A novel enzyme immunoassay specific for ABCA1 protein quantification in human tissues and cells

An in-silico model of lipoprotein metabolism and kinetics for the evaluation of targets and biomarkers in the reverse cholesterol transport pathway

High-density lipoprotein (HDL) is believed to play an important role in lowering cardiovascular disease (CVD) risk by mediating the process of reverse cholesterol transport (RCT). Via RCT, excess cholesterol from peripheral tissues is carried back to the liver and hence should lead to the reduction of atherosclerotic plaques. The recent failures of HDL-cholesterol (HDL-C) raising therapies have initiated a re-examination of the link between CVD risk and the rate of RCT, and have brought into question whether all target modulations that raise HDL-C would be atheroprotective. To help address these issues, a novel in-silico model has been built to incorporate modern concepts of HDL biology, including: the geometric structure of HDL linking the core radius with the number of ApoA-I molecules on it, and the regeneration of lipid-poor ApoA-I from spherical HDL due to remodeling processes. The ODE model has been calibrated using data from the literature and validated by simulating additional experiments not used in the calibration. Using a virtual population, we show that the model provides possible explanations for a number of well-known relationships in cholesterol metabolism, including the epidemiological relationship between HDL-C and CVD risk and the correlations between some HDL-related lipoprotein markers. In particular, the model has been used to explore two HDL-C raising target modulations, Cholesteryl Ester Transfer Protein (CETP) inhibition and ATP-binding cassette transporter member 1 (ABCA1) up-regulation. It predicts that while CETP inhibition would not result in an increased RCT rate, ABCA1 up-regulation should increase both HDL-C and RCT rate. Furthermore, the model predicts the two target modulations result in distinct changes in the lipoprotein measures. Finally, the model also allows for an evaluation of two candidate biomarkers for in-vivo whole-body ABCA1 activity: the absolute concentration and the % lipid-poor ApoA-I. These findings illustrate the potential utility of the model in drug development.

Increased placental phospholipid levels in pre-eclamptic pregnancies

Physiological pregnancy is associated with an increase in lipids from the first to the third trimester. This is a highly regulated response to satisfy energy and membrane demands of the developing fetus. Pregnancy disorders, such as pre-eclampsia, are associated with a dysregulation of lipid metabolism manifesting in increased maternal plasma lipid levels. In fetal placental tissue, only scarce information on the lipid profile is available, and data for gestational diseases are lacking. In the present study, we investigated the placental lipid content in control versus pre-eclamptic samples, with the focus on tissue phospholipid levels and composition. We found an increase in total phospholipid content as well as changes in individual phospholipid classes in pre-eclamptic placental tissues compared to controls. These alterations could be a source of placental pathological changes in pre-eclampsia, such as lipid peroxide insult or dysregulation of lipid transport across the syncytiotrophoblast.

Type 2 diabetes is associated with reduced ATP-binding cassette transporter A1 gene expression, protein and function

Objective

Increasing plasma glucose levels are associated with increasing risk of vascular disease. We tested the hypothesis that there is a glycaemia-mediated impairment of reverse cholesterol transport (RCT). We studied the influence of plasma glucose on expression and function of a key mediator in RCT, the ATP binding cassette transporter-A1 (ABCA1) and expression of its regulators, liver X receptor-α (LXRα) and peroxisome proliferator-activated receptor–γ (PPARγ).

Methods and Results

Leukocyte ABCA1, LXRα and PPARγ expression was measured by polymerase chain reaction in 63 men with varying degrees of glucose homeostasis. ABCA1 protein concentrations were measured in leukocytes. In a sub-group of 25 men, ABCA1 function was quantified as apolipoprotein-A1-mediated cholesterol efflux from 2–3 week cultured skin fibroblasts. Leukocyte ABCA1 expression correlated negatively with circulating HbA1c and glucose (rho = −0.41, p<0.001; rho = −0.34, p = 0.006 respectively) and was reduced in Type 2 diabetes (T2DM) (p = 0.03). Leukocyte ABCA1 protein was lower in T2DM (p = 0.03) and positively associated with plasma HDL cholesterol (HDL-C) (rho = 0.34, p = 0.02). Apolipoprotein-A1-mediated cholesterol efflux correlated negatively with fasting glucose (rho = −0.50, p = 0.01) and positively with HDL-C (rho = 0.41, p = 0.02). It was reduced in T2DM compared with controls (p = 0.04). These relationships were independent of LXRα and PPARγ expression.

