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Kalaivani V, Krishna MS, Kumar AA, Satheesh G, Jaleel A. O-glycan structures in apo(a) subunit of human lipoprotein(a) suppresses the pro-angiogenic activity of galectin-1 on human umbilical vein endothelial cells. FASEB J 2023; 37:e22813. [PMID: 36809652 DOI: 10.1096/fj.202201001rr] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2022] [Revised: 12/29/2022] [Accepted: 01/27/2023] [Indexed: 02/23/2023]
Abstract
Apolipoprotein(a) [apo(a)] is a highly polymorphic O-glycoprotein circulating in human plasma as lipoprotein(a) [Lp(a)]. The O-glycan structures of apo(a) subunit of Lp(a) serve as strong ligands of galectin-1, an O-glycan binding pro-angiogenic lectin abundantly expressed in placental vascular tissues. But the pathophysiological significance of apo(a)-galectin-1 binding is not yet been revealed. Carbohydrate-dependent binding of galectin-1 to another O-glycoprotein, neuropilin-1 (NRP-1) on endothelial cells activates vascular endothelial growth factor receptor 2 (VEGFR2) and mitogen-activated protein kinase (MAPK) signaling. Using apo(a), isolated from human plasma, we demonstrated the potential of the O-glycan structures of apo(a) in Lp(a) to inhibit angiogenic properties such as proliferation, migration, and tube-formation in human umbilical vein endothelial cells (HUVECs) as well as neovascularization in chick chorioallantoic membrane. Further, in vitro protein-protein interaction studies have confirmed apo(a) as a superior ligand to NRP-1 for galectin-1 binding. We also demonstrated that the protein levels of galectin-1, NRP-1, VEGFR2, and downstream proteins in MAPK signaling were reduced in HUVECs in the presence of apo(a) with intact O-glycan structures compared to that of de-O-glycosylated apo(a). In conclusion, our study shows that apo(a)-linked O-glycans prevent the binding of galectin-1 to NRP-1 leading to the inhibition of galectin-1/neuropilin-1/VEGFR2/MAPK-mediated angiogenic signaling pathway in endothelial cells. As higher plasma Lp(a) level in women is an independent risk factor for pre-eclamsia, a pregnancy-associated vascular complication, we propose that apo(a) O-glycans-mediated inhibition of the pro-angiogenic activity of galectin-1 may be one of the underlying molecular mechanism of pathogenesis of Lp(a) in pre-eclampsia.
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Affiliation(s)
- Vasantha Kalaivani
- Diabetes Biology Laboratory, Division of Cardiovascular Diseases and Diabetes Biology, Rajiv Gandhi Centre for Biotechnology, Thiruvananthapuram, India
| | - Mahesh S Krishna
- Diabetes Biology Laboratory, Division of Cardiovascular Diseases and Diabetes Biology, Rajiv Gandhi Centre for Biotechnology, Thiruvananthapuram, India
| | - Asokan Aneesh Kumar
- Diabetes Biology Laboratory, Division of Cardiovascular Diseases and Diabetes Biology, Rajiv Gandhi Centre for Biotechnology, Thiruvananthapuram, India
| | - Gopika Satheesh
- Diabetes Biology Laboratory, Division of Cardiovascular Diseases and Diabetes Biology, Rajiv Gandhi Centre for Biotechnology, Thiruvananthapuram, India
| | - Abdul Jaleel
- Diabetes Biology Laboratory, Division of Cardiovascular Diseases and Diabetes Biology, Rajiv Gandhi Centre for Biotechnology, Thiruvananthapuram, India
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Karthi S, Sukumari-Ramesh S, Geetha M, Appukuttan PS. High glucose removes natural anti-α-galactoside and anti-β-glucoside antibody immune complexes adhering to surface O-glycoproteins of normal platelets and enhances platelet aggregation. Exp Ther Med 2022; 23:82. [PMID: 34934451 PMCID: PMC8652397 DOI: 10.3892/etm.2021.11005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2020] [Accepted: 06/16/2021] [Indexed: 11/06/2022] Open
Abstract
Human natural anti-α-galactoside (anti-Gal) and anti-β-glucoside (ABG) antibodies were previously reported to recognize the serine- and threonine-rich peptide sequences (STPS) of albumin-associated O-glycoproteins (AOP1 and AOP2) as surrogate antigens, forming anti-Gal/ABG-AOP1/AOP2-albumin triplet immune complexes in plasma. Since antibodies in these triplets still possessed unoccupied binding sites, the presence of triplets on human platelets that abound in surface O-glycoproteins was examined. Upon treatment with α-galactosides and β-glucosides, normal platelets freshly isolated from young healthy individuals released triplets identical with plasma triplets according to ELISA