Ca2+-independent phospholipase A2ß-derived PGE2 contributes to osteogenesis.

Bone modeling can be modulated by lipid signals such as arachidonic acid (AA) and its cyclooxygenase 2 (COX2) metabolite, prostaglandin E2 (PGE2), which are recognized mediators of optimal bone formation. Hydrolysis of AA from membrane glycerophospholipids is catalyzed by phospholipases A2 (PLA2s). We reported that mice deficient in the Ca2+- independent PLA2beta (iPLA2ß), encoded by Pla2g6, exhibit a low bone phenotype, but the cause for this remains to be identified. Here, we examined the mechanistic and molecular roles of iPLA2ß in bone formation using bone marrow stromal cells and calvarial osteoblasts from WT and iPLA2ß-deficient mice, and the MC3T3-E1 osteoblast precursor cell line. Our data reveal that transcription of osteogenic factors (Bmp2, Alpl, and Runx2) and osteogenesis are decreased with iPLA2ß-deficiency. These outcomes are corroborated and recapitulated in WT cells treated with a selective inhibitor of iPLA2 ß (10 µM S-BEL), and rescued in iPLA2ß-deficient cells by additions of 10 µM PGE2. Further, under osteogenic conditions we find that PGE2 production is through iPLA2ß activity and that this leads to induction of Runx2 and iPLA2ß transcription. These findings reveal a strong link between osteogenesis and iPLA2ß-derived lipids and raise the intriguing possibility that iPLA2ß-derived PGE2 participates in osteogenesis and in the regulation of Runx2 and also iPLA2ß.

Alterations in ß-Cell Sphingolipid Profile Associated with ER Stress and iPLA2ß: Another Contributor to ß-Cell Apoptosis in Type 1 Diabetes.

Type 1 diabetes (T1D) development, in part, is due to ER stress-induced ß-cell apoptosis. Activation of the Ca2+-independent phospholipase A2 beta (iPLA2ß) leads to the generation of pro-inflammatory eicosanoids, which contribute to ß-cell death and T1D. ER stress induces iPLA2ß-mediated generation of pro-apoptotic ceramides via neutral sphingomyelinase (NSMase). To gain a better understanding of the impact of iPLA2ß on sphingolipids (SLs), we characterized their profile in ß-cells undergoing ER stress. ESI/MS/MS analyses followed by ANOVA/Student's t-test were used to assess differences in sphingolipids molecular species in Vector (V) control and iPLA2ß-overexpressing (OE) INS-1 and Akita (AK, spontaneous model of ER stress) and WT-littermate (AK-WT) ß-cells. As expected, iPLA2ß induction was greater in the OE and AK cells in comparison with V and WT cells. We report here that ER stress led to elevations in pro-apoptotic and decreases in pro-survival sphingolipids and that the inactivation of iPLA2ß restores the sphingolipid species toward those that promote cell survival. In view of our recent finding that the SL profile in macrophages-the initiators of autoimmune responses leading to T1D-is not significantly altered during T1D development, we posit that the iPLA2ß-mediated shift in the ß-cell sphingolipid profile is an important contributor to ß-cell death associated with T1D.

Macrophage polarization is linked to Ca2+-independent phospholipase A2ß-derived lipids and cross-cell signaling in mice.

Phospholipases A2 (PLA2s) catalyze hydrolysis of the sn-2 substituent from glycerophospholipids to yield a free fatty acid (i.e., arachidonic acid), which can be metabolized to pro- or anti-inflammatory eicosanoids. Macrophages modulate inflammatory responses and are affected by Ca2+-independent phospholipase A2 (PLA2)ß (iPLA2ß). Here, we assessed the link between iPLA2ß-derived lipids (iDLs) and macrophage polarization. Macrophages from WT and KO (iPLA2ß-/-) mice were classically M1 pro-inflammatory phenotype activated or alternatively M2 anti-inflammatory phenotype activated, and eicosanoid production was determined by ultra-performance LC ESI-MS/MS. As a genotypic control, we performed similar analyses on macrophages from RIP.iPLA2ß.Tg mice with selective iPLA2ß overexpression in ß-cells. Compared with WT, generation of select pro-inflammatory prostaglandins (PGs) was lower in iPLA2ß-/- , and that of a specialized pro-resolving lipid mediator (SPM), resolvin D2, was higher; both changes are consistent with the M2 phenotype. Conversely, macrophages from RIP.iPLA2ß.Tg mice exhibited an opposite landscape, one associated with the M1 phenotype: namely, increased production of pro-inflammatory eicosanoids (6-keto PGF1α, PGE2, leukotriene B4) and decreased ability to generate resolvin D2. These changes were not linked with secretory PLA2 or cytosolic PLA2α or with leakage of the transgene. Thus, we report previously unidentified links between select iPLA2ß-derived eicosanoids, an SPM, and macrophage polarization. Importantly, our findings reveal for the first time that ß-cell iPLA2ß-derived signaling can predispose macrophage responses. These findings suggest that iDLs play critical roles in macrophage polarization, and we posit that they could be targeted therapeutically to counter inflammation-based disorders.

