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Apolipoprotein biosynthesis and turnover in mammalian small intestine
The mammalian small intestine is a major site (second in total activity only to the liver) for the synthesis and secretion of plasma apolipoproteins, and contributes significantly to overall whole-body lipid dynamics. A prominent feature of the small intestine is its exposure to periodic loads of me...
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| Format: | Thesis |
| Language: | English |
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Division of Medical Biochemistry and Structural Biology
2018
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| _version_ | 1867613486011383808 |
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| access_status_str | Open Access |
| author | Combrinck, Marc Irwin |
| author2 | Gevers, Wieland |
| author_browse | Combrinck, Marc Irwin Gevers, Wieland |
| author_facet | Gevers, Wieland Combrinck, Marc Irwin |
| author_sort | Combrinck, Marc Irwin |
| collection | Thesis |
| description | The mammalian small intestine is a major site (second in total activity only to the liver) for the synthesis and secretion of plasma apolipoproteins, and contributes significantly to overall whole-body lipid dynamics. A prominent feature of the small intestine is its exposure to periodic loads of meals often containing dramatically varying amounts or types of food components, including lipids such as tri-acylglycerols, cholesterol and cholesteryl esters. Since the trans-epithelial transport of most of these latter materials requires the elaboration of particles partially covered by apolipoproteins, the regulation of the biosynthesis or, more correctly, the availability of these proteins is an important and as yet little-understood problem. Previous studies have been conducted on systems which, for one or the other reason, have not permitted the following questions to be satisfactorily or coherently answered: Does the ingestion of fat-containing meals, either acutely or chronically, increase the rate of biosynthesis of intestinal apolipoproteins such as apo B-48, and is this the principal method of matching the "demand" with the supply of this "packaging material" needed for fat transport across the intestinal epithelial cells? Alternatively, does the maintenance of a large steady-state intracellular pool in the face of variations in intracellular apolipoprotein degradation, controlled by acute or chronic lipid ingestion, produce the required "match" between supply and demand for these proteins (as has recently been suggested in studies on liver cells)? An in vitro system was therefore devised whereby sheets of intestinal epithelial cells (enterocytes) were freshly isolated from the jejuna of adult male Syrian golden hamsters and incubated for several hours in a medium supporting steady-state protein synthesis, in a manner which was assumed to be similar to the activity just before the killing of the donor animals. (Hamsters appear on various grounds to be a better small-animal model of human lipoprotein metabolism than the more commonly studied rats). The isolated epithelial cell sheets produced primary apolipoprotein products that could be extracted from the cells or detected in the incubation media, free from the subsequent modifications that they are known to undergo in vivo. Hamsters maintained on a low-fat chow were either studied as such or subjected to a variety of dietary treatments designed to maximize (over short or long time periods) intracellular apolipoprotein requirements for the "packaging" of tri-acylglycerol-rich lipoproteins, especially chylomicrons: acute bolus administration of lipid into the gut; overnight feeding of fat-enriched food; and chronic (six week) fat feeding. Using specific antisera and immuno-precipitation techniques, apo B-48 and two other principal intestinal apolipoproteins were shown to be synthesized in the steady state by intestinal cell sheets derived from control animals and from those subjected to acute or chronic fat-containing diets. Secretion took place, however, only when prior fat exposure of the donor intestines had occurred. Pulse-chase labelling was used to compare the rates of apolipoprotein synthesis, degradation and secretion in the same cell sheet preparations. The rates of apolipoprotein B-48 synthesis did not vary significantly under conditions of low or high trans-epithelial lipid flux, supporting findings derived from in vivo experimental systems. In contrast with data from other systems, however, the biosynthesis of apolipoprotein A-IV was not reproducibly increased on fat challenge. The rates of apo B-48 degradation varied significantly and were markedly reduced under conditions of fat feeding. The experiments permit a choice between the two alternatives mentioned above: Ingestion of fatty foods, either acutely or over long periods of time, does not increase the rates of biosynthesis of apolipoproteins such as apo B-48; but variations in the rate of intracellular degradation of this and probably other apolipoproteins allows the intestinal cells to match their requirements for lipid-transporting molecules to the demands of any given situation, relying in each case on a large steady-state intracellular pool maintained by "constitutive" biosynthesis. Importantly, there seems also to be a specific, possibly related effect of fat feeding on the secretion of lipoproteins into the intestinal extracellular fluid. These conclusions coincide with those obtained by other workers from studies of apolipoprotein B dynamics in isolated hepatocytes and in the hepatoma-derived liver cell line, Hep G2. The mechanisms underlying these phenomena are as yet unresolved. |
| format | Thesis |
| id | oai:open.uct.ac.za:11427/27140 |
| institution | University of Cape Town (South Africa) |
| language | eng |
| last_indexed | 2026-06-10T12:36:54.583Z |
| license_str | Not specified — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from UCTD — University of Cape Town Open Access Repository |
| publishDate | 2018 |
| publishDateRange | 2018 |
| publishDateSort | 2018 |
| publisher | Division of Medical Biochemistry and Structural Biology |
| publisherStr | Division of Medical Biochemistry and Structural Biology |
