Enhanced Colonic Tumorigenesis in Alkaline Sphingomyelinase (NPP7) Knockout Mice

Intestinal alkaline sphingomyelinase (alk-SMase) generates ceramide and inactivates platelet-activating factor (PAF) and is previously suggested to have anticancer properties. The direct evidence is still lacking. We studied colonic tumorigenesis in alk-SMase knockout (KO) mice. Formation of aberrant crypt foci (ACF) was examined after azoxymethane (AOM) injection. Tumor was induced by AOM alone, a conventional AOM/dextran sulfate sodium (DSS) treatment, and an enhanced AOM/DSS method. beta-catenin was determined by immunohistochemistry, PAF levels by ELISA and sphingomyelin metabolites by mass spectrometry. Without treatment, spontaneous tumorigenesis was not identified but the intestinal mucosa appeared thicker in KO than in wild type (WT) littermates. AOM alone induced more ACF in KO mice but no tumors 28 weeks after injection. However, combination of AOM/DSS treatments induced colonic tumors and the incidence was significantly higher in KO than in WT mice. By the enhanced AOM/DSS method tumor number per mouse increased 4.5 times and tumor size 1.8 times in KO compared to WT mice. While all tumors were adenomas in WT mice, 32% were adenocarcinomas in KO mice. Compared to WT mice, cytosol expression of beta-catenin was significantly decreased and nuclear translocation in tumors was more pronounced in KO mice. Lipid analysis showed decreased ceramide in small intestine and increased sphingosine-1-phosphate in both small intestine and colon in nontreated KO mice. PAF levels in feces were significantly higher in the KO mice after AOM/DSS treatment. In conclusion lack of alk-SMase markedly increases AOM/DSS induced colonic tumorigenesis associated with decreased ceramide and increased sphingosine-1-phosphate and PAF levels.