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-g2 Gene

The transcription of many genes involved in lipid metabolism is regulated by the peroxisome proliferator– activated receptor-g (PPAR-g). The Pro12Ala polymorphism in the PPAR-g2 gene has been associated with reduced transcriptional activity in vitro and increased insulin sensitivity in vivo. Although PPAR-g has been demonstrated in human b-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 6 61 mmol/l to 1,732 6 163 mmol/l in the Pro/Pro and from 372 6 46 mmol/l to 1,630 6 96 mmol/l in the X/Ala group (P 5 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 5 0.21). In the Pro/Pro group, second-phase insulin secretion remained unchanged (444 6 67 vs. 471 6 93 pmol/min) and the response to arginine increased from 5,007 6 41 to 6,072 6 732 pmol/min. In contrast, in the X/Ala group, there was a decrease of both second-phase insulin secretion (533 6 58 to 427 6 48 pmol/min, P 5 0.02 vs. Pro/Pro) and in the response to arginine (from 7,518 6 1,306 to 6,458 6 1,040 pmol/min, P 5 0.014 vs. Pro/Pro). The insulin sensitivity index decreased comparably in Pro/Pro and X/Ala (to 71 6 8 vs. 74 6 9% of basal, P 5 0.8). In conclusion, these results provide evidence that the Pro12Ala polymorphism in the PPAR-g2 gene might be involved in a differential regulation of insulin secretion in response to increased FFAs in humans. Diabetes 50:1143–1148, 2001 Lipid and glucose metabolism are inseparable in humans. On the molecular level, the peroxisome proliferator–activated receptor isoform-g (PPARg), a transcription factor, may represent a key mediator between development and metabolism of adipocytes, free fatty acids (FFAs), and glucose homeostasis (1,2). On the one hand, PPAR-g regulates the transcription of genes involved in lipid metabolism (e.g., lipoprotein lipase, fatty acid transport proteins, acyl-CoA synthetase, and malic enzyme) (2); on the other hand, unsaturated FFAs are natural ligands of this receptor (3). Moreover, synthetic ligands of PPAR-g, the thiazolidinediones, possess antidiabetic properties (4,5). PPAR-g1 is expressed in many tissues, including skeletal muscle, whereas the splice variant PPAR-g2 is mainly expressed in adipocytes. Expression of PPAR-g has also been demonstrated in human b-cells (6). This indicates a potential significance of PPAR-g for development and/or metabolism of b-cells. Recently, a reduced transcriptional activity of the proline to alanine exchange in codon 12 (Pro12Ala) in PPAR-g2 has been shown in vitro (7). In humans, an association of the Pro12Ala polymorphism with lower fasting insulin was observed and thought to indicate improved insulin sensitvity (7). Higher insulin sensitivity based on hyperinsulinemic clamps was also reported for severely obese subjects carrying the Ala allele (8). The prevalence of the Pro allele was higher in JapaneseAmericans with type 2 diabetes (7) but was not different in Caucasian populations (9,10). Intriguingly, a significantly higher BMI was reported for subjects carrying the Ala allele (11,12). Moreover, this polymorphism appears to modify the relationship between leptin levels and adipose tissue mass in such a way that, for a given leptin level, the BMI is relatively lower in obese subjects carrying the Ala allele (13). Recently, a reduced risk for type 2 diabetes was demonstrated for carriers of the Ala allele (14). All of these observations cannot easily be incorporated into a unifying schema explaining the functional role of this polymorphism in human obesity and type 2 diabetes. The available data regarding the effect of Pro12Ala in humans were exclusively based on cross-sectional data. However, to understand the pathophysiological relevance of genetic variants, it is essential to also study gene-environment interactions. One environmental factor important for glucose hoFrom the Medizinische Klinik, Abteilung für Endokrinologie, Stoffwechsel und Pathobiochemie, Eberhard-Karls-Universität, Tübingen, Germany. Address correspondence and reprint requests to Dr. Michael Stumvoll, Medizinische Universitätsklinik, Otfried-Müller-Str. 10, 72076 Tübingen, Germany. E-mail: [email protected]. Received for publication 3 July 2000 and accepted in revised form 9 February 2001. FFA, free fatty acid; ISI, insulin sensitivity index; ISR, insulin secretion rate; MANOVA, multivariate analysis of variance; PPAR-g, peroxisome proliferator–activated receptor-g.