Fluorescence flow cytometry of human leukocytes in the detection of LDL receptor defects in the differential diagnosis of hypercholesterolemia

Multiparametric platform for profiling lipid trafficking in human leukocytes

Systematic insight into cellular dysfunction can improve understanding of disease etiology, risk assessment, and patient stratification. We present a multiparametric high-content imaging platform enabling quantification of low-density lipoprotein (LDL) uptake and lipid storage in cytoplasmic droplets of primary leukocyte subpopulations. We validate this platform with samples from 65 individuals with variable blood LDL-cholesterol (LDL-c) levels, including familial hypercholesterolemia (FH) and non-FH subjects. We integrate lipid storage data into another readout parameter, lipid mobilization, measuring the efficiency with which cells deplete lipid reservoirs. Lipid mobilization correlates positively with LDL uptake and negatively with hypercholesterolemia and age, improving differentiation of individuals with normal and elevated LDL-c. Moreover, combination of cell-based readouts with a polygenic risk score for LDL-c explains hypercholesterolemia better than the genetic risk score alone. This platform provides functional insights into cellular lipid trafficking and has broad possible applications in dissecting the cellular basis of metabolic disorders.

B cells interactions in lipid immune responses: implications in atherosclerotic disease

Atherosclerosis is considered as an inflammatory and chronic disorder with an important immunologic component, which underlies the majority of cardiovascular diseases; condition that belongs to a group of noncommunicable diseases that to date and despite of prevention and treatment approaches, they remain as the main cause of death worldwide, with 17.5 million of deaths every year. The impact of lipids in human health and disease is taking center stage in research, due to lipotoxicity explained by elevated concentration of circulating lipids, in addition to altered adipose tissue metabolism, and aberrant intracellular signaling. Immune response and metabolic regulation are highly integrated systems and the proper function of each one is dependent on the other. B lymphocytes express a variety of receptors that can recognize foreign, endogenous or modified self-antigens, among them oxidized low density lipoproteins, which are the main antigens in atherosclerosis. Mechanisms of B cells to recognize, remove and present lipids are not completely clear. However, it has been reported that B cell can recognize/remove lipids through a range of receptors, such as LDLR, CD1d, FcR and SR, which might have an atheroprotector or proatherogenic role during the course of atherosclerotic disease. Pertinent literature related to these receptors was examined to inform the present conclusions.

An improved method on stimulated T-lymphocytes to functionally characterize novel and known LDLR mutations

The main causes of familial hypercholesterolemia (FH) are mutations in LDL receptor (LDLR) gene. Functional studies are necessary to demonstrate the LDLR function impairment caused by mutations and would be useful as a diagnostic tool if they allow discrimination between FH patients and controls. In order to identify the best method to detect LDLR activity, we compared continuous Epstein-Barr virus (EBV)-transformed B-lymphocytes and mitogen stimulated T-lymphocytes. In addition, we characterized both novel and known mutations in the LDLR gene. T-lymphocytes and EBV-transformed B-lymphocytes were obtained from peripheral blood of 24 FH patients and 24 control subjects. Functional assays were performed by incubation with fluorescent LDL followed by flow cytometry analysis. Residual LDLR activity was calculated normalizing fluorescence for the mean fluorescence of controls. With stimulated T-lymphocytes we obtained a better discrimination capacity between controls and FH patients compared with EBV-transformed B-lymphocytes as demonstrated by receiver operating characteristic (ROC) curve analysis (the areas under the curve are 1.000 and 0.984 respectively; P < 0.0001 both). The characterization of LDLR activity through T-lymphocytes is more simple and faster than the use of EBV-transformed B-lymphocytes and allows a complete discrimination between controls and FH patients. Therefore the evaluation of residual LDLR activity could be helpful not only for mutation characterization but also for diagnostic purposes.

