Identification of aberrant forms of alkaline sphingomyelinase (NPP7) associated with human liver tumorigenesis

Abstract

Alkaline sphingomyelinase (alk-SMase) is expressed in the intestine and human liver. It may inhibit colonic tumorigenesis, and loss of function mutations have been identified in human colon cancer. The present study investigates its expression in human liver cancer. In HepG2 liver cancer cells, RT -PCR identified three transcripts with 1.4, 1.2 and 0.4 kb, respectively. The 1.4 kb form is the wild-type cDNA with five translated exons, the 1.2 kb product lacks exon 4 and the 0.4 kb form is a combination of exons 1 and 5. Genomic sequence showed that these aberrant transcripts were products of alternative splicing. Transient expression of the 1.2 kb form showed no alk-SMase activity. In HepG2 cells, the alk-SMase activity is low in monolayer condition and increased with cell polarisation. Coexistence of 1.4 and 1.2 kb forms was also identified in one hepatoma biopsy. GenBank search identified a cDNA clone from human liver tumour, which codes a protein containing full length of alk-SMase plus a 73-amino-acid tag at the N terminus. The aberrant form was translated by an alternative starting codon upstream of the wild-type mRNA. Expression study showed that linking the tag markedly reduced the enzyme activity. We also analysed human liver biopsy samples and found relatively low alkSMase activity in diseases with increased risk of liver tumorigenesis. In conclusion, expression of alk-SMase is changed in hepatic tumorigenesis, resulting in loss or marked reduction of the enzyme function.

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

“…Marked reduction of alk-SMase activity due to the expression of abnormal splicing forms has been reported in human colon and liver cancer cells ( 23,45 ). In the present work with alk-SMase KO mice, we did not fi nd spontaneous tumorigenesis either in the small intestine or colon up to 10 months of age.…”

“…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 ).…”

Resveratrol and its oligomers: modulation of sphingolipid metabolism and signaling in disease

“…Indeed, S1P levels are increased by elevated expression of SK1 in multiple human cancers (Pyne and Pyne 2010). In addition, down-regulation of alkaline sphingomyelinase has been observed in human colon and liver cancers leading to reduction of ceramide level and attenuation of apoptosis (Cheng et al 2007;Duan 2005). Therefore, the interconversion of these sphingolipid metabolites must be considered when assessing the cellular phenotype upon gene knockdown or treatment with pharmacological therapeutic agents.…”

Radiation-induced acid ceramidase confers prostate cancer resistance and tumor relapse

“…Investigations to distinguish differences in sphingolipid profile expression between "normal" versus malignant cells at baseline or in vitro growth conditions have identified higher levels of ceramide expression in cancer tissues and cells as well as species-specific variations (63,64). At the level of enzymatic function, elucidating the control of sphingolipids upon chemotherapeutic or radiotherapeutic challenge may yield novel differences between cell types (65)(66)(67)(68)(69)(70). This offers the tantalizing possibility that a distinct difference in sphingolipid regulation at clinically relevant IR dosage presents targets for therapeutic enhancement.…”