Lipid Transport and Hormonal control

Lipid import

• Free fatty acids taken up by free fatty acid transporters directly from albumin

• Cells have membrane receptors for chylomicrons and VLDL

• Extracellular lipoprotein lipase cleaves TAG and release FA for uptake (found in the capillary, activated by apoC-2)

TAG Metabolism in Tissue

• Store little TAG and doesn't export lipids

Hormone regulation (similar to glucose catabolism)

• Epinephrine, Time of need

  • Stimulates catabolism of FA and TAG

• Insulin, time of plenty

  • Suppress catabolism, promote TAG synthesis

• Intracellular lipases

  • Cleave TAG to release FA

Dietary Lipid absorption

1. Bile salts from the lipid emulsify dietary fats in the small intestine to form mixed micelles

   1. Breaks large globules into smaller droplets for enzymes to act on

2. Pancreatic lipase in the intestinal lumen cleave TAGs into fatty acids and monoacylglycerols

3. Enterocytes in the intestinal mucosa reassembles fatty acids and glycerol into TAG and packaged into chylomicrons, exported by the lymph system, bypasses hepatic portal vein

Free fatty acid transport

• Free fatty acid (non-esterified) is bound to serum albumin most abundant plasma protein)

• Inside cells: fatty acid-binding proteins transport FAs

Lipoprotein TAG delivery into the cell

• Lipoprotein transports FA esterified as TAGs

• TAGs in lipoprotein cannot cross membranes directly

• Lipoprotein lipase (LPL) cleaves extracellular TAGs, releasing FAs for uptake via FA transporters

Lipases enables mobilization/break down lipid esters

Lipase will cleave a carboxyl and it’s R group at each step.

PIP2 is cleaved by phospholipase C to cleave off the ring and phosphate group

Tissue roles

• Muscles (most tissue but neurons)

  • Use FAs for energy by beta-oxidation

• Adipocytes

  • Store FAs as TAGs (import by LPL, no albumin-FFA uptake)

• Liver

  • Central hub to import FAs/lipoproteins and exports VLDL

Lipoprotein particle Overview

• Hydrophobic core of TAG or cholesteryl esters

• Surround by amphiphilic coat of phospholipids and apolipoprotein

• Key lipoproteins are secreted for delivery to tissues

  • Chylomicrons from small intestine

  • VLDL from liver

• Either processed at the surface of recipient cells or taken up by endocytosis for processing in endosome

Lipoproteins

• Specialized macromolecular complexes that transport hydrophobic lipids (triglycerides, cholesterol, and fat-soluble vitamins through the lymphatic system and bloodstream

• Facilitates lipid absorption from diet, endogenous lipid redistribution and cholesterol transport

• Enables lipid homeostatsis and supply energy to tissue

Lipoprotein structure

• Core hydrophobic core (TAG/cholesterol esters)

• Monolayer amphipathic of phospholipids, cholesterol and embedded apolipoproteins

  • Apolipoproteins serve as structural components, enzyme regulatory and receptors recognition signals

Lipoprotein is classified be density

• Chylomicrons<VLDL<LDL<HDL

• Reflect lipid to protein ratio and roles

• Size is inversely proportional to their density

Apolipoprotein

• Different lipoproteins have different apolipoproteins as it is the navigation system

• Tells the particle what enzyme to activate, where to dock, when to deliver or pick up

Chylomicrons: Massive movers of meal-time lipids

• Largest and least dense, specialized for transport of dietary lipids from intestine to peripheral tissues

• Formed in enterocytes of the jejunum and released into the lymphatic system, comprised of mostly triglycerides with some fat-soluble vitamins and cholesterol esters

Surface apolipoprotein

• ApoB-48 and ApoA-1 provides structural stability and support chylomicron assembly

• ApoC-2, acquired from HDL, activates lipoprotein lipase (LPL) at peripheral tissues to drive triglycerides hydrolysis

• ApoE obtained from HDL serves as the ligand for hepatic receptors mediating chylomicron-remnant uptake

• Through LPL hydrolysis of TAGs in muscle and adipose, chylomicrons supply post-meal energy and lipid storage

• Cholesterol-rich remnants are cleared by ApoE-dependent hepatic receptors

Chylomicrons: Delivering dietary lipids and recycling remnants

• It reach muscle, heart and adipose tissue by binding capillary heparan sulfate proteoglycans (HSPGs, sulfated glycoproteins on the endothelial surface) by ApoB-48 and ApoE  positioning them near endothelial lipoprotein lipase

