1. Cholesterol Dr. M. Jawad Hassan

2. Theme: Chest Pain

3. Objectives
1- Structure and Metabolism (absorption, synthesis, and fate) of Cholesterol
2- Regulation of Cholesterol Synthesis
3- Synthesis and fate of bile salts and bile acids
4- Bile salt deficiency (cholelithiasis)

4. Cholesterol sources, biosynthesis and degradation
diet
only found in animal fat
biosynthesis
primarily synthesized in the liver from acetyl CoA
27 C cyclic compound
degradation
only occurs in the liver
cholesterol is converted to bile acids

5. Lipids circulate in the blood in several forms
Cholesterol
Free cholesterol
Cholesterol ester
Phospholipids
Triglycerides
Free fatty acids

6. Serum Cholesterol and CHD in 361,662 U.S. Men
96DEC11
JH/FM/JH/FM
Serum Cholesterol and CHD in 361,662 U.S. Men 18 16 14 12 10 8 6 4 2
6 Year CHD Death Rate
per 1,000 Men
140
160
180
200
220
240
260
280
300
Serum Cholesterol
Martin M. Lancet 1986;11:

9. - synthesis of acetoacetyl CoA
Biosynthesis of cholesterol
- synthesis of acetoacetyl CoA

10. - synthesis of mevalonate
Biosynthesis of cholesterol
- synthesis of mevalonate
rate-limiting step
and step subject to
inhibition by statins

11. synthesis of isopentenyl pyrophosphate
Biosynthesis of cholesterol
synthesis of isopentenyl
pyrophosphate
A monoterpene

12. - synthesis of squalene
Biosynthesis of cholesterol
- synthesis of squalene
a sesquiterpene
a triterpene

13. - synthesis of lanosterol
Biosynthesis of cholesterol
- synthesis of lanosterol

14. Biosynthesis of cholesterol
ACAT inhibitors act
here

15. Biosynthesis summary

16. Conversion of lanosterol to cholesterol involves 19 reactions, catalyzed by enzymes in ER membranes.
Additional modifications yield the various steroid hormones or vitamin D.

17. Many of the reactions involved in converting lanosterol to cholesterol and other steroids are catalyzed by members of the cytochrome P450 enzyme superfamily.
The human genome encodes 57 members of the cyt P450 superfamily, with tissue-specific expression and intracellular localization highly regulated.
Some P450 enzymes are localized in mitochondria.
Others are associated with endoplasmic reticulum membranes.

18. Regulation of Cholesterol Synthesis
1- Sterol-Dependent regulation of gene expression
SREBP-2 (Sterol regulatory element binding protein 2)
Low sterol-SREBP2-SCAP (SREBP2 cleavage-activating protein) association and cleavage in ER
Act as transcription factor for SRE
Sterol abundance—binding to SCAP at sterol sensing domain

19. The SREBP precursor protein is embedded in the endoplasmic reticulum (ER) membrane via two transmembrane a-helices.
The N-terminal SREBP domain, which extends into the cytosol, has transcription factor capability.
The C-terminal domain, also on the cytosolic side of the membrane, interacts with a cytosolic domain of another ER membrane protein SCAP (SREBP cleavage-activating protein).

20. SCAP has a transmembrane sterol-sensing domain homologous to that of HMG-CoA Reductase.
When bound to a sterol, the sterol-sensing domain of SCAP binds the ER membrane protein Insig.
Association with Insig causes the SREBP-SCAP precursor complex to be retained within the ER.
When sterol levels are low, SCAP & Insig do not interact.
This allows the SCAP-SREBP precursor complex to translocate from the ER to the golgi apparatus.

21. Protease S1P (site one protease), an integral protein of golgi membranes, cleaves the SREBP precursor at a site in the lumenal domain.
An intramembrane zinc metalloprotease domain of another golgi protease S2P then catalyzes cleavage within the transmembrane segment of the SREBP precursor, releasing SREBP to the cytosol.
Only the product of S1P cleavage can serve as a substrate for S2P.

22. The released SREBP enters the cell nucleus where it functions as a transcription factor to activate genes for enzymes of the cholesterol synthesis pathway.
Its lifetime in the nucleus is brief, because SREBP is ubiquitinated & degraded.
Diagram (in article by P. J. Espenshade; requires J. Cell Sci. subscription)
Homodimeric DNA-binding domain of SREBP interacting with a sterol regulatory element DNA segment.