results. The resulting denuded platelets, unless pre-treated with fibrinogen or the O-glycan-binding lectin jacalin, recaptured these sugar-extracted triplets in the absence of antibody-specific sugars, suggesting that the triplet antibodies recognized the STPS of O-glycosylated receptors on platelets. Molecular weight of the dominant jacalin-binding subunit on triplet-free platelet membrane was 116 kDa, close to the ~120 kDa reported for the IIb subunit of the most abundant fibrinogen-binding platelet O-glycoprotein, GPIIb/IIIa. Denuded, but not native, platelets underwent slow spontaneous aggregation and rapid ADP-mediated GPIIb/IIIa-dependent aggregation according to spectrophotometric assay. Pre-treatment of denuded platelets with jacalin significantly reduced their ADP-mediated aggregation. Amyloid β (Aβ-42 monomer) was reported to bind triplet O-glycoproteins through their STPS but not to albumin or the antibodies. This peptide bound to the triplets on normal platelets and to surface membrane O-glycoproteins on denuded platelets, suggesting that the surface O-glycoproteins on the normal platelets were engaged and masked by the triplets. The ABG-specific sugar glucose denuded the platelets at concentrations typically reached in diabetic sera, since anti-Gal specific or ABG-specific sugar released the triplets of both the antibodies from the platelets. In conclusion, the present study offered rationale for the presence of anti-Gal/ABG-O-glycoprotein-albumin triplets on normal platelets, for the role of triplets in platelet physiology amidst circulating platelet-activating factors such as ADP, and for platelet vulnerability during diabetes.
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Affiliation(s)
- Sreedevi Karthi
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram, Kerala 695011, India
| | - Sangeetha Sukumari-Ramesh
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram, Kerala 695011, India
| | - Mandagini Geetha
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram, Kerala 695011, India
| | - Padinjaradath Sankunni Appukuttan
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram, Kerala 695011, India
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Apolipoprotein(a), an enigmatic anti-angiogenic glycoprotein in human plasma: A curse or cure? Pharmacol Res 2020; 158:104858. [PMID: 32430285 DOI: 10.1016/j.phrs.2020.104858] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/18/2019] [Revised: 04/09/2020] [Accepted: 04/21/2020] [Indexed: 02/06/2023]
Abstract
Angiogenesis is a finely co-ordinated, multi-step developmental process of the new vascular structure. Even though angiogenesis is regularly occurring in physiological events such as embryogenesis, in adults, it is restricted to specific tissue sites where rapid cell-turnover and membrane synthesis occurs. Both excessive and insufficient angiogenesis lead to vascular disorders such as cancer, ocular diseases, diabetic retinopathy, atherosclerosis, intra-uterine growth restriction, ischemic heart disease, stroke etc. Occurrence of altered lipid profile and vascular lipid deposition along with vascular disorders is a hallmark of impaired angiogenesis. Among lipoproteins, lipoprotein(a) needs special attention due to the presence of a multi-kringle protein subunit, apolipoprotein(a) [apo(a)], which is structurally homologous to many naturally occurring anti-angiogenic proteins such as plasminogen and angiostatin. Researchers have constructed different recombinant forms of apo(a) (rhLK68, rhLK8, RHACK2, KV-11, and AU-6) and successfully exploited its potential to inhibit unwanted angiogenesis during tumor metastasis and retinal neovascularization. Similar to naturally occurring anti-angiogenic proteins, apo(a) can directly interfere with angiogenic signaling pathways. Besides this, apo(a) can also exert its anti-angiogenic effect indirectly by inducing endothelial cell apoptosis, by inhibiting endothelial progenitor cell functions or by upregulating nuclear factors in endothelial cells via apo(a)-bound oxPLs. However, the impact of the anti-angiogenic potential of native apo(a) during physiological angiogenesis in embryos and wounded tissues is not yet explored. In this context, we review the studies so far done to demonstrate the anti-angiogenic activity of apo(a) and the recent developments in using apo(a) as a therapeutic agent to treat impaired angiogenesis during vascular disorders, with emphasis on the gaps in the literature.
