Newly synthesized cholesterol

It is equally difficult to define in quantitative terms how much newly synthesized cholesterol is present in the intestinal lymph under different physiological circumstances and to determine where such sterol comes from. In theory, such newly synthesized cholesterol could be derived from the intestinal mucosal cells directly or from a more remote organ after either being secreted into the lumen of the bowel or after being delivered to the intestinal villi in plasma lipoproteins. 

in summary, it may be concluded that much of the cholesterol synthesized in the intestine is apparently used for local purposes. Under circumstances where there is no triglyceride absorption taking place essentially no newly synthesized sterol of intestinal origin can be detected in the lymphatic outflow from the gut. During active triglyceride absorption, however, the rate of sterol synthesis increases markedly in the intestinal absorptive cells, and a portion of this newly synthesized cholesterol is incorporated into chylomicrons and other intestinal lipoproteins and delivered into the lymph. Thus, both the rate of sterol synthesis by the intestine and the rate of entry of this sterol into the body pools is partially dictated by the rate of triglyceride absorption. 

On the basis of the data reviewed in this chapter, it seems likely that there are functionally distinct pools of cholesterol in the intestinal epithelial cell that serve different metabolic functions. These pools are illustrated diagrammatically in the model of an epithelial cell shown in Fig. 14. Pool A is defined as having been derived largely from the uptake of luminal unesterified cholesterol (arrow 1) and serves as a major substrate for the CoA-dependent esterification reaction (arrow 2). The cholesterol esters that result from this reaction are incorporated into the hydro-phobic core of the chylomicron particle. Following cholesterol feeding there is a marked increase in apparent ACAT activity in the intestinal epithelium that seems to be related to an increase in the amount of intracellular cholesterol available to the enzyme under the in vitro conditions of the assay rather than to an increase in the 

Pools A and B also appear to be functionally distinct with respect to the role of the sterol in these pools to effectively regulate the rate of cholesterol synthesis within the intestinal epithehal cell. For example, cholesterol feeding in the rat and in man 

The relative importance of each of these contributions to pool C is likely to be different in epithelial cells located at different points along the villus-crypt axis. The fact that cholesterol derived from synthesis and from the uptake of LDL is critically important for membrane formation and differentiation is suggested by the finding that 70-80% of total mucosal sterol synthetic activity and LDL transport activity are localized to the immature cells of the lower villus and crypt regions in both the proximal and distal intestine. In the mature absorptive cells of the upper villus in the jejunum, where most sterol absorption takes place, the rate of cholesterol synthesis appears to be suppressed. In the absence of fat absorption, cholesterol newly synthesized in these cells apparently is sloughed into the lumen and not reabsorbed. However, with active triglyceride absorption cholesterol synthesis in these cells is increased and a portion of this sterol appears in the intestinal lymph. Only under this condition does pool B apparently supply sterol for lipoprotein formation. 

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Although apoE HDL particles are formed by astrocytes in vitro, the brain contents of apoE knockout (-/-) were not found to differ in lipid content in comparison to those obtained from normal animals [14]. A probable explanation is that newly synthesized cholesterol can be transported from astrocyte ER to plasma membrane via an alternative route that employs caveolae to form apoAl-HDL [15]. 

Another heat shock protein-immunophilin chaperone complex is involved in the trafficking of cholesterol.124 A macromolecular complex consisting of FKBP52, caveolin (a 22 kDa protein that plays a role in regulating cholesterol concentration), CyPA, CyP-40, and cholesterol, transports newly synthesized cholesterol from the ER to caveolae. In cells expressing the complex, treatment with either CsA or rapamycin disrupted the complex and interfered with the rapid transport of cholesterol. 

The changes in activity and/or properties of HMG-CoA reductase reported above are, however, accompanied by large changes in enzyme quantity typical of such feeding regimens. These would appear to overshadow any effects due to membrane compositional changes. However, the hypothesis that cholesterol directly feedback-inhibits its own synthesis by altering the catalytic activity of HMG-CoA reductase remains attractive since newly synthesized cholesterol appears preferentially in the smooth endoplasmic reticulum that harbors HMG-CoA reductase [115],... 

Fig. 12. Appearance of newly synthesized cholesterol in mesenteric lymph of the rat. The experimental animals had indwelling lymph fistulae and were infused intraintestinally with a glucose-amino acid-electrolyte solution. One group of these animals was also administered intravenously chylomicrons containing lOS mg of cholesterol, while another group was infused intraduodenally with com oil. 24 h after the initial surgery, each animal was administered [ H]water intravenously and the secretion of labeled cholesterol in intestinal lymph was followed for 18 h. The amount of [ HJwater incorporated into cholesterol was used to calculate the amount of newly synthesized cholesterol present in lymph. These values are expressed as the nmoles of newly synthesized cholesterol secreted into the lymph each hour. The data points represent means +1 S.E.M.

Fig. 13. The relationship between the appearance rate of newly synthesized cholesterol in mesenteric lymph and the rate of hepatic cholesterol synthesis in the same animal. These studies were carried out as described in the legend to Fig. 12 but contained an additional group of animals that were fed different amounts of cholesterol to alter rates of hepatic cholesterol synthesis.

