Endoplasmic reticulum (ER) stress is one of the main molecular events underlying pancreatic beta cell (PBC) failure, apoptosis, and a decrease in insulin secretion. Recent studies have highlighted the fundamental role of A2a adenosine receptor (A2aR) in potentiation of insulin secretion and proliferation of PBCs. However, possible protective effects of A2aR signaling against ER stress have not been elucidated yet. Thus, in the present study, we aimed to investigate the effects of A2aR activation in MIN6 beta cells undergoing tunicamycin (TM)‐mediated ER stress. A2aR expression and activity were evaluated using real‐time polymerase chain reaction and measurement of the cyclic adenosine monophosphate (cAMP), protein kinase A (PKA), phospho‐protein kinase B or Akt (p‐Akt)/Akt, and phospho‐Cyclic adenosine monophosphate response element‐binding protein/CREB levels in response to a specific agonist (CGS 21680). Survival and proliferation in TM and CGS 21680 cotreated cells were evaluated using 3‐(4,5‐Dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT), annexin V–fluorescein isothiocyanate (FITC)/propidium iodide staining, colony formation, and 5‐bromo‐2′‐deoxyuridine (Brdu) assays. In addition, the effects of A2aR stimulation on insulin secretion were evaluated using the enzyme‐linked immunosorbent assay. B‐cell lymphoma 2 (Bcl‐2), phospho‐eukaryotic Initiation Factor 2α (p‐eIF2α)/eIF2α, growth arrest and DNA‐damage‐inducible 34 (GADD34), X‐box binding protein 1 (XBP‐1), spliced X‐box binding protein 1 (XBP‐1s), immunoglobulin heavy‐chain‐binding protein (BIP), and CCAAT‐enhancer‐binding protein homologous protein (CHOP) levels were evaluated using western blotting. Our results showed a decrease in A2aR expression and p‐Akt/Akt and p‐CREB/CREB levels in TM‐pretreated cells. We also mentioned that CGS 21680 effectively increased cell survival, proliferation, and insulin secretion in TM‐treated cells. The antiapoptotic effects were possibly mediated through Bcl‐2 upregulation. Our western blotting results indicated that A2aR effectively downregulated p‐eIF2α/eIF2α, XBP‐1, XBP‐1s, BIP, and CHOP levels, whereas GADD34 was upregulated. Altogether, the present study revealed that A2aR signaling through PKA/Akt/CREB mediators alleviated TM cytotoxicity effects in MIN6 beta cells. Thus, the stimulation of this receptor was seen as a new approach to control ER stress in the PBC cells.

Chronic exposure to high glucose induces endoplasmic reticulum (ER) stress in pancreatic beta cells (PBCs). The previous evidence showed that adenosine modulate PBCs viability and insulin secretion. The aim of this study was to evaluate possible involvement of adenosine in protection of MIN6 β‐cells from Tunicamycin (Tu)‐induced ER stress. MIN6 cells were cotreated with Tu and different concentrations of adenosine. Cell viability, proliferation, and apoptosis were evaluated using 3‐[4,5‐dimethylthiazol‐2‐yl]‐2,5 diphenyl tetrazolium bromide (MTT), 5‐bromo‐2′‐deoxyuridine (Brdu), and colony formation assays. Caspase‐12 activity was assayed using the fluorometric method. Thioflavin T (ThT) staining was used for the evaluation of protein aggregation. Insulin secretion was evaluated using specific an ELISA kit. Ca2+ mobilization assayed using Fura2/AM probe. BIP, CHOP, XBP‐1, and XBP‐1s expression in both messenger RNA (mRNA) and protein levels were evaluated using the reverse transcription‐polymerase chain reaction (RT‐PCR) and Western blot analysis, respectively. Bcl‐2, p‐eIF2α/eIF2α, and GADD34 levels also determined with Western blot analysis. Adenosine protected MIN6 cells against Tu‐induced ER stress in a dose‐dependent manner and increased their proliferation. Decreased caspase‐12 activity and upregulated Bcl‐2 protein may explain antiapoptotic effects of adenosine. ThT staining indicated an attenuated aggregation of misfolded proteins. Adenosine effectively increased insulin secretion in Tu‐treated cells. BIP, CHOP, XBP1, and sXBP1 expression were decreased significantly in cotreated cells, indicating alleviation of ER stress. However, adenosine potentiated the expression of GADD34 and decreased p‐eIF2α/eIF2α ratio. Adenosine increased cytosolic Ca 2+ levels, which may promote adenosine triphosphate (ATP) synthesis in mitochondria, helping ER to preserve protein hemostasis. Taken together, adenosine upregulated Bcl‐2 and GADD34 to protect PBCs against Tu‐induced apoptosis and increase Insulin secretion.

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