Polarization of Macrophages toward M2 Phenotype Is Favored by Reduction in iPLA2ß (Group VIA Phospholipase A2).

Macrophages are important in innate and adaptive immunity. Macrophage participation in inflammation or tissue repair is directed by various extracellular signals and mediated by multiple intracellular pathways. Activation of group VIA phospholipase A2 (iPLA2ß) causes accumulation of arachidonic acid, lysophospholipids, and eicosanoids that can promote inflammation and pathologic states. We examined the role of iPLA2ß in peritoneal macrophage immune function by comparing wild type (WT) and iPLA2ß-/- mouse macrophages. Compared with WT, iPLA2ß-/- macrophages exhibited reduced proinflammatory M1 markers when classically activated. In contrast, anti-inflammatory M2 markers were elevated under naïve conditions and induced to higher levels by alternative activation in iPLA2ß-/- macrophages compared with WT. Induction of eicosanoid (12-lipoxygenase (12-LO) and cyclooxygenase 2 (COX2))- and reactive oxygen species (NADPH oxidase 4 (NOX4))-generating enzymes by classical activation pathways was also blunted in iPLA2ß-/- macrophages compared with WT. The effects of inhibitors of iPLA2ß, COX2, or 12-LO to reduce M1 polarization were greater than those to enhance M2 polarization. Certain lipids (lysophosphatidylcholine, lysophosphatidic acid, and prostaglandin E2) recapitulated M1 phenotype in iPLA2ß-/- macrophages, but none tested promoted M2 phenotype. These findings suggest that (a) lipids generated by iPLA2ß and subsequently oxidized by cyclooxygenase and 12-LO favor macrophage inflammatory M1 polarization, and (b) the absence of iPLA2ß promotes macrophage M2 polarization. Reducing macrophage iPLA2ß activity and thereby attenuating macrophage M1 polarization might cause a shift from an inflammatory to a recovery/repair milieu.

Group VIA Phospholipase A2 (iPLA2ß) Modulates Bcl-x 5'-Splice Site Selection and Suppresses Anti-apoptotic Bcl-x(L) in ß-Cells.

Diabetes is a consequence of reduced ß-cell function and mass, due to ß-cell apoptosis. Endoplasmic reticulum (ER) stress is induced during ß-cell apoptosis due to various stimuli, and our work indicates that group VIA phospholipase A2ß (iPLA2ß) participates in this process. Delineation of underlying mechanism(s) reveals that ER stress reduces the anti-apoptotic Bcl-x(L) protein in INS-1 cells. The Bcl-x pre-mRNA undergoes alternative pre-mRNA splicing to generate Bcl-x(L) or Bcl-x(S) mature mRNA. We show that both thapsigargin-induced and spontaneous ER stress are associated with reductions in the ratio of Bcl-x(L)/Bcl-x(S) mRNA in INS-1 and islet ß-cells. However, chemical inactivation or knockdown of iPLA2ß augments the Bcl-x(L)/Bcl-x(S) ratio. Furthermore, the ratio is lower in islets from islet-specific RIP-iPLA2ß transgenic mice, whereas islets from global iPLA2ß(-/-) mice exhibit the opposite phenotype. In view of our earlier reports that iPLA2ß induces ceramide accumulation through neutral sphingomyelinase 2 and that ceramides shift the Bcl-x 5'-splice site (5'SS) selection in favor of Bcl-x(S), we investigated the potential link between Bcl-x splicing and the iPLA2ß/ceramide axis. Exogenous C6-ceramide did not alter Bcl-x 5'SS selection in INS-1 cells, and neutral sphingomyelinase 2 inactivation only partially prevented the ER stress-induced shift in Bcl-x splicing. In contrast, 5(S)-hydroxytetraenoic acid augmented the ratio of Bcl-x(L)/Bcl-x(S) by 15.5-fold. Taken together, these data indicate that ß-cell apoptosis is, in part, attributable to the modulation of 5'SS selection in the Bcl-x pre-mRNA by bioactive lipids modulated by iPLA2ß.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases.

