Cholesterol Dr. M. Jawad Hassan

Theme: Chest Pain

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)

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

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

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;11:933-936

- synthesis of acetoacetyl CoA Biosynthesis of cholesterol - synthesis of acetoacetyl CoA

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

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

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

- synthesis of lanosterol Biosynthesis of cholesterol - synthesis of lanosterol

Biosynthesis of cholesterol ACAT inhibitors act here

Biosynthesis summary

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

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.

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

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).

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.

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.

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.

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

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

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

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)

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

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

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)

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

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

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

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)

Mixed micelle formed by bile salts, triacylglycerols And pancreatic lipase

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.

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)

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