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The secretion of insulin from β-cells depends on extracellular factors, in particular glucose and other small molecules, some of which act on G-protein-coupled receptors. Fatty acids (FAs) have been discussed as exogenous secretagogues of insulin for decades, especially after the FA receptor GPR40 (G-protein-coupled receptor 40) was discovered. However, the role of FAs as endogenous signaling factors has not been investigated until now. In the present work, we demonstrate that lowering endogenous FA levels in β-cell medium by stringent washing or by the application of FA-free (FAF) BSA immediately reduced glucose-induced oscillations of cytosolic Ca 2+ ([Ca 2+ ] i oscillations) in MIN6 cells and mouse primary β-cells, as well as insulin secretion. Mass spectrometry confirmed BSA-mediated removal of FAs, with palmitic, stearic, oleic, and elaidic acid being the most abundant species. [Ca 2+ ] i oscillations in MIN6 cells recovered when BSA was replaced by buffer or as FA levels in the supernatant were restored. This was achieved by recombinant lipase-mediated FA liberation from membrane lipids, by the addition of FA-preloaded FAF-BSA, or by the photolysis of cell-impermeant caged FAs. Our combined data support the hypothesis of FAs as essential endogenous signaling factors for β-cell activity and insulin secretion., (© 2018 by the American Diabetes Association.)

ATP has been previously identified as an autocrine signaling factor that is co‐released with insulin to modulate and propagate β‐cell activity within islets of Langerhans. Here, we show that β‐cell activity and insulin secretion essentially rely on the presence of extracellular ATP. For this, we monitored changes of the intracellular Ca2+ concentration ([Ca2+]i oscillations) as an immediate read‐out for insulin secretion in live cell experiments. Extensive washing of cells or depletion of extracellular ATP levels by recombinant apyrase reduced [Ca2+]i oscillations and insulin secretion in pancreatic cell lines and primary β‐cells. Following ATP depletion, [Ca2+]i oscillations were stimulated by the replenishment of ATP in a concentration‐dependent manner. Inhibition of endogenous ecto‐ATP nucleotidases increased extracellular ATP levels, along with [Ca2+]i oscillations and insulin secretion, indicating that there is a constant supply of ATP to the extracellular space. Our combined results demonstrate that extracellular ATP is essential for β‐cell activity. The presented work suggests extracellular ATPases as potential drug targets for the modulation of insulin release. We further found that exogenous fatty acids compensated for depleted extracellular ATP levels by the recovery of [Ca2+]i oscillations, indicating that autocrine factors mutually compensate for the loss of others. Thereby, our results contribute to a more detailed and complete understanding of the general role of autocrine signaling factors as a fundamental regulatory mechanism of β‐cell activity and insulin secretion. [ABSTRACT FROM AUTHOR]

Pharmacological treatment of pancreatic β cells targeting cannabinoid receptors 1 and 2 (CB1 and CB2) has been shown to result in significant effects on insulin release, possibly by modulating intracellular calcium levels ([Ca2+] i). It is unclear how the interplay of CB1 and CB2 affects insulin secretion. Here, we demonstrate by the use of highly specific receptor antagonists and the recently developed photo-releasable endocannabinoid 2-arachidonoylglycerol that both receptors have counteracting effects on cytosolic calcium oscillations. We further show that both receptors are juxtaposed in a way that increases [Ca2+] i oscillations in silent β cells but dampens them in active ones. This study highlights a functional role of CB1 and CB2 acting in concert as a compensator/attenuator switch for regulating β cell excitability. • The endocannabinoid 2-AG activates CB1 and CB2 • Photo-activatable endocannabinoid dissects receptor function • CB1/CB2 act as a compensator/attenuator switch for regulating β cell excitability Laguerre et al. highlight the role of co-activated cannabinoid receptors 1 and 2 in regulating β cell activity. Whereas CB1 activation is required to maintain intracellular [Ca2+] i oscillations, CB2 activation has a dampening role. Both receptors act in concert as a compensator/attenuator switch for regulating intracellular calcium levels and insulin secretion. [ABSTRACT FROM AUTHOR]