Macrophage ABCG1 expression and regulation in type 2 diabetes.

摘要

Patients with Type 2 diabetes are at increased risk for the development of atherosclerosis and other forms of cardiovascular disease. In the early stages of atherosclerotic development, monocytes transmigrate into the subendothelial space and differentiate into macrophage foam cells. Scavenger receptors and ATP Binding Cassette (ABC) Transporters play an important role in foam cell formation as they regulate the influx and efflux of oxidized lipids. The ATP Binding Cassette (ABC) Transporters, ABCA1 and ABCG1, play an integral role in maintaining cellular cholesterol homeostasis by facilitating cholesterol efflux. Here, we show that ABCG1 mRNA and protein is downregulated in Type 2 diabetes. Macrophages isolated from two mouse models of Type 2 diabetes, the Leprdb/db (db/db) and KKay (KK) were used to confirm downregulation of ABCG1 in a diabetic environment and that the decrease in mRNA and protein were not due to genetic abnormalities inherent of the mice. We show that this decrease in ABCG1 leads to a decrease in cholesterol efflux to HDL, leading to increased intracellular cholesterol accumulation in the mouse macrophage. siRNA studies targeting ABCG1 were performed to confirm that the observed phenotype was due to ABCG1. Furthermore, using C57BL/6J (B6) macrophages incubated in hyperglycemic (25mM) glucose, we were able to recreate the diabetic phenotype with respect to ABCG1 in vitro. The decrease in ABCG1 mRNA in a hyperglycemic environment suggested regulation at the transcriptional level. Therefore, the ABCG1a promoter was cloned and transfected into HEK293t cells grown in either high (25mM) or low (5mM) glucose chronically. Luciferase assays showed a 30% decrease in promoter activity when placed in the hyperglycemic environment 48hrs post-transfection. Furthermore, deletion and mutation constructs narrowed the glucose responsiveness to an E-box located just upstream of the +1 start site of exon 1. Additionally, translational research in the human population found that patients with Type 2 diabetes also had decreased protein expression of ABCG1, decreased cholesterol efflux to HDL, and increased cholesterol ester accumulation within their macrophages. These studies identify ABCG1 as a potential drug target, whereby strategies to upregulate ABCG1 expression and function in Type 2 diabetes could have therapeutic potential for limiting accelerated vascular disease in Type 2 Diabetes.