largest lipid content (TAG)
as lipoproteins get progressively bigger, there is less protein and more lipid
therefore in ultracentrifugation, HDL would be at the bottom (most dense! duh!)
what is the origin and function of each type of lipoprotein:
a. HDL
b. LDL
c. VLDL
d. chylomicrons
a. HDL (liver, intestine): return excess cholesterol to liver
b. LDL (liver): distribute cholesterol from liver
c. VLDL (liver): distribute TAG from liver
d. chylomicrons (small intestine): distribute dietary TAG
[TAG = triacylglycerols]
where are chylomicrons assembled, and what is their function?
assembled in enterocytes (intestinal mucosal cells)
carry TAGs, cholesterol, cholesteryl esters absorbed from diet
important for exogenous (dietary) lipid metabolism
how are cholesterol and other sterols imported into enterocytes and incorporated into chylomicrons?
- NPC1L1 imports cholesterol and other sterols into enterocytes
- MTTP (microsomal triglyceride transfer protein) chaperones sterols into chylomicrons
- chylomicrons leave via lymphatics
ABCG5/G8 pumps “xeno”sterols (from plants, etc) out of enterocytes - we don’t keep them
what is the mechanism and use of Ezetimibe?
Ezetimibe: inhibits NPC1L1 (sterol transporter in enterocytes)
used to treat high plasma cholesterol levels, mainly in conjunction with statins (HMGR inhibitors)
chylomicrons (assembled in enterocytes) transport lipid-soluble vitamins, which are:
DAKE
vit D, A, K, E
the main apoprotein associated with chylomicrons is
apoB48
fill in the blank for the metabolism of chylomicrons (CM):
1. CM are produced in ______ carrying Apo__ and transporting TAGs
2. in plasma, ___ transfers ApoE and ApoC-II to CM
3. ApoC-II activates __________, which is attached to tissue surfaces and stimulated by insulin
4. TAGs are hydrolyzed and ApoC-II returns to HDL
5. CM particles shrink and become _____, which bind to ____ receptors in liver
- CM are produced in ENTEROCYTES carrying ApoB48 and transporting TAGs
- in plasma, HDL transfers ApoE and ApoC-II to CM
- ApoC-II activates LIPOPROTEIN LIPASE, which is attached to tissue surfaces and stimulated by insulin
- TAGs are hydrolyzed and ApoC-II returns to HDL
- CM particles shrink and become CHYLOMICRON REMNANTS, which bind to ApoE receptors in liver
—> hydrolytically degraded in lysosomes (this is how liver receives dietary cholesterol)
what is the respective function of the following apoproteins in chylomicron metabolism?
a. ApoB48
b. ApoC-II
c. ApoE
a. ApoB48: main apoprotein associated with chylomicrons
b. ApoC-II: activates lipoprotein lipase (LPL), which hydrolyzes TAGs from chylomicron
c. ApoE: major apoprotein of remnant lipoproteins, binds LDL receptors in hepatocytes to initiate hydrolytic degradation in lysosomes
what is ApoB48 (main apoprotein associated with chylomicrons) derived from?
ApoB100 mRNA undergoes RNA editing to become ApoB48, which lacks LDL receptor binding domain
therefore, chylomicrons are taken up by liver through ApoE interaction (with LDL receptor) instead
[no other known case of RNA editing in human physiology, idk weird]
why are blood samples taken after fasting? what would it look like if the patient had eaten?
