Ying Wong

Serum advanced glycation end products (AGEs) are associated with insulin resistance

In addition to the important role of advanced glycation end products (AGEs) in the pathogenesis of diabetic vascular complications, recent data suggest that advanced glycation end products can also impair insulin action in vitro. We have investigated whether circulating advanced glycation end products are associated with insulin resistance in human subjects independent of metabolic parameters.

Effects of atorvastatin on serum soluble receptors for advanced glycation end-products in type 2 diabetes

OBJECTIVE The receptor for advanced glycation end-products (RAGE) plays an important role in the pathogenesis of diabetic complications and atherosclerosis. Interfering with the activation of RAGE by using a soluble form of the receptor (sRAGE) ameliorates the vascular complications of diabetes in animal models. We have investigated whether statin can influence the expression of sRAGE and esRAGE (a splice variant of sRAGE) in vitro and in vivo. METHODS THP-1 cells were incubated with atorvastatin in vitro and sRAGE and esRAGE in the medium was measured by Western immunoblot. Serum levels of sRAGE and esRAGE were measured by ELISA in archived serum samples from a previous randomized double-blind placebo-controlled clinical trial that explored the cardiovascular effects of atorvastatin in hypercholesterolemic Chinese type 2 diabetic patients. RESULTS sRAGE and esRAGE were induced by atorvastatin in a time- and dose-dependent manner in THP-1 cells. In the diabetic patients, there was a significant increase in serum sRAGE (p<0.05) and esRAGE (p<0.01) in the atorvastatin group at 6-month, but no change in placebo group. Serum esRAGE was higher in atorvastatin group than placebo group [median 240.5pg/ml (interquartile range 186.5-377.3) vs 194.8pg/ml (124.1-347.9) respectively, p<0.01] at 6-month, whereas the differences in sRAGE did not reach statistical significance (p=0.051). There was a correlation between the increase of serum esRAGE and reduction of serum LDL (r=-0.36, p=0.001). CONCLUSIONS Statins are known to have pleiotropic effects and we have shown that atorvastatin can increase circulating esRAGE levels in type 2 diabetic patients.

Soluble lectin-like oxidized low density lipoprotein receptor-1 in type 2 diabetes mellitus

The lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) can be proteolytically cleaved and released as soluble forms (sLOX-1). We have determined serums LOX-1 in type 2 diabetes and evaluated the effect of glucose and advanced glycation end products (AGEs) on sLOX-1 in vitro and in vivo. Endothelial cells were incubated with glucose or AGEs, and sLOX-1 in cell medium was measured. Serum sLOX-1 was measured in 219 diabetic patients and 187 controls by ELISA. The effect of lowering glucose and AGEs on sLOX-1 was determined in 38 poorly controlled diabetic patients after improvement in glycemic control. Incubation of endothelial cells with AGE-BSA led to a dose-dependent increase in sLOX-1, whereas the effect of glucose on sLOX-1 was less marked. Serum sLOX-1 was 9% higher in diabetic patients compared with controls (P < 0.01). In the poorly controlled patients, serum sLOX-1 decreased by 12.5% after improvement in glycemic control (P < 0.05). The magnitude of reduction in sLOX-1 correlated with the improvement in hemoglobin A1c and AGEs but not with the reduction in oxidized LDL.XXX sLOX-1 level is increased in type 2 diabetes. Both glucose and AGEs are important determinants of LOX-1 expression, and lowering glucose and AGEs leads to a reduction in sLOX-1.

Cellular cholesterol efflux to serum is impaired in diabetic nephropathy

Cholesterol efflux from cells is an early step of reverse cholesterol transport (RCT) and the capacity of serum to induce cellular cholesterol efflux has recently been shown to be an independent predictor of coronary artery atherosclerosis. Our aim is to evaluate the capacity of serum to induce ATP‐binding cassette transporter A1 (ABCA1) and scavenger receptor class B type I (SR‐BI) mediated cholesterol efflux in type 2 diabetic patients with nephropathy.

Plasma phospholipid transfer protein activity and small, dense LDL in type 2 diabetes mellitus.

