Reactive oxygen and nitrogen species disturb Ca²⁺oscillations in insulin-secreting MIN6 β-cells

Abstract

Keywords: calcium pumps, mathematical modeling, pulsatile insulin secretion, photosensitizer, ROS/RNS Abbreviations: AlClPc, Aluminum Phthalocyanine Chloride; CICR, calcium-induced calcium release; CuFL, selective turn-on fluorescent NO reporter formed by a Cu(II) complex of a fluorescein modified with an appended metal-chelating ligand (FL); ER, endoplasmic reticulum; IP 3 R, inositol triphospate receptor; MitoSOX, mitochondrial superoxide indicator; NO, nitric oxide; PMCA, plasma membrane Ca 2C ATPase; RNS, reactive nitrogen species; ROS, reactive oxygen species; RyR, ryanodine receptor; SERCA, Sarcoplasmic/endoplasmic reticulum Ca 2C ATPase; TEA, tetraethylammonium; Tg, thapsigargin.Disturbances in pulsatile insulin secretion and Ca 2C oscillations in pancreatic b-cells are early markers of diabetes, but the underlying mechanisms are still incompletely understood. Reactive oxygen/nitrogen species (ROS/RNS) are implicated in reduced b-cell function, and ROS/RNS target several Ca 2C pumps and channels. Thus, we hypothesized that ROS/RNS could disturb Ca 2C oscillations and downstream insulin pulsatility. We show that ROS/RNS production by photoactivation of aluminum phthalocyanine chloride (AlClPc) abolish or accelerate Ca 2C oscillations in the MIN6 b-cell line, depending on the amount of ROS/RNS. Application of the sarcoplasmic/endoplasmic reticulum Ca 2C ATPase (SERCA) inhibitor thapsigargin modifies the Ca 2C response to high concentrations of ROS/RNS. Further, thapsigargin produces effects that resemble those elicited by moderate ROS/RNS production. These results indicate that ROS/RNS interfere with endoplasmic reticulum Ca 2C handling. This idea is supported by theoretical studies using a mathematical model of Ca 2C handling adapted to MIN6 cells. Our results suggest a putative link between ROS/RNS and disturbed pulsatile insulin secretion. IntroductionInsulin is secreted from the pancreatic b-cells in distinct pulses 1,2 and disturbed pulsatile insulin release has been suggested to be an early marker of diabetes. 3 These bursts of insulin are driven by underlying oscillations in electrical activity and cytosolic Ca 2C levels, which periodically trigger Ca 2C -regulated exocytosis. 4 Importantly, secretory pulses are more effective than constant insulin levels in lowering plasma glucose levels. 3,5,6 In particular, hepatic signaling is highly sensitive to whether insulin is released in pulses, 7 leading to the suggestion that insulin resistance in the liver is secondary to disturbed pulsatile insulin release. 8 Thus, a deeper understanding of oscillatory b-cell activity and its disturbance has clinical relevance.Disturbed insulin secretion and diabetes have been associated with b-cell damage caused by reactive oxygen species (ROS). 9 However, ROS have also been shown to exert a stimulating short-term effect on insulin release. 10 Hence, ROS may act as a double-edged sword: moderate and short-term elevations play a positive signaling role augmenting insulin secretion, but prolonged exposure to elevated ROS...

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