Faculty Bibliography

The effect of cholesterol esterification on the distribution of apoA-IV in human plasma was investigated. Human plasma was incubated in the presence or absence of the lecithin:cholesterol acyltransferase (LCAT) inhibitor 5,5-dithiobis(2-nitrobenzoic acid) (DTNB) and immediately fractionated by 6% agarose column chromatography. Fractions were monitored for apoA-IV, apoE, and apoA-I by radioimmunoassay (RIA). Incubation resulted in an elevated plasma concentration of cholesteryl ester and in an altered distribution of apoA-IV. After incubation apoA-IV eluted in the ordinarily apoA-IV-poor fractions of plasma that contain small VLDL particles, LDL, and HDL2. Inclusion of DTNB during the incubation resulted in some enlargement of HDL; however, both cholesterol esterification and lipoprotein binding of apoA-IV were inhibited. Addition of DTNB to plasma after incubation and prior to gel filtration had no effect on the apoA-IV distribution when the lipoproteins were immediately fractionated. Fasting plasma apoE was distributed in two or three peaks; in some plasmas there was a small peak that eluted with the column void volume, and, in all plasmas, there were larger peaks that eluted with the VLDL-LDL region and HDL2. Incubation resulted in displacement of HDL apoE to larger lipoproteins and this effect was observed in the presence or absence of DTNB. ApoA-I was distributed in a single broad peak that eluted in the region of HDL and the gel-filtered distribution was unaffected by incubation either in the presence or absence of DTNB. Incubation of plasma that was previously heated to 56 degrees C to inactivate LCAT resulted in no additional movement of apoA-IV onto lipoproteins, unless purified LCAT was present during incubation. The addition of heat-inactivated LCAT to the incubation, had no effect on movement of apoA-IV. These data suggest that human apoA-IV redistribution from the lipoprotein-free fraction to lipoprotein particles appears to be dependent on LCAT action. The mechanism responsible for the increased binding of apoA-IV to the surface of lipoproteins when LCAT acts may involve the generation of 'gaps' in the lipoprotein surface due to the consumption of substrate from the surface and additional enlargement of the core. ApoA-IV may bind to these 'gaps,' where the packing density of the phospholipid head groups is reduced

Apolipoprotein A-IV (apoA-IV) synthesis rates were measured in vivo in rat enterocytes by immunoprecipitation after administration of [3H]leucine into in situ loops of jejunum and ileum. Basal apoA-IV synthesis rates (percent total protein synthesis) were significantly higher in jejunal enterocytes (2.05 +/- 0.54%) compared with ileal enterocytes (0.48 +/- 0.32%) from the same fasted animals. After an acute triglyceride bolus, significant and sustained elevations of apoA-IV synthesis rates were seen in both jejunal and ileal enterocytes with maximal effects noted at 4-6 h. Animals fed diets containing 30% wt/wt triglyceride as saturated (SF) or polyunsaturated (UF) fats for 6 wk had similarly increased rates of apoA-IV synthesis in jejunal enterocytes with both SF (3.73 +/- 0.83%) and UF (3.33 +/- 0.64%) but no change in ileal enterocytes. By contrast, animals consuming a fat-free diet for 3 wk had jejunal apoA-IV synthesis rates indistinguishable from basal values (2.40 +/- 0.45%). Translatable intestinal mRNA levels for pre-apoA-IV after triglyceride increased in parallel to synthesis rates with a 50% increase in jejunum and a 350% increase in ileum observed at 4-6 h. These results suggest that apoA-IV synthesis by rat small intestine increases in response to acute and chronic dietary triglyceride, is maintained in the absence of dietary triglyceride, and may be under pretranslational control

A sensitive and rapid immunological detection method was used to screen for apolipoprotein A-IV variants. Antibodies to human lymph chylomicron or plasma apolipoprotein A-IV, and plasma apolipoprotein A-I were raised in rabbits. Antibodies to apolipoprotein A-I or apolipoprotein A-IV were shown to be monospecific to their respective antigens by reactivity against human chylomicron apolipoproteins by immunoblot analysis. Plasma samples were obtained from dyslipidemic subjects from the Lipid Research Clinic of Columbia University. The plasma samples were isoelectrically focused (pH 4-6) on slab gels. Plasma proteins were then transferred to nitrocellulose paper for immunoblotting. Apolipoprotein A-IV polymorphism was determined by specific immunological detection of apolipoprotein A-IV. Identical apolipoprotein A-IV isoprotein patterns were observed when either antibodies to lymph or plasma apolipoprotein A-IV were used for immunoblotting. All the dyslipidemic plasma samples screened contained the two major and one or two minor isoproteins of normal plasma. In two instances, new apolipoprotein A-IV variants having an additional isoform were detected. One subject was hypertriglyceridemic (triacylglycerols = 342 mg/dl, cholesterol = 251 mg/dl) and had an additional major acidic apolipoprotein A-IV isoform. Another subject with mild hypocholesterolemia (triacylglycerols = 209 mg/dl, cholesterol = 120 mg/dl) was found to have additional major and minor basic apolipoprotein A-IV isoforms. The specificity of this technique allows detection of polymorphism of apolipoproteins of similar isoelectric points by use of a single dimension isoelectric focusing gel. This technique also demonstrated the presence of altered apolipoprotein A-I isoforms in the plasma of a patient with Tangier disease. These isoforms were previously identified as isoforms 2 and 4 of normal plasma by use of two-dimensional gel electrophoresis. However, by use of this new technique and careful evaluation of previously published two-dimensional gels, we now identify these apolipoprotein A-I isoforms as being more acidic than those of normal plasma