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2ß) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

Dysfunctional mitochondrial bioenergetics and oxidative stress in Akita(+/Ins2)-derived ß-cells.

Insulin release from pancreatic ß-cells plays a critical role in blood glucose homeostasis, and ß-cell dysfunction leads to the development of diabetes mellitus. In cases of monogenic type 1 diabetes mellitus (T1DM) that involve mutations in the insulin gene, we hypothesized that misfolding of insulin could result in endoplasmic reticulum (ER) stress, oxidant production, and mitochondrial damage. To address this, we used the Akita(+/Ins2) T1DM model in which misfolding of the insulin 2 gene leads to ER stress-mediated ß-cell death and thapsigargin to induce ER stress in two different ß-cell lines and in intact mouse islets. Using transformed pancreatic ß-cell lines generated from wild-type Ins2(+/+) (WT) and Akita(+/Ins2) mice, we evaluated cellular bioenergetics, oxidative stress, mitochondrial protein levels, and autophagic flux to determine whether changes in these processes contribute to ß-cell dysfunction. In addition, we induced ER stress pharmacologically using thapsigargin in WT ß-cells, INS-1 cells, and intact mouse islets to examine the effects of ER stress on mitochondrial function. Our data reveal that Akita(+/Ins2)-derived ß-cells have increased mitochondrial dysfunction, oxidant production, mtDNA damage, and alterations in mitochondrial protein levels that are not corrected by autophagy. Together, these findings suggest that deterioration in mitochondrial function due to an oxidative environment and ER stress contributes to ß-cell dysfunction and could contribute to T1DM in which mutations in insulin occur.

Vitamin D and calcium-sensing receptor polymorphisms differentially associate with resting energy expenditure in peripubertal children.

Given that calcium metabolism is influenced by genes and is tightly linked to energy-utilizing pathways, this study evaluated the association of single nucleotide polymorphisms (SNPs) in the vitamin D receptor (VDR) and calcium-sensing receptor (CASR) with resting energy expenditure (REE). In 273 boys and girls, 7-12 years of age, cross-sectional REE was measured via indirect calorimetry, body composition by DXA, and dietary measures by 24-h recall. SNPs for VDR Cdx-2 (rs11568820) and CASR A986S (rs1801725) were genotyped using the Illumina Golden Gate assay. Multiple linear regression models were used to determine the association between SNPs and REE. African American carriers of the 'A' VDR Cdx2 allele had increased levels of REE in the overall sample, and this association was apparent among participants with an adiposity level of <25 % and 30 % body fat in males and females, respectively. For CASR, an association between carriers of the 'A' allele and REE was observed only in those in the upper median of calcium intake. VDR and CASR variants are associated with REE in children and are influenced by levels of calcium intake and adiposity. Our results bring awareness to mechanisms underlying the regulation of REE and biological and dietary influential factors.

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes.

Impaired glucose tolerance (IGT) and ß-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from ß-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within ß-cells by the ß-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 ß) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the ß-cell function is reduced. Interestingly, while ß-cell-specific deletion of iPLA 2 ß reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 ß in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 ß and C1ql3) pathways and how they may affect ß-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring ß-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

Fatty acid-mediated signaling as a target for developing type 1 diabetes therapies.

