Although most of the incretin effects on glucose metabolism

Although most of the incretin effects on MB05032 metabolism can be secondary to their action on insulin/glucagon secretion, some evidence strongly suggests that they could also act directly on liver glucose utilization and production [4], [13], [22], [27], [34], [44]. Supporting this concept, Ayala et al. [1] using a model with controlled circulating insulin levels have recently demonstrated that GLP-1 receptor participates directly in regulation of hepatic production and muscle uptake of glucose. Using the hyperinsulinemic-euglycemic clamp model, other authors have also demonstrated that acute infusion of des-F-sitagliptin induces insulin-mediated suppression of endogenous glucose production, mainly at hepatic level, with no differences in glucose disposal rate [12]. The effect of GLP-1 and GIP occurred via activation of specific G-protein coupled receptors for both compounds (GLP-1 receptor and GIP receptor) expressed in most body tissues [4], [22]. Although the effect of incretins at hepatic level has been clearly shown, whether these effects occur through or independently of GLP-1 receptor is still a matter of discussion [39], [40], [41]. While it has been shown that radiolabeled GLP-1 binds to hepatic membranes [43] and GLP-1 receptor has recently been identified in human and rodent hepatocytes [18], [37], the latter results have been seriously challenged [31]. On the other hand, it has been postulated that GLP-1 receptor activation could directly reduce liver steatosis [11], [28] probably acting via insulin- [18] and AMPK-signaling pathway [2]. Additionally, Svegliati-Baroni et al. [37] also demonstrated that GLP-1 receptors in hepatocytes were reduced in patient with NASH. Our current in vitro results obtained using HepG2 cells, suggest that exendin-4 exerts its preventive effect on F-induced glucokinase increased activity, by acting directly on liver cells, interacting mainly with GLP-1 receptor. However, we cannot rule out the participation of other mechanisms in this effect. In this regard, it has been reported that exendin-9 could inhibit insulin secretion even in the absence of high levels of circulating GLP-1 [8], [35], thereby suggesting that exendin-9 is an inverse agonist of the GLP-1 receptor [5]. This concept implies tonic regulation via ligand-independent activity of the GLP-1 receptor inhibited by exendin-9 [5]. This phenomenon, however, was not observed in our model since GK activity level reported in HepG2 cells cultured in presence of fructose was not modified by co-incubation with exendin-9 alone. The beneficial effect of DPP-4 inhibitors and GLP-1 receptor agonists on glucose metabolism and β-cell mass/function in different models of T2DM has been largely and consistently documented [14], [15], [29], [33]. However, their effect on liver carbohydrate metabolism, particularly glucokinase activity, in an animal model with characteristics that resemble those recorded in human pre-diabetes is not conclusive. Our data is the first demonstration that the preventive effect of both exendin-4 and des-F-sitagliptin on fructose-induced high liver glucokinase activity, could be partly due to their direct effect on liver cells. Since exendin-9 blocks the exendin-4 preventive effect in HepG2 cells, the latter effect may be mediated through exendin-4 interaction with liver GLP-1 receptor. This assumption is supported by the current demonstration of the presence of glucokinase in these cells and the blocking effect of exendin-9 on that of exendin-4 on glucokinase activity (Fig. 4C).
Conclusions
Authors contribution
Acknowledgments This study was partially supported by an unrestricted grant provided by (PIP0371) Consejo Nacional de Investigaciones Científicas y Técnicas -CONICET- Argentina (grant PIP0371) and by an unrestricted grant provided by Merck, Sharp & Dohme, Argentina. The authors manifest no conflict of interest.
Introduction