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John J, Cherian K, Abraham T, Appukuttan PS. Low reactivity of tumor MUC1-binding natural anti-α-galactoside antibody is a risk factor for breast cancer. Exp Biol Med (Maywood) 2020; 245:254-265. [PMID: 31955603 DOI: 10.1177/1535370220901493] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Natural plasma anti-α-galactoside antibody (anti-Gal) reactivity was reported to vary inversely with the individual’s lipoprotein(a) [Lp(a)] size. Since MUC1 mucin over-expressed in tumors bear surrogate peptide ligands for anti-Gal, we examined if high anti-Gal reactivity in small size/high titer Lp(a) individuals correlated with lower incidence of breast cancer. Newer protocol for size determination revealed that Lp(a) in controls were significantly smaller than in breast cancer patients ( P = 0.0023; n = 46 in either group). Activity per unit plasma volume and specific reactivity (reactivity per unit immunoglobulin) of anti-Gal were significantly lower in cancer patients ( P = 0.0033). Specific reactivity lower than the mean of controls was a risk factor for breast cancer with odds ratio (OR) 3.2 (95% confidence interval [CI]: 1.368–7.557). Immunochemical staining using fluorescein isothiocyanate-labeled anti-Gal revealed absolute inactivity towards normal cells and strong recognition of cancer cells by the antibody. O-Glycosylation of MUC1, though more frequent than in normal cells, was incomplete in tumor cells as revealed by binding of the O-glycan-specific lectin jacalin, accounting for the access of anti-Gal to its peptide ligand in cancer MUC1. As tumor advanced and MUC1 with increasing affinity for anti-Gal was synthesized by the tumor, the specific reactivity of circulating anti-Gal also increased, apparently due to antigenic stimulation or affinity maturation by the proliferating MUC1, indicating that pre-cancer anti-Gal reactivity in patients should have been much lower than measured after detection of cancer and that lower reactivity of the antibody is a stronger risk factor for breast cancer than indicated by the OR above. Reactivity towards a given group of tumor MUC1 antigens increased in proportion to anti-Gal specific reactivity. Results suggested tumor-specific MUC1 as likely target for anti-Gal-mediated anti-cancer defense and offer infusion of small Lp(a) or high reactivity anti-Gal as possible immunopotentiation measures.Impact statementThis paper offers a molecular explanation for the positive correlation of individuals’ lipoprotein(a) [Lp(a)] size with breast cancer incidence, found more pronounced using interference-free assays. It established unambiguously the marked affinity of human anti-Gal antibody towards cancer phenotype of the cell surface MUC1 and inertness towards normal cell MUC1. This selectivity enabled small Lp(a) molecules, known to produce higher specific reactivity anti-Gal by affinity maturation, to achieve more efficient immune defense so that women with specific reactivity lower than the mean value of normal subjects ran cancer risk with odds ratio (OR) above 3.2. However, increasing O-glycosylation and decreasing O-glycan length of MUC1 with tumor advance increased anti-Gal specific reactivity, indicating antigenic stimulation and/or affinity maturation of the antibody by tumor MUC1. Thus, pre-cancer anti-Gal specific reactivity should be lower than that measured on detection and the above OR actually higher. Results suggest small Lp(a) and high specific reactivity anti-Gal infusions as therapeutic options.
