**6. Metabolic pathways involved in cholesterol homeostasis**

#### **6.1. Biosynthesis**

Cholesterol levels in the body derived from *de novo* biosynthesis and diet. The majority of cholesterol utilized by healthy adults is synthesized in the liver, which accounts for about 70% of the daily cholesterol. Virtually all cells containing nucleus are capable of cholesterol synthesis, which occurs in endoplasmic reticulum and the cytosol. Biosynthesis of cholesterol generally takes place in the endoplasmic reticulum of hepatic cells and begins with acetyl-CoA, which is mainly derived from fatty acid oxidation reaction in the mitochondria. The conversion of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to mevalonate by HMG-CoA reductase is the rate-limiting step of cholesterol biosynthesis and is under strict regulatory control. Thus, HMG-CoA reductase is one of important targets of cholesterol lowering drugs. The develop‐ ment of statin drugs is a very successful story of discovering and applying HMG-CoA reductase inhibitor to lower hypercholesterolemia.

#### **6.2. Absorption**

Dietary cholesterol is absorbed within the lumen of the small intestine. Bile salts produced from cholesterol in the liver interact with phospholipids to produce a biliary micelle that is transported via bile into the lumen. Dietary cholesterol in the lumen is easily incorporated into the micelles and together with the biliary cholesterol can be absorbed into the enterocytes. In the enterocytes, absorbed cholesterol is esterified by acyl-coenzyme A:cholesterol acyltrans‐ ferase 2 (ACAT2), which is found in both the intestine and liver. Reducing the absorption of cholesterol of dietary ad billiary sources has become another key area in cholesterol research and product development. A typical example is plant sterols/stanols that have long found as effective inhibitors of cholesterol absorption. These molecules inhibit cholesterol absorption by competitively inhibiting with cholesterol for incorporation into micelles. Recently, inhibi‐ tors, such as ezetimibe, that block the absorption of cholesterol into the enterocytes through suppressing the activity of cholesterol transporters have also been used to reduce absorption of dietary cholesterol.

#### **6.3. Transport**

instance, defects in the essential components of lipid transportation and metabolism inherited from family. Examples include familial defect in LDL receptor or apo B-100 (diminished LDL clearance and hypercholesterolemia), familial lipoprotein lipase deficiency (hypertriglyceridemia), and combination of multiple unknown defect and

**2.** Secondary hyperlipidemia arises due to other underlying causes, such as sedentary lifestyle coupled with the excessive dietary intakes of saturated fat, cholesterol and transfats, in addition to many other disease conditions and drug uses. These factors include obesity, diabetes mellitus, hyperhomocystinemia, smoking, alcohol intake, chronic kidney disease, hypothyroidism, primary biliary cirrhosis and other cholestatic liver diseases, and drugs, such as thiazides, β-blockers, retinoids, estrogen and progesterons,

Cholesterol levels in the body derived from *de novo* biosynthesis and diet. The majority of cholesterol utilized by healthy adults is synthesized in the liver, which accounts for about 70% of the daily cholesterol. Virtually all cells containing nucleus are capable of cholesterol synthesis, which occurs in endoplasmic reticulum and the cytosol. Biosynthesis of cholesterol generally takes place in the endoplasmic reticulum of hepatic cells and begins with acetyl-CoA, which is mainly derived from fatty acid oxidation reaction in the mitochondria. The conversion of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to mevalonate by HMG-CoA reductase is the rate-limiting step of cholesterol biosynthesis and is under strict regulatory control. Thus, HMG-CoA reductase is one of important targets of cholesterol lowering drugs. The develop‐ ment of statin drugs is a very successful story of discovering and applying HMG-CoA

Dietary cholesterol is absorbed within the lumen of the small intestine. Bile salts produced from cholesterol in the liver interact with phospholipids to produce a biliary micelle that is transported via bile into the lumen. Dietary cholesterol in the lumen is easily incorporated into the micelles and together with the biliary cholesterol can be absorbed into the enterocytes. In the enterocytes, absorbed cholesterol is esterified by acyl-coenzyme A:cholesterol acyltrans‐ ferase 2 (ACAT2), which is found in both the intestine and liver. Reducing the absorption of cholesterol of dietary ad billiary sources has become another key area in cholesterol research and product development. A typical example is plant sterols/stanols that have long found as effective inhibitors of cholesterol absorption. These molecules inhibit cholesterol absorption by competitively inhibiting with cholesterol for incorporation into micelles. Recently, inhibi‐ tors, such as ezetimibe, that block the absorption of cholesterol into the enterocytes through

known familial defects (combined hyperlipidemia).

188 Using Old Solutions to New Problems - Natural Drug Discovery in the 21st Century

reductase inhibitor to lower hypercholesterolemia.

