The expression and metabolism of low density lipoprotein receptors in familial hypercholesterolaemia

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dc.contributor.advisorVan der Westhuyzen, Deneys Ren_ZA
dc.contributor.authorFourie, Anne Madeleineen_ZA
dc.date.accessioned2018-01-31T13:47:16Z
dc.date.available2018-01-31T13:47:16Z
dc.date.issued1989en_ZA
dc.description.abstractThe expression of two phenotypically-contrasting LDL receptor mutations was characterized in cultured fibroblasts from the genetically-homozygous Afrikaner subjects, FH1a and lb, and FH3a and 3b, respectively. Surface receptor expression and functional activity were studied by ligand (¹²⁵I-LDL) and monoclonal antibody (¹²⁵I-IgG-C7) binding, and c35s]-methionine pulse-chase experiments were used to analyze biosynthesis, processing and degradation of IgG-C7- immunoprecipitable mutant receptors. Cells from the "receptor-negative" subjects, FH3a and 3b exhibited reduced, but significant (40-60% of normal) LDL receptor synthesis rates. Newly-synthesized precursors were processed slowly (t½ 1.5 hours versus normal t½ of approximately 15 minutes) to mature receptors which reached the cell-surface, but were rapidly degraded thereafter with a half-life of approximately 1.7 hours (normal value 12.6 hours) thus representing a new type of LDL receptor defect. Lysosomotropic weak bases such as ammonium chloride partially inhibited rapid degradation of the mutant receptors, suggesting the involvement of proteolysis in acidic compartments such as lysosomes or endosomes. Fibroblasts from FH1a and lb exhibited normal synthesis rates of LDL receptor precursors that were processed at a severely reduced rate (t½ approximately 5 hours) to functionally heterogeneous mature surface receptors. Onethird of the receptors (20% of normal levels) bound ¹²⁵I-LDL with normal affinity at 4°C and 37°C, whereas the majority were able to recognize only ¹²⁵I-IgG-C7, and apparently showed defective internalisation and subsequent degradation of the bound IgG-C7 at 37°C. The existence of the two receptor populations was further supported by selective intracellular trapping and degradation of only the active, LDL-binding population, in the presence of ammonium chloride and LOL. The abnormal form predominated even in newly-synthesized receptors and reached a maximum of 50-70% of normal levels after 48 hours of upregulation. Upregulation kinetics and degradation rates (t½ = 10-11 hours) of both functionally-active and abnormal receptor populations were similar to normal. A progressive increase in apparent molecular weight of the slowly-processed precursor receptors suggested a possible role for abnormal glycosylation in the formation of both "normal" and abnormal conformations of the same receptor molecule.en_ZA
dc.identifier.apacitationFourie, A. M. (1989). <i>The expression and metabolism of low density lipoprotein receptors in familial hypercholesterolaemia</i>. (Thesis). University of Cape Town ,Faculty of Health Sciences ,Division of Medical Biochemistry & Structural Biology. Retrieved from http://hdl.handle.net/11427/27174en_ZA
dc.identifier.chicagocitationFourie, Anne Madeleine. <i>"The expression and metabolism of low density lipoprotein receptors in familial hypercholesterolaemia."</i> Thesis., University of Cape Town ,Faculty of Health Sciences ,Division of Medical Biochemistry & Structural Biology, 1989. http://hdl.handle.net/11427/27174en_ZA
dc.identifier.citationFourie, A. 1989. The expression and metabolism of low density lipoprotein receptors in familial hypercholesterolaemia. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Fourie, Anne Madeleine AB - The expression of two phenotypically-contrasting LDL receptor mutations was characterized in cultured fibroblasts from the genetically-homozygous Afrikaner subjects, FH1a and lb, and FH3a and 3b, respectively. Surface receptor expression and functional activity were studied by ligand (¹²⁵I-LDL) and monoclonal antibody (¹²⁵I-IgG-C7) binding, and c35s]-methionine pulse-chase experiments were used to analyze biosynthesis, processing and degradation of IgG-C7- immunoprecipitable mutant receptors. Cells from the "receptor-negative" subjects, FH3a and 3b exhibited reduced, but significant (40-60% of normal) LDL receptor synthesis rates. Newly-synthesized precursors were processed slowly (t½ 1.5 hours versus normal t½ of approximately 15 minutes) to mature receptors which reached the cell-surface, but were rapidly degraded thereafter with a half-life of approximately 1.7 hours (normal value 12.6 hours) thus representing a new type of LDL receptor defect. Lysosomotropic weak bases such as ammonium chloride partially inhibited rapid degradation of the mutant receptors, suggesting the involvement of proteolysis in acidic compartments such as lysosomes or endosomes. Fibroblasts from FH1a and lb exhibited normal synthesis rates of LDL receptor precursors that were processed at a severely reduced rate (t½ approximately 5 hours) to functionally heterogeneous mature surface receptors. Onethird of the receptors (20% of normal levels) bound ¹²⁵I-LDL with normal affinity at 4°C and 37°C, whereas the majority were able to recognize only ¹²⁵I-IgG-C7, and apparently showed defective internalisation and subsequent degradation of the bound IgG-C7 at 37°C. The existence of the two receptor populations was further supported by selective intracellular trapping and degradation of only the active, LDL-binding population, in the presence of ammonium chloride and LOL. The abnormal form predominated even in newly-synthesized receptors and reached a maximum of 50-70% of normal levels after 48 hours of upregulation. Upregulation kinetics and degradation rates (t½ = 10-11 hours) of both functionally-active and abnormal receptor populations were similar to normal. A progressive increase in apparent molecular weight of the slowly-processed precursor receptors suggested a possible role for abnormal glycosylation in the formation of both "normal" and abnormal conformations of the same receptor molecule. DA - 1989 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 1989 T1 - The expression and metabolism of low density lipoprotein receptors in familial hypercholesterolaemia TI - The expression and metabolism of low density lipoprotein receptors in familial hypercholesterolaemia UR - http://hdl.handle.net/11427/27174 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/27174
dc.identifier.vancouvercitationFourie AM. The expression and metabolism of low density lipoprotein receptors in familial hypercholesterolaemia. [Thesis]. University of Cape Town ,Faculty of Health Sciences ,Division of Medical Biochemistry & Structural Biology, 1989 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/27174en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDivision of Medical Biochemistry and Structural Biology
dc.publisher.facultyFaculty of Health Sciencesen_ZA
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherCell receptorsen_ZA
dc.subject.otherLipoproteinsen_ZA
dc.subject.otherHypercholesteremiaen_ZA
dc.subject.otherHypercholesterolemia, Familialen_ZA
dc.subject.otherReceptors,en_ZA
dc.titleThe expression and metabolism of low density lipoprotein receptors in familial hypercholesterolaemiaen_ZA
dc.typeDoctoral Thesis
dc.type.qualificationlevelDoctoral
dc.type.qualificationnamePhDen_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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