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Affiliation(s)
- Jessy John
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695 011, India
| | - Kurian Cherian
- Division of Surgical Oncology, Regional Cancer Centre, Thiruvananthapuram 695 011, India
| | - Thomas Abraham
- Division of Clinical Laboratory Services, Regional Cancer Centre, Thiruvananthapuram 695 011, India
| | - Padinjaradath S Appukuttan
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695 011, India
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Karthi S, Sumitha KC, Geetha M, Appukuttan PS. Amyloid β Binds to Albumin-Associated Lrp-Like Plasma O-Glycoproteins: Albumin Prevents Inhibition of Binding by LDL. Protein Pept Lett 2019; 26:869-878. [PMID: 37020364 DOI: 10.2174/0929866526666190722151027] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2019] [Revised: 06/16/2019] [Accepted: 07/18/2019] [Indexed: 11/22/2022]
Abstract
<P>Background: Albumin was reported to engage nearly 95% of plasma Amyloid β (Aβ)
and to reverse Aβ fibril formation in brain.
</P><P>
Objective: Since O-glycosylated LRP family of receptors capture Aβ in brain we compared Aβ
binding to electrophoretically purified albumin and to O-glycoproteins AOP1 and AOP2 that
adhere noncovalently to plasma albumin.
</P><P>
Methods: Strength of Aβ-protein interaction was measured as fluorescence increase in Fluorescentlabeled
Aβ (F-Aβ) resulting from conformational changes. Alternatively, differential segregation of
free and protein-bound Aβ in Density Gradient Ultracentrifugation (DGUC) was also examined.
</P><P>
Results: Fluorescence enhancement in F-Aβ was significantly greater by AOP1 and AOP2 than by
known Aβ reactants α -synuclein and β -cyclodextrin, but nil by albumin. In DGUC Aβ migrated
with the O-glycoproteins but not with albumin. Free O-glycoproteins unlike their albumin-bound
forms were blocked by LDL from capturing F-Aβ. Associated albumin did not affect Aβ binding of
O-glycoproteins. De-O-glycosylation of AOP1/AOP2 enhanced their Aβ binding showing that
peptide sequences at O-glycosylated regions were recognized by Aβ. Unlike albumin, AOP1 and
AOP2 were immunologically cross-reactive with LRP. Albumin sample used earlier to report
albumin-Aβ interaction contained two O-glycoproteins cross-reactive with human LRP and equal in
size to human AOP1 or AOP2.
