Fig 2: Liver-specific Dpp4 overexpression causes increased plasma DPP4 activity and reduced glucose-stimulated GLP-1 levels. Male wild-type (WT, open circles) and Dpp4-transgenic (Dpp4-Liv-Tg, black circles) received a high-fat diet until 30 weeks of age. (A, B) Relative Dpp4 mRNA expression (A) and protein content (B) in various tissues (n = 3–4). DPP4 western blots are depicted with two different exposure times. Liver DPP4 is quantified with tubulin as loading control (n = 4). (C) Immunohistochemical staining of liver sections for DPP4 (green) and E-cadherin (red). Nuclei were stained with TO-PRO® 3 iodide (blue). Scale bar, 10 or 30 μm, respectively. (D) Plasma DPP4 activity in vena cava (v.cava) (n = 7–9). (E) Plasma (active) GLP-1 levels in portal vein (v.portae) and vena cava 15 min after oral glucose bolus (n = 8). Liv, liver; gWAT, gonadal white adipose tissue; sWAT, subcutaneous white adipose tissue; BAT, brown adipose tissue; SM, skeletal muscle (quadriceps). All data are represented as mean ± SEM. *p < 0.05, **p < 0.01. ns, not significant.

Fig 3: NAFLD and insulin resistant subjects show elevated plasma DPP4 activity. (A–C) Liver fat content (A), insulin sensitivity (B), and plasma DPP4 activity (C) of age- and sex-matched subjects with or without (control) NAFLD. Data are represented as mean ± SEM (n = 75–77). One-tailed t-test was performed to test whether liver fat, insulin sensitivity index, and plasma DPP4 activity are increased in NAFLD subjects. *p < 0.05, ***p < 0.001.

Fig 4: Increased DPP4 levels cause hepatic insulin resistance. (A, B) Basal (white bars) and insulin-stimulated (100 nmol/l, 10 min, gray bars) Akt-phosphorylation after 48 h treatment with various concentrations of recombinant human (rhDPP4) or mouse (rmDPP4) DPP4 in HepG2 cells (A) and murine primary hepatocytes (B) (n = 3–6), respectively. (C) Relative insulin-stimulated Akt-phosphorylation in primary mouse hepatocytes, 48 h after infection with adenoviruses coding for DPP4 (Ad-Dpp4) or GFP (Ad-Gfp) (n = 4). (D) Relative insulin-stimulated Akt-phosphorylation in primary hepatocytes isolated from wild-type (WT) and Dpp4-transgenic (Dpp4-Liv-Tg) mice, 48 h after isolation (n = 3). (E) Akt-phosphorylation in livers of 30-week-old WT and Dpp4-Liv-Tg mice 15 min after NaCl (−) or insulin injection (+; 1.25 IU/kg body weight) (n = 3). (F) Insulin tolerance test (1.25 IU/kg body weight) and area under the curve (AUC) for blood glucose in 22-week-old WT and Dpp4-Liv-Tg mice (n = 7–9). All data are represented as mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001.

Fig 5: Hepatocyte-specific Dpp4 overexpression induces adiposity and adipose tissue inflammation. Male wild-type (WT, open circles) and Dpp4-transgenic (Dpp4-Liv-Tg, black circles) mice were fed a high-fat diet until 30 weeks of age. (A–C) Development of body weight (A), fat mass (B), and lean mass (C). (D) Tissue weight of gonadal (gWAT) and subcutaneous white adipose tissue (sWAT), and brown adipose tissue (BAT). (E) Relative expression of the macrophage marker F4/80 (Emr1) and proinflammatory cytokines (TNFα (Tnf), MCP1 (Ccl2) and IL6 (Il6)) in gWAT of 30-week-old mice. (F) Immunohistochemical staining of F4/80 in gWAT. All data are represented as mean ± SEM (n = 7–9). *p < 0.05, **p < 0.01.