Cellular cholesterol efflux in diabetes mellitus

Abstract

Cellular cholesterol efflux is the first step of the reverse cholesterol transport pathway that transfers extrahepatic cholesterol to the liver for further metabolism. Recent studies have suggested that cholesterol efflux capacity is associated with cardiovascular disease. The process of cellular cholesterol efflux is determined by three known intracellular cholesterol transporters, which are adenosine triphosphate-binding cassette A1 (ABCA1), ABCG1, and scavenger receptor class B type I (SR-BI), and by extracellular cholesterol acceptors, including high density lipoprotein (HDL) and its precursor apolipoprotein AI (apo AI). My hypothesis is that both the intracellular and extracellular components of cellular cholesterol efflux would be impaired in patients with type 2 diabetes and that impaired cellular cholesterol efflux in the kidney might contribute to the pathogenesis of diabetic nephropathy.

In this research, the extracellular component of the process of cellular cholesterol efflux was determined by measuring the capacity of serum to induce ABCG1- and SR-BI-mediated cholesterol efflux and the intracellular component by evaluating macrophage cholesterol efflux and the expression of the three cholesterol transporters, ABCA1, ABCG1, and SR-BI. The potential role of cholesterol transporters in nephropathy was investigated using a diabetic mouse model.

The capacity of serum to induce ABCG1- and SR-BI mediated cholesterol efflux in the ex vivo studies with human ABCG1-transfected CHO-K1 and Fu5AH cells respectively was significantly reduced in patients with type 2 diabetes (n=264) compared with non-diabetic control subjects (n=275), and the greatest abnormalities were seen in diabetic patients with nephropathy. SR-BI-mediated cholesterol efflux, but not ABCG1-mediated cholesterol efflux, was found to be significantly correlated with serum amyloid A (SAA), an acute phase reactant. The level of SAA was significantly increased in diabetic patients, albeit to a much lesser degree than that seen in acute inflammatory response. Binding assay showed that the lipid-free SAA inhibited the binding of HDL to SR-BI resulting in a reduction in efflux of cellular cholesterol.

Macrophage cholesterol efflux was evaluated using monocyte-derived macrophages from type 2 diabetic patients (n=77) and healthy controls (n=58). Macrophage cholesterol efflux to apo AI and to HDL was significantly reduced in diabetic patients, and the expression of ABCG1 in macrophages was decreased. The magnitude of reduction in macrophage cholesterol efflux was largest in those diabetic subjects with poor glycaemic control or with chronic kidney disease.

In the kidney, both human primary mesangial and tubular (HK-2) cells expressed ABCA1, ABCG1 and SR-BI. High glucose concentration downregulated the expression of these cholesterol transporters in human mesangial and tubular cells, and impaired cholesterol efflux from these cells in vitro. Male DBA/2J mice were injected with streptozotocin to induce T1DM and nephropathy in in vivo studies. The expression of all three transporters in the kidneys was significantly reduced and this was accompanied by renal lipid accumulation.

In conclusion, both the intracellular and extracellular components of the cellular cholesterol efflux process were impaired in patients with type 2 diabetes mellitus, and these abnormalities were potentially atherogenic. Cellular cholesterol efflux was also impaired in the kidney, and might contribute to the development of diabetic nephropathy.