• Binding positions the particle where LPL is anchored on the endothelial surface, ApoC-2 activates LPL and initiates TAG hydrolysis

• LPL release free Fas for uptake and use by nearby tissue, particle shrinks into a cholesterol-enriched remnant

• Chylomicron remnants

  • Retains ApoB-48 and ApoE, ApoE binds hepatic receptors enabling receptor-mediated endocytosis

  • Remnants are lysosomal degraded and release cholesterol, phospholipids and TAGs

  • Cholesterol is repackaged into VLDL stored or converted to bile acids, linking dietary lipid absorption ot systemic lipid homeostasis

• 90% TAGs

VLDL: Transport TAGs to peripheral tissue

• Newly synthesized/excess dietary TAGs/cholesterol returned to liver by chylomicron remnants are packaged into VLDL

• Transport and redistribute lipids to peripheral tissues when it isn't needed for energy or biosynthesis of liver

• Operates continuously to redistribute lipids as need

• Composition

  • Made of TAGs, cholesterol, phospholipids

  • ApoB100 is the structural backbone for VLDL assembly and secretion

  • ApoC-2 activates LPL to hydrolyze TAGs in peripheral tissues

  • ApoE mediates remnant re-uptake by the liver

• Assembly

  • Takes place in the hepatocytes ER

  • ApoB100 is added intracellular

  • ApoC-2 and ApoE join later in circulation

• Leaner in TAGs and richer in cholesterol esters

  • Dual role in cholesterol transports

  • 55-65% TAGs and 10-20% cholesterol esters

VLDL production reflects dietary intake

• Liver packages fat and excess carbohydrate into VLDLs

• Carbs is first store as glycogen but once glycogen is full, surplus is converted into fatty acids and stored as TAGs

LDL: Primary cholesterol carrier

• Retain ApoB-100 as the only apolipoprotein, the same one from the start of VLDL

• Cholesterol rich from both dietary and synthesiszed, cholesterol esters is 40% of hydrophbic core

• Function

  • Major cholesterol carrier in the blood

  • Delivers cholesterol to extrahepatic tissues for membrane synthesis, steroid production and other functions

• Uptake

  • ApoB-100 mediated LDL receptor uptake for excess LDL to return to the liver

  • Receptor mediated endocytosis for recycling or conversion to bile acids

• Elevated LDL is associated with atherosclerosis, taken up by macrophages to form foam cells = bad cholesterol

HDL Biology: Formation, maturation, protective

Structure and Composition

• High-density lipoproteins (HDLs) are the densest and most protein-rich lipoprotein

• Contains 40-55% protein mostly ApoA-1 were relatively little TAG and a small core of cholesterol esters that expands

• Begin as small, disc-shaped, lipid-deficient particles

• Ideal for scavenging excess cholesterol, HDL's sterling reputation as good cholesterol

Formation and maturation

• HDL synthesis begins in the liver as nascent, protein-rich particles with a half-life of several days

• As it progressively accumulate free cholesterol from peripheral cells, macrophages (foam cells), chylomicron remnants and VLDL remnants

• The surface of lecithin-cholesterol acyl transferase (LCAT) converts free cholesterol and phosphatidylcholine into cholesteryl esters

• Esters migrate to HDL core, transforms disc-shaped HDL into the spherical HDL particles characteristics of the mature form

Enterohepatic circulation and cholesterol recycling

Process of reusing cholesterol, circulation of cholesterol > bile salts, recycle bile salts without having to make new bile

• Bile is Produced in liver from cholesterol, stored in gallbladder and released into duodenum of small intestine during digestion

• In ileum of small intestine, most bile acids are reabsorbed into hepatic portal blood and transported back to liver

• Allows recycled cholesterol to be converted to bile salts, facilitates fat digestion and absorption while conserving cholesterl through enterohepatic circulation

Intrahepatic duct pathway

• Bile is transported by intrahepatic ductules that merge into right/left ducts

• They join to form the common hepatic duct, connects to cystic duct form gallbladder to form common bile duct

Exogenous cholesterol pathway

• It reach muscle, heart and adipose tissue by binding capillary heparan sulfate proteoglycans (HSPGs, sulfated glycoproteins on the endothelial surface) by ApoB-48 and ApoE  positioning them near endothelial lipoprotein lipase

• Binding positions the particle where LPL is anchored on the endothelial surface, ApoC-2 activates LPL and initiates TAG hydrolysis

• LPL release free Fas for uptake and use by nearby tissue, particle shrinks into a cholesterol-enriched remnant