Modifications in low-density lipoprotein receptor expression affects Cyclosporin A cellular uptake and cytotoxicity

The purpose of this study was to test the effect of modulating the expression of the human low-density lipoprotein receptor (LDLr) in human embryonic kidney (293T) cells on Cyclosporin A (CsA) cellular uptake and CsA-mediated cytotoxicity. LDLr expression was modulated using RNA interference (RNAi) and an LDLr overexpression plasmid. One of the small-interfering RNA (siRNA) constructs, LDLr-792, showed a 60% decrease in LDLr protein expression. The downregulation effect was specific as transfection with an annexin V (AxV) siRNA construct did not decrease LDLr expression levels. AxV and ABCA1 expression levels were not affected in the cells transfected with LDLr-792 (LDLr(LOW) cells) compared to the controls. At a functional level, fluorescent low-density lipoprotein (LDL) (DiI-LDL) internalization in the LDLr(LOW) cells was decreased (30%) compared to control cells. We tested the dose-dependent cytotoxicity induced by CsA using a respiration assay. We found a decrease in CsA-mediated cytotoxicity in the range of CsA doses studied (1-10 microg/mL) in the LDLr(LOW) cells compared to the pSHAG-transfected cells, reaching a statistical significance at 10 microg/mL CsA. At higher CsA doses we found a significant decrease in LDLr expression. When the control and LDLr(LOW) cells were treated with another cytotoxic drug, gentamycin, there was no difference in the cell viability, suggesting that this effect is specific for CsA. We confirmed the association of LDLr expression levels with CsA uptake by overexpressing the LDLr. The LDLr overexpressing cells showed an enhanced uptake of radiolabelled CsA. Taken together these results suggest that CsA internalization and cytotoxicity are affected by the LDL receptor expression levels.

Model system for phenotypic characterization of sequence variations in the LDL receptor gene

BACKGROUND

Sequence variations in the LDL receptor (LDLR) gene cause defects of LDLR protein production and function through different molecular mechanisms. Here we describe a cell model system for the phenotypic characterization of sequence variations in the LDLR gene. Well-known sequence variations belonging to LDLR classes 2 to 5 (p.G565V, p.I161D, p.Y828C, and p.V429M) were studied in CHO and HepG2 cells.

METHODS

Expression of LDLR protein on the cell surface was detected by use of fluorescence-conjugated antibodies against the LDLR and the LDLR activity was measured by incubating the cells with fluorescently labeled and radiolabeled LDL. The intracellular locations of the LDLR mutants and wild-type were also investigated.

RESULTS

The class 2A p.G565V sequence variant exhibited an intracellular distribution of LDLR with no active receptors on the cell surface. Both the class 3 p.I161D and class 4 p.Y828C sequence variants gave surface staining but had a reduced ability to bind or internalize LDL, respectively. By determining the intracellular locations of the receptors we were able to visualize the accumulation of the class 5 p.V429M sequence variant in endosomes by means of a specific marker, as well as confirming that the class 4 p.Y828C variant was not localized in clathrin-coated pits. Flow cytometry allowed us quantitatively to determine the amount and activity of receptors. To confirm the results of binding and cell association of fluorescently labeled LDL analyzed by flow cytometry, assays using 125I-labeled LDL were performed. In addition to a useful and valid alternative to radiolabeled LDL, the unique properties of fluorescently labeled LDL allowed a variety of detection technologies to be used.

CONCLUSIONS

This new approach enables phenotypic characterization of sequence variations in the LDLR gene. The assays developed may be valuable for confirming the pathogenicity of novel missense sequence variations found throughout the LDLR gene.

Suitability of LLC-PK1 pig kidney cells for the study of drug action on renal cell cholesterol uptake: Identification and characterization of low-density lipoprotein receptors

INTRODUCTION

The purpose of this study was to identify and characterize the presence of low-density lipoprotein receptors (LDLr) in LLC-PK(1) cells.

METHODS

LLC-PK(1) cells were assessed for the presence of LDLr by conducting dose-response, LDL specific binding and competitive studies with DiI-LDL, and Western blot and RT-polymerase chain reaction (PCR) analyses. Assay conditions with IgG-C7, a monoclonal antibody (mAb) to the LDLr, were optimized, including temperature, preincubation time, and concentration in LLC-PK(1) cells.

RESULTS

LLC-PK(1) cells express LDL receptors as determined by LDL specific and competitive binding studies and Western blot and RT-PCR analysis (specific binding 0.5 ng DiI-LDL/mug of cellular protein).

