Adipocytes Flashcards
two types of adipocytes:
brown adipose tissue (BAT)
white adipose tissue (WAT)
How does WAT store energy?
As synthesized triglyceride in the form of lipid droplets in its cytoplasm
Where does WAT’s FA come from?
- from chylomicrons and VLDLs
- must hydrolyze FA from TG in the chylomicron or VLDL to absorb it
Why does WAT need FA from chylomicrons and VLDL?
Human WAT can’t synthesize FA
How does glucose get into WAT?
GLUT 4
How does WAT make TG?
WAT makes glycerol-3-phosphate from glucose for TG synthesis
How does WAT release FA from TG?
- adipocyte triglyceride lipase (ATGL)
- hormone sensitive lipase (HSL)
- monoglyceride lipase
- perilipin
4 ways insulin increases FA storage in WAT
1) increasing LPL production in WAT
2) increasing amound of GLUT4 on WAT membrane
3) decreasing ATGL/HSL activity
4) decreasing perilipin phosphorylation, which decreases HSL access to lipid droplet
What cells express GLUT4?
adipocyte and muscle only
What regulates GLUT4?
insulin
How does insulin regulate GLUT4?
- regulates AMOUNT of GLUT4 but not activity of the channel
- causes GLUT4 containing vesicles to fuse with plasma membrane
why do adipocytes take up glucose?
1) use as fuel
2) make glycerol for TG synthesis
oxidative capacity of adipocytes?
BAT - high - lots of mitochondria
WAT - low
lipid droplets in adipocytes?
BAT - many small droplets
WAT - one large droplet
main jobs of two types of adipocytes?
BAT - fatty acid oxidation “uncoupled” from ATP production due to high expression of uncoupling protein (UCP). Produces heat rather than ATP
WAT - storage and release of FA into circulation for energy
new evidence for BAT?
May be important in adults, amount of BAT is inversely correlated with obesity
new evidence for WAT?
belly fat causes more risk of obesity-associated problems such as diabetes type II than peripheral fat (legs, butt, arms)
Two ways adipocytes receive FA
1) from chylomicrons - from intestines (diet)
2) VLDL - from liver (synthesized from excess glucose)
FA uptake is governed by concentration gradient of FA, created by:
- LPL activity creates “source” outside cell
- TG synthesis inside the cell creates “sink”
4 steps of TG synthesis inside adipocyte
1) fatty acyl CoA ligase converts FA to acyl-CoA
2) glucose entry via GLUT4
3) production of glycerol-3-phosphate from glucose (glucose to DHAP, DHAP to G3P)
4) coupling of fatty acyl-CoA with G3P
3 steps in coupling of fatty acyl-CoA to glycerol-3-phosphate to form TG
1) two esterifications of acyl-CoA onto G3P to form phosphatidic acid
2) dephosphorylation of phophatidic acid to diacylglycerol
3) final esterification of a third acyl-CoA onto diacylglycerol
catabolic pathways in adipocytes
- fat mobilization (lipolysis)
- mainly to supply other tissues (heart, skeletal muscle) with FA for fuel
How is adipocyte triglyceride lipase (ATGL) regulated?
Through insulin and its counter-stimulatory hormones
How is hormone sensitive lipase (HSL) regulated?
- activated by PKA-dependent phophorylation (adrenaline activates, insulin inhibits)
how is perilipin regulated?
- PKA-dependent phosphorylation causes conformational change (adrenaline activates, insulin inhibits)
what is perilipin?
protein that surrounds lipid droplet and must move out of the way to allow lipases to access TG
What is attached to FA in blood stream?
albumin
what is the fate of the glycerol from TG breakdown in adipocytes?
- leaves cell through aquaporin-7 channel
- goes to liver to get turned into glucose by gluconeogenesis
what is the major adipocyte derived hormone?
leptin
