Lipid Droplets, Lipoproteins Flashcards
(27 cards)
What are the storage forms of lipids?
- Cholesterol esters (CE)
- synthesized by ACAT (enzyme) in ER membranes
- ACAT1 and ACAT2 -> convert cholesterol to cholesterol esters when cholesterol levels rise in the ER.
- highly insoluble - Triglycerides (TG)
- synthesized by DGAT1, DGAT2 in ER membranes
- DGAT (enzyme catalyzing final step in the production of triglycerides)
- DGAT1 remains in ER membrane.
- DGAT2 relocates to lipid droplets when cells are loaded with triglycerides, allowing direct incorporation into existing droplets and helping reduce ER stress.
- highly insoluble
**both must be sequestered into lipoprotein particles or lipid droplets.
** these storage forms are found within lipid droplets.
What are lipid droplets?
- cytoplasmic organelles composed of a core of triglycerides and cholesterol esters surrounded by a phospholipid monolayer.
- set of proteins known as PAT proteins are located on the surface of lipid droplets: perilipin, ADRP, Tip47
What is the formation and growth of lipid droplets?
- likely originate from ER membrane, where both DGAT and ACAT enzymes are located.
- they can grow by acquiring more lipids (TG, CE) from the ER.
- fusing with other lipid droplets.
- surrounded by a monolayer of phospholipids and associated proteins (e.g., PAT protein)
- often physically associate with mitochondria, ER, and peroxisomes, aiding lipid transfer for beta-oxidation or membrane synthesis.
How is the production of cholesterol ester regulated?
- ACAT is activated when ER cholesterol concentrations rise, converting free cholesterol into cholesterol esters.
- Cholesterol esters can be stored in lipid droplets or packed into lipoproteins (e.g., in hepatocytes or enterocytes).
- Low ER cholesterol triggers cholesterol synthesis and uptake, while high levels promote storage—helping maintain stable ER cholesterol levels.
How are lipid droplets distributed among cells?
- Adipocytes (fat cells).
primary site of lipid storage
single large lipid droplet
long term energy storage - Hepatocytes (Liver Cells)
multiple smaller lipid droplets
involved in lipid metabolism and lipoprotein synthesis
can accumulate excess lipids in conditions like fatty liver disease - Mammary Gland Epithelial Cells
many lipid droplets
contribute to milk fat production - Macrophages/ Foam Cells
macrophages can uptake large amount of lipids leading to formation of foam cells - involved in the formation of atherosclerotic plaques.
How are lipid droplets visualized?
- lipid droplets are relatively easy to isolate.
1. lyse cells
2. pellet debris
3. float LDs on a sucrose gradient
however, LDs often remain attached to organelles like the ER and mitochondria, leading to contamination.
What are analytical methods to visualize lipid droplets?
- Thin Layer Chromatography
- Protein analysis
- LDs are dissolved in organic solvents
- proteins can be extracted for western blotting, proteomics, or other biochemical techniques.
What are PAT proteins?
- examples: ADRP, Perilipin
- Can be phosphorylated by PKA (Protein Kinase A)
function:
- Act as a scaffold on the surface of lipid droplets.
- Control access of lipases to the droplet core.
- Provide structural stability to the LD.
how is HSL activated? (hormone-sensitive lipase)
- β-adrenergic receptor activation → Gs protein → cAMP → PKA
- PKA phosphorylates HSL and PAT proteins
What is the function of phosphorylates HSL?
- translocates to lipid droplets
- binds to phosphorylated PAT proteins
- Hydrolyzes triglycerides to release free fatty acids (FFA).
- Has minor cholesterol esterase activity.
What is the function of ATGL (adipose triglyceride lipase)?
- binds to PAT proteins
- Initiates lipolysis by converting triglycerides into diglycerides.
What is the fate of released fatty acids and cholesterol?
