Elevated hepatic DPP4 activity promotes insulin resistance and non-alcoholic fatty liver disease

Baumeier, Christian; Schlüter, Luisa; Saussenthaler, Sophie; Laeger, Thomas; Rödiger, Maria; Alaze, Stella Amelie; Fritsche, Louise; Häring, Hans-Ulrich; Stefan, Norbert; Fritsche, Andreas; Schwenk, Robert Wolfgang; Schürmann, Annette

We have recently shown that early alterations in hepatic Dpp4 are associated with insulin resistance resulting in later liver steatosis [23] . As studies on primary human adipocytes and skeletal muscle cells discovered direct effects of soluble DPP4 on the insulin sensitivity of these cells [18] , we tested whether elevated DPP4 levels also affect hepatic insulin sensitivity. Human HepG2 cells were treated with various concentrations of recombinant human DPP4 (rhDPP4) and analyzed for their insulin responsiveness. Insulin-stimulated Aktphosphorylation was unaffected with low dose of rhDPP4 (75e150 ng/ ml) but completely blunted with a dosage of 300e500 ng/ml ( Figure 6A) . Also, in primary murine hepatocytes obtained from 12week-old lean WT mice, 500 ng/ml recombinant mouse DPP4 (rmDPP4) reduced the insulin-stimulated Akt-phosphorylation by 15% ( Figure 6B ). Moreover, adenoviral-mediated overexpression of fulllength Dpp4 (Ad-Dpp4) [23] in primary hepatocytes led to a reduction of insulin sensitivity by 30%, when compared to Ad-Gfp infected control cells ( Figure 6C ). Primary hepatocytes obtained from 12-weekold standard diet fed WT and Dpp4-Liv-Tg mice did not differ in lipid content (data not shown) but revealed differences in insulin responsiveness, as hepatocytes from Dpp4-Liv-Tg mice showed an 18% lower Akt-phosphorylation than those of WT controls ( Figure 6D ). Taken together, soluble DPP4 reduced the insulin sensitivity of human and mouse liver cells, suggesting a direct role of DPP4 in hepatic insulin signaling. To test hepatic insulin sensitivity in WT and Dpp4-Liv-Tg mice, we injected insulin into 30-week-old animals and sacrificed them 15 min later. Figure 6E shows that insulin-stimulated Akt-phosphorylation was markedly lower in livers of Dpp4-Liv-Tg mice when compared to WT littermates. In agreement, insulin tolerance test at 22 weeks of age confirmed the impaired insulin sensitivity of Dpp4-Liv-Tg mice ( Figure 6F) , however, at a time-point when fatty liver was already induced ( Figure 4A ). Fasting insulin levels as well as HOMA-IR (homeostatic model assessment for insulin resistance) were numerically increased in Dpp4-Liv-Tg mice but did not reach statistical significance (Table 1) . Finally, we considered leptin to adiponectin ratio as another measure for systemic insulin resistance [31] and found increased ratio in Dpp4-Liv-Tg mice at 30 weeks of age (Table 1) . Thus, overexpression of Dpp4 in livers of diet-induced obesity mice impairs the hepatic insulin sensitivity.