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

The present data demonstrate that (i) subjects suffering from NAFLD exhibit elevated plasma DPP4 activity and that (ii) hepatocyte-specific overexpression of Dpp4 contributes to elevated plasma activity of the enzyme, which, in turn, causes diminished glucose-induced active GLP-1 levels. Moreover, overexpression of Dpp4 in the liver (iii) promotes the development of hepatic insulin resistance and NAFLD and (iv) enhances adipose tissue expansion and inflammation under obese conditions. NAFLD patients are known to have increased DPP4 mRNA levels in the liver [9] , and this associates with the degree of hepatic steatosis in obese subjects [23] . In addition, we have recently shown that obesityprone mice reveal an increased expression of Dpp4 already at the age of 6 weeks, proceeding in hepatic steatosis later in life [23] . However, it was not clear whether elevated DPP4 levels in the liver are causal for the later onset of NAFLD. Here, we show for the first time that the overexpression of Dpp4 in hepatocytes of HFD fed C57BL/6J mice promotes the development of fatty liver. Thus, dysregulation of Dpp4 expression by e.g. epigenetic mechanisms [23] can lead to perturbations in hepatic metabolism and finally to elevated ectopic lipid accumulation in the long-term. Although we show increased hepatic steatosis as a consequence of Dpp4 overexpression, our data indicate that the elevated hepatic lipid accumulation in Dpp4-Liv-Tg mice is a secondary effect of alterations in (i) hepatic insulin sensitivity and (ii) energy homeostasis via interference in the incretin axis. Dpp4-Liv-Tg mice display severe hepatic insulin resistance at 30 weeks of age, and insulin tolerance test indicates impaired insulin sensitivity already at a younger age. The fact that human HepG2 cells and murine primary hepatocytes showed impaired insulin sensitivity in response to DPP4 leads to the conclusion that DPP4 has a direct incretin-independent effect on hepatic insulin signaling. Insulinstimulated Akt-phosphorylation was blunted by the treatment with recombinant DPP4 as well as by adenoviral and transgenic overexpression of Dpp4. Importantly, dose of recombinant protein inducing insulin resistance (500 ng/ml) was in a physiological range found in serum of obese and insulin resistant subjects [18, 19] . The observed effects on insulin sensitivity were independent of the lipid content, excluding lipotoxicity-induced insulin resistance. This is in line with recently published data showing improved insulin sensitivity and lower fat content in HepG2 cells after siRNA-mediated suppression of DPP4 [32] . Furthermore, studies in primary human adipocytes showed insulin resistance by the administration of recombinant DPP4 [18] and improved insulin sensitivity when DPP4 was downregulated [20] . Moreover, long-term DPP4 inhibition improved insulin sensitivity and reduced liver fat content in animals with diet-induced hepatic steatosis and insulin resistance [16, 33] . Thus, DPP4 has an autocrine effect on hepatic insulin signaling which might contribute to later accumulation of ectopic fat in the liver. GLP-1 is known to be involved in energy homeostasis by its anorexic action in the brain. Here we show that Dpp4-Liv-Tg mice exhibited lower levels of active GLP-1 in the periphery, whereas portal vein concentrations were not affected. This clearly demonstrates that despite normal secretion from intestinal L-cells, hepatic Dpp4 overexpression leads to a substantial reduction of acti