Downregulation of a human colonic sialyltransferase by a secondary bile acid and a phorbol ester

Fecal constituents such as bile acids and increased sialylation of membrane glycoproteins by α-2,6-sialyltransferase (HST6N-1) may contribute to colorectal tumorigenesis. We hypothesized that bile acids and phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA)] would upregulate HST6N-1 in colonic cells. However, deoxycholate (DOC) (300 μmol/l), a secondary bile acid, and TPA (20 ng/ml) decreased expression of an ~100-kDa glycoprotein bearing α-2,6-linked sialic acid in a colon cancer cell line (T84) in vitro. HST6N-1 mRNA levels were reduced ~80% by treatment (≤24 h) with DOC or TPA but not by cholate, a primary bile acid. Treatment (24 h) with DOC or TPA decreased activity of this enzyme to 30% and 13% of control, respectively. These effects of DOC and TPA were transcriptional and were mediated by Ca²⁺ and protein kinase C, respectively. Thus DOC and TPA both downregulated, and did not upregulate, α-2,6-sialyltransferase expression in vitro, but by different transduction pathways. As colorectal tumors grow, their progressive removal from the fecal milieu that normally downregulates this enzyme may favor invasion and metastasis.