Sammy W. M. Shiu

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.

LDL subfractions in acromegaly: Relation to growth hormone and insulin-like growth factor-I

Acromegaly is associated with changes in lipoprotein metabolism and an excess in cardiovascular mortality. We have examined low density lipoprotein (LDL) subfraction distribution in 24 patients with active acromegaly and in controls matched for age, sex and body mass index. LDL was subfractionated by density gradient ultracentrifugation. The concentration of small dense LDL-III was significantly higher in the acromegalic patients compared to the controls (94.2 +/- 44.9 versus 67.2 +/- 30.4 mg/dl, P < 0.05) and there was a concomitant reduction in the intermediate subfraction LDL-II (124.8 +/- 31.3 versus 149.9 +/- 30.0 mg/dl, P < 0.05). Univariate analysis showed that both growth hormone (GH) and insulin-like growth factor (IGF)-I correlated with LDL-III and inversely with LDL-II. Acromegalic patients were found to have lower hepatic lipase (HL) and lipoprotein lipase (LPL) activities than controls (HL: 13.29 +/- 6.56 versus 21.58 +/- 7.27 micromol FFA released/ml/h, P < 0.001: LPL: 7.22 +/- 3.04 versus 11.53 +/- 7.85 micromol FFA released/ml/h, P < 0.05) whereas plasma cholesteryl ester transfer protein (CETP) activity was significantly increased (8.15 +/- 1.81 versus 5.54 +/- 1.86 pmol/microl/h, P < 0.001). Both GH and IGF-I were significantly associated with HL, LPL and CETP activities. Multivariate analysis on this relatively small sample size showed that in normal subjects, triglyceride and HL activity were the major determinants of LDL-III. In contrast, GH and HDL were the main determinants in acromegaly, accounting for 32 and 24% in the variability of LDL-III respectively. In conclusion, GH excess has a direct effect on LDL subfraction distribution.

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.

Roles of hepatic lipase and cholesteryl ester transfer protein in determining low density lipoprotein subfraction distribution in Chinese patients with non-insulin-dependent diabetes mellitus

Patients with non-insulin-dependent diabetes mellitus (NIDDM) are known to have abnormalities in their low density lipoprotein (LDL) subclass pattern with a preponderance of small dense LDL. The present study was performed to define the roles of lipolytic enzymes (hepatic and lipoprotein lipase) and cholesteryl ester transfer protein (CETP) in determining the distribution of LDL subfractions in these patients. LDL subfractions were measured by density gradient ultracentrifugation in 137 patients with NIDDM (75 male, 62 female) and 140 matched controls (80 male, 60 female). The male diabetic patients had a lower concentration of LDL-I (P < 0.01) and a higher concentration of LDL-III than the controls (P < 0.01). In the female diabetic patients, both LDL-I (P < 0.001) and LDL-II concentrations (P < 0.05) were significantly lower than the controls whereas LDL-III was increased (P < 0.001). Hepatic lipase (HL) was significantly increased in both the male and female diabetic patients (P < 0.01, P < 0.05, respectively) compared to their controls. No significant changes were seen in plasma lipoprotein lipase (LPL) and CETP activity. On multivariate analysis, plasma triglyceride (TG), CETP and HL accounted for 10, 5 and 3% of the variability in LDL-III, respectively, in the diabetic patients (adjusted R2 = 0.18, P = 0.0003). Our findings would support the hypothesis that plasma triglyceride influences LDL particles through a cycle of lipid exchange via the action of CETP. LDL become enriched in triglyceride and are then acted on by HL to produce a population of small dense lipid-poor LDL.

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.

Effects of gender, hepatic lipase gene polymorphism and type 2 diabetes mellitus on hepatic lipase activity in Chinese

Genetic variation in the hepatic lipase (HL) gene (LIPC) promoter is an important determinant of HL activity in Caucasians. As HL activity is increased in patients with type 2 diabetes mellitus, we have investigated whether the -514 C-to-T polymorphism acted independently of type 2 diabetes to regulate HL activity. The frequency of this polymorphism and its effect on plasma HL activity and lipids were examined in 203 Chinese patients with type 2 diabetes and 205 controls. The frequency of the T allele was 0.343 and 0.376 in male and female diabetic patients, respectively, compared with 0.371 and 0.372 in male and female controls. The effect of LIPC genotype on HL activity was similar between men and women, and between diabetic patients and non-diabetic controls, with the lowest HL activity being found in those subjects with the TT genotype. On multivariate analysis, gender, LIPC genotype, the presence of type 2 diabetes and body mass index were independent predictors of HL activity, accounting for 22, 9, 5 and 3%, respectively, of the variance in HL activity (whole model adjusted R(2)=0.39, P<0.0001). The T allele was associated with higher high-density lipoprotein in the controls but not in the diabetic patients, and no associations were found between LIPC genotype and low-density lipoprotein subfractions in either groups. In conclusion, despite the higher frequency of the T allele in Chinese than in Caucasians, gender was the best predictor for HL activity, with LIPC gene polymorphism and type 2 diabetes making relatively smaller contributions to the variation in HL activity.

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.