Peroxisomal dysfunction is often associated with mitochondrial abnormalities for unknown reasons. We found that peroxisomal loss upon Pex19 mutation in Drosophila results in Hnf4 hyperactivation with free fatty acid accumulation and mitochondrial damage as a consequence. Genetic reduction of Hnf4 in Pex19 mutants improves all phenotypes, including lethality.

Maintenance of metabolic homeostasis requires adaption of gene regulation to the cellular energy state via transcriptional regulators. Here, we identify a role of ceramide synthase (CerS) Schlank, a multiple transmembrane protein containing a catalytic lag1p motif and a homeodomain, which is poorly studied in CerSs, as a transcriptional regulator. ChIP experiments show that it binds promoter regions of lipases lipase3 and magro via its homeodomain. Mutation of nuclear localization site 2 (NLS2) within the homeodomain leads to loss of DNA binding and deregulated gene expression, and NLS2 mutants can no longer adjust the transcriptional response to changing lipid levels. This mechanism is conserved in mammalian CerS2 and emphasizes the importance of the CerS protein rather than ceramide synthesis. This study demonstrates a double role of CerS Schlank as an enzyme and a transcriptional regulator, sensing lipid levels and transducing the information to the level of gene expression.

Edited by Laszlo NagyDrosophila Ceramide Synthase (CerS) Schlank regulates both ceramide synthesis and fat metabolism. Schlank contains a catalytic lag1p motif and, like many CerS in other species, a homeodomain of unknown function. Here, we show that the Drosophila CerS Schlank is imported into the nucleus and requires two nuclear localization signals (NLSs) within its homeodomain and functional Importin-b import machinery. Expression of Schlank variants containing the homeodomain without functional lag1p motif rescued the fat metabolism phenotype of schlank mutants whereas a variant with a mutated NLS site did not rescue. Thus, the homeodomain of Schlank is involved in the regulation of lipid metabolism independent of the catalytic lag1p motif.Keywords: ceramide synthase; Drosophila; homeodomain; importin; lipid homeostasis; nuclear function Ceramides and sphingolipids are key intermediates in diverse cellular processes for growth, stress resistance, apoptosis, neurodegeneration, insulin function, and lipid homeostasis [1][2][3][4][5].In synthesis of ceramide, Ceramide Synthases (CerSs) acylate a sphingoid long-chain base (LCB) with a fatty acyl-CoA of distinct length to generate (dihydro) ceramide, the backbone of all complex sphAbbreviations aa, amino acid; ASAH1, acid ceramidase; ATF2, activating transcription factor 2; cDNA, complementary DNA; CerSs, ceramide synthases; dsRNA, double-stranded RNA; ER, endoplasmic reticulum; Gal4, galactose-responsive transcription factor; GFP, green fluorescent protein; GlcT-1, glucosylceramide synthase; H215D, change in a histidine at position 215 of schlank into glutamate; INM, inner nuclear membrane;