In the past few years
In the past few years we and others have reported that GLP-1 may also be secreted by sub-populations of alpha myosin inhibitor in human pancreatic islets (Marchetti et al., 2012, Ellingsgaard et al., 2011), which could contribute to beta cell function and survival (Marchetti et al., 2012, Ellingsgaard et al., 2011, Chambers et al., 2017, Masur et al., 2005). For instance, islets isolated from T2D seem to release more GLP-1 than non-diabetic (ND) islets ex-vivo, which was hypothesized to represent a compensatory mechanism to restore beta cell health (Marchetti et al., 2012). Similarly, GLP-1 release from alpha cells is upregulated in the Psammomys obesus gerbil during the development of diabetes (Hansen et al., 2011). On top of this, it has been recently described that DPP-4 is expressed in human pancreatic alpha cells (Omar et al., 2014, Segerstolpe et al., 2016, Blodgett et al., 2015) and that its inhibition improves ND beta cell function (Shah et al., 2013). However, the role of DPP-4 in human T2D islet cells remains largely unknown. Preliminary observations have shown that DPP-4 gene expression (as assessed by microarray analysis) did not differ between ND and T2D islets (Omar et al., 2014); nevertheless, DPP-4 activity has been reported to be lower in T2D human islet lysates (Omar et al., 2014). Against this background, we presently carried out a comprehensive assessment of expression and role of DPP-4 in human islet cells, performing morphological, ultrastructural, survival, functional and molecular studies in human ND and T2D islets under several different experimental conditions, including the use of a DPP-4 inhibitor. Major findings of our study are the demonstration of the presence of DPP-4 in both alpha and beta human islet cells, with reduced expression in T2D islets, and the evidence that DPP-4 inhibition has direct beneficial effects on human T2D beta cells.
Materials and methods
Discussion The present study confirms that DPP-4, a proteolytic enzyme that inactivates multiple substrates (including incretin hormones), is expressed in human pancreatic alpha cells (Omar et al., 2014, Bramswig et al., 2013). By performing electron microscopy immunogold analysis, we found that in ND islets around 90% of alpha cells are positive for DPP-4 (Fig. 2A). The intracellular localization of the enzyme has been shown previously by electron microscopy in pig alpha cells, mainly in association with the glucagon granules (Poulsen et al., 1993, Grondin et al., 1999). Importantly, the results of our present confocal and electron microscopy evaluations demonstrate that DPP-4 is also present in approximately 25% of insulin containing cells (Fig. 2A). Accordingly, we found that the enzyme is expressed in EndoC-βH1 cells, a human beta cell line shown to possess analogous physiological characteristics to primary human beta cells (Solimena et al., 2017, Gurgul-Convey et al., 2015, Andersson et al., 2015). In previous work, it has been observed that DPP-4 is also expressed in primary rodent beta cells and INS-1832/13 cells (Liu et al., 2014). Overall, these findings demonstrate that DPP-4 is expressed not only in human alpha, but also beta cells. Recent work performed with single islet cells has not reported the expression of DPP-4 in non-diabetic beta cells (Segerstolpe et al., 2016, Blodgett et al., 2015). Whether these discrepancies are due to beta cell heterogeneity, low grade DPP-4 expression in beta cells or differences in the techniques used (including the low coverage inherent to single cell RNAseq, that usually detects around 7–10.000 genes) remains to be elucidated. The pathophysiological role of DPP-4 in endocrine pancreatic islet cells is still uncertain, but several possibilities can be envisaged. Some substrates of DPP-4, such as peptide YY, are expressed in the embryonic pancreas and may play a role in endocrine cell differentiation (Jackerott and Larsson, 1997, Lukinius et al., 1992). In addition, DPP-4 mediates cellular adhesion by its interaction with fibronectin, which is also involved in organ development and organization (Shimomura et al., 2006). Furthermore, since islet cells (in particular alpha cells) are known to produce GLP-1 under certain circumstances (Marchetti et al., 2012, Ellingsgaard et al., 2011), DPP-4 might contribute to modulate intra-islet GLP-1 concentrations.