Intestinal apoproteins during fat absorption.

A B S T R A C T The effect of the microtubule inhibitor colchicine on the metabolism of 125I-low density lipoprotein (LDL) by cultured human skin fibroblasts and aortic medial cells was studied in vitro. Colchicine did not alter the binding of LDL to cell surface receptors. However, the rate of LDL endocytosis was reduced to 58% of that expected. Despite diminished endocytosis, LDL was found to accumulate within the cells to 165% of that expected, whereas the release of LDL protein degradation products into the medium was reduced to 34% of control, findings consistent with a reduced rate of intracellular LDL breakdown. Colchicine did not alter cell content of the acid protease which degrades LDL, nor did [3H]colchicine accumulate in lysosomal fractions. However, colchicine did alter the intracellular distribution of both fibroblast lysosomes and endosomes. After colchicine, lysosomes tended to accumulate in the perinuclear region, whereas endosomes were found at the cell periphery. These findings are consistent with the hypothesis that ingested LDL is less available to lysosomal enzymes in the presence of colchicine. The actions of colchicine appear to be a result of destruction of cell microtubules. Lumicolchicine, a mixture of colchicine isomers which (unlike the parent compound) does not bind to the subunit of microtubules, was without effect.The uptake and degradation of LDL by cultured cells consists ofboth a receptor-specific component and nonspecific pinocytosis. Important differences must exist between these processes because even large amounts of LDL taken up and degraded by the nonspecific route fail to regulate key aspects of intracellular cholesterol metabolism. Colchicine selectively inhibited receptormediated LDL degradation. No effect was demonstrable on the nonspecific degradation of LDL by familial hypercholesterolemia fibroblasts grown in

Section: Resultsmentioning

confidence: 90%

Role of Microtubules in Low Density Lipoprotein Processing by Cultured Cells

Ostlund¹,

Pfleger²,

Schonfeld³

1979

“…The triglyceride concentration was measured using a commercial kit (#701882; Boehringer Mannheim Corp.) and the radioactivity in the triglyceride fraction was determined using TLC (19). To determine intestinal triglyceride production, three control and five high expressor Triton WR 1339-treated female mice were fed by gastric tube 100 ,Ci ofglycerol-[3H]trioleate mixed with 100 1d ofcorn oil (25). Blood samples were drawn 30, 45, 60, and 90 min thereafter and the amounts of radioactivity in serum triglycerides were determined as described above.…”

Section: Methodsmentioning

confidence: 99%

Mechanism of hypertriglyceridemia in human apolipoprotein (apo) CIII transgenic mice. Diminished very low density lipoprotein fractional catabolic rate associated with increased apo CIII and reduced apo E on the particles.

Aalto‐Setälä¹,

Fisher²,

Chen³

et al. 1992

422

312

Hypertriglyceridemia is common in the general population, but its mechanism is largely unknown. In previous work human apo CIII transgenic (HuCIIITg) mice were found to have elevated triglyceride levels. In this report, the mechanism for the hypertriglyceridemia was studied. Two different HuCIIITg mouse lines were used: a low expresser line with serum triglycerides of -280 mg/dl, and a high expressor line with serum triglycerides of -1,000 mg/dl. Elevated triglycerides were mainly in VLDL. VLDL particles were 1.5 times more triglyceride-rich in high expressor mice than in controls. The total amount of apo CIII (human and mouse) per VLDL particle was 2 and 2.5 times the normal amount in low and high expressors, respectively. Mouse apo E was decreased by 35 and 77% in low and high expresser mice, respectively. Under electron microscopy, VLDL particles from low and high expresser mice were found to have a larger mean diameter, 55.2±16.6 and 58.2±17.8 nm, respectively, compared with 51.0±13.4 nm from control mice. In in vivo studies, radiolabeled VLDL fractional catabolic rate (FCR) was reduced in low and high expresser mice to 2.58 and 0.77 pools/h, respectively, compared with 7.67 pools/h in controls, with no significant differences in the VLDL production rates. In an attempt to explain the reduced VLDL FCR in transgenic mice, tissue lipoprotein lipase (LPL) activity was determined in control and high expresser mice and no differences were observed. Also, VLDLs obtained from control and high expresser mice were found to be equally good substrates for purified LPL

“…The small intestine is a source of apoA-I in humans (20,34) as well as the rat (35)(36)(37). Studies in chyluric subjects show that the human small intestine is a quantitatively significant source of phospholipid and apoA-I (21).…”

Section: Incubation Of Chylomicrons In Serummentioning

confidence: 99%

Metabolic Fate of Chylomicron Phospholipids and Apoproteins in the Rat

Tall¹,

Green²,

Glickman³

et al. 1979

210

A B S T R A C T To study the metabolic fate of chylomicron phospholipid and apoproteins, 15 mg of doubly labeled ([3H]leu, [32P]phospholipid) rat mesenteric lymph chylomicrons were injected as an intravenous bolus into conscious rats. The specific radioactivity, composition, pool size, and morphology of the plasma lipoproteins were determined after 2-60 min. After injection of chylomicrons, there was a rapid transfer of radioactivity into high density lipoproteins (HDL). At peak specific activity in HDL (2-5 min), 35% of injected apoprotein and 25% of phospholipid radioactivity were recovered in HDL (d 1.063-1.21 g/ml), with smaller recoveries in other lipoproteins and liver. There was an initial rapid rise of 32P specific activity in HDL and d 1.02-1.063 lipoproteins (low density lipoproteins [LDL]), but whereas LDL specific activity subsequently converged with that of d < 1.02 lipoproteins, HDL specific activity decayed more rapidly than LDL or d < 1.02 lipoproteins.Lipolysis of chylomicrons was associated with a transfer of phospholipid mass into LDL and HDL. At 5 min, 80% of injected triglyceride had been lipolyzed and there was a significant increase in phospholipid mass in LDL and a smaller increase in HDL. At 10 min,