• Chylomicron remnants

  • Retains ApoB-48 and ApoE, ApoE binds hepatic receptors enabling receptor-mediated endocytosis

  • Remnants are lysosomal degraded and release cholesterol, phospholipids and TAGs

  • Cholesterol is repackaged into VLDL stored or converted to bile acids, linking dietary lipid absorption ot systemic lipid homeostasis

• 90% TAGs

Endogenous Cholesterol Pathway

VLDLs shed Tags in circulation to become IDLs then processed into LDLs for cholesterol delivery

• In blood, VLDL TAGs are hydrolyzed by LPL in adipose, muscle and adrenal galnds, release Fas for energy or storage

  • LPL is activated by ApoC-2

• As TAGs are removed, VLDL shrinks into an IDL and then a LDL as more TAGs and Apolipoproteins are lost

• Smaller size allows for easy penetration to peripheral tissues or return it to liver for recycling for endogenous cholesterol pathway

Reverse cholesterol Transport

• HDL mediates reverse cholesterol transport by removing excess cholesterol from plaque macrophages to the liver for excretion or bile acid synthesis

• Reduces atherogenic burden

• Foam cells are macrophages that have engulfed oxidized LDL in the arterial wall becomes packed with cholesterol esters and lipids

• Accumulation marks the early stages of atherosclerotic plaque formation in arteries

• HDL's role

  • Interacts with foam cell transporters to remove cholesterol and phospholipids, carrying it back to the liver for excretion or bile acid conversion and reducing foam cell lipid load

• Outcome

  • Foam cells are de-foam becoming less lipid laden and reduces inflammatory and plaque promoting behavior

LDL is considered bad because of the associated risk with atherosclerosis

• LDL is taken up by macrophages to turn to foam cells

• Modified/oxidized LDL taken by macrophage > foam cells

• Foam cells accumulate in arterial walls >plaque formation (early atherosclerosis)

HDL: Good cholesterol

• Removes excess cholesterol by reverse cholesterol transport, reducing plaque burden

• Protective roles

  • Scavenges cholesterol from tissues, foam cells and remnants

  • Defoams macrophage to decrease inflammation/plaque growth

Foam formation

• LDL is taken up by macrophages to turn to foam cells

• Modified/oxidized LDL taken by macrophage > foam cells

• Foam cells accumulate in arterial walls >plaque formation (early atherosclerosis)

Cholesterol ester transfer protein: shuffling cholesterol and shaping atherosclerotic risk

• Cholesterol ester transfer protein CETP is a plasma protein that moves neutral lipids (cholesteryl esters and TAGs) between lipoproteins, shifting cholesterol from HDL to VLDL and produce cholesterol-rich LDL

• VLDL drives LDL formation and the distribution of cholesterol among lipoproteins is a key determinant of atherosclerosis risk

Mice experiment

• Lack CETP

• When given CETP and fed high fat, high cholesterol diets, maintain similar cholesterol levels but HDL decrease and LDL rises, results in accelerated atherosclerotic lesion area

Conclusion about CETP

• Cholesterol distribution critically influences cardiovascular risk rather than total plasma cholesterol

• Favoring HDL relative to LDL is protective

• Inhibition of CETP  didn't demonstrates benefit in heart disease

LDL receptor-mediated endocytosis

• Occurs in all cells especially liver, adrenal and steroidogenic tissues, they need regulated supply of cholesterol

• LDL bind to receptor by ApoB-100, it becomes internalized by Cathrin coated

• Delivered to endosomes and then to lysosomes where cholesteryl ester are hydrolyzed to free cholesterol

• LDL receptor is recycled to cell surface in vesicles

• Defects in LDL receptors = high LDL levels and familial hypercholesterolemia

Hormonal control of adipocyte FA metabolism

• Adipocyte cells normally responds to insulin by

  • Suppressing FA mobilization by inhibiting hormone-sensitive lipase (HSL)

    • Stops TAG from breaking down into FFA to leave

  • Enhancing FA extraction from VLDL by stimulating lipoprotein lipase (LPL)

  • Enhancing glucose import (GLUT) and FA synthesis by acetyl-CoA carboxylase (ACC)

    • Stimulates FFA synthesis to make into TAG

• Glucagon/epinephrine stimulates adipocytes

  • PKA activation by hormones stimulates hormone sensitive lipase to bind lipid droplets and release FAS for export

  • PKA phosphorylates perilipin, releases CGI-58 to activate lipase ATGL to mobilize fats for energy