Background Phospholipid transfer protein (PLTP) and cholesteryl ester transfer protein (CETP) remodel circulating lipoproteins and play a role in the antiatherogenic reverse cholesterol transport pathway. The present study determined whether abnormalities in the LDL subfraction pattern in type 2 diabetic patients were related to changes in lipid transfer proteins.

Determinants of leukocyte adenosine triphosphate–binding cassette transporter G1 gene expression in type 2 diabetes mellitus

Cellular cholesterol efflux is regulated by cholesterol transporters including adenosine triphosphate-binding cassette transporter A1 (ABCA1), ABCG1, and scavenger receptor class B type I (SR-BI). We have investigated whether the expression of these transporters is affected by type 2 diabetes mellitus and the association with glycemic indexes and oxidized low-density lipoprotein (oxLDL). Messenger RNA of ABCA1, ABCG1, and SR-BI in peripheral monocytes was measured in 30 diabetic patients and 30 matched controls. Plasma oxLDL and advanced glycation end products (AGEs) were assayed by enzyme-linked immunosorbent assay. Cellular cholesterol efflux from monocytes to serum was determined in a subgroup chosen randomly. The expression of ABCG1 was decreased in diabetic patients (P < .05), whereas the levels of ABCA1 and SR-BI were comparable between the 2 groups. Fasting glucose, hemoglobin A(1c), AGEs, and oxLDL were all significantly increased in diabetic patients. There was an inverse correlation between serum AGEs and ABCG1 (r = -0.44, P < .05) that remained significant after adjusting for potential confounding factors. No associations between fasting glucose, hemoglobin A(1c), plasma lipids, or oxLDL and the expression of ABCG1, ABCA1, or SR-BI were found. Cholesterol efflux from monocytes to standard serum or autologous serum was significantly impaired in diabetic patients, and the reduction in efflux to autologous serum correlated with the expression of ABCG1 (r = 0.60, P < .05). The expression of ABCG1 in monocytes is reduced in type 2 diabetes mellitus and is partly related to serum AGEs concentration. The reduction in ABCG1 is associated with impairment in cholesterol efflux and may contribute to accelerated foam cell formation in diabetic patients.

ABCG1 mediated oxidized LDL-derived oxysterol efflux from macrophages.

OBJECTIVES The uptake of oxidized LDL (oxLDL) by macrophages is a key initial event in atherogenesis, and the removal of oxidized lipids from artery wall via reverse cholesterol transport is considered antiatherogenic. The aims of this study were to investigate the pathways mediating the removal of oxysterols from oxLDL-loaded macrophages, and the subsequent uptake of the oxysterols by hepatocytes. METHODS LDL was labeled with [3H]cholesterol, and LDL-[3H]cholesterol was oxidized by copper using a standard method. [3H]oxysterol formation in oxLDL was analyzed by thin layer chromatography. oxLDL-[3H]sterol was incubated with macrophages, allowing the uptake of [3H]sterol by macrophages. [3H]sterol efflux from macrophages mediated by ATP binding cassette transporters (ABCA1, ABCG1), or scavenger receptor class B type I (SR-BI) was measured. The subsequent uptake of the [3H]sterol by hepatocytes was also determined. RESULTS 7-Ketocholesterol was the major oxysterol formed in oxLDL, and it was significantly higher in oxLDL compared with that in native LDL (naLDL). oxLDL-derived sterol efflux to HDL from macrophages was significantly increased compared with naLDL-derived sterol, and it was mainly mediated by ABCG1, but not by ABCA1 or SR-BI. Moreover, although HDL dose-dependently induced sterol efflux from macrophages, only the exported sterol by ABCG1 pathway was efficiently taken up by hepatocytes. CONCLUSIONS ABCG1 mediates oxysterol efflux from oxLDL-loaded macrophages, and the exported oxysterol by ABCG1 pathway can be selectively taken up by hepatocytes.

Impact of serum amyloid A on cellular cholesterol efflux to serum in type 2 diabetes mellitus.