DISCUSSION

Taken together, these findings confirm the presence of LDL receptors on LLC-PK1 cells and support the appropriateness of using these cells in studies involving renal cell cholesterol uptake and metabolism.

Comparative analysis of foetal calf and human low density lipoprotein: relevance for pharmacodynamics of photosensitizers

The effects of differences in lipoprotein content on the distribution of the novel hydrophobic photosensitizer n-butyl-3-[18-(2-butylcarbamoyl-ethyl)-3,7,12,17-tetramethyl-18,13-divinyl-22,24-dihydro-porphin-2-yl]propionamide (PP-N-3) and haematoporphyrin ester (HpE), a relatively hydrophilic photosensitizer, in human (HS) and foetal calf sera (FCS), were investigated. The binding characteristics of human and foetal calf low-density lipoprotein (LDL) were characterised using a human fibroblast line (Vag 12). The uptake into cells of HpE and PP-N-3 was also examined. A comparison of the lipoprotein content, composition and receptor-binding characteristics of foetal calf and human serum was also carried out. LDL content was measured directly using sequential ultracentrifugation to isolate LDL. In our study, we found haematoporphyrin ester to bind to human very low-density lipoprotein (VLDL), LDL and high-density lipoprotein (HDL) in the ratio 2:31:65. In the case of PP-N-3 this ratio was 56:10:33. As VLDL was not detected in foetal calf serum, only binding to LDL and HDL was observed. Using the sequential ultracentrifugation technique, foetal calf serum was found to contain LDL which in turn did bind to human LDL receptors. The uptake of PP-N-3 and HpE in the presence of low density lipoprotein from foetal calf serum (FC-LDL) was not significantly different to values observed in the presence of human serum low density lipoprotein (HS-LDL).

The ATP binding cassette transporter A1 contributes to the secretion of interleukin 1beta from macrophages but not from monocytes

Deficiency of ABCA1 causes high density lipoprotein deficiency and macrophage foam cell formation in Tangier disease. ABCA1 was also postulated to mediate the secretion of IL-1beta from monocytes and macrophages. We investigated the contribution of ABCA1 to IL-1beta secretion from human monocytes and macrophages of normal donors and Tangier disease patients. Neither an anti-ABCA1 antisense oligonucleotide nor ABCA1 deficiency interfered with LPS-induced secretion of IL-1beta from full blood or freshly isolated monocytes. By contrast, anti-ABCA1 antisense oligonucleotides decreased the LPS-induced secretion of IL-beta from macrophages by 30-50%. The secretion of the precursor pro-IL-1beta and TNFalpha was not inhibited. Compared to normal macrophages, LPS-stimulated Tangier disease macrophages secreted less IL-1beta relative to TNFalpha. Also the spontaneous secretion of IL-1beta by Tangier macrophages was lower than by control cells. We conclude that IL-1beta is secreted from monocytes by an ABCA1-independent pathway and from macrophages by ABCA1-dependent and -independent pathways.

Electrothermal atomic absorption spectrometric diagnosis of familial hypercholesterolemia

We have developed a new nonradioactive assay to identify human low-density lipoprotein receptor defects. It is based on the incubation of cultured cells with colloidal gold-LDL conjugates and quantitation of the gold associated with the cells by electrothermal atomic absorption spectrometry. After an oxidative treatment with nitric and hydrochloric acids, the biological matrix interferes neither with the gold recovery nor with the gold measurements, which are linear, at least from 0.15 to 3 ng of gold. When cells expressing a functional LDL receptor are incubated with increasing amounts of colloidal-gold LDL conjugates, the obtained saturation curve parallels that described when [125I]LDL is used as ligand. Moreover, this new assay allows us to clearly distinguish among fibroblasts from normal subjects or from heterozygous or homozygous patients of familial hypercholesterolemia, a very common autosomal disease. The assay is easy to perform, is sensitive, and avoids the use of radioactive compounds. Therefore, it could be successfully employed in the clinical diagnosis of this disease. Furthermore, since the methodology developed here can be applied to quantify the association of other gold-conjugated ligands to cells, it could have a widespread use in a variety of clinical and basic research studies.

