Cholesterol absorption and turnover in hypercholesterolemic dogs

Measurement of intestinal cholesterol absorption by plasma and fecal dual-isotope ratio, mass balance, and lymph fistula methods in the mouse: an analysis of direct versus indirect methodologies

The rate of intestinal cholesterol (Ch) absorption is an important criterion for quantitation of Ch homeostasis. However, studies in the literature suggest that percent Ch absorption, measured usually by a fecal dual-isotope ratio method, spans a wide range, from 20% to 90%, in healthy inbred mice on a chow diet. In the present study, we adapted four standard methods, one direct (lymph collection) and three indirect (plasma and fecal dual-isotope ratio, and sterol balance) measurements of Ch absorption and applied them to mice. Our data establish that all methodologies can be valid in mice, with all methods supporting the concept that gallstone-susceptible C57L mice absorb significantly more Ch (37 +/- 5%) than gallstone-resistant AKR mice (24 +/- 4%). We ascertained that sources of error in the literature leading to marked differences in Ch absorption efficiencies between laboratories relate to a number of technical factors, most notably expertise in mouse surgery, complete solubilization and delivery of radioisotopes, appropriate collection periods for plasma and fecal samples, and total extraction of radioisotopes from feces. We find that all methods provide excellent interexperimental agreement, and the ranges obtained challenge previously held beliefs regarding the spread of intestinal Ch absorption efficiencies in mice. The approaches documented herein provide quantifiable methodologies for exploring genetic mechanisms of Ch absorption, and for investigating the assembly and secretion of chylomicrons, as well as intestinal lipoprotein metabolism in mice.

Expression and purification of recombinant cynomolgus monkey cholesteryl ester transfer protein from Chinese hamster ovary cells

Cholesteryl ester transfer protein (CETP) mediates the transfer of cholesteryl ester from high- and low-density lipoproteins to triglyceride-rich lipoproteins, and reciprocally mediates triglyceride transfer. The gene for cynomolgus monkey CETP was expressed in serum-free CHO culture with 2 micrograms/ml insulin as its only exogenous protein supplement. Cell growth was facilitated by immobilizing the CHO cells in alginate beads. Recombinant CETP (rCETP) was purified 176-fold with a three-step protocol resulting in a 60% final yield as measured by a fluorescent CETP activity assay. Typically, 3.4 mg of rCETP was purified from 1700 ml of media by affinity-gel chromatography involving Reactive Red 120 (RR120) followed by concanavalin A Sepharose 4B and rechromatography on RR120. SDS-PAGE shows a single broad band of M(r) ranging from 68,000 to 74,000 which immunoreacts in Western blot analysis. Amino acid analysis and protein sequencing of the purified protein agree with the theoretical amino acid composition and sequence of cynomolgus CETP.

Apo B metabolism in the cynomolgus monkey: evidence for post-transcriptional regulation

Previous studies have shown that hepatic apo B mRNA levels do not increase in animals fed high cholesterol diets, even though plasma apo B concentrations increase markedly. As a result, it has been suggested that the diet-induced increase in plasma apo B levels was due solely to an inhibited clearance of those lipoproteins. The present study was undertaken to test that hypothesis. Hepatic apo B mRNA levels were measured in liver biopsies taken from five male cynomolgus monkeys before and twice after, they began to consume a high cholesterol diet. The diet had no effect on hepatic apo B mRNA levels, even though it caused a 7-fold increase in the plasma apo B levels. However, measurements of the apo B secretion rate in eight separate monkeys (four chow-fed and four cholesterol-fed) by isotope dilution showed that apo B secretion by the liver was increased 4-fold in the cholesterol-fed monkeys. These data, taken together, indicate that apo B secretion is not regulated by the rate at which the apo B gene is transcribed, but at some point further along in the secretion pathway.

Molecular cloning, sequence, and expression of cynomolgus monkey cholesteryl ester transfer protein. Inverse correlation between hepatic cholesteryl ester transfer protein mRNA levels and plasma high density lipoprotein levels

A cDNA clone containing the coding region for cynomolgus monkey cholesteryl ester transfer protein (CETP) was isolated by the polymerase chain reaction with primers based on the human CETP cDNA sequence and cDNA synthesized from liver poly (A+) RNA. Analysis of that cDNA indicated that the nucleotide and amino acid sequences of cynomolgus monkey CETP were greater than 95% homologous with the human sequences. A fragment of the cDNA was used to develop an internal-standard/RNAse protection assay that allowed precise quantification of CETP mRNA levels. Analysis of total RNA from various tissues with this assay revealed that the liver and thoracic aorta expressed high levels of CETP mRNA; the mesenteric fat, adrenal gland, spleen, and abdominal aorta had low but detectable levels of the mRNA; and the brain, kidney, intestine, and skeletal muscle had undetectable levels of that mRNA. When the monkeys were made hypercholesterolemic by a high-fat, high-cholesterol (HFHC) diet, hepatic levels of CETP mRNA increased from 1.6 +/- 0.4 pg/micrograms total RNA (mean +/- SEM) to 4.1 +/- 0.8 pg/micrograms (p less than 0.005); mesenteric fat CETP mRNA increased from 0.4 +/- 0.1 pg/micrograms total RNA to 5.3 +/- 2.2 pg/micrograms (p less than 0.05); and plasma CET activity increased approximately fourfold. The CETP mRNA levels in the thoracic and abdominal aortas were not significantly increased in monkeys fed the HFHC diet, even though those animals had gross atherosclerosis. The apoprotein E mRNA levels, however, were markedly increased in the aortas of monkeys with atherosclerosis, with the largest increase occurring in the abdominal aorta. Taken together, these data suggest that lipid deposition in the artery was not accompanied by increased expression of the CETP gene in that tissue. Statistical analysis showed that a strong, negative correlation existed between hepatic CETP mRNA levels and both high density lipoprotein cholesterol (r = -0.85, p less than 0.001) and apoprotein A-I (r = -0.84, p less than 0.001). These data suggest that HFHC diet-induced changes in high density lipoprotein metabolism may be linked to altered expression of a function CETP gene.