Identification of one exon deletion of intestinal alkaline sphingomyelinase in colon cancer HT-29 cells and a differentiation-related expression of the wild-type enzyme in Caco-2 cells.

Sphingomyelin (SM) metabolism in the gut has been implicated in colonic tumorigenesis. Intestinal alkaline sphingomyelinase (alk-SMase) hydrolyses SM in the intestinal content and at the brush border. The enzyme activity is decreased in the tissues of human colorectal tumours. This study examines whether site or chain-mutation of alk-SMase occurs in colon cancer HT-29 cells and Caco-2 cells. Total RNA was isolated and the cDNA of alk-SMase was amplified by RT-PCR. The size of the cDNA from HT-29 cells was smaller than that of the wild-type cDNA. DNA sequencing identified a deletion of exon 4 in alk-SMase cDNA in HT-29 cells. No mutation in genomic alk-SMase DNA from exon 3 to 5 was identified. The exon 4 deletion was caused by a shift of RNA splice site in chromosome 17q25. In Caco-2 cells, no mutation of alk-SMase cDNA was identified. Transient expression in COS-7 cells showed that the enzyme from the cDNA in HT-29 cells had little alk-SMase activity whereas that in Caco-2 cells was as active as the wild-type alk-SMase. The deleted region included residue His353, which is predicted to form a substrate-binding site of alk-SMase. H353A substitution resulted in a protein with no alk-SMase activity. In monolayer cultured Caco-2 cells and HT-29 cells the alk-SMase activities were low. However, to culture the cells under polarizing conditions increased alk-SMase activity and reduced SM level in Caco-2 cells. The alk-SMase activity varied in parallel with alkaline phosphatase activity. In conclusion, we identified an inactive deletion in alk-SMase in HT-29 cells, and a differentiation-related expression of the enzyme in Caco-2 cells. The results provide a molecular mechanism related to previous findings of reduced alk-SMase activity in human colon cancers.