</P><P>
Conclusion: Unlike albumin, albumin-bound O-glycoproteins, immunologically cross-reactive
with LRP, bind plasma Aβ. These O-glycoproteins are potential anti-amyloidogenic therapeutics if
they inhibit Aβ aggregation as other Aβ reactants do. Circulating immune complexes of albuminbound
O-glycoproteins with O-glycoprotein-specific natural antibodies can bind further to LRP-like
membrane proteins and are possible O-glycoprotein transporters to tissues.</P>
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Affiliation(s)
- Sreedevi Karthi
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695011, Kerala, India
| | - K. C. Sumitha
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695011, Kerala, India
| | - Mandagini Geetha
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695011, Kerala, India
| | - Padinjaradath S. Appukuttan
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695011, Kerala, India
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John J, Kalaivani V, Geetha M, Appukuttan PS. Activity of MUC1 cancer antigen-binding plasma anti-α-galactoside antibody correlates inversely with size of autologous lipoprotein(a). Exp Biol Med (Maywood) 2019; 244:893-900. [PMID: 31397607 DOI: 10.1177/1535370219855002] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Variation in ligand-binding affinity of natural plasma anti-α-galactoside antibody (anti-Gal) is a plausible reason for differing anti-cancer defense among individuals since serine- and threonine-rich peptide sequences (STPS) in the cancer-specific MUC-1 antigen are surrogate ligands for this antibody. As affinity of a natural antibody could be modulated by systemic antigens by processes including affinity maturation, we examined the contribution of the size of lipoprotein(a) [Lp(a)], an efficient autologous anti-Gal-binding macromolecule that possesses variable numbers of STPS due to genetically determined size polymorphism, towards the specific activity (activity per unit mass) of anti-Gal. Binding of purified Lp(a) to FITC-labeled anti-Gal, measured in terms of increase in fluorescence of the latter, was inhibited by LDL in proportion to Lp(a) size presumably because LDL molecules also bind noncovalently and in proportion to Lp(a) size at the O-glycosylated and STPS-rich region of Lp(a). For the same reason, circulating forms of smaller Lp(a) which carried fewer or no noncovalently attached LDL molecules were more efficient ligands for the antibody than the same number of larger ones ( P < 0.0001). Result suggested that smaller Lp(a), with their STPS ligands less obstructed by adhering LDL, would be more effective systemic antigens for anti-Gal. In confirmation of this, the specific activity of anti-Gal decreased with Lp(a) size (r − 0.5443; P < 0.0001) but increased with Lp(a) concentration (r 0.6202; P < 0.0001) among 73 normal plasma samples. IgG to IgM ratio, an index of immunoglobulin class switching characteristic of affinity maturation, was decidedly higher for anti-Gal in small Lp(a) individuals than in their large Lp(a) counterparts ( P = 0.0014). Results indicated that modulation of activity of anti-Gal by Lp(a) size may account for the lower incidence of cancer reported in people carrying more plasma Lp(a) which are generally smaller as well.
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Affiliation(s)
- Jessy John
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695011, India
| | - Vasantha Kalaivani
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695011, India
| | - Mandagini Geetha
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695011, India
| | - Padinjaradath S Appukuttan
- Department of Biochemistry, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram 695011, India
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Sreedevi K, Subramanian SP, Mandagini G, Appukuttan PS. Anti-α-galactoside and Anti-β-glucoside Antibodies are Partially Occupied by Either of Two Albumin-bound O-glycoproteins and Circulate as Ligand-binding Triplets. Immunol Invest 2018; 48:222-241. [PMID: 30081721 DOI: 10.1080/08820139.2018.1502299] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
Two heavily O-glycosylated proteins and albumin co-purified with anti-α-galactoside (anti-Gal), the chief xenograft-rejecting antibody and anti-β-glucan (ABG) antibody isolated from human plasma by affinity chromatography on respective ligand-bearing matrices. Both antibodies and O-glycoproteins co-purified with plasma albumin eluted from albumin-specific matrix. Using components of affinity-purified antibody samples separated by electrophoresis binding of either albumin or antibody to the affinity matrix of the other or binding of O-glycoprotein to either matrix was ruled out. Enzyme-linked immunoassay and ligand-induced fluorescence enhancement of fluorolabeled antibody showed that O-glycoproteins occupied sugar-binding sites of anti-Gal and ABG. Neither antibody recognized albumin. O-Glycoprotein-albumin complexes free in plasma, or released from antibodies by specific sugars, were captured on microwell-coated O-glycan-specific lectin jacalin and detected using labeled anti-albumin. We conclude that circulating anti-Gal and ABG form protein triplets in which either O-glycoprotein bridges between antibody and albumin by binding simultaneously to both. Bound albumin restricted O-glycoprotein occupation on antibodies enabling triplets to bind other ligands using spared binding sites. Free anti-Gal and ABG were undetectable in plasma. Jacalin treatment, but not de-O-glycosylation of O-glycoproteins abolished their recognition by anti-Gal or ABG indicating that antibodies recognized serine- and threonine-rich peptide sequences that underlie the O-glycans and are reported surrogate ligands for anti-Gal. The albumin- and antibody-binding O-glycoproteins AOP1 and AOP2 were single polypeptide proteins of size 107 kDa and 98 kDa, containing 54% and 51% carbohydrate respectively and conformed to no known plasma protein in properties. Prospects of triplet-mediated modulations in autologous tissues expressing antibody ligands are discussed.
