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Introduction: Short-chain fatty acids (SCFA), generated in the colon by bacterial fermentation of dietary fiber, protect against colorectal cancer. Among the SCFAs, butyrate is believed to be most relevant to colonic health. Butyrate induces differentiation of colonocytes and promotes apoptosis in colonic tumor cells. The ability of butyrate to induce apoptosis in tumor cells is related to its function as an inhibitor of histone deacetylases (HDACs). To do this, butyrate must enter the colonocyte. SLC5A8 is a Na⁺-coupled transporter for SCFA expressed predominantly in the colon. SLC5A8 has been shown to be silenced in colon cancer and re-expression of the transporter in colon cancer cell lines leads to apoptosis. However the underlying mechanism of tumor suppression is unknown. Hypothesis: SLC5A8-mediated concentrative entry of butyrate into colon cancer cells is responsible for tumor cell-specific induction of apoptosis. Methods:Expression of SLC5A8 was compared in paired samples of human tumor and normal colon tissue, colon tissue from Apc^Min/+ mice, a model for intestinal/colon cancer, and wild type mice, and several colon cancer and normal colon cell lines. Mature oocytes from X.laevis, injected with human SLC5A8 cRNA, were used for electrophysiological studies to characterize SLC5A8-mediated butyrate transport. The Na⁺-activation kinetics of butyrate-induced currents was analyzed by measuring the butyrate-specific currents with increasing amounts of Na⁺. Normal and colon cancer cell lines transfected with pcDNA or SLC5A8 cDNA and treated with or without butyrate were used to evaluate apoptosis, HDAC activity, and expression of pro- and anti-apoptotic genes.
Results: SLC5A8 transports butyrate via a Na⁺-dependent electrogenic process. Na⁺-activation of the transport process exhibits sigmoidal kinetics, indicating involvement of more than 1 Na⁺ in the activation process. SLC5A8 is silenced in colon cancer in humans, in a mouse model of colon cancer, and in colon cancer cell lines. The tumor-associated silencing of SLC5A8 involves DNA methylation by DNA methyltransferase 1. Re-expression of SLC5A8 in colon cancer cells leads to apoptosis, but only in the presence of butyrate. SLC5A8-mediated entry of butyrate into cancer cells is associated with inhibition of histone deacetylation. Changes in gene expression in SLC5A8/butyrate-induced apoptosis include upregulation of pro-apoptotic genes and downregulation of anti-apoptotic genes. Also, expression of phosphatidylinositol-3-kinase subunits is affected differentially. Conclusion:These studies show that SLC5A8 mediates the tumor-suppressive effects of the SCFA butyrate in the colon.

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