Alkaline Sphingomyelinase; A Potential Inhibitor in Colorectal Carcinogenesis

Sammanfattning: Sphingomyelin (SM) is present both in eucaryotic cells and the diet. Sphingomyelinase (SMase) hydrolyses SM and generates ceramide that has been implicated in the regulation of cell growth, differentiation and apoptosis. Previously a SMase with alkaline pH optimum was identified in the intestinal tract. The thesis studied the distribution pattern, the species difference and the clinical implications of this enzyme. Unlike neutral and acid SMase, which are present in most tissues, alkaline SMase is specifically located to the intestinal tract. The distribution pattern in human intestine was similar to other species such as rats, mice and hamsters, the highest values were demonstrated in the jejunum and lower in the colon. Man was the only species examined with an additional alkaline SMase in the bile. After giving SM orally to ileostomists, both SM and ceramide were accumulated in the stoma output indicating that the colonic mucosa is exposed to ceramide. When the activities of three SMases were determined in patients with human colorectal carcinoma (CRC), all SMases were reduced in cancer tissue compared to the surrounding mucosa, but the most predominant reduction being for alkaline SMase by (75%, p=0.004). Furthermore, the enzyme activity was markedly decreased by 90% (p<0.001) in adenoma tissue and in flat mucosa of patients with familial adenomatous polyposis compared to controls. These results suggested a potential link to APC mutations. However, when the activities of the enzyme were studied in the Min mouse (germline APC mutations) and human CRC tissue (Somatic APC mutations), no direct link was identified. Finally, we studied the effect of a purified rat intestinal alkaline SMase on growth in human HT-29 colon carcinoma cells and non-transformed rat intestinal IEC-6 cells. Alkaline SMase inhibited cell proliferation of HT-29 cells but not IEC-6 cells. In contrast, a bacterial neutral SMase had no effect. Alkaline SMase did not induce apoptosis. The antiproliferative effect was accompanied by SM hydrolysis and ceramide production. Alkaline SMase did not induce apoptosis whereas C2-ceramide and sulindac did. In conclusion, alkaline SMase is a gut specific enzyme with antiproliferative properties. The enzyme may be important in cell growth regulation and its reduction may facilitate the development of colorectal cancer.