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Affiliation(s)
- Karthi Sreedevi
- a Department of Biochemistry , Sree Chitra Tirunal Institute for Medical Sciences and Technology , Thiruvananthapuram - India
| | - Sabarinath P Subramanian
- a Department of Biochemistry , Sree Chitra Tirunal Institute for Medical Sciences and Technology , Thiruvananthapuram - India.,b Department of Technologies for the Advancement of Science , Presently at Institute for Stem Cell Biology and Regenerative Medicine (Instem), NCBS-TIFR,GKVK POST , Bangalore , India
| | - Geetha Mandagini
- a Department of Biochemistry , Sree Chitra Tirunal Institute for Medical Sciences and Technology , Thiruvananthapuram - India
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Sheela B, George G, Mandagini G, Appukuttan PS. Plasma anti-α-galactoside antibody mediates lipoprotein(a) binding to macrophages. Glycoconj J 2016; 33:953-961. [DOI: 10.1007/s10719-016-9713-z] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2016] [Revised: 07/01/2016] [Accepted: 07/12/2016] [Indexed: 01/27/2023]
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Antigen-Induced Activation of Antibody Measured by Fluorescence Enhancement of FITC Label at Fc. J Fluoresc 2015; 25:1493-9. [DOI: 10.1007/s10895-015-1640-z] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2015] [Accepted: 08/03/2015] [Indexed: 10/23/2022]
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Immunopathology of desialylation: human plasma lipoprotein(a) and circulating anti-carbohydrate antibodies form immune complexes that recognize host cells. Mol Cell Biochem 2015; 403:13-23. [PMID: 25633186 DOI: 10.1007/s11010-015-2332-3] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2014] [Accepted: 01/16/2015] [Indexed: 10/24/2022]
Abstract
Human plasma lipoprotein(a) [Lp(a)], the dominant lipoprotein in atherosclerotic plaques, contains an apo(a) subunit of variable size linked to the apoB subunit of a low-density lipoprotein (LDL) molecule. Circulating lipoprotein immune complexes (ICs) assayed by ELISA using microplate-coated anti-apo(a) or anti-apoB antibody for capture and peroxidase-labelled anti-human immunoglobulins as probe consisted mostly of Lp(a) despite several-fold excess of LDL over Lp(a) in plasma. Microplate coating of plasma lipoprotein IC and probing with antibodies to apo(a) and apoB also revealed negligible presence of LDL compared to Lp(a). Peanut agglutinin specific to desialylated O-glycans bound significantly more to Lp(a) recovered after urea dissociation of IC than to free Lp(a). Plasma lipoproteins separated by ultracentrifugation and desialylated by neuraminidase formed IC with naturally occurring antibodies in normal plasma. These de novo ICs agglutinated desialylated but not normal human RBC in proportion to the polyagglutinin antibody titre of plasma used, suggesting availability of multiple unoccupied binding sites on the participating antibodies even after IC formation. Agglutination was inhibitable by galactosides and decreased 4-8 fold if precursor lipoprotein was selectively depleted of Lp(a), showing agglutinating ICs were contributed mainly by desialylated Lp(a) and galactose-specific antibodies. IC was 2 fold more agglutinating if lipoproteins used contained smaller rather than larger Lp(a) molecules of the same number. Small size/high plasma concentration Lp(a) phenotype and neuraminidase-releasing diseases including diabetes are risk factors for vascular disorders. Results suggest a possible route of Lp(a) attachment to vascular cells that offer terminal galactose on surface glycans following desialylation.
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