Serine palmitoyltransferase-mediated de novo sphingolipid biosynthesis is required for normal insulin production and glucose tolerance

ABSTRACT Aims/Hypothesis The importance for normal insulin secretion of ceramide synthesis is unclear. De novo ceramide synthesis requires serine palmitoyl transferase, SPT2, encoded by Sptl2 . Methods We generated β-cell-selective Sptl2 null mice by crossing animals with floxed alleles to mice expressing Cre recombinase from the Ins1 locus. Metabolic phenotyping, transcriptomic, functional analyses and histology were performed using standard approaches. Results Islets from Sptlc2 Δ Ins1 mice displayed marked alterations in ceramide and sphingomyelin levels: ceramide content: p=0.016 and p=0.109; sphingomyelin content: p=0.016 and p=0.004 in Sptlc2 Δ Ins1 vs Sptlc2 CTL mice under regular and high fat diet, respectively, despite compensatory increases in the expression of enzymes in the salvage and sphingomyelinase pathways. Correspondingly, profound abnormalities were observed in glucose-regulated insulin secretion and glucose tolerance in vivo , both on a regular chow and high fat diet. These changes were associated with a drastic (∼80%) lowering in β-cell numbers, and a more minor increase in delta cell numbers. They were also preserved in animals maintained on a ketogenic diet, consistent with a cell autonomous effect on the β-cell. Despite normal glucose-regulated intracellular calcium dynamics and insulin secretion, marked transcriptomic changes were observed in Sptlc2 Δ Ins1 mouse islets, with affected GO terms including lysosome organisation and regulation of autophagy. Consistent with roles for compromised SPT2 function in diseased β-cells, Sptl2 expression in Balbc and DBA2J mouse islets was lowered by a high fat-diet. Moreover, SPTLC2 mRNA tended to be lower, and SPTLC1 mRNA was significantly decreased, in islets from human subjects with type 2 diabetes versus normoglycemic individuals. Conclusions Preserved de novo ceramide synthesis is required to maintain normal β-cell mass and thus insulin secretion in mice. Therapeutic approaches which seek to target this process systemically using pharmacological SPT2 inhibitors should thus be treated with caution. Research in context - What is already known about this subject? Ceramides are key components of sphingolipid metabolism. Excess ceramide levels contribute to lipotoxicity and β-cell apoptosis. α-cell-restricted deletion of Cers2 , which is responsible for the synthesis of very long ceramide chains, alters the insulin content of pancreatic islets and modifies glucose tolerance. Deletion of Cers 5 or 6 , responsible for the synthesis of the long chains, has no effect. - What is the key question? What is the importance of de novo ceramide synthesis in β-cells for the normal regulation of insulin production and glucose homeostasis? - What are the new findings? Inhibition of the de novo ceramide synthesis pathway in β-cells, achieved by selective deletion of Sptlc2 , encoding subunit 2 of the serine palmitoyltransferase (SPT) enzyme, induces a major alteration of glucose tolerance and insulin secretion. This is accompanied by a drastic reduction in β-cell mass and islet insulin content. The remaining islets of Sptlc2 Δ Ins1 display normal glucose-regulated intracellular calcium dynamics and insulin secretion despite imbalances in ceramide and sphingomyelin levels and substantial transcriptomic changes. Expression of SPTLC1 , which encodes the other subunit of the SPT heterodimer, is reduced in islets from humans with type 2 diabetes, and a trend is observed towards lowered SPTLC2 expression. Taken together, these findings highlight the importance of de novo ceramide synthesis for normal β-cell survival and function - How might this impact on clinical practice in the foreseeable future? By suppressing insulin production, global blockade or inhibition of SPT2, achieved with pharmacological approaches which seek to rescue insulin sensitivity in T2D, may be deleterious for glucose tolerance.

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