Islet inflammation is an important etiopathology of type 2 diabetes; however, the underlying mechanisms are not well defined. Using complementary experimental models, we discovered RIPK3-dependent IL1B induction in β cells as an instigator of islet inflammation. In cultured β cells, ER stress activated RIPK3, leading to NF-kB–mediated proinflammatory gene expression. In a zebrafish muscle insulin resistance model, overnutrition caused islet inflammation, β cell dysfunction, and loss in an ER stress–, ripk3-, and il1b-dependent manner. In mouse islets, high-fat diet triggered the IL1B expression in β cells before macrophage recruitment in vivo, and RIPK3 inhibition suppressed palmitate-induced β cell dysfunction and Il1b expression in vitro. Furthermore, in human islets grafted in hyperglycemic mice, a marked increase in ER stress, RIPK3, and NF-kB activation in β cells were accompanied with murine macrophage infiltration. Thus, RIPK3-mediated induction of proinflammatory mediators is a conserved, previously unrecognized β cell response to metabolic stress and a mediator of the ensuing islet inflammation.
Key points Tetraspanin (TSPAN) proteins regulate many biological processes, including intracellular calcium (Ca2+) handling. TSPAN‐7 is enriched in pancreatic islet cells; however, the function of islet TSPAN‐7 has not been identified. Here, we characterize how β‐cell TSPAN‐7 regulates Ca2+ handling and hormone secretion. We find that TSPAN‐7 reduces β‐cell glucose‐stimulated Ca2+ entry, slows Ca2+ oscillation frequency and decreases glucose‐stimulated insulin secretion. TSPAN‐7 controls β‐cell function through a direct interaction with L‐type voltage‐dependent Ca2+ channels (CaV1.2 and CaV1.3), which reduces channel Ca2+ conductance. TSPAN‐7 slows activation of CaV1.2 and accelerates recovery from voltage‐dependent inactivation; TSPAN‐7 also slows CaV1.3 inactivation kinetics. These findings strongly implicate TSPAN‐7 as a key regulator in determining the set‐point of glucose‐stimulated Ca2+ influx and insulin secretion. Abstract Glucose‐stimulated insulin secretion (GSIS) is regulated by calcium (Ca2+) entry into pancreatic β‐cells through voltage‐dependent Ca2+ (CaV) channels. Tetraspanin (TSPAN) transmembrane proteins control Ca2+ handling, and thus they may also modulate GSIS. TSPAN‐7 is the most abundant islet TSPAN and immunostaining of mouse and human pancreatic slices shows that TSPAN‐7 is highly expressed in β‐ and α‐cells; however, the function of islet TSPAN‐7 has not been determined. Here, we show that TSPAN‐7 knockdown (KD) increases glucose‐stimulated Ca2+ influx into mouse and human β‐cells. Additionally, mouse β‐cell Ca2+ oscillation frequency was accelerated by TSPAN‐7 KD. Because TSPAN‐7 KD also enhanced Ca2+ entry when membrane potential was clamped with depolarization, the effect of TSPAN‐7 on CaV channel activity was examined. TSPAN‐7 KD enhanced L‐type CaV currents in mouse and human β‐cells. Conversely, heterologous expression of TSPAN‐7 with CaV1.2 and CaV1.3 L‐type CaV channels decreased CaV currents and reduced Ca2+ influx through both channels. This was presumably the result of a direct interaction of TSPAN‐7 and L‐type CaV channels because TSPAN‐7 coimmunoprecipitated with both CaV1.2 and CaV1.3 from primary human β‐cells and from a heterologous expression system. Finally, TSPAN‐7 KD in human β‐cells increased basal (5.6 mM glucose) and stimulated (45 mM KCl + 14 mM glucose) insulin secretion. These findings strongly suggest that TSPAN‐7 modulation of β‐cell L‐type CaV channels is a key determinant of β‐cell glucose‐stimulated Ca2+ entry and thus the set‐point of GSIS.
SUMMARYBariatric surgery, such as vertical sleeve gastrectomy (VSG), causes high rates of type 2 diabetes remission and remarkable increases in postprandial glucagon-like peptide-1 (GLP-1) secretion. GLP-1 plays a critical role in islet function by potentiating glucose-stimulated insulin secretion; however, the mechanisms remain incompletely defined. Therefore, we applied a murine VSG model to an inducible β cell-specific GLP-1 receptor (GLP-1R) knockout mouse model to investigate the role of the β cell GLP-1R in islet function. Our data show that loss of β cell GLP-1R signaling decreases α cell GLP-1 expression after VSG. Furthermore, we find a β cell GLP-1R-dependent increase in α cell expression of the prohormone convertase required for the production of GLP-1 after VSG. Together, the findings herein reveal two concepts. First, our data support a paracrine role for α cell-derived GLP-1 in the metabolic benefits observed after VSG. Second, we have identified a role for the β cell GLP-1R as a regulator of α cell proglucagon processing.
This article is part of a themed section on Molecular Mechanisms Regulating Perivascular Adipose Tissue - Potential Pharmacological Targets? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.20/issuetoc.
Objective Elevations in pancreatic α-cell intracellular Ca 2+ ([Ca 2+ ] i ) lead to glucagon (GCG) secretion. Although glucose inhibits GCG secretion, how lactate and pyruvate control α-cell Ca 2+ handling is unknown. Lactate enters cells through monocarboxylate transporters (MCTs) and is also produced during glycolysis by lactate dehydrogenase A (LDHA), an enzyme expressed in α-cells. As lactate activates ATP-sensitive K + (K ATP ) channels in cardiomyocytes, lactate may also modulate α-cell K ATP . Therefore, this study investigated how lactate signaling controls α-cell Ca 2+ handling and GCG secretion. Methods Mouse and human islets were used in combination with confocal microscopy, electrophysiology, GCG immunoassays, and fluorescent thallium flux assays to assess α-cell Ca 2+ handling, V m , K ATP currents, and GCG secretion. Results Lactate-inhibited mouse (75 ± 25%) and human (47 ± 9%) α-cell [Ca 2+ ] i fluctuations only under low-glucose conditions (1 mM) but had no effect on β- or δ-cells [Ca 2+ ] i . Glyburide inhibition of K ATP channels restored α-cell [Ca 2+ ] i fluctuations in the presence of lactate. Lactate transport into α-cells via MCTs hyperpolarized mouse (14 ± 1 mV) and human (12 ± 1 mV) α-cell V m and activated K ATP channels. Interestingly, pyruvate showed a similar K ATP activation profile and α-cell [Ca 2+ ] i inhibition as lactate. Lactate-induced inhibition of α-cell [Ca 2+ ] i influx resulted in reduced GCG secretion in mouse (62 ± 6%) and human (43 ± 13%) islets. Conclusions These data demonstrate for the first time that lactate entry into α-cells through MCTs results in K ATP activation, V m hyperpolarization, reduced [Ca 2+ ] i , and inhibition of GCG secretion. Thus, taken together, these data indicate that lactate either within α-cells and/or elevated in serum could serve as important modulators of α-cell function.
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