fasting blood sample is taken so that it doesn’t contain chylomicrons, which give a “lactescent” (milky) appearance
fill in the blank regarding VLDL metabolism:
1. VLDL are assembled in _____ with apoprotein ___
2. HDL donates Apo__ and Apo___
3. ____ activates ______ and TAGS are removed
4. VLDL without TAGS become ___
5. give remaining TAGS to HDL in exchange for _____ via _____
6. with cholesterol return to liver at LDL receptor OR remodeled to ___ (mainly cholesterol, no TAGS)
- VLDL are assembled in HEPATOCYTES with apoprotein B100
- HDL donates ApoE and ApoC-II
- ApoC-II activates LIPOPROTEIN LIPASE and TAGS are removed
- VLDL without TAGS become IDL (intermediate density lipoproteins)
- IDL give remaining TAGS to HDL in exchange for CHOLESTEROL via CETP (cholesteryl ester transfer protein)
- IDL with cholesterol return to LIVER at LDL receptor OR remodeled to LDL (mainly cholesterol, no TAGS)
the main transporter of cholesterol in the body
LDL = low density lipoprotein, has long half-life
LDL receptors are mostly in liver, but also found in other tissues that take up cholesterol
what would be the consequence of a mutation in MTTP (microsomal triglyceride transfer protein)?
MTTP (chaperone protein in ER) is required for incorporation of apoB48 into chylomicrons and apoB100 into VLDL
abetalipoproteinemia (ABL): mutation in MTTP —> failure to absorb cholesterol and lipid-soluble vitamins (DAKE)
—> vitamin deficiency (DAKE), steatorrhea (fat in stool), non-alcoholic fatty liver disease, acanthocytes (RBC look spiny, like suns), hypocholesterolemia
abetalipoproteinemia (ABL) - cause and consequences?
abetalipoproteinemia (ABL): mutation in MTTP (chaperone protein in ER) is required for incorporation of apoB48 into chylomicrons and apoB100 into VLDL
—> failure to absorb cholesterol and lipid-soluble vitamins (DAKE)
—> vitamin deficiency (DAKE), steatorrhea (fat in stool), non-alcoholic fatty liver disease, acanthocytes (RBC look spiny, like suns), hypocholesterolemia
*treat with low-fat diet and vitamin supplements
which apoproteins are associated with IDL (intermediate density lipoproteins)?
removal of TAGs (triglycerides) from VLDL = IDL, enriched in cholesterol
IDL particles contain ApoB100 and ApoE
which lipoprotein is considered the “bad” cholesterol?
LDL (low density lipoprotein): carries majority of cholesterol in circulation, mainly delivers to extra-hepatic tissues, returns excess to the liver
predominant apolipoprotein associated is B100
elevated levels of LDL cholesterol and apolipoprotein B100 are strongly associated with risk of atherosclerotic CV events
how is expression of LDL receptor regulated for cholesterol clearance?
- LDL receptor (LDLR) are found in Clathrin-coated pits in membrane
- LDL binding induces endocytosis
- LDL is metabolized and LDLR recycles to the cell membrane
high cholesterol: downregulates transcription of LDLR gene AND PCSK9 (protease) inhibits LDLR recycling (causes degradation in lysosome)
low cholesterol levels: SREBP2-SCAP binds COP-II and is taken to nuclear SRE (sterol response element) —> HMG CoA reductase and LDL receptor gene are transcribed
failure of LDL to bind to its receptor results in uncontrolled synthesis of cholesterol because synthesis of which protein is not repressed?
a. ACAT
b. HMG CoA Reductase
c. ApoB100
d. lipoprotein lipase
b. HMG CoA Reductase
if LDL can’t bind its receptor, then intracellular levels of cholesterol will be low, and HMGR transcription will be increased
recall HMGR is rate-limiting enzyme in cholesterol synthesis
explain why use of statins causes an increase in the number of LDL receptors on hepatocytes?
statins competitively inhibitors HMG CoA reductase to inhibit de novo cholesterol synthesis
therefore, intracellular cholesterol synthesis is decreased, so LDL receptor transcription is increased
this actually helps, because with more LDL receptors, more cholesterol is cleared from the blood!
how would plasma levels of cholesterol look different in a patient with high levels of PCSK9 vs low levels?