Flow cytometric assessment of LDL receptor activity in peripheral blood mononuclear cells compared to gene mutation detection in diagnosis of heterozygous familial hypercholesterolemia

BACKGROUND

Studies indicate that human peripheral blood mononuclear cells mirror low-density lipoprotein (LDL) receptor activity of other cells in the body. To measure LDL receptor activity in patients with heterozygous familial hypercholesterolemia (FH), we prepared peripheral blood mononuclear cells from individuals with molecularly verified LDL receptor defective (Trp66-Gly mutation, n = 18) or receptor negative (Trp23-stop mutation, n = 17) heterozygous FH and from healthy individuals (n = 24).

METHODS

The cells were stimulated to express maximum LDL receptor by preincubation in lipoprotein-free medium. They were then incubated at 4 degrees or 37 degrees C with fluorescently conjugated LDL (DiI-LDL). T-lymphocytes and monocytes were identified by fluorescently conjugated monoclonal antibodies. DiI-LDL bound (at 4 degrees C) or internalized (at 37 degrees C) by the cells was measured using flow cytometry. Knowing the LDL receptor gene mutation of the FH patients allowed us to compare the diagnostic capability of our functional assay with the DNA diagnosis.

RESULTS

The diagnostic accuracy did not allow our assay to be used for diagnosis of individual cases of heterozygous FH.

CONCLUSIONS

We suggest that our two-color fluorescence flow cytometry assay can be used to characterize functionally gene mutations causing LDL receptor dysfunction in patients with heterozygous FH.

Expression of low-density lipoprotein receptors in peripheral blood and tonsil B lymphocytes

B lymphocytes, purified from peripheral leucocytes from young normolipaemic humans, expressed and internalized low-density lipoprotein receptors (LDLR). The expression was assessed by a monoclonal anti-LDLR. The internalization of LDL was assessed by LDL labelled with 125I (125I-LDL) and 1,1'-dioctadecyl-3,3,3',3' tetramethyl-indocarboxycyanine perchlorate (LDL-DiI). The expression of LDLR, assessed by anti-LDLR, was: 38 +/- 8% (n = 5) for fresh purified cells, 60 +/- 10% (n = 12) for non-stimulated cells, 79 +/- 5% (n = 10) for IL-2 (100 U/ml)-stimulated cells and 95 +/- 5% (n = 8) for pokeweed mitogen (PWM) (1:200 dilution)-stimulated cells. The optimal concentrations of agonist were 100 U/ml of IL-2, and 1:200 dilution of PWM. IL-2 and PWM increased the internalization of LDL-DiI by 1.5-fold. The internalization of LDL-DiI was maximal at 60 microg of protein/ml (48 +/- 8%). Scatchard analysis revealed a Kd of 3.2 +/- 0.22 x 10(-8) M and 2180 +/- 190 binding sites in non-stimulated cells, a Kd of 7.73 +/- 0.36 x 10(-9) M and 12,500 +/- 430 binding sites for IL-2 (100 U/ml)-stimulated cells, and a Kd of 7.2 +/- 0.43 x 10(-9) M and 13,250 +/- 450 binding sites for PWM (1:200 dilution)-stimulated cells. Lineweaver-Burk analysis of LDL binding (LDL-DiI) revealed that the apparent Kd for non-stimulated cells was 1.3 +/- 0.11 x 10(-8) M, and 9.2 +/- 0.2 x 10(-9) M and 7.5 +/- 0.25 x 10(-9) M for IL-2- and PWM-stimulated cells, respectively. B lymphocytes from tonsils also showed a high expression of LDLR assessed with anti-LDLR (70 +/- 6%). The high expression of LDLR and the avid internalization of LDL suggest that LDL may be important for B cell physiological responses.

