Islet transplantation is a promising treatment for selected patients with type 1 diabetes mellitus (T1DM). Hypoxia and oxidative stress are major causes of damage to transplanted islets. Mesenchymal stem cells (MSCs) have been shown to enhance cell survival mainly through paracrine secretion. However, mechanisms of action underlying the protective effects of MSCs on islets have not been fully elucidated. In this study, we investigated whether human umbilical cord-derived MSCs (huc-MSCs) could inhibit hypoxia and ROS-related cell death of neonatal porcine islet cell clusters (NICCs) and further determined the underlying molecular mechanisms. NICCs were cultured in vitro under normoxic and hypoxic (1% O2) conditions with or without MSC-conditioned medium (MSC-CM). Apoptosis of NICCs was evaluated by the AO/EB staining and Annexin V/PI flow cytometry analysis. Total and mitochondrial ROS production was detected by fluorometric assays. Western blot and the ERK pathway inhibitor, PD98059, were used to assess the possible pathways involved. The results showed that MSC-CM suppressed hypoxia-induced oxidative stress and cell death of NICCs. MSC-CM also activated several pro-survival pathways in NICCs under hypoxic conditions. Furthermore, MSC-secreted exosomes and IL-6 partially recapitulated the multifunctional benefits of MSC-CM. This study showed that huc-MSCs protected NICCs from hypoxia-induced cell death by regulating the cell redox state and cell signaling pathways. This increased understanding may enable MSCs to become a more promising adjuvant cell therapy for islet transplantation. Impact statement The utilization of mesenchymal stem cells (MSCs) is a promising approach to serve as adjuvant therapy for islet transplantation. But the inability to translate promising preclinical results into sound therapeutic effects in human subjects indicates a lack of key knowledge of MSC-islet interactions that warrant further research. Hypoxia and oxidative stress are critical factors which lead to a tremendous loss of islet grafts. However, previous studies mainly focused on other aspects of MSC protection such as inducing revascularization, enhancing insulin secretion, and reducing islet apoptosis. In this study, we aim to investigate whether MSC can protect islet cells from hypoxic damage by inhibiting ROS production and the potential underlying pathways involved. We also explore the effects of MSC-derived exosomes and IL-6 on hypoxia-injured islets. Our data provide new molecular targets for developing MSC applications, and this may ultimately promote the efficiency of clinical islet transplantation.

Background The dysfunction of islet grafts is generally attributed to hypoxia‐induced damage. Mesenchymal stem cells (MSCs) are currently thought to effectively protect cells from various risk factors via regulating autophagy. In our study, we investigated if human umbilical cord‐derived MSCs could ameliorate hypoxia‐induced apoptosis in porcine islets by modulating autophagy, and we explored the underlying mechanisms. Methods Neonatal porcine islet cell clusters (NICCs) were cultured with human umbilical cord‐derived MSC conditioned medium (huc‐MSC‐CM) and RPMI‐1640 medium (control) under hypoxic conditions (1% O2) in vitro. NICCs were treated with 3‐methyladenine (3‐MA) and chloroquine (CQ) to examine the role of huc‐MSC‐CM in regulating autophagy. Finally, the levels of several cytokines secreted by huc‐MSCs were detected by ELISAs, and the corresponding inhibitors were applied to investigate which cytokine mediates the protective effects of huc‐MSC‐CM. The effects of huc‐MSC‐CM on NICCs viability and autophagy were examined using AO/PI staining, flow cytometry analysis, transmission electron microscopy (TEM) and confocal fluorescence microscopy analysis. The insulin secretion of NICCs was tested with an insulin immunoradiometric assay kit. Results Compared to the control, the huc‐MSC‐CM treatment improved the viability of NICCs, inhibited apoptosis, increased autophagic activity and the levels of PI3K class III and phosphorylated Akt, while the ratio of phosphorylated mTOR/mTOR was reduced. These changes were reversed by CQ and 3‐MA treatments. High concentrations of IL‐6 were detected in hu‐MSC‐CM. Furthermore, recombinant IL‐6 pre‐treatment exerted similar effects as huc‐MSC‐CM, and these effects were reversed by a specific inhibitor of IL‐6 (Sarilumab). Conclusions Our results demonstrated that huc‐MSC‐CM improved islet viability and function by increasing autophagy through the PI3K/Akt/mTOR pathway under hypoxic conditions. Additionally, IL‐6 plays an important role in the function of huc‐MSC‐CM.