GIP Overexpression and Diet-Induced Obesity Figure 4. Het GIP Tg mice retain responsiveness to GIP. A. Plasma GIP levels. After 13 weeks of LF or HF diet feeding, circulating GIP levels were determined in Het GIP Tg and WT littermates (n = 5,6/group). B and D. Effects of GIP on glycemic excursions in LF (B)- and HF (D) diet-fed Het GIP Tg and WT littermates. After 13 weeks of feeding, WT and Het GIP Tg mice (n = 5,6/group) were fasted for 4 hours, and PBS or GIP (24 nmol/kg) were administered immediately prior to glucose loading (2 g/kg). Blood glucose levels were measured at 0, 15, 30, 60 and 120 min following the glucose challenge. C and E. Quantification of AUC for the total glycemic excursions in B and D. All data represent the mean 6 S.E.M. and significance was tested using ANOVA with a Newman-Keuls post hoc test, where ** represents p,0.05 vs WT littermates or ndicated group. doi:10.1371/journal.pone.0040156.g004 biogenesis is reduced by p38 MAPK activation [42], a target of GIP signaling. The marked reduction in expression of genes coding for proinflammatory proteins and enzymes involved in mitochondrial biogenesis was unexpected, since previous studies showed that GIP stimulated adipocyte resistin release [16], an effect that has been interpreted as pro-inflammatory. However, it is possible that resistin responses to GIP are unrelated to inflammatory effects, but associated with resistin’s metabolic action on the adipocyte. Expression levels of adipocyte enzyme genes involved in fat metabolism were also decreased in Het GIP Tg mice. Down- regulation of Ppargc1a and Acly would be expected to reduce fatty acid synthesis, whereas reduced Pck1 expression would lower rates of glyceroneogenesis and reesterification [43,44]. In contrast Pparg expression was increased. Apart from Pparg these changes were also unexpected because in vitro treatment of 3T3-L1 adipocytes and human or rodent adipocytes with GIP has been shown to induce lipogenesis [15–17,21]. However, lipogenic responses to GIP are insulin-dependent and, since IRS-1 was down-regulated and IRS-2 increased, the synergistic effects of these hormones may be altered. It is therefore likely that reduced steatosis and adipocyte macrophage infiltration plus altered WAT signaling all contribut- ed to the improved metabolic profiles of the Het GIP Tg mice on the HF diet, including the improved insulin sensitivity. We next examined whether altered physical activity, energy expenditure or PLoS ONE | www.plosone.org 7 July 2012 | Volume 7 | Issue 7 | e40156