Cholesterol synthesis and breakdown Dr. Carolyn K. Suzuki 1

22 To compare and contrast the different mechanisms by which cholesterol biosynthesis are regulated. To predict whether intracellular cholesterol synthesis will be up- or down-regulated in response to energy availability as influenced by diet, hormones and exercise. To distinguish the different mechanisms by which plasma cholesterol levels are controlled by clinically adminstrered pharmacological agents. OBJECTIVES

Cholesterol in cellular membranes 3

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6 26 hydrocarbon tail steroid nucleus A C D B cholesterol Cholesterol 27 carbons all derived from acetate C-3 hydroxyl group C-17 side chain with 8 carbons Sources in the body synthesized primarily in liver and intestine not required in diet intestinal uptake from diet Elimination converted into bile acids and bile salts in liver stored in gall bladder, secreted into intestine small % excreted in feces 21 3 cholesterol ester fatty acid A CD B Cholesterol esters esterification at C-3 with fatty acid primary form transported in plasma packaged in lipoprotein particles (e.g. LDL, HDL)

cytosol mitochondria peroxisomes A. Synthesis of HMG CoA 7 2 acetyl CoA (2C) 2 acetoacetyl CoA (4C) 3-hydroxy-3-methylglutaryl CoA (6C) (HMG CoA) thiolase HMG CoA synthase CoA acetyl CoA

endoplasmic reticulum B. Synthesis of mevalonic acid HMG CoA (6C) HMG CoA reductase 2 NADPH 2 NADP + CoA Mevalonic acid (6C) RATE LIMITING STEP 8

C. Cholesterol synthesis 9

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The transcription factor regulating cholesterol synthesis genes is SREBP- sterol responsive element binding protein 11

SCAP- SREBP Cleavage Activating Protein a transmembrane protein has a sterol sensing domain binds to SREBP in the ER when ER sterols are low, SCAP-SREBP move to the Golgi Protease 1 and Protease 2- localized to the Golgi responsible for the two step cleavage of SREBP resulting in soluble, cytosolic SREBP Mature, proteolytically-processed SREBP translocates from the Golgi to the nucleus activates the expression of cholesterol synthesis genes SREBP- Sterol Regulatory Element Binding Protein a transmembrane protein has a DNA binding domain has a SCAP interacting domain Sterol-dependent regulation of cholesterol synthesis genes 12

nucleus 13

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SCAP and SREBP are transported to the Golgi when sterol levels are low step #1 15

step #2 step #3 SREBP release 16

SREBP translocates to nucleus step #4 transcriptional activation of sterol responsive element (SRE) controlled genes nucleus 17

When intracellular cholesterol is low 18

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Cholesterol-dependent degradation of HMG CoA reductase 20

(AMP kinase) AMP kinase is different from cAMP-dependent kinase (PKA) 21

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Statins competitively inhibit HMGR- they mimic the transient intermediate mevadyl CoA 24

Vytorin (ezetimibe + simvastatin) ezetimibe administered in combination with a simavastatin (i.e. a statin) further reduces total cholesterol levels as compared to statin alone blocks cholesterol absorption in the intestine and cholesterol synthesis in the liver permits reduced doses of statins, which have side effects Zetia (ezetimibe) Mechanism of action- acts at small intestine brush border does not enter the bloodstream, no side effects inhibits absorption of cholesterol does not block absorption of triglycerides or fat-soluble vitamins 25

Reduction of LDL-cholesterol across all tested statins Mean % change in LDL-C from baseline 26

cholesterol is the precursor of bile acids and bile salts synthesized in the liver stored in the gall bladder secreted into intestine aids digestion by emulsifying dietary lipids making them accessible to pancreatic lipases aids intestinal absorption of fat-soluble vitamins (A, D, E, K) ~95% are reabsorbed in ileum and returned to liver ~5% of bile salts are excreted in feces Bile acids and bile salts Excretion of bile salts is the principal mechanism for eliminating cholesterol from the body Enterohepatic circuit synthesis in the liver storage in the gall bladder secretion into intestine re-circulation to liver 27

28 cholic acid - + cholesterol rate limiting step Primary bile acids are formed from cholesterol cholic acid chenodeoxycholic acid 7-  -hydroxylase hydroxylation of C7 addition of OH group 7-  -hydroxycholesterol

Primary bile acids 7-  -hydroxylase cholic acid - + cholesterol 29

Hypercholesterolemia is often treated with “sequestrants” that bind bile acids in the intestine. These compounds: prevent reabsorbtion of bile acids increase conversion of cholesterol to bile acids increase bile salt elimination in feces Dietary fiber also sequesters bile acids Secondary bile acids Secondary bile acids Primary bile acids 7-  -hydroxylase cholic acid - + cholesterol 30 Increased elimination of cholesterol from the body

Secondary bile acids Primary bile acids Bile salts- glycine or taurine conjugated to bile acids in liver 5% lost in feces Recent R & D efforts focusing on bile acid receptors as drug targets for treating liver disease, liver cancer, metabolic disease.

Review- you tell me !!!! How many carbons are there in cholesterol? Which carbons are the business ends of the cholesterol molecule? When cholesterol levels are high, HMG CoA reductase is regulated by which of the following mechanisms? When cholesterol levels are high, HMG CoA reductase is regulated by which of the following mechanisms? What organ STORES bile acids and bile salts?