Localization of dipeptidyl peptidase-4 (CD26) to human pancreatic ducts and islet alpha cells
Introduction
Dipeptidyl peptidase-4 (DPP-4) is a serine exopeptidase that cleaves X-proline dipeptides from the N-terminus of polypeptides. DPP-4 is identical with the leukocyte surface antigen CD26 and adenosine deaminase complexing protein 2 [1], [2], [3]. It is expressed on the surface of many cell types and has multiple functions, including the processing of peptide hormones, chemokines and neuropeptides, binding of adenosine deaminase and co-stimulation of T cells [4], [5], [6], [7]. In addition, DPP-4/CD26 is a marker of cancer stem cells and has been implicated in malignant transformation [8].
DPP-4 plays a major role in glucose metabolism [9]. It degrades the incretin hormones glucagon-like peptide 1 (GLP-1) and gastric inhibitory peptide (GIP) that are released from the gut into the bloodstream in response to ingested glucose [10], [11]. Both incretins enhance glucose-stimulated insulin secretion, suppress glucagon secretion and reduce gastric emptying, but have short half-lives due to their degradation by DPP-4 [4], [10], [11]. DPP-4 inhibitors that increase incretin concentrations and extend their duration of action are a new class of oral hypoglycaemic drugs [9].
Earlier studies detected DPP-4 in the exocrine pancreas of rabbits [12] and in the endocrine pancreas of mice [13] and pigs [14]. In pig islets, DPP-4 was detected in secretory granules of alpha cells [13], [14]. The cellular localization of DPP-4 in the human pancreas has been poorly documented but may have implications for understanding the physiology of incretins. Dinjens et al. in 1989 [15], reported DPP-4 expression in the luminal membranes of intra- and inter-lobular pancreatic ducts but not in islets or pancreatic acini. In 2013 Bramswig et al. [16] detected DPP-4 gene expression by RNA-Seq in human glucagon-secreting alpha cells. At the same time Omar et al. [17] and ourselves [18], [19] investigated DPP-4 expression in the human pancreas by immunochemistry. Recently, Omar et al. [20] reported the expression of DPP-4 in islets. Here, we localize DPP-4 expression to ducts and alpha cells in human pancreas tissue obtained from cadaveric donors without diabetes and with type 1 and 2 diabetes.
Section snippets
Pancreatic tissue processing
Human pancreata were obtained through the Australian Islet Transplant Consortium and trained coordinators of DonateLife, from heart-beating, brain-dead donors, with informed written consent of next-of-kin. Copies of the consent documents were retained by the next-of-kin, DonateLife and the researchers. The Human Research Ethics Committees of St. Vincent's Institute of Medical Research, Western Sydney Local Health District and the Walter and Eliza Hall Institute of Medical Research approved the
Islet and non-islet cells express DPP-4
The characteristics of the organ donors are provided in Table 1. DPP-4 was detected by immunohistochemistry in islet and non-islet cells from 13 donors without diabetes (example shown in Fig. 2A–C). 85% of islets expressed DPP-4.
By flow cytometry, we confirmed the expression of DPP-4 on both islet and duct-rich cells (Fig. 1C). DPP-4 was preferentially expressed on islet cells with a higher side scatter (SSC), equivalent to Hpi1+ cells, and on duct-rich cells with a lower SSC, equivalent to Hpd1
Discussion
Earlier studies investigating the pancreatic expression of DPP-4 reported remarkable differences between mammalian species [12], [13], [14], [15], [16]. In mice, DPP-4 was found on islet and ductal cells [13]. In rabbits, DPP-4 was expressed on acinar cells and intercalated ducts but not in islets [12], [13], [14], [15], [16], whereas in pigs DPP-4 was detected only in secretory granules of alpha cells [14].
Since DPP-4 is a target of a new generation of oral hypoglycemic drugs, we addressed the
Conflict of interest statement
All authors declare that they have no conflicts of interest. We confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the
Acknowledgements
P.A. was the recipient of EFSD Albert Renold Travel Fellowship from the European Federation for the Study of Diabetes. This research was supported by a NHMRC-JDRF Special Program grant (465199). The authors are grateful to Mrs. Alana Neale for technical assistance and to Dr. Matthias Hundt for advice about image analysis. The work was made possible through Victorian State Government Operational Infrastructure Support and Australian National Health and Medical Research Council Research Institute
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