PCSK9: proteasome that inhibits LDL receptor recycling to the PM by facilitating its degradation in lysosomes
someone with high levels of PCSK9 would have higher plasma levels of cholesterol because there would be less LDLR for uptake
therefore, PCSK9 inhibitors are useful in treating hypercholesterolemia
what are 3 gene mutations that commonly cause familial hypercholesterolemia (FH)?
- defects in LDL receptor synthesis/function - most common
- defects in ApoB100 (which is associated with LDL and interacts with receptor)
- increased PCSK9 (proteasome that inhibits LDLR recycling to surface)
describe the role of LDL in atherosclerosis
circulating LDL is protected from oxidation by circulating antioxidants (vit. E, vit. C, glutathione peroxidase)
but in endothelial injury, LDL attaches to intima and becomes oxidized —> Ox-LDL are phagocytized by macrophages (via scavenger receptor A) —> macrophages become foam cells which accumulate and lead to atherosclerosis
fill in the blank regarding HDL metabolism:
1. HDL is associated with Apo___ and Apo___
2. HDL takes up cholesterol from non-hepatic tissue via ___ membrane transporter
3. cholesterol is esterified by ___, which is activated by ____
4. HDL exchangers cholesterol for TAGS in VLDL/IDL via ____
5. HDL returns to liver where scavenger receptor ___ and hepatic lipase uptake cholesterol and TAGs
6. HDL re-enters circulation
- HDL is associated with ApoA-I and ApoA-II
- HDL takes up cholesterol from non-hepatic tissue via ABCA1 membrane transporter
- cholesterol is esterified by LCAT, which is activated by ApoA-I
- HDL exchangers cholesterol for TAGS in VLDL/IDL via CETP
- HDL returns to liver where scavenger receptor SR-B1 and hepatic lipase uptake cholesterol and TAGs
- HDL re-enters circulation
which lipoprotein is considered “good” cholesterol?
HDL (high-density): picks up cholesterol from peripheral tissue and returns it to liver (reverse cholesterol transport)
ABCA1 membrane transporter in non-hepatic peripheral tissues causes cholesterol efflux to HDL, which is then esterified to cholesteryl esters via LCAT, and brought back to liver through SR-B1 receptor
HDL can pick up cholesterol from foam macrophages in atherosclerosis
how is cholesterol transported from
1. diet
2. liver
3. peripheral tissues
- dietary cholesterol - packaged into chylomicrons, remnants of which are taken up by liver
- liver cholesterol (de novo or diet) - packaged into VLDL, which turns into IDL, which then becomes LDL, which is taken up by peripheral tissues and liver
- cholesterol in peripheral tissues - carried away by HDL and returned to liver
what is the function of each in cholesterol metabolism?
a. ApoA-I
b. ApoA-II
a. ApoA-I: major protein of HDL, activates LCAT (for esterification)
b. ApoA-II: primarily in HDL, activates hepatic lipase activity
hypoalphalipoproteinemia
low plasma HDL
hypo-alpha-lipoprotein-emia
recall that HDL associates with alpha apoproteins (ApoA-I, ApoA-II)
what are the 2 possible genetic causes of familial hyperchylomicronemia (Type I hyperlipidemia)?
- LPL (lipoprotein lipase) deficiency
- ApoC-II deficiency (associated with HDL)
autosomal recessive
fasting triglycerides are very high (>1000mg/dL) - blood plasma appears very milky - but cholesterol levels can be normal
what are the clinical manifestations of familial hyperchylomicronemia (Type I hyperlipidemia)? (3)
- milky fasting plasma (very high triglycerides)
- recurrent pancreatitis causing abdominal pain (chylomicrons obstruct pancreatic capillaries)
- eruptive cutaneous xanthomas (rash)
autosomal recessive but rare, early onset (infancy/childhood)
What are the possible genetic causes of familial hypercholesterolemia (FH)/ Type IIa Hyperlipidemia? (4)
most common form of inherited high cholesterol, autosomal dominant
- LDL receptor mutation (majority)
- ApoB100 mutation: reduced binding of LDL to LDL-R
- PCSK9 mutation: GOF, enhanced LDL-R degradation
- LDL adaptor protein mutation: autosomal recessive
What are the clinical manifestations of familial hypercholesterolemia (FH)/ Type IIa Hyperlipidemia?