A coupling appears to exist between the rate-limiting enzyme in the biosynthesis of cholesterol, HMG-CoA reductase, and cholesterol 7a-hydroxylase. The two enzymes seem to be located close to each other on the endoplasmic reticulum membrane [66], and the two activities covariate under most conditions. Results from both in vivo and in vitro experiments show that newly synthesized cholesterol is the preferred substrate for cholesterol 7a-hydroxylase. In an early study by Staple and Gurin, it was shown that the bile acids in bile had a higher radioactivity than cholesterol after administration of labelled acetate to rats [67]. Bjorkhem and Danielsson found that the specific radioactivity of 7a-hydroxycholesterol was higher than that of cholesterol after incubation of labelled mevalonate with the 10000 x g supernatant fluid of a rat liver homogenate [50]. Balasubramaniam et al. showed that 7a-hydroxycholesterol isolated from the livers of rats after intravenous administration of labelled cholesterol had a lower specific radioactivity than cholesterol [58]. Cronholm and collaborators measured the incorporation of isotope from [1- H2]-,... 

There is a close relation between cholesterol 7a-hydroxylase and HMG-CoA reductase. The two enzymes are located near each other on the endoplasmic reticulum and newly synthesized cholesterol seems to be the preferred substrate for the cholesterol 7a-hydroxylase. 

Since very little bile acids are transported via the lymph [224], the increased cholesterol 7a-hydroxylase activity observed after lymphatic drainage [225] is not due to depletion of bile acids. In view of the fact that the synthesis of cholesterol increases after lymphatic drainage, it is possible that the increased 7a-hydroxylation in this case may be due to increased supply of newly synthesized cholesterol. 

Turley, S. D., Burns, D. K., and Dietschy, J. M., Preferential utilization of newly synthesized cholesterol for brain growth in neonatal lambs, Am J Physiol 274 (1998) E1099-1105. 

Transport of cholesterol to and from the plasma membrane. Following its synthesis at the ER, cholesterol is transported throughout the cell and becomes enriched in the plasma membrane [7]. The transport of newly synthesized cholesterol to the plasma membrane has been examined in tissue culture cells using pulse-chase experiments with either the rapid plasma membrane isolation procedure (M. Kaplan, 1985), caveolae isolation (A. Uittenbogaard,... 

Important insights into the mechanism of cholesterol transport have come from LDL metabolism in cells from individuals with NPC disease [30]. In NPC fibroblasts, cholesterol transport from the lysosomal compartment to the plasma membrane is markedly retarded compared to that in normal fibroblasts (L. Liscum, 1999 E. Blanchette-Mackie, 2000). The transport defect results in the accumulation of cholesterol in lysosomes and endosomes. NPC cells also have impaired regulation of acyl-CoA cholesterol acyltransferase, 3-hydroxy-3-methylglutaryl-Co A reductase, and LDL receptor levels, in response to LDL (Chapter 14). In contrast, the transport of newly synthesized cholesterol from the ER to the plasma membrane of NPC fibroblasts is essentially identical to that found for normal cells. These findings localize one abnormality of NPC disease to cholesterol export from the lysosomes to other organelles. 

The causal relationship between low bile acid production and high serum cholesterol in familial hypercholesterolemia remains unknown. It can be postulated that they are not associated initially, particularly because augmented conversion of cholesterol to bile acids is not able to normalize the serum cholesterol level. On the other hand, it can be speculated that cholestyramine treatment, for instance, stimulates cholesterol production by hepatocytes so that subnormal amounts of newly synthesized cholesterol are utilized for bile acid synthesis, a relatively large amount being released as lipoproteins into the bloodstream so that blood cholesterol still remains high. 

Recently Samuel and McNamara showed - with a triple-lumen intubation technique in sic volunteers - that isotopic exchange between luminal and mucosal (newly synthesized) cholesterol did not occur [23]. In addition this careful study indicated the possibility of a differential absorption of endogenous and exogenous cholesterol over a 100-200 cm segment of small bowel. 

The small intestine plays an active role in cholesterol syn-thesis[40] thus, as luminal cholesterol is taken up by the entero-cytes it mixes with the newly synthesized cholesterol and probably with the cholesterol which enters into the mucosal cells from serum lipoproteins. Cholesterol in the mucosal cell - either from the lumen or newly synthesized - can be disposed in three main ways First, it may be lost in the lumen with esfoliation of enterocytes or with enteric secretions. Second, it may be incorporated into structural component of the cellular membrane or of organelles. Finally,... 

I. Bjorkhem and A. Lewenhaupt, Preferential utilization of newly synthesized cholesterol as substrate for bile acid biosynthesis. J Biol Chem 254 5252 (1979). 

Mitropoulos et al.[l] showed that newly synthesized cholesterol is preferentially converted to bile acids. On the other hand Schwarz et al. and Portman et al. showed the preferential conversion of HDL-cholesterol to bile acids[2,3] in man and squirrel-monkeys. Although many anatomical features present in vivo are altered when parenchymal cells are cultured, these hepatocytes afford a direct approach to examining how bile acid synthesis is regulated. 

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