INTRODUCTION: Type 1 diabetes (T1D) is an autoimmune disease in which pro-inflammatory and cytotoxic signaling drive the death of the insulin-producing ß cells. This complex signaling is regulated in part by fatty acids and their bioproducts, making them excellent therapeutic targets. AREAS COVERED: We provide an overview of the fatty acid actions on ß cells by discussing how they can cause lipotoxicity or regulate inflammatory response during insulitis. We also discuss how diet can affect the availability of fatty acids and disease development. Finally, we discuss development avenues that need further exploration. EXPERT OPINION: Fatty acids, such as hydroxyl fatty acids, ω-3 fatty acids, and their downstream products, are druggable candidates that promote protective signaling. Inhibitors and antagonists of enzymes and receptors of arachidonic acid and free fatty acids, along with their derived metabolites, which cause pro-inflammatory and cytotoxic responses, have the potential to be developed as therapeutic targets also. Further, because diet is the main source of fatty acid intake in humans, balancing protective and pro-inflammatory/cytotoxic fatty acid levels through dietary therapy may have beneficial effects, delaying T1D progression. Therefore, therapeutic interventions targeting fatty acid signaling hold potential as avenues to treat T1D.

Loss of Brain Angiogenesis Inhibitor-3 (BAI3) G-Protein Coupled Receptor in Mice Regulates Adaptive Thermogenesis by Enhancing Energy Expenditure.

Effective energy expenditure is critical for maintaining body weight (BW). However, underlying mechanisms contributing to increased BW remain unknown. We characterized the role of brain angiogenesis inhibitor-3 (BAI3/ADGRB3), an adhesion G-protein coupled receptor (aGPCR), in regulating BW. A CRISPR/Cas9 gene editing approach was utilized to generate a whole-body deletion of the BAI3 gene (BAI3-/-). In both BAI3-/- male and female mice, a significant reduction in BW was observed compared to BAI3+/+ control mice. Quantitative magnetic imaging analysis showed that lean and fat masses were reduced in male and female mice with BAI3 deficiency. Total activity, food intake, energy expenditure (EE), and respiratory exchange ratio (RER) were assessed in mice housed at room temperature using a Comprehensive Lab Animal Monitoring System (CLAMS). While no differences were observed in the activity between the two genotypes in male or female mice, energy expenditure was increased in both sexes with BAI3 deficiency. However, at thermoneutrality (30 °C), no differences in energy expenditure were observed between the two genotypes for either sex, suggesting a role for BAI3 in adaptive thermogenesis. Notably, in male BAI3-/- mice, food intake was reduced, and RER was increased, but these attributes remained unchanged in the female mice upon BAI3 loss. Gene expression analysis showed increased mRNA abundance of thermogenic genes Ucp1, Pgc1α, Prdm16, and Elov3 in brown adipose tissue (BAT). These outcomes suggest that adaptive thermogenesis due to enhanced BAT activity contributes to increased energy expenditure and reduced BW with BAI3 deficiency. Additionally, sex-dependent differences were observed in food intake and RER. These studies identify BAI3 as a novel regulator of BW that can be potentially targeted to improve whole-body energy expenditure.

Carmofur prevents cell cycle progression by reducing E2F8 transcription in temozolomide-resistant glioblastoma cells.

Sphingolipid metabolism is dysregulated in many cancers, allowing cells to evade apoptosis through increased sphingosine-1-phosphate (S1P) and decreased ceramides. Ceramidases hydrolyze ceramides to sphingosine, which is phosphorylated by sphingosine kinases to generate S1P. The S1P allows cells to evade apoptosis by shifting the equilibrium away from ceramides, which favor cell death. One tumor type that exhibits a shift in the sphingolipid balance towards S1P is glioblastoma (GBM), a highly aggressive brain tumor. GBMs almost always recur despite surgical resection, radiotherapy, and chemotherapy with temozolomide (TMZ). Understanding sphingolipid metabolism in GBM is still limited, and currently, there are no approved treatments to target dysregulation of sphingolipid metabolism in GBM. Carmofur, a derivative of 5-fluorouracil, inhibits acid ceramidase (ASAH1), a key enzyme in the production of S1P, and is in use outside the USA to treat colorectal cancer. We find that the mRNA for ASAH1, but not other ceramidases, is elevated in recurrent GBM. When TMZ-resistant GBM cells were treated with carmofur, decreased cell growth and increased apoptosis were observed along with cell cycle perturbations. RNA-sequencing identified decreases in cell cycle control pathways that were specific to TMZ-resistant cells. Furthermore, the transcription factor and G1 to S phase regulator, E2F8, was upregulated in TMZ-resistant versus parental GBM cells and inhibited by carmofur treatment in TMZ-resistant GBM cells, specifically. These data suggest a possible role for E2F8 as a mediator of carmofur effects in the context of TMZ resistance. These data suggest the potential utility of normalizing the sphingolipid balance in the context of recurrent GBM.

