Intestinal epithelial cell accumulation of the cancer preventive polyphenol ellagic acid--extensive binding to protein and DNA

Abstract

Ellagic acid (EA), a polyphenol present in many berries, has been demonstrated to be preventive of esophageal cancer in animals both at the initiation and promotion stages. To be able to extrapolate these findings to humans we have studied the transcellular absorption and epithelial cell accumulation of [14C]EA in the human intestinal Caco-2 cells. The apical (mucosal) to basolateral (serosal) transcellular transport of 10 microM [14C]EA was minimal with a P(app) of only 0.13 x 10(-6)cm/s, which is less than for the paracellular transport marker mannitol. In spite of observations of basolateral to apical efflux, Caco-2 cell uptake studies showed high accumulation of EA in the cells (1054+/-136 pmol/mg protein), indicating facile absorptive transport across the apical membrane. Surprisingly, as much as 93% of the cellular EA was irreversibly bound to macromolecules (982+/-151 pmol/mg protein). To confirm the irreversible nature of the binding to protein, Caco-2 cells treated with 10 microM [14C]EA were subjected to SDS-PAGE analysis. This resulted in radiolabeled protein bands trapped in the stacking gel, consistent with [14C]EA-crosslinked proteins. Treatment of Caco-2 cells with 10 microM [14C]EA also revealed irreversible binding of EA to cellular DNA as much as five times higher than for protein (5020+/-773 pmol/mg DNA). Whereas the irreversible binding to protein required oxidation of EA by reactive oxygen species, this did not seem to be the case with the DNA binding. The avid irreversible binding to cellular DNA and protein may be the reason for its highly limited transcellular absorption. Thus, EA appears to accumulate selectively in the epithelial cells of the aerodigestive tract, where its cancer preventive actions may be displayed.