High levels of bile acids in the colon may correlate with an increased risk of colon cancer, but the underlying mechanisms are not known. Proteoglycan structures have been shown to change when human colon cells differentiate in vitro. The expression of [(35)S]sulphated molecules was used as a phenotypic marker to study the effects of bile acids on the human-colon-carcinoma cell line CaCo-2. [(35)S]sulphated compounds were isolated from the medium of cell fractions of cells metabolically labelled with [(35)S]sulphate in the absence and presence of cholic acid, deoxycholic acid, chenodeoxycholic acid and lithocholic acid (LA). Labelled molecules were analysed by gel chromatography, HPLC and SDS/PAGE in combination with chemical and enzymic methods. The expression of (35)S-labelled proteoglycans was not affected by any of the bile acids tested. However, the level of sulphated metabolites increased 7-18-fold in different experiments during a 22 h labelling period in the presence of an LA concentration of 10 microg/ml (26.6 nmol/ml) compared with controls. Further analyses showed that this was due, at least in part, to the sulphation of LA itself. This sulphation of LA was a rapid process followed by secretion back to the medium. Brefeldin A did not reduce the sulphation of LA, indicating that this conversion takes place in the cytosol, rather than in the Golgi apparatus of the CaCo-2 cells. LA in colon may be sulphated efficiently by the colonocytes to reduce the toxic effects of this particular bile acid. Sulphation may possibly be an important protective mechanism in the colon.
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