A proteomic meta-analysis refinement of plasma extracellular vesicles.

Extracellular vesicles play major roles in cell-to-cell communication and are excellent biomarker candidates. However, studying plasma extracellular vesicles is challenging due to contaminants. Here, we performed a proteomics meta-analysis of public data to refine the plasma EV composition by separating EV proteins and contaminants into different clusters. We obtained two clusters with a total of 1717 proteins that were depleted of known contaminants and enriched in EV markers with independently validated 71% true-positive. These clusters had 133 clusters of differentiation (CD) antigens and were enriched with proteins from cell-to-cell communication and signaling. We compared our data with the proteins deposited in PeptideAtlas, making our refined EV protein list a resource for mechanistic and biomarker studies. As a use case example for this resource, we validated the type 1 diabetes biomarker proplatelet basic protein in EVs and showed that it regulates apoptosis of ß cells and macrophages, two key players in the disease development. Our approach provides a refinement of the EV composition and a resource for the scientific community.

Carbonylation induces heterogeneity in cardiac ryanodine receptor function in diabetes mellitus.

Heart failure and arrhythmias occur at 3 to 5 times higher rates among individuals with diabetes mellitus, compared with age-matched, healthy individuals. Studies attribute these defects in part to alterations in the function of cardiac type 2 ryanodine receptors (RyR2s), the principal Ca(2+)-release channels on the internal sarcoplasmic reticulum (SR). To date, mechanisms underlying RyR2 dysregulation in diabetes remain poorly defined. A rat model of type 1 diabetes, in combination with echocardiography, in vivo and ex vivo hemodynamic studies, confocal microscopy, Western blotting, mass spectrometry, site-directed mutagenesis, and [(3)H]ryanodine binding, lipid bilayer, and transfection assays, was used to determine whether post-translational modification by reactive carbonyl species (RCS) represented a contributing cause. After 8 weeks of diabetes, spontaneous Ca(2+) release in ventricular myocytes increased ~5-fold. Evoked Ca(2+) release from the SR was nonuniform (dyssynchronous). Total RyR2 protein levels remained unchanged, but the ability to bind the Ca(2+)-dependent ligand [(3)H]ryanodine was significantly reduced. Western blotting and mass spectrometry revealed RCS adducts on select basic residues. Mutation of residues to delineate the physiochemical impact of carbonylation yielded channels with enhanced or reduced cytoplasmic Ca(2+) responsiveness. The prototype RCS methylglyoxal increased and then decreased the RyR2 open probability. Methylglyoxal also increased spontaneous Ca(2+) release and induced Ca(2+) waves in healthy myocytes. Treatment of diabetic rats with RCS scavengers normalized spontaneous and evoked Ca(2+) release from the SR, reduced carbonylation of RyR2s, and increased binding of [(3)H]ryanodine to RyR2s. From these data, we conclude that post-translational modification by RCS contributes to the heterogeneity in RyR2 activity that is seen in experimental diabetes.

Role of calcium-independent phospholipase A(2)ß in human pancreatic islet ß-cell apoptosis.

Death of ß-cells due to apoptosis is an important contributor to ß-cell dysfunction in both type 1 and type 2 diabetes mellitus. Previously, we described participation of the Group VIA Ca(2+)-independent phospholipase A(2) (iPLA(2)ß) in apoptosis of insulinoma cells due to ER stress. To examine whether islet ß-cells are similarly susceptible to ER stress and undergo iPLA(2)ß-mediated apoptosis, we assessed the ER stress response in human pancreatic islets. Here, we report that the iPLA(2)ß protein is expressed predominantly in the ß-cells of human islets and that thapsigargin-induced ER stress promotes ß-cell apoptosis, as reflected by increases in activated caspase-3 in the ß-cells. Furthermore, we demonstrate that ER stress is associated with increases in islet iPLA(2)ß message, protein, and activity, iPLA(2)ß-dependent induction of neutral sphingomyelinase and ceramide accumulation, and subsequent loss of mitochondrial membrane potential. We also observe that basal activated caspase-3 increases with age, raising the possibility that ß-cells in older human subjects have a greater susceptibility to undergo apoptotic cell death. These findings reveal for the first time expression of iPLA(2)ß protein in human islet ß-cells and that induction of iPLA(2)ß during ER stress contributes to human islet ß-cell apoptosis. We hypothesize that modulation of iPLA(2)ß activity might reduce ß-cell apoptosis and this would be beneficial in delaying or preventing ß-cell dysfunction associated with diabetes.

