Fate of indole-3-carbinol in cultured human breast tumor cells.

Indole-3-carbinol (I3C), a natural component of Brassica vegetables, is a promising cancer preventive agent that can reduce the incidence of tumors in reproductive organs when administered in the diet. Here we report on the metabolic fate of radiolabeled I3C in MCF-7 cells. I3C was surprisingly inert to metabolism by these cells with a half-life in medium of approximately 40 h. [(3)H]I3C levels in media declined at a similar rate whether incubation was with cultured cells or in cell-free medium. Neither [(3)H]I3C nor its modified products accumulated in MCF-7 cells and only low levels of intact I3C were detected in cellular fractions. In contrast, I3C represented over 30% of the radioactivity in media even after 72 h. In cytosolic fractions, the 3-(cystein-S-ylmethyl) and 3-(glutathion-S-ylmethyl) conjugates of [(3)H]I3C were the primary conversion products identified after 16 h, representing approximately 50% and approximately 15% of the radioactivity in these fractions, respectively. The reaction of I3C with thiols appears to be nonenzymatic since the cysteine conjugate is produced when I3C is incubated in cell-free medium containing additional cysteine. Both cellular and extracellular proteins were nonspecifically modified with [(3)H]I3C. In medium, proteins are radiolabeled even in the absence of cells, indicating again that enzymatic activation was not required. I3C was also oxidized to indole-3-carboxaldehyde and indole-3-carboxylic acid in culture medium independent of cells. Unexpectedly, 3,3'-diindolylmethane (DIM), an I3C product with in vitro and in vivo biological activity, was detected in cellular fractions and appeared to accumulate in the nucleus, representing approximately 40% of this fraction after 72 h treatment. These findings suggest that MCF-7 cells do not vigorously metabolize I3C and that the major route of reaction is with cellular thiols such as glutathione and proteins. The accumulation of DIM in the nucleus suggests that this product may have a role in the cellular biological activities of I3C.3