APETx2 receptor Interestingly we observed that T D islet cel
Interestingly, we observed that T2D islet APETx2 receptor have lower DPP-4 expression, as compared to ND cells (Fig. 1, Fig. 2). Previously, a reduced activity of this peptidase was observed in human T2D islet lysates (Omar et al., 2014). As mentioned above, GLP-1 is also produced by islet cells (alpha cells in particular) (Marchetti et al., 2012, Ellingsgaard et al., 2011). Given the beneficial effect of GLP-1 on beta cells (Campbell and Drucker, 2013, Seghieri et al., 2013, Vilsbøll et al., 2003, Nauck and Meier, 2016, Ahrén et al., 2002, Chia and Egan, 2008, Muscelli et al., 2012, Dalle et al., 2013, Puddu et al., 2013), one might speculate that reduced expression and activity of DPP-4 represents an attempt to protect beta cells under stressful conditions by increasing local active GLP-1 concentrations. Accordingly, the use of DPP-4 inhibitors has been shown to exert direct beneficial effects on beta cell function and survival. Previous works demonstrated that linagliptin protected isolated ND human islets from the damage induced by elevated glucose, palmitate, cytokines or H2O2 (Shah et al., 2013). In the present study, we confirm that a different DPP-4 inhibitor, MK-0626, directly prevented beta cell death and dysfunction caused by cytokines in both primary human islets and the human cell line EndoC-βH1 (Fig. 3 and Supplemental Fig. 2). We tested cytokines since islet inflammation can play a role in both type 1 and type 2 diabetes development and progression (Eizirik et al., 2012b, Marchetti, 2016). Of more importance, our data showed that DPP-4 inhibition directly improves function, survival and ultrastructure of human diabetic beta cells (Fig. 4). It has been previously shown that cytokine exposure does not apparently affect the expression of DPP-4 in isolated human islets (Eizirik et al., 2012a). Since DPP-4 activity may change independent from its expression (Omar et al., 2014), the beneficial effects of DPP-4 inhibition may not be necessarily accompanied by changes in the expression of the enzyme. The reasons for these direct beneficial effects of DPP-4 inhibition on islet function are not fully clear. DPP-4 inhibitors are effective tools for the treatment of T2D, supposedly due to their action to sustain circulating GLP-1 concentrations (Campbell and Drucker, 2013, Seghieri et al., 2013, Vilsbøll et al., 2003, Nauck and Meier, 2016, Ahrén et al., 2002, Ahrén and Foley, 2016, Deacon and Holst, 2013). However, recent work shows that inhibition of DPP-4 may also be directly associated with protective actions in different cells. Thus, chemically-induced toxicity on human kidney 2 cells (HK-2) is attenuated by the DPP-4 inhibitor diprotin A, via regulation of the ROS/p38MAPK/ERK and PI3K-AKT pathways and NFKB downstream signaling (Wang et al., 2014). In cultured mouse proximal tubular cells, linagliptin, sitagliptin, vildagliptin and alogliptin all inhibit the increased expression of MCP-1, a proinflammatory chemokine induced by free fatty acids (Tanaka et al., 2016). In addition, DPP-4 inhibition reduces hypoxia-induced apoptosis in rat neonatal cardiomyocytes, possibly via STAT3 signaling (Kubota et al., 2016) and represses foam cell formations in THP-1 macrophages via the inhibition of PKC activity (Dai et al., 2014). Finally, linagliptin has been reported to exert neuroprotection against stroke in GLP-1R−/− mice (Darsalia et al., 2016). Our results show that the direct protective effects of DPP-4 inhibition on human beta cells are at least in part independent from GLP-1 action and that DPP-4 inhibition was associated with reduced expression of NFKB1, a gene encoding a DNA binding subunit of the NF-kappa-B (NFKB) protein complex (Fig. 3). Accordingly, several genes downstream NFKB1 activation, involved in inflammatory reactions (Ortis et al., 2006, Meyerovich et al., 2016), were upregulated in human islets exposed to cytokines, which was prevented by DPP-4 inhibition (Supplemental Fig. 3). This anti-inflammatory action is in line with recent data showing that sitagliptin reduced lipopolysaccharide-induced apoptosis in rat insulinoma (RINm) cells by suppressing NFKB activation (Hu et al., 2017), and that the drug reduced cytokine and chemokine expression in islets isolated from C57Bl/6J obese mice (Dobrian et al., 2011).