Crucial role of alkaline sphingomyelinase in sphingomyelin digestion: a study on enzyme knockout mice

Abstract

and dietary products such as milk, eggs, meat, and fi sh. Our daily intake of SM is about 300 mg ( 1 ). SM in the diet has been shown to inhibit the colonic tumorigenesis in mice treated with chemical carcinogens ( 2 ), to promote the development of the intestinal mucosa in new born rats ( 3 ), and to inhibit cholesterol absorption in the gut ( 4,5 ). Because most of these effects can be reproduced or linked to ceramide, a hydrolytic product of SM ( 6, 7 ), it is important to study the enzymes that hydrolyze SM and generate ceramide in the gut.More than 40 years ago, Nilsson ( 8 ) identifi ed a type of enzyme in the intestinal mucosa that hydrolyses SM and generates ceramide at alkaline pH. This enzyme was thereafter named alkaline sphingomyelinase (alk-SMase) ( 9 ). Due to the fi ndings of important biological effects of sphingolipids in the last two decades ( 10-12 ), we performed several studies on alk-SMase. We have purifi ed the protein ( 13,14 ), cloned the gene ( 15, 16 ), and found potential implications of the enzyme in SM digestion ( 17 ), cell proliferation ( 18 ), colonic infl ammation ( 19 ), and cholesterol absorption ( 7 ). We also found decreased alk-SMase activity in colonic diseases such as colon cancer and colitis (20)(21)(22), and identifi ed inactive abnormal splicing forms of the enzyme in human colon and liver cancer cells ( 23,24 ).Besides alk-SMase, other SMases such as acid and neutral SMases were also identifi ed in the intestinal tract ( 9 ). A cloning study showed that alk-SMase shares no structural similarities with either acid or neutral SMase but belongs to the ectonucleotide pyrophosphatase/phosphodiesterase (NPP) family ( 15 ). Being a novel member of this family, alk-SMase is also called NPP7. Manuscript received 16 November 2010 and in revised form 20 December 2010. Published, JLR Papers in Press, December 21, 2010 DOI 10.1194 Abbreviations: alk-SMase, alkaline sphingomyelinase; APN, aminopeptidase N; BSSL, bile salt stimulated lipase; Cre-LoxP, Cre-recombinase-Locus of X over P1; ES, embryonic stem; KO, knockout; NPP, ectonucleotide pyrophosphatase/phosphodiesterase; TG, triglyceride; WT, wild-type.

Dietary Milk Sphingomyelin Reduces Systemic Inflammation in Diet-Induced Obese Mice and Inhibits LPS Activity in Macrophages

“…Although dietary SM is known to inhibit cholesterol absorption, luminal cholesterol can reciprocally inhibit SM digestion [16]. Feeding mice an HFD has also been shown to reduce the expression of the key enzyme in SM digestion, alkaline sphingomyelinase [46,47]. Therefore, it is possible that the use of a high-fat lard diet with added cholesterol in this study influenced ileal and colonic exposure to SM and bioactive metabolic products (ceramide, sphingosine, and sphingosine-1-phosphate), which in turn could affect colon inflammation [48,49].…”

Cellular function and molecular structure of ecto-nucleotidases

“…It hydrolyzes lysophosphatidic acid to form monoacylglycerol and phosphorylcholine but not lysophosphatidic acid, identifying it as a lysophospholipase C. It also hydrolyzes efficiently glycerophosphorylcholine and sphingosylphosphorylcholine [388]. NPP7 was shown to possess alkaline sphingomyelin phosphodiesterase (sphingomyelinase) activity, generating ceramide from sphingomyelin in the intestinal tract [389,390]. Interestingly, its catalytic activity is inhibited by ATP but not by ADP or AMP.…”

Enhanced Colonic Tumorigenesis in Alkaline Sphingomyelinase (NPP7) Knockout Mice

“…Alk-SMase KO mice were generated by the Cre-LoxP system as described previously (10). In brief, two loxP sites are inserted to flank the exon 2 of ENPP7 located on chromosome 11 (Ensembl Gene ID:ENSMUSG 00000046697).…”

“…The key enzyme in the gut that hydrolyses sphingomyelin to ceramide is alkaline sphingomyelinase (alk-SMase; ref. 10), which was discovered, purified, and cloned in our group (11,12). Cloning studies reveal that alk-SMase shares no structural similarities with other SMases, but belongs to the nucleotide pyrophosphatase phosphodiesterase (NPP) family and is now also called NPP7 (11,13,14).…”

“…Direct evidence is, however, still lacking. We here examined the tumorigenesis in alk-SMase knockout (KO) mice generated in our group by the Cre-LoxP system (10) and show that lack of alk-SMase significantly enhanced colon cancer susceptibility induced by carcinogens and inflammation.…”