OBJECTIVE Serum amyloid A (SAA) is an acute phase response protein and has apolipoprotein properties. Since type 2 diabetes is associated with chronic subclinical inflammation, the objective of this study is to investigate the changes in SAA level in type 2 diabetic patients and to evaluate the relationship between SAA and the capacity of serum to induce cellular cholesterol efflux via the two known cholesterol transporters, scavenger receptor class B type I (SR-BI) and ATP-binding cassette transporter G1 (ABCG1). METHODS 264 patients with type 2 diabetes mellitus (42% with normoalbuminuria, 30% microalbuminuria, and 28% proteinuria) and 275 non-diabetic controls were recruited. SAA was measured by ELISA. SR-BI and ABCG1-mediated cholesterol efflux to serum were determined by measuring the transfer of [(3)H]cholesterol from Fu5AH rat hepatoma cells expressing SR-BI and from human ABCG1-transfected CHO-K1 cells to the medium containing the tested serum respectively. RESULTS SAA was significantly increased in diabetic patients with incipient or overt nephropathy. Both SR-BI and ABCG1-mediated cholesterol efflux to serum were significantly impaired in all three groups of diabetic patients (p < 0.01). SAA inversely correlated with SR-BI-mediated cholesterol efflux (r = -0.36, p < 0.01) but did not correlate with ABCG1-mediated cholesterol efflux. Stepwise linear regression analysis showed that HDL, the presence or absence of diabetes, and log(SAA) were significant independent determinants of SR-BI-mediated cholesterol efflux to serum. CONCLUSION SAA was increased in type 2 diabetic patients with incipient or overt nephropathy, and SAA was associated with impairment of SR-BI-mediated cholesterol efflux to serum.

Plasma apolipoprotein E concentration is an important determinant of phospholipid transfer protein activity in type 2 diabetes mellitus.

Phospholipid transfer protein (PLTP) transfers phospholipids between lipoproteins and plays an important role in HDL metabolism. PLTP exists as a high‐activity and a low‐activity form in the circulation. In vitro studies have shown that apolipoprotein (apo) E is involved in maintaining PLTP in the active form, while the low‐activity form is associated with apo AI. We have therefore investigated whether plasma apo AI, B and E concentrations are important determinants of plasma PLTP activity in type 2 diabetes, a condition associated with increased plasma PLTP activity.

Glycoxidized LDL increases lectin-like oxidized low density lipoprotein receptor-1 in diabetes mellitus

AIMS LDL is subjected to glycoxidation in diabetes mellitus. We have evaluated the effect of glycoxidized LDL on lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) expression in endothelial cells in vitro, as well as the relationship between glycoxidzied LDL and LOX-1 in type 2 diabetic patients with and without microalbuminuria in vivo. METHODS Endothelial cells were incubated with modified LDL including glycoxidized LDL, oxidized LDL (oxLDL), glycated LDL, and acetylated LDL, and cellular LOX-1 and the soluble forms of LOX-1 (sLOX-1) in cell medium was measured. Glycoxidized LDL in diabetic patients was determined by measuring the glycoxidation product N(epsilon)-(carboxymethyl)lysine (CML) in apolipoprotein (apo) B. Serum oxLDL and sLOX-1 was determined by ELISA. RESULTS Only glycoxidized LDL and oxLDL significantly increased LOX-1 expression (p<0.05) and the production of sLOX-1 (p<0.05), and the effect of glycoxidized LDL was greater than that of oxLDL. Both normoalbuminuric (n=110) and microalbuminuric (n=91) patients had higher serum apoB-CML than controls (n=105) (p<0.01), but oxLDL was only elevated in the microalbuminuric patients (p<0.05). Serum sLOX-1 was significantly increased in both groups of patients compared to controls (p<0.01). Serum sLOX-1 correlated with apoB-CML (r=0.36, p<0.001) but not with oxLDL. The relationship between sLOX-1 and apoB-CML was independent of HbA1c, age, gender, BMI and smoking status. CONCLUSION Glycoxidized LDL was more potent than oxLDL in inducing LOX-1 in vitro. Serum concentration of apoB-CML, a marker of glycoxidized LDL, was increased in type 2 diabetic patients with and without microalbuminuria, and this was associated with an increase in serum sLOX-1.