The Increase of Apolipoprotein A-V During Postprandial Lipemia Parallels the Response of Triglyceride-Rich Lipoproteins in Type 2 Diabetes: No relationship between apoA-V and postheparin plasma lipolytic activity

The present study cohort was comprised of 39 men and 8 women with type 2 diabetes who were enrolled in the previously published nateglinide study with the same inclusion and exclusion criteria (6). A total of 17 patients were treated with diet alone, 20 with sulfonylurea, and 11 with metformin. The study protocol was approved by the ethics committee of the Helsinki University Hospital, and all patients gave informed consent.

The test meal was served after an overnight 12-h fast, and it contained 63 g fat (polyunsaturated fat/saturated fat ratio 0.08), 490 mg cholesterol, 25 g carbohydrate, and 35 g protein as previously described (6). Blood samples were drawn from a catheter placed in an antecubital vein before the meal and at 3, 4, 6, and 8 h. Concentrations of cholesterol, triglycerides, and HDL cholesterol, as well as apoB-100 and apoB-48 concentrations in the plasma lipoprotein fractions isolated by density-gradient ultracentrifugation, were analyzed as previously described (7). Postheparin LPL and hepatic lipase were analyzed by an established method (8). ApoA-V was analyzed by enzyme-linked immunosorbent assay (9). ApoC-III was determined by using a commercially available kit (10).

Data were analyzed using SPSS software (version 13.0; SPSS, Chicago, IL). Postprandial responses over the 8-h period were calculated as incremental areas under the curve (IAUCs) as previously described (11). The significance of the postprandial response of different variables was calculated by using ANOVA of repeated measurements. Spearman rank correlations were calculated to study the associations between variables, and P value <0.05 (two tailed) was considered statistically significant.

The patients were middle-aged (62.8 ± 8.5 years [mean ± SD]), overweight (BMI 28.4 ± 3.4 kg/m²), and mildly dyslipidemic (total cholesterol 5.3 ± 0.7 mmol/l, triglyceride 1.8 ± 0.6 mmol/l, and HDL cholesterol 1.31 ± 0.28 mmol/l). The mean A1C level averaged 7.6 ± 1.0%, and the mean fasting glucose level averaged 10.1 ± 2.7 mmol/l. ApoA-V and apoAC-III concentrations averaged 268 ± 98 ng/ml (range 119–557) and 3.91 ± 1.8 mg/dl (1.0–9.7), respectively.

The responses of both apoA-V and apoC-III closely paralleled the postprandial triglyceride response, particularly in VLDL1 fraction (Figure). The IAUC of apoA-V during oral fat load was positively associated with IAUCs of serum triglyceride (r = 0.571, P > 0.001), chylomicron triglyceride (r = 0.520, P < 0.001), VLDL1-triglyceride (r = 0.691, P < 0.001), chylomicron apoB-48 (r = 0.525, P < 0.001), and VLDL1-apoB-100 (r = 0.558, P < 0.001). Similarly, the IAUC of apoC-III showed significantly positive correlations with these lipoprotein parameters (data not shown). Importantly, the IAUC of apoA-V correlated significantly with that of apoC-III (r = 0.562, P < 0.001). Postheparin plasma LPL activity negatively correlated with IAUCs of VLDL1 triglycerides, VLDL1-cholesterol, and VLDL1- apoB-100 (data not shown).

Fasting ApoA-V concentration did not correlate with postheparin plasma LPL or hepatic lipase activities. The concentration of apoC-III showed no relationship with LPL or hepatic lipase activities. The baseline concentration of apoC-III correlated significantly with triglyceride in fasting plasma (r = 0.866, P < 0.001), chylomicrons (r = 0.769, P < 0.001), VLDL1 (r = 0.888, P < 0.001), and VLDL2 (r = 0.741, P < 0.001). No relationship existed between IAUC of apoA-V and postheparin plasma LPL or hepatic lipase activity.

Despite the lack of correlation between the fasting apoA-V and lipid parameters, the postprandial increase of apoA-V paralleled those of plasma and VLDL1 triglyceride as well as apoC-III, reaching its maximum at 4 h after oral fat load. Since apoA-V is associated with VLDL and chylomicron particles (12), the increase of apoA-V may reflect the increase of VLDL particles during the postprandial phase in analogy with apoC-III overproduction (2). Another potential explanation is that apoA-V production in the liver is increased as a regulatory mechanism to cover the increased need of postprandial lipolysis. Both hypotheses are consistent with the two recent reports showing elevated apoA-V concentrations in patients with severe hypertriglyceridemia (13–14).

Although apoA-V has been reported to play an important role in lipolysis (3,15–16), we found no association between postprandial apoA-V values and LPL activity. This could be explained by the fact that ApoA-V is present in plasma at very low concentrations, far lower than other lipoprotein levels (14). In contrast, animal studies (3,13) have used transgenic models manifesting higher concentrations of apoA-V than seen physiologically in plasma. To summarize, apo A-V response to oral fat load closely parallels those of TRLs and apo CIII in individuals with type 2 diabetes.

This study was supported by grants from the Finnish Cardiovascular Research Foundation, Special State Grants for health science research, The Biomedicum Helsinki Foundation, The Finnish Medical Society Duodecim, The Paulo Foundation, the Sigrid Juselius Foundation, and Aarne Koskelo Foundation.

We thank Helinä Perttunen-Nio, Hannele Hilden, and Virve Naatti for their excellent technical assistance.