FH: elevated LDL and serum cholesterol but normal TAGs
—> accelerated ischemic heart disease (MIs in childhood for homozygotes)
—> tendon xanthomas: lipid collects in tendons
—> xanthelasoma: lipids accumulate around eyes
—> corneal arcus: lipids accumulate around cornea
what is the cause of familial dysbetalipoproteinemia (type III hyperlipidemia)? what is the consequence of this?
ApoE deficiency: major protein of remnant lipoproteins, binds LDL receptor
—> accumulation of remnants of chylomicrons and VLDLS/IDLs
both TAGs and cholesterol levels are increased
what are the clinical manifestations of familial dysbetalipoproteinemia (type III hyperlipidemia)?
ApoE deficiency: major protein of remnant lipoproteins, binds LDL receptor —> accumulation of remnants of chylomicrons and VLDLS/IDLs, both TAGs and cholesterol levels are increased
—> palmar crease xanthomas (rash)
—> tuberous xanthomas (knees, elbows)
—> onset in adulthood
—> increased risk of atherosclerosis
what manifests from ApoA-I or LCAT deficiency?
ApoA-I: associated with HDL, activator of LCAT
LCAT: synthesized by liver, esterifies cholesterol and promotes formation of HDL
—> very low HDL count, non-esterified cholesterol accumulates and deposits in tissues
what is the cause of Tangier Disease and what manifests because of it?
Tangier disease: absence of ABCA1 (transporter for cholesterol efflux in peripheral tissues so HDL can pick it up)
—> very low HDL and ApoA-I levels
—> yellow tonsils due to cholesterol accumulation
A patient presents with yellow tonsils and very low HDL. Which of these conditions is most likely:
a. hypoalphalipoproteinemia
b. tangier disease
c. LCAT deficiency
d. familial dysbetalipoproteinemia
b. tangier disease
Tangier disease: absence of ABCA1 (transporter for cholesterol efflux in peripheral tissues so HDL can pick it up)
—> very low HDL and ApoA-I levels
—> yellow tonsils due to cholesterol accumulation
A patient presents with early onset corneal opacifications and very low HDL. Which of these conditions is most likely:
a. hypoalphalipoproteinemia
b. tangier disease
c. LCAT deficiency
d. familial dysbetalipoproteinemia
c. LCAT deficiency
LCAT: synthesized by liver, esterifies cholesterol and promotes formation of HDL
—> very low HDL count, non-esterified cholesterol accumulates and deposits in tissues
A patient presents with palmar crease xanthoma. Both TAGs and total cholesterol are elevated. Which of these conditions is most likely:
a. hypoalphalipoproteinemia
b. tangier disease
c. LCAT deficiency
d. familial dysbetalipoproteinemia
d. familial dysbetalipoproteinemia
aka Apo-E deficiency (Type III hyperlipidemia): major protein of remnant lipoproteins, binds LDL receptor —> accumulation of chylomicrons remnants, VLDLS/IDLs, TAGS, and cholesterol
A 7yo patient presents with elevated LDL and serum cholesterol, but relatively normal TAGs. Tendon xanthomas are noted on bilateral kneecaps. Which of these conditions is most likely:
a. familial hypercholesterolemia
b. tangier disease
c. LCAT deficiency
d. familial dysbetalipoproteinemia
a. familial hypercholesterolemia
FH: elevated LDL and serum cholesterol but normal TAGs
—> accelerated ischemic heart disease (MIs in childhood for homozygotes)
—> tendon xanthomas: lipid collects in tendons
—> xanthelasoma: lipids accumulate around eyes
—> corneal arcus: lipids accumulate around cornea
A 2yo patient presents with eruptive cutaneous xanthomas and pancreatitis. Blood samples taken show milky plasma, though the patient hasn’t been fed since the night before. Cholesterol levels are normal but TAGs are measured at 1200mg/dL. Which of these conditions is most likely:
a. familial hypercholesterolemia
b. tangier disease
c. familial hyperchylomicronemia
d. familial dysbetalipoproteinemia
c. familial hyperchylomicronemia (Type I hyperlipidemia): autosomal recessive but rare, early onset (infancy/childhood)
normal cholesterol but HIGH TAGs and chylomicrons
- milky fasting plasma (very high triglycerides)
- recurrent pancreatitis causing abdominal pain (chylomicrons obstruct pancreatic capillaries)
- eruptive cutaneous xanthomas (rash)
what is the cause of familial combined hyperlipidemia (FCHL)?