Mutation screening of the LDLR gene and ApoB gene in patients with a phenotype of familial hypercholesterolemia and normal values in a functional LDL receptor/apolipoprotein B assay

Mutations in the LDL receptor (LDLR) or the apolipoprotein B-100 genes causing familial hypercholesterolemia (FH) and familial defective apolipoprotein B-100 (FDB), two of the most frequent inherited diseases, are the underlying genetic defects in a small proportion of patients suffering from premature atherosclerotic heart disease. Consequently, secure diagnostic tools for these conditions allowing early preventive measures are needed. Since clinical and biochemical diagnosis often is inaccurate, assays analyzing patient LDLR function and LDL affinity have been established. These assays are, however, not able clearly to differentiate between suspected FH/FDB samples and normal controls. To evaluate if this may be caused by other hitherto undescribed genetic defects or to failure of the functional assays, we undertook denaturing gradient gel electrophoresis based mutation screening of the LDLR gene and the codon 3456 3553 region of the apolipoprotein B gene in six French FH/FDB patients with normal outcomes on functional assays. In all six patients, pathogenic LDLR mutations were found, including three previously undescribed mutations, suggesting that failure of the functional assays explains the normal results found in some phenotypic FH/FDB patients and illustrating the need for DNA based screening techniques for routine genetic diagnosis in FH/FDB.

Flow cytometry with a monoclonal antibody to the low density lipoprotein receptor compared with gene mutation detection in diagnosis of heterozygous familial hypercholesterolemia

We used a fluorescence flow cytometry assay with a monoclonal low density lipoprotein (LDL) receptor-specific antibody to detect LDL receptor expression on blood T lymphocytes and monocytes. We prepared peripheral blood mononuclear cells from patients with genetically verified LDL receptor-defective (Trp66-Gly mutation, n = 17) or receptor-negative (Trp23-stop mutation, n = 17) heterozygous familial hypercholesterolemia (FH) and from healthy individuals (n = 24). The cells were stimulated to express the maximum amount of LDL receptor by preincubation in lipoprotein-deficient medium. A dual-labeling technique allowed flow cytometric analysis of LDL receptor expression on cells identified by fluorescently conjugated surface marker antibodies. Knowing the LDL receptor gene mutation of the FH patients allowed us to compare the diagnostic capability of this functional assay with the DNA diagnosis and to validate the assay with molecular genetics instead of clinical indices of heterozygous FH. T lymphocytes expressed more LDL receptors and gave better diagnostic results than monocytes, and cells from patients with either the Trp66-Gly or the Trp23-stop mutation had variable but significantly reduced LDL receptor expression. The data indicate that this fluorescence flow cytometry assay is unsuitable for diagnosis of individual cases of heterozygous FH but that it may be useful for functionally characterizing mutations in the LDL receptor gene.

LDL receptor activity in human leukocyte subtypes: regulation by insulin

OBJECTIVES

LDL receptors of leukocytes play a key role in lipoprotein uptake, immunoregulation and the pathogenesis of atherosclerosis. Numerous studies with different methods of low reliability yielded conflicting results of its regulation in leukocyte subtypes.

DESIGN AND METHODS

LDL receptors of human leukocytes were measured with use of the monoclonal antibody C-7. Specific C-7 binding was detected by FACS analysis using phycoerythrin-anti-mouse-IgG. Parallel incubations with FITC-labelled anti-LEU 4 (CD 3), anti-LEU 12 (CD 19) and anti-MY 4 (CD 14) antibodies were used to distinguish C-7 binding of specific cell types (T-, B-lymphocytes and monocytes).

RESULTS

In contrast to monocytes, T and B-lymphocytes freshly isolated from healthy blood donors had no detectable binding capacity for C-7. After 24 and 48 h incubation of cells in a lipid-free medium, lymphocytes acquired some C-7 binding, albeit still much less than monocytes. Incubation with insulin for 24 h in a concentration of 0.5 microgram/mL led to an increase in C-7 binding for monocytes (up to 180%). Saturation experiments with the ligand suggests an increase in the number of receptors. In contrast the same insulin concentration inhibited C-7 binding of B- and T-lymphocytes by 35%.

CONCLUSIONS

FACS analysis using monoclonal antibodies seems to be a feasible method for the investigation of lipid metabolism in leukocytes. The LDL receptor expression and its regulation by insulin differs in circulating monocytes and lymphocytes.

