Overexpression of sphingosine-1-phosphate lyase protects insulin-secreting cells against cytokine toxicity

Abstract

Increasing evidence suggests a crucial role of inflammation in cytokine-mediated β-cell dysfunction and death in type 1 diabetes mellitus, although the mechanisms are incompletely understood. Sphingosine 1-phosphate (S1P) is a multifunctional bioactive sphingolipid involved in the development of many autoimmune and inflammatory diseases. Here, we investigated the role of intracellular S1P in insulin-secreting INS1E cells by genetically manipulating the S1P-metabolizing enzyme S1P lyase (SPL). The expression of was down-regulated by cytokines in INS1E cells and rat islets. Overexpression of SPL protected against cytokine toxicity. Interestingly, the SPL overexpression did not suppress the cytokine-induced NFκB-iNOS-NO pathway but attenuated calcium leakage from endoplasmic reticulum (ER) stores as manifested by lower cytosolic calcium levels, higher expression of the ER protein Sec61a, decreased dephosphorylation of Bcl-2-associated death promoter (Bad) protein, and weaker caspase-3 activation in cytokine-treated (IL-1β, TNFα, and IFNγ) cells. This coincided with reduced cytokine-mediated ER stress, indicated by measurements of CCAAT/enhancer-binding protein homologous protein () and immunoglobulin heavy chain binding protein () levels. Moreover, cytokine-treated SPL-overexpressing cells exhibited increased expression of prohibitin 2 (Phb2), involved in the regulation of mitochondrial assembly and respiration. SPL-overexpressing cells were partially protected against cytokine-mediated ATP reduction and inhibition of glucose-induced insulin secretion. siRNA-mediated suppression resulted in effects opposite to those observed for SPL overexpression. Knockdown of partially reversed beneficial effects of SPL overexpression. In conclusion, the relatively low endogenous Spl expression level in insulin-secreting cells contributes to their extraordinary vulnerability to proinflammatory cytokine toxicity and may therefore represent a promising target for β-cell protection in type 1 diabetes mellitus.

Sphingolipids in Type 1 Diabetes: Focus on Beta-Cells

“…The deficiency of SPL promotes tumor growth and survival [86][87][88]. In neurons [89][90][91][92][93][94][95] and pancreatic beta-cells [21] intracellular S1P is involved in cytotoxic effects (see below). The effects of S1P are mediated by cell surface receptors and by activation of intracellular targets [1][2][3].…”

“…Moreover the SK2-mediated formation of S1P in the nucleus has been associated with changes in the epigenetic signature of cells due to the ability of S1P to directly bind and inhibit the histone deacetylases HDAC1 and HDAC2 [100,101]. Finally, many studies reported calcium as a possible second messenger in the intracellular action of S1P [21,89,93].…”

“…Most data available regarding the sphingolipid pathway in pancreatic beta-cells has been gained from murine and rodent models (RINm5F, INSE cells, MIN6 cell lines and mouse models). The extensive qRT-PCR analysis revealed the presence of all enzymes involved in the SL pathway in rodent beta-cells [21]. Pancreatic beta-cells are characterized by an imbalance of the enzymatic capacity for S1P formation (SK1 and SK2) and degradation (SPL) [21], which seems to play a particularly important role for cytokine toxicity (see Section 7).…”

“…Interestingly, our preliminary observations indicate that a number of the sphingolipid pathway enzymes might be affected by this specific RNase, a mechanism that could contribute to cytokine effects on beta-cell sphingolipidome. Finally, proinflammatory cytokines dysregulate the beta-cell function by shutting down insulin biosynthesis and by disrupting glucose-stimulated insulin secretion (GSIS) [21,41,[51][52][53].…”

Brain Cancer-Activated Microglia: A Potential Role for Sphingolipids

“…A b b r e v i a t i o n s : C o e n z y m e A : C o A ; s e r i n e palmitoyltransferase: SPT; 3-keto-Dihydrosphingosine: KDS; 3-ketodihydrosphingosine reductase: KDSR; dihydrosphingosine: dhSPH; dihydro ceramide: dhCer; (dihydro)ceramide synthase: CerS; (dihydro) ceramide desaturase: DES; DAG: diacylglycerol; phosphatidylcholine: PC; sphingomyelin synthase: SMS; sphingomyelinase: aSMase; ceramide kinase: CerK; ceramide-1P Cer-1P; ceramide-1P: phosphatase: C1PP, sphingosine: Sph; glucosylceramide synthase: G l u C e r ; a c i d -β -g l u c o s y l c e r a m i d a s e : a c i d -β -G l u C e r a s e ; galactosylceramide: GalCer; galactosylceramide synthase: GalCerS; galactosylceramide galactosylceramide: GalCer; ceramidase: GalCer ceramidase; glucosylceramide: GluCer; lactosylceramide: LacCer; ganglioside M3: GM3; ganglioside D3: GD3; ganglioside D2: GD2; globotriaosylceramide 3: Gb3; sphingosine kinase: SPHK; sphingosine-1-phosphate phosphatase: S1PP; sphingosine-1-phosphate lyase: S1PL; CoA synthase: CoAS; aldehyde dehydrogenase: ALDH. increasing number of scientific studies have now shown a neurotoxic effect of S1P [17] and a cytotoxic effect in pancreas β-cells [18]. A large number of extracellular signals or stimuli elevates intracellular Cer (heat shock, ionizing radiation, oxidative stress, tumor necrosis factor (TNF)-α, nitric oxide (NO), etc.)…”