- Fatty Acids:
- Sent to mitochondria or peroxisomes for β-oxidation
- Transferred to ER for phospholipid synthesis - Cholesterol:
- Retrieved possibly by HSL or other neutral lipases.
- May require autophagy in certain cells (e.g., macrophages).
What is lipophagy?
A type of autophagy where lipid droplets are engulfed and degraded in lysosomes.
What is the process of lipophagy?
- Lipid droplets are targeted by autophagosomes.
- Delivered to lysosomes for degradation.
- Lysosomal lipases (e.g., lysosomal acid lipase - LAL) hydrolyze triglycerides and cholesterol esters.
- Releases free fatty acids and cholesterol.
What experimental tools are used to study lipophagy?
- Block lysosomal degradation: Use weak bases to neutralize lysosomal pH.
- Block autophagy: Use inhibitors like 3-methyladenine or siRNA knockdown of autophagy-related proteins.
What are lipoprotein particles?
- particles that transport lipids in the bloodstream.
Chylomicrons (lipoprotein particle)
- formed in intestinal enterocytes after a fat-containing meal.
- main role is to transport dietary triglycerides and cholesterol from the intestines to peripheral tissues (muscles, adipose)
- contains ApoB-48
Lifecycle:
- Nascent chylomicrons receive ApoC-II and ApoE from HDL in circulation.
- ApoC-II activates Lipoprotein Lipase (LPL) to release fatty acids to tissues.
- Remnants return to liver for uptake via ApoE recognition.
VLDL (Very Low-Density Lipoprotein)
- formed in the liver.
- transports endogenously synthesized triglycerides to peripheral tissues.
- contains ApoB-100.
Lifecycle:
- receives ApoC-II and ApoE from HDL.
- LPL hydrolyzes triglycerides → becomes IDL, then LDL.
IDL (Intermediate-Density Lipoprotein)
- formed from VLDL after triglyceride loss.
- Transitional lipoprotein; some IDL is taken up by the liver, the rest is converted to LDL.
- Still carries ApoB-100 and ApoE.
LDL (Low-Density Lipoprotein)
- formed from IDL after further triglyceride loss.
- Delivers cholesterol to cells via LDL receptors.
- ApoB-100 (required for receptor binding).
- High LDL = “bad cholesterol,” linked to atherosclerosis.
HDL (High-Density Lipoprotein)
- formed in liver and intestines
- main role - reverse cholesterol transport - collects excess cholesterol from tissues and returns it to the liver.
- ApoA-1
- ABCA1 transporter helps HDL collect cholesterol from cells.
- LCAT (lecithin-cholesterol acyltransferase) esterifies cholesterol, trapping it in the core of HDL.
What are the key types of apolipoproteins?
- ApoA-I
Found on HDL.
Activates LCAT.
Required for reverse cholesterol transport. - ApoB-48
Found only on chylomicrons.
Synthesized in the intestine via mRNA editing.
Lacks the LDL receptor binding domain (unlike ApoB-100). - ApoB-100
Found on VLDL, IDL, LDL.
Synthesized in the liver.
Required for LDL receptor binding. - ApoE
Found on chylomicron remnants and IDL.
Required for remnant clearance by liver via LDL receptor–related proteins. - ApoC-II
Donated by HDL to chylomicrons/VLDL.
Activates Lipoprotein Lipase (LPL).
What is a genetic disease of the LDL receptor?
- Familial Hypercholesterolemia (FH)
- Mutations in the LDL receptor gene (LDLR) or ApoB-100.
- Reduced LDL uptake by cells → elevated plasma LDL.
- Autosomal dominant.
- Early-onset atherosclerosis, tendon xanthomas, high LDL cholesterol levels.
What is the function of the ABCA1 transporter?
- Facilitates cholesterol efflux from cells to nascent HDL.
- required for the initial formation of HDL particles.
- mutation causes Tangier disease -> very low HDL levels, cholesterol accumulation in tissues.