    • Break down TAGs

Hormone Control of hepatocyte FA metabolism

Glucagon/epinephrine

• Suppresses FA synthesis and TAG storage and reduce VLDL export

  • VLDL primarily delivers lipids to adipocytes during times of plenty instead of tissues during times of need

  • Less malonyl-CoA means more active carnitine acetyl-transferase 1

Hormone Control of hepatocyte FA metabolism

Insulin

• Initially suppresses VLDL export just after a meal when glucose is abundant

• Simultaneously stimulates FA and TAG synthesis

• Newly synthesize Fas and TAG accumulate and insulin indrectly promotes VLDL export

New FA synthesis is restricted to times of plenty

• Glucagon suppresses FA synthesis, even though FAs are valuable fuels in times of need

• In adipocytes, glucagon mobilize FA from Tag while inhibiting new synthesis

• In liver glucagon mobilizes glucose form glycogen and synthesizes it form lactate and amino acids, but the carbon isn't used for synthesis

  • It prioritizes blood glucose maintenance over fat storage

Why is de novo FA synthesis reserved for times of plenty

• Has more anabolic cost than glucose synthesis

  • Lactate to glucose synthesis assembly 6C at a time, uses NTP but no net hydride consumption

  • Lactate to FA synthesis commits to assembly of 16 C at a time using lots of hydride, 1 hydride is several ATP

• FA is ketogenic carbon that can't address a glucose shortage = fatty acids can't be used to make glucose

Summary

• Liver and adipocyte work as a team

• Insulin drives hepatic FA synthesis and TAG packaging for VLDL-mediated delivery to adpocytes

• Glucagon direct liver to import and catabolize FA from adipocytes

  • OAA depletion by gluconeogenesis leads to ketogenesis

Lipid transport across 3 tissues

Import of lipid

• Times of need = myocyte transport

  • FFA and chylomicrons > ATP

• Times of Plenty

  • Adipocytes (VLDL and chylomicron > TAG store)

  • Hepatocyte (FFA and chylomicrons > VLDL)

Export lipid

• Times of need

  • Adipocytes (FFA)

• Times of plenty

  • Hepatocyte (VLDL to adipocyte)

Synthesize lipid

• Times of need

  • No FA synthesis: glucagon inhibits ACC

• Times of plenty

  • Adipocytes (Glucose > FA > TAG store)

  • Hepatocyte (Glucose > FA >TAG >VLDL)

• Catabolize lipid

  • Times of need

    • Myocyte (FA >ATP)

    • Adipocyte (TAG >FA)

  • Times of plenty

    • No FA catabolism, insulin by malonyl-COA inhibits carnitine shuttle

Adipocytes, lipid storage

• Store and mobilize FA for tissues in need

Lipid uptake

• Adipocytes import FAs liberated from chylomicrons and VLDL by LPL activity

• Do not take up free FA boudn to albumin

• All fatty acids are re-esterified into TAG for storage in cell

Lipid export

• Intracellular lipase liberate FA from stored TAG, release into bloodstream bound to albumin for delivery

Hepatocytes, energy use and lipid metabolism/synthesis

• Store little TAG in normal conditions

• Newly formed TAG is packaged with apoB-100, phospholipids and cholesterol into nascent VLDL secretion

• Constantly imports FAs - albumin bound free FAs or from chylomicron remnants -  channels into beta-oxidation, phospholipid synthesis and re-esterification into TAG

• Liver clears circulating lipoproteins by receptor-mediated endocytosis and exports HDL precursors, positions as central hub for lipid uptake, packaging and redistribution

Transport to myocytes, energy use

• It reach muscle, heart and adipose tissue by binding capillary heparan sulfate proteoglycans (HSPGs, sulfated glycoproteins on the endothelial surface) by ApoB-48 and ApoE  positioning them near endothelial lipoprotein lipase

• Binding positions the particle where LPL is anchored on the endothelial surface, ApoC-2 activates LPL and initiates TAG hydrolysis

• LPL release free Fas for uptake and use by nearby tissue, particle shrinks into a cholesterol-enriched remnant

• Chylomicron remnants

  • Retains ApoB-48 and ApoE, ApoE binds hepatic receptors enabling receptor-mediated endocytosis

  • Remnants are lysosomal degraded and release cholesterol, phospholipids and TAGs

  • Cholesterol is repackaged into VLDL stored or converted to bile acids, linking dietary lipid absorption ot systemic lipid homeostasis

• 90% TAGs