In recent studies (1985. J. Lipid Res. 26:368-379 and 1986. J. Lipid Res. 27:30-39) we characterized aspects of synthesis of rat intestinal apolipoproteins (apo) A-I and B-48 in vivo, and their short term regulation by dietary and biliary lipid flux. We now report studies extending these observations to the effects on intestinal apoA-I and apoB-48 metabolism of sustained (3 or 6 weeks) isocaloric intake of diets containing 0-30% (by weight) triglyceride, the latter as either butter fat (saturated) or corn oil (polyunsaturated). Additional studies were conducted to determine, separately, the effects of perturbations of intestinal mucosal cholesterol flux and hypothyroidism on intestinal apoA-I and apoB-48 metabolism. Intestinal synthesis (% total protein) of apoA-I and apoB-48 was not influenced by either dietary triglyceride quantity or quality (saturated vs. polyunsaturated fat); the values that were obtained were strictly comparable to those of both chow-fed animals and animals maintained for 3 weeks on fat-free chow. Intestinal apoA-I synthesis was not influenced by either acute or chronic perturbations of mucosal cholesterol flux. Hypothyroid rats demonstrated a 50% suppression of jejunal apoA-I synthesis. Intestinal synthesis of apoB-48, by contrast, appeared to undergo regulation by chronic (but not acute) perturbations of mucosal cholesterol flux. Maneuvers that augmented intestinal cholesterol uptake (particularly hypothyroidism) appeared to suppress intestinal apoB-48 synthesis by over 40%, while Surfomer (AOMA) administration reduced cholesterol absorption (control, 54 +/- 7%; AOMA, 26 +/- 8%; P less than 0.0005) and resulted in a 24% increase in apoB-48 synthesis by jejunal enterocytes. Intracellular intestinal lipoproteins demonstrated marked cholesteryl ester enrichment of the triglyceriderich lipoprotein fractions in hypercholesterolemic, hypothyroid rats. When all the groups were compared, cholesterol absorption (used as an index of mucosal cholesterol uptake) was negatively correlated with jejunal apoB-48 synthesis (r = -0.92, P less than 0.05). The data suggest that regulation of rat intestinal apoA-I and apoB-48 metabolism is independent of triglyceride flux. It is further concluded that an important regulatory effect of mucosal cholesterol flux can be demonstrated on enterocyte apoB-48 synthesis. Finally, the data suggest the additional possibility that circulating levels of thyroid hormone may exert an independent effect on the expression of rat intestinal apolipoproteins A-I and B-48

The synthesis of apolipoprotein B (apoB) was examined in human fetal and adult intestine and liver. Intestine and liver were minced and then incubated with [3H]leucine, homogenized, and subjected to immunoprecipitation with antiserum that recognized both apoB-100 and apoB-48 (forms of apoB found in low density lipoproteins and in chylomicrons, respectively). Immunoprecipitates of fetal and adult liver contained radioactivity in a single apoB-100 peak when examined by NaDodSO4/polyacrylamide gel electrophoresis. Intestine from fetuses at 11 weeks of gestation incorporated radioactivity mainly into apoB-100, with little incorporation into apoB-48. Sixteen-week fetal intestine showed both apoB-100 and apoB-48, whereas adult intestine incorporated radioactivity only into apoB-48. Pulse-chase experiments with 11- and 16-week fetal intestine showed no evidence for the conversion of apoB-100 to apoB-48. Incubation of intestinal homogenates with fetal liver apoB-100 did not result in the conversion of apoB-100 to smaller forms of apoB. A cDNA probe to hepatic apoB-100 identified a single, 18-kilobase transcript in poly(A)+ RNA from fetal and adult liver and fetal intestine of all ages. These studies define the developmental pattern of apoB synthesis in human fetal and adult liver and intestine. No evidence could be found for the conversion of apoB-100 to apoB-48. The finding of a single mRNA transcript despite the form of apoB synthesized in each tissue is discussed

Apolipoprotein B (apoB) synthesis rates have been determined, in vivo, in rat enterocytes. Following intralumenal administration of a pulse of [3H]leucine, newly synthesized apoB was quantitated by specific immunoprecipitation and compared to [3H]leucine incorporation into total, trichloroacetic acid-insoluble protein. ApoB synthesis rates were determined after acute administration of either 0.1 or 1 g of triglyceride to fasting animals. No differences were found at any time from 90 min to 6 hr after challenge and values were not different from the basal values established in fasted controls. Animals rechallenged with triglyceride after 8 days' intake of fat-free chow also failed to demonstrate a change in intestinal apoB synthesis rate. By contrast, enterocyte content of apoB appeared to fall, temporarily, with the onset of active triglyceride flux. Groups of animals were then subjected to external bile diversion for 48 hr, a maneuver designed to remove all lumenal sources of lipid. Jejunal apoB synthesis rates fell by 43% (from 0.76% +/- 0.14 to 0.43% +/- 0.12, P less than 0.001), a change that was completely prevented by continuous replacement with 10 mM Na taurocholate. The suppression of jejunal apoB synthesis, induced by prolonged bile diversion, was reversed after 14 hr, but not 8 hr, of intralumenal perfusion with 10 mM Na taurocholate. The addition of micellar fatty acid-monoolein to the perfusate for 4 hr produced no further change in apoB synthesis. Ileal apoB synthesis rates fell by 70% (from 0.61% +/- 0.15 to 0.18% +/- 0.10, P less than 0.001) following 48 hr external bile diversion, a change that was only partially prevented by continuous bile salt replacement. These results suggest that jejunal apoB synthesis demonstrates bile salt dependence but not regulation by acute triglyceride flux. The data further suggest that key aspects of the regulation of apoB synthesis by cellular lipid flux may be mediated independently in jejunal and ileal enterocytes