Deletion of Gdf15 Reduces ER Stress-induced Beta-cell Apoptosis and Diabetes.

Endoplasmic reticulum (ER) stress contributes to pancreatic beta-cell apoptosis in diabetes, but the factors involved are still not fully elucidated. Growth differentiation factor 15 (GDF15) is a stress response gene and has been reported to be increased and play an important role in various diseases. However, the role of GDF15 in beta cells in the context of ER stress and diabetes is still unclear. In this study, we have discovered that GDF15 promotes ER stress-induced beta-cell apoptosis and that downregulation of GDF15 has beneficial effects on beta-cell survival in diabetes. Specifically, we found that GDF15 is induced by ER stress in beta cells and human islets, and that the transcription factor C/EBPß is involved in this process. Interestingly, ER stress-induced apoptosis was significantly reduced in INS-1 cells with Gdf15 knockdown and in isolated Gdf15 knockout mouse islets. In vivo, we found that Gdf15 deletion attenuates streptozotocin-induced diabetes by preserving beta cells and insulin levels. Moreover, deletion of Gdf15 significantly delayed diabetes development in spontaneous ER stress-prone Akita mice. Thus, our findings suggest that GDF15 contributes to ER stress-induced beta-cell apoptosis and that inhibition of GDF15 may represent a novel strategy to promote beta-cell survival and treat diabetes.

Targeting Acid Ceramidase Inhibits Glioblastoma Cell Migration through Decreased AKT Signaling.

Glioblastoma (GBM) remains one of the most aggressive cancers, partially due to its ability to migrate into the surrounding brain. The sphingolipid balance, or the balance between ceramides and sphingosine-1-phosphate, contributes to the ability of GBM cells to migrate or invade. Of the ceramidases which hydrolyze ceramides, acid ceramidase (ASAH1) is highly expressed in GBM samples compared to non-tumor brain. ASAH1 expression also correlates with genes associated with migration and focal adhesion. To understand the role of ASAH1 in GBM migration, we utilized shRNA knockdown and observed decreased migration that did not depend upon changes in growth. Next, we inhibited ASAH1 using carmofur, a clinically utilized small molecule inhibitor. Inhibition of ASAH1 by carmofur blocks in vitro migration of U251 (GBM cell line) and GBM cells derived from patient-derived xenografts (PDXs). RNA-sequencing suggested roles for carmofur in MAPK and AKT signaling. We found that carmofur treatment decreases phosphorylation of AKT, but not of MAPK. The decrease in AKT phosphorylation was confirmed by shRNA knockdown of ASAH1. Our findings substantiate ASAH1 inhibition using carmofur as a potential clinically relevant treatment to advance GBM therapeutics, particularly due to its impact on migration.

Extracellular vesicles in ß cell biology: Role of lipids in vesicle biogenesis, cargo, and intercellular signaling.

BACKGROUND: Type 1 diabetes (T1D) is a complex autoimmune disorder whose pathogenesis involves an intricate interplay between ß cells of the pancreatic islet, other islet cells, and cells of the immune system. Direct intercellular communication within the islet occurs via cell surface proteins and indirect intercellular communication has traditionally been seen as occurring via secreted proteins (e.g., endocrine hormones and cytokines). However, recent literature suggests that extracellular vesicles (EVs) secreted by ß cells constitute an additional and biologically important mechanism for transmitting signals to within the islet. SCOPE OF REVIEW: This review summarizes the general mechanisms of EV formation, with a particular focus on how lipids and lipid signaling pathways influence their formation and cargo. We review the implications of EV release from ß cells for T1D pathogenesis, how EVs and their cargo might be leveraged as biomarkers of this process, and how EVs might be engineered as a therapeutic candidate to counter T1D outcomes. MAJOR CONCLUSIONS: Islet ß cells have been viewed as initiators and propagators of the cellular circuit giving rise to autoimmunity in T1D. In this context, emerging literature suggests that EVs may represent a conduit for communication that holds more comprehensive messaging about the ß cells from which they arise. As the field of EV biology advances, it opens the possibility that intervening with EV formation and cargo loading could be a novel disease-modifying approach in T1D.

