Cholesteryl ester biosynthesis

There are two forms of the hydrolytic enzyme, one of which is specific for short chain esters like retinyl acetate, even though this ester does not occur naturally The other has maximum activity with retinyl palmitate as substrate but also hydrolyses other long chain esters. As in the hydrolysis of cholesteryl esters, the enzyme is not just a non-specific esterase, but has quite definite specificity for retinyl esters. In vitamin A deficiency, the activity of the enzyme increases one hundred fold. The esterification enzyme resembles the low energy cholesteryl esterase in that neither ATP nor coenzyme A appear to take part in the reaction nor are free fatty acids or acyl-CoA thiolesters incorporated into retinyl esters. One of the major problems in this area of research is to identify the acyl donor, which may be, as in plasma cholesteryl ester biosynthesis, a phospholipid. 

Of the various lipid components of the lipoproteins, only the biosynthesis of cholesteryl esters has not yet been mentioned. Cholesteryl ester is the storage form of cholesterol in cells. It is synthesized from cholesterol and acyl-CoA by acyl-CoA cholesterol acyltransferase (ACAT) (fig. 20.13), which is located on the cytosolic surface of hepatic endoplasmic reticulum. Acylation of the 3 hydroxyl group of cholesterol eliminates the polarity of cholesterol and facilitates the packing of cholesterol as its ester in the core of the lipoprotein or for storage in lipid droplets within cells. 

Biosynthesis of cholesteryl esters. The acyl-CoA cholesterol acyltransferase involved in cholesteryl ester synthesis is located on the cytosolic surface of liver endoplasmic reticulum. 

MZ Kuo, RJ Zielinski, JI Cialdella, CK Marschke, MJ Dupui s, GP Li, DA Klooster-man, CH Spilman, VP Marshall. Discovery, isolation, structure elucidation and biosynthesis of U-106305, a cholesteryl ester transfer protein inhibitor from UC11136. J Am Chem Soc 117 10629-10634, 1995. 

Fig. 2. Pathway of biosynthesis of the glucocorticoid, cortisol, in the adrenal cortex. Cholesterol, from stores in cholesteryl esters or from other sources (see text) is converted via mitochondrial cytochrome P-450SCC (cholesterol side-chain cleavage enzyme) to pregnenolone, which then is successively converted by the microsomal enzymes cytochrome P-450,7 (17a-hydroxylase), 3 j8-hydroxysteroid dehydrogenase/ isomerase and cytochrome P-450c2, (21-hydroxylase) to 11-deoxycortisol, followed by conversion by the mitochondrial cytochrome P-450ll(3 (11/3-hydroxylase) to cortisol. The short-term action of ACTH in stimulation of steroidogenesis is to increase the availability of cholesterol for conversion by cytochrome P-450scc. From Ref. 9.

Simultaneously, some of the phospholipids and the apo A apolipoproteins are transferred from the chylomicron particle onto HDL. The newly formed particle, the chylomicron remnant, contains 80% to 90% of the triglyceride content of the original chylomicron. Because of the presence of apo B-48 and apo E on its surface, the chylomicron remnant can be recognized by specific hepatic remnant receptors and internalized by endocytosis. The components of the particle are then hydrolyzed in the lysosomes. The cholesterol released can form bile acids, be incorporated into newly synthesized lipoprotein, or be stored as cholesteryl ester. Furthermore, the cholesterol from these remnants can down regulate HMG-CoA reductase, the rate-hmiting enzyme of cholesterol biosynthesis (see earlier section on cholesterol synthesis). 

There are conflicting data on whether the availability of cholesterol and/or cholesteryl esters directly influences apo B secretion. Several studies have suggested that cholesterol supply can regulate VLDL secretion. For example, VLDL production in animals and man is decreased by statin treatment, and inhibition of cholesterol synthesis by a statin, an inhibitor of the rate-limiting step of cholesterol biosynthesis (Chapter 14), reduced VLDL secretion in perfused rat livers (M. Heimberg, 1990) and primary hepatocytes. However, this effect of statins can perhaps be ascribed to increased expression of LDL receptors rather than to a reduction in cholesterol synthesis (Section 7.1). Depletion of cholesterol in rodent hepatocytes by the ABCAl-dependent lipidation of apo A1 (Chapter 19) also decreases VLDL secretion (R. Lehner, 2004). Furthermore, the secretion of apo BlOO-containing VLDLs is increased in primary hepatocytes derived from Niemann-Pick Cl-deficient mice. Niemann-Pick Cl-deficiency causes a severe defect in trafficking of unesterified cholesterol out of the lysosomal/endosomal pathway and consequently, Niemann-Pick Cl-deficient hepatocytes accumulate 5- to 10-fold more unesterified cholesterol than do wild-type hepatocytes. In hepatocytes from Niemann-Pick Cl-deficient mice, cholesterol synthesis is increased and the rate of cholesterol esterification and the amount of the transcriptionally active form of SREBP-1 are also increased (J.E. Vance, 2007). However, because of multiple alterations in lipid metabolism in these hepatocytes, increased VLDL secretion cannot be attributed specifically to increased synthesis of cholesterol or cholesteryl esters. 

The LDL receptor is a key component in the feedback-regulated maintenance of cholesterol homeostasis [1]. In fact, as an active interface between extracellular and intracellular cholesterol pools, it is itself subject to regulation at the cellular level (Fig. 2). LDL-derived cholesterol (generated by hydrolysis of LDL-bome cholesteryl esters) and its intracellularly generated oxidized derivatives mediate a complex series of feedback control mechanisms that protect the cell from over-accumulation of cholesterol. First, (oxy)sterols suppress the activities of key enzymes that determine the rate of cellular cholesterol biosynthesis. Second, the cholesterol activates the cytoplasmic enzyme acyl-CoA cholesterol acyltransferase, which allows the cells to store excess cholesterol in re-esterified form. Third, the synthesis of new LDL receptors is suppressed, preventing further cellular entry of LDL and thus cholesterol overloading. The coordinated regulation of LDL receptors and cholesterol synthetic enzymes relies on the sterol-modulated proteolysis of a membrane-bound transcription factor, SREBP, as described in Chapter 14. 

Inside the cell, cholesteryl esters are hydrolysed by cholesteryl ester hydrolase. Incorporation of cholesterol into the endoplasmic reticulum membranes serves to inhibit hydroxymethylglutaryl-CoA reductase, the rate limiting enzyme in cholesterol biosynthesis (section 7.5). Thereby, an abundant supply of cholesterol in the plasma is able to suppress its own endogenous biosynthesis and ensure that excessive amounts do not accumulate. The importance of this pathway for cholesterol homeostasis... 

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