Adipocytes

Question 1

two types of adipocytes:

Answer 1

brown adipose tissue (BAT)

white adipose tissue (WAT)

Question 2

How does WAT store energy?

Answer 2

As synthesized triglyceride in the form of lipid droplets in its cytoplasm

Question 3

Where does WAT’s FA come from?

Answer 3

• from chylomicrons and VLDLs

- must hydrolyze FA from TG in the chylomicron or VLDL to absorb it

Question 4

Why does WAT need FA from chylomicrons and VLDL?

Answer 4

Human WAT can’t synthesize FA

Question 5

How does glucose get into WAT?

Answer 5

GLUT 4

Question 6

How does WAT make TG?

Answer 6

WAT makes glycerol-3-phosphate from glucose for TG synthesis

Question 7

How does WAT release FA from TG?

Answer 7

• adipocyte triglyceride lipase (ATGL)
• hormone sensitive lipase (HSL)
• monoglyceride lipase
• perilipin

Question 8

4 ways insulin increases FA storage in WAT

Answer 8

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

Question 9

What cells express GLUT4?

Answer 9

adipocyte and muscle only

Question 10

What regulates GLUT4?

Answer 10

insulin

Question 11

How does insulin regulate GLUT4?

Answer 11

• regulates AMOUNT of GLUT4 but not activity of the channel

- causes GLUT4 containing vesicles to fuse with plasma membrane

Question 12

why do adipocytes take up glucose?

Answer 12

1) use as fuel

2) make glycerol for TG synthesis

Question 13

oxidative capacity of adipocytes?

Answer 13

BAT - high - lots of mitochondria

WAT - low

Question 14

lipid droplets in adipocytes?

Answer 14

BAT - many small droplets

WAT - one large droplet

Question 15

main jobs of two types of adipocytes?

Answer 15

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

Question 16

new evidence for BAT?

Answer 16

May be important in adults, amount of BAT is inversely correlated with obesity

Question 17

new evidence for WAT?

Answer 17

belly fat causes more risk of obesity-associated problems such as diabetes type II than peripheral fat (legs, butt, arms)

Question 18

Two ways adipocytes receive FA

Answer 18

1) from chylomicrons - from intestines (diet)

2) VLDL - from liver (synthesized from excess glucose)

Question 19

FA uptake is governed by concentration gradient of FA, created by:

Answer 19

• LPL activity creates “source” outside cell

- TG synthesis inside the cell creates “sink”

Question 20

4 steps of TG synthesis inside adipocyte

Answer 20

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

Question 21

3 steps in coupling of fatty acyl-CoA to glycerol-3-phosphate to form TG

Answer 21

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

Question 22

catabolic pathways in adipocytes

Answer 22

• fat mobilization (lipolysis)

- mainly to supply other tissues (heart, skeletal muscle) with FA for fuel

Question 23

How is adipocyte triglyceride lipase (ATGL) regulated?

Answer 23

Through insulin and its counter-stimulatory hormones

Question 24

How is hormone sensitive lipase (HSL) regulated?

Answer 24

• activated by PKA-dependent phophorylation (adrenaline activates, insulin inhibits)

Question 25

how is perilipin regulated?

Answer 25

• PKA-dependent phosphorylation causes conformational change (adrenaline activates, insulin inhibits)

Question 26

what is perilipin?

Answer 26

protein that surrounds lipid droplet and must move out of the way to allow lipases to access TG

Question 27

What is attached to FA in blood stream?

Answer 27

albumin

Question 28

what is the fate of the glycerol from TG breakdown in adipocytes?

Answer 28

• leaves cell through aquaporin-7 channel

- goes to liver to get turned into glucose by gluconeogenesis

Question 29

what is the major adipocyte derived hormone?

Answer 29

leptin