**6. Metabolic pathways involved in cholesterol homeostasis**

and glucocorticoids.

**6.1. Biosynthesis**

**6.2. Absorption**

Chylomicrons deliver absorbed dietary and biliary cholesterol from the enterocytes to the liver. During the process, triacylglycerols are released with assistance of lipoprotein li‐ pase and taken up by adipose tissues and muscle, the remnants of the lipoprotein then delivered to, and taken up by, the liver through interaction with the chylomicron rem‐ nant receptor. In the liver, absorbed cholesterol, together with synthesized cholesterol and cholesterol transported back from peripheral tissues and LDL receptor-mediated up‐ take go through several metabolic pathways and secreted out from different outputs. One of them is the secretion in VLDL back into the bloodstream. VLDL removes triacyl‐ glycerols and cholesteryl esters from the liver and distributes them throughout the body. Endothelial Lipoprotein lipase remove the majority of fatty acids from both the VLDL and IDL, thus increasing the cholesterol and cholesteryl ester concentrationsand apoB-100 results in LDL. LDL is the primary plasma carrier of cholesterol, which can be taken up by the liver and other tissues via receptor-mediated endocytosis. The cytoplasmic domain of LDL receptor facilitates the formation of coated pits which is the receptor-rich regions of the membrane. The ligand binding domain of the receptor recognizes apo-B100 on LDL, resulting in the formation of a clathrin-coated vesicle that buds from the inner sur‐ face of the cell membrane. ATP-dependent proton pumps lower the pH inside the vesicle resulting in dissociation of LDL from its receptor. After loss of the clathrin coat, the vesi‐ cles fuse with lysozomes, resulting in peptide and cholesteryl ester enzymatic hydrolysis. On the other hand, HDL is the small and rich in lipoproteins. The HDL protein particle accumulates cholesteryl esters by the esterification of cholesterol with lecithin:cholesterol acyl-transferase (LCAT). In the plasma, these particles undergo aseries of remodeling steps involving two HDL-associated proteins:phospholipid transfer protein (PLTP) and CETP. The primary role of PLTP is in thetransfer of surface remnants, which contain apolipoproteinsand phospholipids originating from triglyceride-rich lipoproteins,to pre-ß– HDL. PLTP has also been implicatedin mediating fusion of HDL particles to generate pre-ß–HDLand CE-rich HDL. CETP promotesboth transfer and exchange of hydrophobic lipids, CE, and triacylglycerols between lipoproteins. HDL can acquire cholesterol from cell membranes and transfer cholesteryl esters to VLDL and LDL via the transferase ac‐ tivity of apoD. More importantly, HDL can return to the liver where cholesterol is re‐ moved by reverse cholesterol transport, thus serving as a scavenger of free cholesterol. Scavenging activity of HDL initiates by accepting cholesterol from tissues in smaller HDL3 via the ATP-binding Cassette transporter -1 (ABC-1). The cholesterol in HDL3 is then esterified by LCAT, increasing the size of the particles to form the less dense HDL2. The cycle is completed by the reformation of HDL3 either after selective delivery of cho‐ lesteryl esters to the liver via the scavenger receptor-B1 or by the hydrolysis of HDL2 phospholipid and triacylglycerol by hepatic lipase. HDL2 concentration is inversely relat‐ ed to the incidence of coronary atherosclerosis. The enzyme cholesterol esterase controls the hydrolysis of these stored cholesterol esters, yielding bioavailable cholesterol and fat‐ ty acids.

#### **6.4. Excretion of cholesterol**

About 1 g of cholesterol is eliminated from the body per day, approximately equivalent to the amount that absorbed cholesterol and synthesize cholesterol. Approximately, half is excreted in the feces after conversion to bile acids in liver, and the remainder is excreted as cholesterol. Bile acids serve to remove unwanted cholesterol from the body and to aid in lipid digestion in the intestine. 7α-hydroxylase, the rate limiting enzyme of bile acid biosynthesis converts cholesterol into 7-hydroxycholesterol. 7-hydroxycholesterol is converted to one of the two primary bile acids, cholic acid and chenodeoxycholic acid. Bile acids are then delivered to the intestines where they aid in the absorption of lipids. Some of bile acids are modified to form secondary bile acids (lithocholic acid and deoxycholic acid) in the intestine by intestinal bacteria. However, the majority of bile acids delivered to intestine are recycled by re-absorp‐ tion in the ileum and returned to the liver by enterohepatic circulation. In liver, glyco- and tauroconjugate bile acids are formed and stored in gall bladder, from where they are released into the intestinal lumen for aid fat/lipids digestion and absorption.
