Ceramide production pathways in the intestinal epithelium. Schematic depiction of the major pathways controlling ceramide levels in the intestinal epithelial cells: de novo synthesis, hydrolysis of complex sphingolipids (for example, sphingomyelin hydrolysis) and the salvage pathway. SPT, serine palmitoyltransferase; 3KSR, 3-ketosphinganine reductase; CERS, ceramide synthase; DES1, dihydroceramide desaturase 1; aCDase, acid ceramidase; nCDase, neutral ceramidase; aSMase, acid sphingomyelinase; nSMase, neutral sphingomyelinase; alk-SMase, alkaline sphingomyelinase; SMS, sphingomyelin synthase; TAG, triacylglyceride.