Spontaneous development of endoplasmic reticulum stress that can lead to diabetes mellitus is associated with higher calcium-independent phospholipase A2 expression: a role for regulation by SREBP-1.

Our recent studies indicate that endoplasmic reticulum (ER) stress causes INS-1 cell apoptosis by a Ca(2+)-independent phospholipase A(2) (iPLA(2)beta)-mediated mechanism that promotes ceramide generation via sphingomyelin hydrolysis and subsequent activation of the intrinsic pathway. To elucidate the association between iPLA(2)beta and ER stress, we compared beta-cell lines generated from wild type (WT) and Akita mice. The Akita mouse is a spontaneous model of ER stress that develops hyperglycemia/diabetes due to ER stress-induced beta-cell apoptosis. Consistent with a predisposition to developing ER stress, basal phosphorylated PERK and activated caspase-3 are higher in the Akita cells than WT cells. Interestingly, basal iPLA(2)beta, mature SREBP-1 (mSREBP-1), phosphorylated Akt, and neutral sphingomyelinase (NSMase) are higher, relative abundances of sphingomyelins are lower, and mitochondrial membrane potential (DeltaPsi) is compromised in Akita cells, in comparison with WT cells. Exposure to thapsigargin accelerates DeltaPsi loss and apoptosis of Akita cells and is associated with increases in iPLA(2)beta, mSREBP-1, and NSMase in both WT and Akita cells. Transfection of Akita cells with iPLA(2)beta small interfering RNA, however, suppresses NSMase message, DeltaPsi loss, and apoptosis. The iPLA(2)beta gene contains a sterol-regulatory element, and transfection with a dominant negative SREBP-1 reduces basal mSREBP-1 and iPLA(2)beta in the Akita cells and suppresses increases in mSREBP-1 and iPLA(2)beta due to thapsigargin. These findings suggest that ER stress leads to generation of mSREBP-1, which can bind to the sterol-regulatory element in the iPLA(2)beta gene to promote its transcription. Consistent with this, SREBP-1, iPLA(2)beta, and NSMase messages in Akita mouse islets are higher than in WT islets.

Evidence for proteolytic processing and stimulated organelle redistribution of iPLA(2)beta.

Over the past decade, important roles for the 84-88kDa Group VIA Ca(2+)-independent phospholipase A(2) (iPLA(2)beta) in various organs have been described. We demonstrated that iPLA(2)beta participates in insulin secretion, insulinoma cells and native pancreatic islets express full-length and truncated isoforms of iPLA(2)beta, and certain stimuli promote perinuclear localization of iPLA(2)beta. To gain a better understanding of its mobilization, iPLA(2)beta was expressed in INS-1 cells as a fusion protein with EGFP, enabling detection of subcellular localization of iPLA(2)beta by monitoring EGFP fluorescence. Cells stably-transfected with fusion protein expressed nearly 5-fold higher catalytic iPLA(2)beta activity than control cells transfected with EGFP cDNA alone, indicating that co-expression of EGFP does not interfere with manifestation of iPLA(2)beta activity. Dual fluorescence monitoring of EGFP and organelle Trackers combined with immunoblotting analyses revealed expression of truncated iPLA(2)beta isoforms in separate subcellular organelles. Exposure to secretagogues and induction of ER stress are known to activate iPLA(2)beta in beta-cells and we find here that these stimuli promote differential localization of iPLA(2)beta in subcellular organelles. Further, mass spectrometric analyses identified iPLA(2)beta variants from which N-terminal residues were removed. Collectively, these findings provide evidence for endogenous proteolytic processing of iPLA(2)beta and redistribution of iPLA(2)beta variants in subcellular compartments. It might be proposed that in vivo processing of iPLA(2)beta facilitates its participation in multiple biological processes.