Conclusions

ABCA1 expression and protein concentrations in leukocytes, as well as function in cultured skin fibroblasts, are reduced in T2DM. ABCA1 protein concentration and function are associated with HDL-C levels. These findings indicate a glycaemia- related, persistent disruption of a key component of RCT.

Type 2 diabetes is associated with reduced ATP-binding cassette transporter A1 gene expression, protein and function

OBJECTIVE

Increasing plasma glucose levels are associated with increasing risk of vascular disease. We tested the hypothesis that there is a glycaemia-mediated impairment of reverse cholesterol transport (RCT). We studied the influence of plasma glucose on expression and function of a key mediator in RCT, the ATP binding cassette transporter-A1 (ABCA1) and expression of its regulators, liver X receptor-α (LXRα) and peroxisome proliferator-activated receptor-γ (PPARγ).

METHODS AND RESULTS

Leukocyte ABCA1, LXRα and PPARγ expression was measured by polymerase chain reaction in 63 men with varying degrees of glucose homeostasis. ABCA1 protein concentrations were measured in leukocytes. In a sub-group of 25 men, ABCA1 function was quantified as apolipoprotein-A1-mediated cholesterol efflux from 2-3 week cultured skin fibroblasts. Leukocyte ABCA1 expression correlated negatively with circulating HbA1c and glucose (rho = -0.41, p<0.001; rho = -0.34, p = 0.006 respectively) and was reduced in Type 2 diabetes (T2DM) (p = 0.03). Leukocyte ABCA1 protein was lower in T2DM (p = 0.03) and positively associated with plasma HDL cholesterol (HDL-C) (rho = 0.34, p = 0.02). Apolipoprotein-A1-mediated cholesterol efflux correlated negatively with fasting glucose (rho = -0.50, p = 0.01) and positively with HDL-C (rho = 0.41, p = 0.02). It was reduced in T2DM compared with controls (p = 0.04). These relationships were independent of LXRα and PPARγ expression.

CONCLUSIONS

ABCA1 expression and protein concentrations in leukocytes, as well as function in cultured skin fibroblasts, are reduced in T2DM. ABCA1 protein concentration and function are associated with HDL-C levels. These findings indicate a glycaemia-related, persistent disruption of a key component of RCT.

Identification of ABCA1 and ABCG1 in milk fat globules and mammary cells--implications for milk cholesterol secretion

The ATP-binding cassette (ABC) transporters ABCA1 and ABCG1 play an important role in cellular cholesterol homeostasis, but their function in mammary gland (MG) tissue remains elusive. A bovine MG model that allows repeated MG sampling in identical animals at different functional stages was used to test whether 1) ABCA1 and ABCG1 protein expression and subcellular localization in mammary epithelial cells (MEC) change during the pregnancy-lactation cycle, and 2) these 2 proteins were present in milk fat globules (MFG). Expression and localization in MEC were investigated in bovine MG tissues at the end of lactation, during the dry period (DP), and early lactation using immunohistochemical and immunofluorescence approaches. The presence of ABCA1 and ABCG1 in MFG isolated from fresh milk was determined by immunofluorescence. The ABCA1 protein expression in MEC, expressed as arbitrary units, was higher during the end of lactation (12.2±0.24) and the DP (12.5±0.22) as compared with during early lactation (10.2±0.65). In contrast, no significant change in ABCG1 expression existed between the stages. Throughout the cycle, ABCA1 and ABCG1 were detected in the apical (41.9±24.8 and 49.0±4.96% of cows, respectively), basal (56.2±28.1 and 54.6±7.78% of cows, respectively), or entire cytoplasm (56.8±13.4 and 61.6±14.4% of cows, respectively) of MEC, or showed combined localization. Unlike ABCG1, ABCA1 was absent at the apical aspect of MEC during early lactation. Immunolabeling experiments revealed the presence of ABCA1 and ABCG1 in MFG membranes. Findings suggest a differential, functional stage-dependent role of ABCA1 and ABCG1 in cholesterol homeostasis of the MG epithelium. The presence of ABCA1 and ABCG1 in MFG membranes suggests that these proteins are involved in cholesterol exchange between MEC and alveolar milk.