| record_format | dspace |
| source_str | UCTD — University of Cape Town Open Access Repository |
| spelling | oai:open.uct.ac.za:11427/27140 Apolipoprotein biosynthesis and turnover in mammalian small intestine Combrinck, Marc Irwin Gevers, Wieland Medical Biochemistry Apolipoproteins - biosynthesis Intestine, Small - metabolism The mammalian small intestine is a major site (second in total activity only to the liver) for the synthesis and secretion of plasma apolipoproteins, and contributes significantly to overall whole-body lipid dynamics. A prominent feature of the small intestine is its exposure to periodic loads of meals often containing dramatically varying amounts or types of food components, including lipids such as tri-acylglycerols, cholesterol and cholesteryl esters. Since the trans-epithelial transport of most of these latter materials requires the elaboration of particles partially covered by apolipoproteins, the regulation of the biosynthesis or, more correctly, the availability of these proteins is an important and as yet little-understood problem. Previous studies have been conducted on systems which, for one or the other reason, have not permitted the following questions to be satisfactorily or coherently answered: Does the ingestion of fat-containing meals, either acutely or chronically, increase the rate of biosynthesis of intestinal apolipoproteins such as apo B-48, and is this the principal method of matching the "demand" with the supply of this "packaging material" needed for fat transport across the intestinal epithelial cells? Alternatively, does the maintenance of a large steady-state intracellular pool in the face of variations in intracellular apolipoprotein degradation, controlled by acute or chronic lipid ingestion, produce the required "match" between supply and demand for these proteins (as has recently been suggested in studies on liver cells)? An in vitro system was therefore devised whereby sheets of intestinal epithelial cells (enterocytes) were freshly isolated from the jejuna of adult male Syrian golden hamsters and incubated for several hours in a medium supporting steady-state protein synthesis, in a manner which was assumed to be similar to the activity just before the killing of the donor animals. (Hamsters appear on various grounds to be a better small-animal model of human lipoprotein metabolism than the more commonly studied rats). The isolated epithelial cell sheets produced primary apolipoprotein products that could be extracted from the cells or detected in the incubation media, free from the subsequent modifications that they are known to undergo in vivo. Hamsters maintained on a low-fat chow were either studied as such or subjected to a variety of dietary treatments designed to maximize (over short or long time periods) intracellular apolipoprotein requirements for the "packaging" of tri-acylglycerol-rich lipoproteins, especially chylomicrons: acute bolus administration of lipid into the gut; overnight feeding of fat-enriched food; and chronic (six week) fat feeding. Using specific antisera and immuno-precipitation techniques, apo B-48 and two other principal intestinal apolipoproteins were shown to be synthesized in the steady state by intestinal cell sheets derived from control animals and from those subjected to acute or chronic fat-containing diets. Secretion took place, however, only when prior fat exposure of the donor intestines had occurred. Pulse-chase labelling was used to compare the rates of apolipoprotein synthesis, degradation and secretion in the same cell sheet preparations. The rates of apolipoprotein B-48 synthesis did not vary significantly under conditions of low or high trans-epithelial lipid flux, supporting findings derived from in vivo experimental systems. In contrast with data from other systems, however, the biosynthesis of apolipoprotein A-IV was not reproducibly increased on fat challenge. The rates of apo B-48 degradation varied significantly and were markedly reduced under conditions of fat feeding. The experiments permit a choice between the two alternatives mentioned above: Ingestion of fatty foods, either acutely or over long periods of time, does not increase the rates of biosynthesis of apolipoproteins such as apo B-48; but variations in the rate of intracellular degradation of this and probably other apolipoproteins allows the intestinal cells to match their requirements for lipid-transporting molecules to the demands of any given situation, relying in each case on a large steady-state intracellular pool maintained by "constitutive" biosynthesis. Importantly, there seems also to be a specific, possibly related effect of fat feeding on the secretion of lipoproteins into the intestinal extracellular fluid. These conclusions coincide with those obtained by other workers from studies of apolipoprotein B dynamics in isolated hepatocytes and in the hepatoma-derived liver cell line, Hep G2. The mechanisms underlying these phenomena are as yet unresolved. 2018-01-30T14:00:28Z 2018-01-30T14:00:28Z 1994 Doctoral Thesis Doctoral PhD http://hdl.handle.net/11427/27140 eng application/pdf Division of Medical Biochemistry and Structural Biology Faculty of Health Sciences University of Cape Town |
| spellingShingle | Medical Biochemistry Apolipoproteins - biosynthesis Intestine, Small - metabolism Combrinck, Marc Irwin Apolipoprotein biosynthesis and turnover in mammalian small intestine |
| thesis_degree_str | Doctoral |
| title | Apolipoprotein biosynthesis and turnover in mammalian small intestine |
| title_full | Apolipoprotein biosynthesis and turnover in mammalian small intestine |
| title_fullStr | Apolipoprotein biosynthesis and turnover in mammalian small intestine |
| title_full_unstemmed | Apolipoprotein biosynthesis and turnover in mammalian small intestine |
| title_short | Apolipoprotein biosynthesis and turnover in mammalian small intestine |
| title_sort | apolipoprotein biosynthesis and turnover in mammalian small intestine |
| topic | Medical Biochemistry Apolipoproteins - biosynthesis Intestine, Small - metabolism |
| url | http://hdl.handle.net/11427/27140 |
| work_keys_str_mv | AT combrinckmarcirwin apolipoproteinbiosynthesisandturnoverinmammaliansmallintestine |