Intracellular role for sphingosine kinase 1 in intestinal adenoma cell proliferation

Sphingosine kinase (Sphk) enzymes are important in intracellular sphingolipid metabolism as well as in the biosynthesis of sphingosine 1-phosphate (S1P), an extracellular lipid mediator. Here, we show that Sphk1 is expressed and is required for small intestinal tumor cell proliferation in Ape(Min/+) mice. Adenoma size but not incidence was dramatically reduced in Apc(Min/+) Sphk(-/-) mice. Concomitantly, epithelial cell proliferation in the polyps was significantly attenuated, suggesting that Sphk1 regulates adenoma progression. Although the SIP receptors (S1P1R, S1P2R, and S1P3R) are expressed, polyp incidence or size was unaltered in Apc(Min/+) S1p2r(-/-), Apc(4Min/+) S1p3r(-/-), and Apc(Min/+) S1p1r(+/-) bigenic mice. These data suggest that extracellular SIP signaling via its receptors is not involved in adenoma cell proliferation. Interestingly, tissue sphingosine content was elevated in the adenomas of Apc(Min/+) Sphk1(-/-) mice, whereas S1P levels were not significantly altered. Concomitantly, epithelial cell proliferation and the expression of the G(1)/S cell cycle regulator CDK4 and c-myc were diminished in the polyps of Apc(Min/+) Sphk1(-/-) mice. In rat intestinal epithelial (RIE) cells in vitro, Sphk1 overexpression enhanced cell cycle traverse at the G(1)/S boundary. In addition, RIE cells treated with sphingosine but not C6-ceramide exhibited reduced cell proliferation, reduced retinoblastoma protein phosphorylation, and cyclin-dependent kinase 4 (Cdk4) expression. Our findings suggest that Sphk1 plays a critical role in intestinal tumor cell proliferation and that inhibitors of Sphk1 may be useful in the control of intestinal cancer.