ACP Broad Sheet no 151: September 1997. Investigation of dyslipidaemias

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Surface expression of low density lipoprotein receptor in EBV-transformed lymphocytes: characterization and use for studying familial hypercholesterolemia

The objectives of the present study were to characterize the surface expression of low density lipoprotein receptor (LDL-R) in Epstein-Barr virus transformed lymphocytes (EBV-L) and to determine the applicability of the cellular system for the study of familial hypercholesterolemia. The EBV-L subsets and LDL-R expression were determined by immuno-cytofluorimetry. The LDL-R expression in EBV-L which consisted of mostly B cells was no different among antigenic subsets. EBV-L cultured in lipoprotein deficient serum demonstrated a 9.3-fold higher LDL-R expression than primary lymphocytes. Lovastatin caused an additional 1.9-and 1.4-fold increase in EBV-L and primary lymphocytes respectively. This difference in lovastatin response is statistically significant (paired t-test, P < 0.001). 54% of the high LDL-R expression in EBV-L was related to the changes in proliferation measured as stimulation index (SI). LDL and lovastatin modulated the LDL-R expression without affecting SI. FH subjects demonstrated 2% (homozygote, n = 1) and 44.6 +/- 12.3% (heterozygotes, n = 35) in LDL-R expression of controls (n = 30). This maintenance of the FH phenotype and the intrinsically high LDL-R expression in EBV-L make the cellular system suitable for the study of FH as well as the regulation of LDL-R.

Single-strand conformation polymorphism analysis with high throughput modifications, and its use in mutation detection in familial hypercholesterolemia

The identification of the specific mutation causing an inherited disease in a patient is the framework for the development of a rationale for therapy and of DNA-based tests for screening relatives. We present here a review of the single-strand conformational polymorphism (SSCP) method, which allows DNA fragments that have been amplified with specific primers and PCR to be scanned rapidly for any sequence variation. The general principles of the method are described, as are the major factors that must be considered in developing an optimal SSCP strategy, namely the length of the PCR fragment and the temperature of the gel run. Options for sample denaturing gel characteristics and detection of DNA fragments are discussed. In addition, several modifications are presented that have been developed for high-throughput mutational analysis. The application of these techniques to screen for mutations in the LDL receptor gene in patients with familial hypercholesterolemia are described.

Determination of LDL- and scavenger-receptor activity in adherent and non-adherent cultured cells with a new single-step fluorometric assay

Lipoproteins labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) are widely used to visualize LDL-and scavenger-receptor activity in cultured cells. The purpose of this study was to evaluate a new single-step fluorometric assay with high sensitivity for the quantitative determination of the LDL- or scavenger-receptor activity in adherent and non-adherent cells. We used an aqueous solution of 1 g/l SDS dissolved in 0.1 M NaOH to lyse the cells after incubation with DiI-LDL or DiI-acetylated LDL. This allows for the first time the determination of fluorescence intensity and cell protein in the same sample without prior lipid extraction of the fluorochrome. Fluorescence of the cell lysates was determined in microtiter plates with excitation-emission set at 520 and 580 nm, respectively. This rapid method demonstrates high specificity for determining the LDL- and scavenger-receptor activity in cultured cells (e.g., human skin fibroblasts from patients with and without familial hypercholesterolemia; human U-937 monocyte and murine P388 D1 macrophage cell lines). The validity of our fluorescence assay is demonstrated by comparison of cellular uptake and metabolism of lipoproteins labeled with both, DiI and 125iodine. The rapidity and accuracy of this assay allows its routine application for studying receptor-mediated lipoprotein uptake in various cell types.

