Liver fructokinase: a new target for GLP-1 receptor agonists

FRANCINI F; MASSA ML; CASTRO MC; POLO MP; GAGLIARDINO JJ

Congreso; 49th European Association for the Study of Diabetes Annual Meeting; 2013

Background: While exendin-4 (GLP-1R agonist) and the DPP-IV inhibitors effects upon islet of Langerhans, CNS and stomach have been widely studied, little is known about their effects upon liver glucokinase (GK) and fructokinase (FK). On the other hand, the presence and function of a GLP-1R in this organ is controversial. Aim: to evaluate the effect of exendin-4 and sitagliptin upon fructose (Fr)-induced changes upon GK and FK activity in in vivo and in vitro models. Materials and methods: Adult male Wistar rats received for 3 weeks a standard commercial diet, without (C) or with 10% Fr (w/v) in the drinking water (F). C and F were daily co-treated with either sitagliptin (115 mg/day per rat) (CS and FS) or exendin-4 (0.35 nmol/kg of body weight) (CE and FE). After treatments, we measured: 1) Serum glucose (G) (enzymatic method), insulin (I) (RIA) and triglyceride (TG) (enzymatic method) levels; 2) GK activity in cytosolic (CF) and nuclear (DNF) fractions (enzyme-coupled photometric assay), 3) GK and FK gene (qPCR) and protein expression (Western blot [Wb]), 4) liver TG content and 5) FK activity.  In parallel, human HepG2 cells were incubated for 72 either with Fr 1.5 mM alone, or plus exendin-4 (5 nM), exendin-9 (100 nM) and both.  After culture we measured in these cells GK and FK activity. Results: In vivo study   C CE CS F FE FS G (mM) 4.5±0.1 4.2±0.1 4.5±0.2 4.9±0.2 4.2±0.2 4.7±0.3 TG (mM) 0.47±0.01 0.48±0.06 0.63±0.09 1.14±0.12?? 0.72±0.09* 0.67±0.05** I (ng/ml) 0.30±0.02 0.25±0.03 0.27±0.03 1.09±0.28? 0.30±0.05* 0.31±0.05* Values are means ± SEM (n =20). TG: ?? F vs. C, P < 0.0001; * FE vs. F, P < 0.003; ** FS vs. F, P < 0.001.  I: ? F vs. C, P < 0.02; * FE or FS vs. F, P <0.03. F induced a significant increase of total GK activity in rat livers that was prevented by administration of either exendin-4 or sitagliptin. These animals also present a significantly increase in FK activity and in liver TG content. Co-administration of Exendin-4 and sitagliptin to F rats prevented all these changes.In HepG2 cells, incubation with F reproduced the above mentioned changes, i.e. an increase in GK and FK activity that was reverted by Exendin-4 addition to the culture medium. The preventive effect of Exendin 4 was blunted by co-treatment with Exendin-9. Conclusions: Both Exendin 4 and sitagliptin co-administration to F rats prevent the development of all changes induced by Fr both in vivo (rats) and in vitro (HepG2 cells) conditions.  Such preventive effect could be mediated throughout inhibition of FK using a GLP-1 receptor pathway.