Abstract
Extracellular ATP is an important short-range signaling molecule that promotes various physiological responses virtually in all cell types, including pancreatic β-cells. It is well documented that pancreatic β-cells release ATP through exocytosis of insulin granules upon glucose stimulation. We hypothesized that glucose might stimulate ATP release through other non-vesicular mechanisms. Several purinergic receptors are found in β-cells and there is increasing evidence that purinergic signaling regulates β-cell functions and survival. One of the receptors that may be relevant is the P2X7 receptor, but its detailed role in β-cell physiology is unclear. In this study we investigated roles of the P2X7 receptor and pannexin-1 in ATP release, intracellular ATP, Ca2+ signals, insulin release and cell proliferation/survival in β-cells. Results show that glucose induces rapid release of ATP and significant fraction of release involves the P2X7 receptor and pannexin-1, both expressed in INS-1E cells, rat and mouse β-cells. Furthermore, we provide pharmacological evidence that extracellular ATP, via P2X7 receptor, stimulates Ca2+ transients and cell proliferation in INS-1E cells and insulin secretion in INS-1E cells and rat islets. These data indicate that the P2X7 receptor and pannexin-1 have important functions in β-cell physiology, and should be considered in understanding and treatment of diabetes.
Highlights
Pancreatic β-cells are the only source of insulin in the body, and they have a key role in whole body metabolic homeostasis
Accumulation of ATP into vesicles is thought to occur via vesicular nucleotide transporter, VNUT/SLC17A9, and knockdown of VNUT leads to diminished glucose-responsive ATP release, though described effects on insulin release are disparate[8,9]
The receptor is highly polymorphic and recent studies show that several single nucleotide polymorphisms (SNPs) in the receptor are associated with osteoporosis, multiple myeloma, leukemia, pain and bipolar diseases[28,29,30,31,32]
Summary
Pancreatic β-cells are the only source of insulin in the body, and they have a key role in whole body metabolic homeostasis. There are two potential sources of extracellular ATP for stimulating β-cells: ATP co-releases with transmitters from nerve terminals, and ATP released from insulin-containing granules[2,3,4,5] The latter process is well investigated and it has been shown that ATP is stored in vesicles and upon release can reach local concentrations in micromolar range[2,3,4,5]. (b) Representative Western blot of P2X7R and Panx[1] expression in INS-1E cells grown in increasing glucose concentrations. (c,d) Western blot quantification of P2X7R and Panx[1] (isoform A and D) expression in INS-1E cells grown in increasing glucose concentrations. A knockout of the P2X7R prevented streptozotocin-induced type-1 diabetes in mice[45], and targeting the receptor in T-cells with the inhibitor, oxidized-ATP, delayed islet allograft rejection[46]
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