familial combined hyperlipidemia: increased ApoB100 synthesis in liver —> increased VLDL production, and therefore high LDL (which are derived from VLDL) - “combined lipidemia”
premature risk for atherosclerosis
what is the mechanism of Bile Acid Resins (BAR)?
Bile Acid Resins/ Sequestrants: bind bile acids in liver and prevent reabsorption —> increased bile elimination in feces, decreased plasma cholesterol
*note that dietary fiber also sequesters bile acids
Which of the following apoproteins is an activator of lipoprotein lipase?
a. ApoA-I
b. ApoB-100
c. ApoC-II
d. ApoE
c. ApoC-II: activates LPL
a. ApoA-I: major lipoprotein of HDL
b. ApoB-100: major lipoprotein of LDL, VLDL
d. ApoE: transferred by HDL to chylomicrons and VLDL
A 40yo M, BMI of 25, has a history of elevated cholesterol with normal triglycerides and HDL. FHx is significant for a father with a similar history who died of a heart attack at age 48. A potential mutation in this patient would be which of the following proteins?
a. LCAT
b. CETP
c. ABCA1
d. ApoE
e. LDL receptor
patient is presenting with familial hypercholesteremia
mutation in LDL receptor is most likely - reduces cholesterol uptake from plasma
Pt is an 8yo M presenting with orange-colored tonsils, very low HDL levels, and hepatosplenomegaly. What is the most likely diagnosis?
Tangier disease: absence of ABCA1 (transporter for cholesterol efflux in peripheral tissues so HDL can pick it up)
—> very low HDL and ApoA-I levels
—> yellow tonsils due to cholesterol accumulation
Statins are competitive inhibitors of HMG CoA reductase, which converts HMG CoA to…..
mevalonate
why might a patient take phytosterols to reduce their risk of heart disease?
phytosterols reduce absorption of dietary cholesterol
Pt is a 40yo F presenting with elevated LDL and triglycerides. She is diagnosed with type II familial hypercholesterolemia due to a mutated LDL receptor. Which of the following would result?
a. triglycerides in chylomicrons cannot be degraded
b. VLDL levels in serum increase
c. HDL level in serum increases
d. VLDL cannot be converted to IDL
e. cellular HMG CoA reductase activity is not inhibited
e. cellular HMG CoA reductase activity is not inhibited
mutation LDL receptor means cholesterol cannot be taken up into hepatocytes - hepatocytes will be driven towards more de novo cholesterol synthesis, for which HMGR is the rate-limiting enzyme
A 25yo F presents with anemia, corneal opacities, and kidney insufficiency. She is found to have a mutation in an enzyme involved in converting cholesterol to cholesterol esters. What is her diagnosis?
LCAT deficiency
A 16yo M presents with moderate to severe epigastric pain. PE reveals eruptive xanthoma and hepatosplenomegaly. Blood samples taken show milky plasma. What condition does this patient have?
hypertriglyceridemia - high levels of chylomicrons are causing plasma to appear milky, obstructing capillary beds to cause organ pain, and depositing in tissue to cause xanthoma.
