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Loss of functional β-cell mass caused by lipotoxicity is a key pathogenic factor in the development of type 2 diabetes mellitus (T2DM). We have previously reported that sphingosine kinase (SK)1 is an endogenous protector of β-cells against lipotoxicity. The current study reports that SK2, another isoform of SK, is a crucial mediator of lipotoxicity in β-cells. Exposure of β-cells to palmitatic acid (PA), a saturated free fatty acid, resulted in a nearly 2-fold increase in SK2 expression, which paralleled the induction of cell death in a similar dose- and time-dependent fashion. Silencing SK2 expression by its specific small interfering RNAs significantly inhibited PA-induced cell death and caspase-3 activation, whereas overexpression of SK2 promoted lipotoxicity in β-cells. Mechanistically, upon exposure to PA, endogenous SK2 was shuttled from the nucleus to the cytoplasm, where it interacted with B-cell lymphoma-extra-large (Bcl-xL), leading to mitochondrial apoptotic pathway activation and cell death. By blocking SK2 translocation and its interaction with Bcl-xL, either the nuclear export signal mutant (L423A/L425A) or the BH3 domain mutant (L219A) of SK2 significantly attenuated β-cell lipotoxicity. Furthermore, SK2 deficiency in mice significantly prevented the loss of β-cell mass, preserved insulin production, and ameliorated the diabetic phenotype in an established T2DM model induced by feeding a high-fat diet accompanied by administration of streptozotocin. These findings provide the first evidence, in vitro and in vivo, of a critical role for SK2 in mediating β-cell lipotoxicity and the progression of diabetes.-Song, Z., Wang, W., Li, N., Yan, S., Rong, K., Lan, T., Xia, P. Sphingosine kinase 2 promotes lipotoxicity in pancreatic β-cells and the progression of diabetes.