23. 2- Sterol-accelerated enzyme degradation
sterol itself act as sterol sensing integral protein
High sterol levels accelerate binding of insig and proteosomal degradation of HMG CoA Reductase

24. 3- Sterol independent phosphorylation / dephosphorylation
Adenosine monophosphate –activated protein kinase (AMPK) and phosphoprotein phosphatase
Phosphorylated form is inactive
AMPK activated by AMP
Cholesterol synthesis decreased with decrease in ATP availability

25. 4- Hormonal Regulation
Increase in insulin or thyroid hormones favors up-regulation of expression of HMG co A reductase gene
Cortisol and glucagon have decreasing effect

26. Pathways Affecting Cholesterol Balance
Liver
(Intake)
(Excretion)
ABCA1
HMG CoA Reductase
(Esterification)
(Synthesis)
Key point: Multiple processes are involved in cholesterol absorption and synthesis.
Dietary cholesterol can be reduced simply by decreasing intake. Bile acid sequestrants such as cholestyramine and colestipol bind to bile acids, blocking their normal resorption. This leads to increased hepatic conversion of cholesterol to bile acids to maintain the bile acid pool, with a resultant decrease in plasma cholesterol concentrations. [Illingworth 2000, page 33] [Knopp 1999, page 504] Plant stanol esters, such as sitostanol, are structurally similar to cholesterol but are poorly absorbed. They compete with cholesterol for incorporation into micelles, reducing micellar cholesterol and increasing cholesterol excretion. [Cater 2000, page 122][Homan 1997, page 33] [Hallikainen 2000, page 772] [Ikeda 1988, page 1573]
(Bile Acids)
(Micellar Cholesterol)
(Uptake)

27. 5- ANTI-CHOLESTEROL DRUGS
They are reversible competitive inhibitors of HMG CoA Reductase: (The Statins)
Simvastatin(Zocor)
Lovastatin(Mevacor)
Atorovostatin(Lipitor)
Fluvastatin(Lescol)

28. Degradation of cholesterol Bile acids and salts
formed from cholesterol in the liver
stored in the gall bladder in bile as bile salts
utilized during digestion of fats and other lipid substances (act as detergents, emulsification)
rate limiting step is the conversion of cholesterol to 7-alpha cholesterol by 7-a-hydroxylase

29. Bile acids
cholic acid is the bile acid found in the largest amount in bile
cholic acid and chenodeoxycholic acid are referred to as primary bile acids
bile acids are converted to either glycine or taurine conjugates (in humans the ratio of glycine to taurine conjugates is 3:1)

30. Approximate composition of bile salts
Glycocholate – 24%
Glycochenodeoxycholate – 24%
Taurocholate – 12%
Taurochenodeoxycholate – 12%
Glycodeoxycholate- 16%
Taurodeoxycholate – 8%
Various lithocholate – 4%

31. Bile acids
fat digestion products are absorbed in the first 100 cm of small intestine
the primary and secondary bile acids are reabsorbed almost exclusively in the ileum returning to the liver by way of the portal circulation (98 to 99%)
this is known as the entero-hepatic circulation
less than 500 mg a day escapes re-absorption and is excreted in the feces

32. Bile salts
detergent character of bile salts is due to the hydrophobic-hydrophilic nature of the molecules
the presence of hydroxyl (or sulfate) and the terminal carboxyl group on the tail gives the molecule its hydrophilic face
the steroid ring with its puckered plane provides the hydrophobic face

33. Function of bile salts
emulsification of fats due to detergent activity
aid in the absorption of fat-soluble vitamins (especially vitamin K)
accelerate the action of pancreatic lipase
have choleretic action –stimulate the liver to secrete bile
stimulate intestinal motility
keep cholesterol in solution (as micelles)

34. Mixed micelle formed by bile salts, triacylglycerols
And pancreatic lipase

36. Entero-Hepatic Circulation
Entero-hepatic circulation of bile salts (excretion into intestines and re-absorption in to liver)
Bile acid squestrants (cholestyramine), binds bile acids in gut, prevent their re-absorption and promote excretion.
Treatment of Hypercholesterolemia (removal of bile acids promote synthesis of bile acids in liver, so more cholesterol removal).
Dietary fiber has same effect.

37. Cholelithioasis Gross malabsorption of bile acids from intestine
Obstruction of Biliary tract
Sever hepatic dysfunction
Increase biliary cholesterol excretion with the use of fibrates (up regulation of fatty acid beta oxidation)

38. Thank You