Effect of Experimental Elevation of Free Fatty Acids on Insulin Secretion and Insulin Sensitivity in Healthy Carriers of the Pro12Ala Polymorphism of the Peroxisome Proliferator–Activated Receptor-γ₂ Gene

Abstract

The transcription of many genes involved in lipid metabolism is regulated by the peroxisome proliferator–activated receptor-γ (PPAR-γ). The Pro12Ala polymorphism in the PPAR-γ₂ gene has been associated with reduced transcriptional activity in vitro and increased insulin sensitivity in vivo. Although PPAR-γ has been demonstrated in human β-cells, it is unknown whether the Pro12Ala polymorphism plays a role in insulin secretion. Moreover, it is also unknown if and how the effect of free fatty acids (FFAs) on insulin secretion and insulin sensitivity is modulated by the presence of this polymorphism. We therefore performed hyperglycemic clamps (8 mmol/l, 140 min, 5 g arginine bolus at min 120) in 10 healthy subjects with the (X/Ala) polymorphism and in 10 subjects without the polymorphism (Pro/Pro) basally and after 5 h infusion of Intralipid plus heparin. FFA concentrations increased from 473 ± 61 μmol/l to 1,732 ± 163 μmol/l in the Pro/Pro and from 372 ± 46 μmol/l to 1,630 ± 96 μmol/l in the X/Ala group (P = 0.68). Basally, neither insulin sensitivity nor insulin secretion were significantly different between the two groups. During infusion of Intralipid, first-phase insulin secretion remained unchanged in both groups (P = 0.21). In the Pro/Pro group, second-phase insulin secretion remained unchanged (444 ± 67 vs. 471 ± 93 pmol/min) and the response to arginine increased from 5,007 ± 41 to 6,072 ± 732 pmol/min. In contrast, in the X/Ala group, there was a decrease of both second-phase insulin secretion (533 ± 58 to 427 ± 48 pmol/min, P = 0.02 vs. Pro/Pro) and in the response to arginine (from 7,518 ± 1,306 to 6,458 ± 1,040 pmol/min, P = 0.014 vs. Pro/Pro). The insulin sensitivity index decreased comparably in Pro/Pro and X/Ala (to 71 ± 8 vs. 74 ± 9% of basal, P = 0.8). In conclusion, these results provide evidence that the Pro12Ala polymorphism in the PPAR-γ₂ gene might be involved in a differential regulation of insulin secretion in response to increased FFAs in humans.