Standardization of a flow cytometric method for measurement of low-density lipoprotein receptor activity on blood mononuclear cells

Flow cytometric methods for measurement of low-density lipoprotein (LDL) receptor activity on peripheral blood mononuclear cells (PBMC) may be used to identify patients with familial hypercholesterolemia (FH). However, cellular LDL receptor activities measured in FH heterozygotes may overlap with those of healthy subjects. Analytical variation is probably responsible for some of this overlap. We have examined several technical details that may affect analytical variation. In each analysis, we included one standard and two control cell preparations. These were cells isolated from three donors and stored in aliquots at -135 degrees C. Use of standard cells reduced between-series analytical variation of the controls by approximately 50%. Preincubation-conditions used to induce the maximum number of receptors, the concentration of fluorochrome 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine-perchlorate (DiI)-LDL, labelling time, and conditions during storage of labelled cells before flow cytometry were also examined in order to reduce analytical variation. Having standardized the assay, we found among 20 healthy subjects a median receptor activity of 100% vs. 51% among 26 patients who fulfilled clinical criteria for FH. However, four of the patients showed distinctly normal receptor activities, which may suggest either the presence of some other biochemical defect or that in vivo dysfunctional receptors may be measured as normal in some patients with our assay.

Insulin binding to human cultured lymphocytes measured by flow cytometry using three ligands

The binding of insulin to cultured IM-9 human lymphocytes was studied by flow cytometry using FITC-insulin and biotinylated insulins coupled to streptavidin-phycoerythrin (N alpha beta 1-biotinylinsulin (B-insulin) and N alpha B1-(biotinyl-epsilon-aminocaproyl)insulin (NBC-insulin)). The reference methods were 125I-insulin binding and the insulin-antiinsulin antibody complexes for flow cytometry. There was a close correlation between 125I-insulin binding and increase in fluorescence for B-insulin, NBC-insulin, and insulin-anti-insulin antibody complexes, but not for FITC-insulin. NBC-insulin gave the largest increase in fluorescence (79 +/- 9 channels) and the the insulin-antiinsulin antibody complexes the smallest (34 +/- 2 channels) (P < 0.05). FITC-insulin and B-insulin gave similar results: 47 +/- 6 and 59 +/- 6 channels. The concentration reducing 125I-insulin binding by 50% was 1.1 x 10(-9) M for native insulin, 2.7 x 10(-9) M for B-insulin, 3.3 x 10(-9) M for NBC-insulin, and 6.6 x 10(-9) M for FITC-insulin (P < 0.05). Nonspecific binding was low for B-insulin and NBC-insulin but reached 75% for 10(-6) M FITC-insulin. These results suggest that B-insulin and NBC-insulin are suitable ligands for insulin binding studies using flow cytometry. This two-step procedure is easier than the insulin-antiinsulin antibody complex technique. Its poor affinity, specificity, and sensitivity make FITC-insulin less suitable.

High density lipoprotein deficiency with xanthomas. A defect in reverse cholesterol transport caused by a point mutation in the apolipoprotein A-I gene

A 7-yr-old girl with high density lipoprotein (HDL) deficiency and xanthomas has been identified in a Turkish kindred with repetitive consanguinity. She has severely reduced HDL-cholesterol and no apolipoprotein (apo) A-I. ApoA-II is reduced, whereas apoA-IV and apoC-III are normal. ApoB and low density lipoprotein (LDL)-cholesterol are increased. This is reflected in hypercholesterolemia. VLDL and IDL particles are low, and serum triglycerides are normal. The genetic defect could be identified as a base insertion into the third exon of the apoA-I gene. This leads to a nonsense peptide sequence beginning at amino acid 5 of the mature plasma protein and early termination of translation. The patient is homozygous for this mutation. Pedigree analysis indicated an autosomal dominant inheritance with no evidence of another genetic defect of lipoprotein metabolism in the kindred. In HDL deficiency, HDL binding to leukocytes was increased compared to normal. In the postprandial state, binding of labeled HDL3 to leukocytes is unchanged. This is in contrast to results with postprandially isolated leukocytes from controls or Tangier patients, which have a reduced binding capacity for HDL3. These results indicate that postprandial HDL precursors may compete the binding of labeled HDL3. The metabolic consequences of HDL deficiency were analyzed. There is only a small number of HDL-like particles containing apoA-II, apoA-IV, apoE, and lecithin/cholesteryl acyl transferase. The C-apolipoproteins were normal in the proband. Due to the lack of HDL they can only associate with apoB-containing particles, where they may interfere with cellular uptake. Thus, pure apoA-I deficiency leads to a complex metabolic derangement.