Human Caco-2 cells have been established as a model system for intestinal biotransformation and permeability. When grown on Transwell polycarbonate filters they develop morphologic and biochemical characteristics of enterocytes with well separated apical and basolateral surfaces. In addition, Caco-2/TC-7 cells have proven to be useful to study regulation of human UDP-glucuronosyltransferases (UGTs) by Ah receptor agonists and antioxidant-type inducers such as beta-naphthoflavone (BNF) and t-butylhydroquinone (TBHQ). In the present investigation, formation and transport of 4-methylumbelliferone glucuronide was studied in intact Caco-2 cell monolayers. The following results were obtained: when loaded with 50-200 microM MUF either apically or basolaterally, MUF-GA was the major metabolite which was mostly released (80%) at the basolateral surface, probably via the multidrug resistance protein isoform MRP3; MUF sulfate formation was low (5 +/-2%). Pretreatment of cells with 80 microM TBHQ or 50 microM BNF for 72 hr before addition of 100 microM MUF enhanced basolateral secretion of MUF-GA 1.4- and 1.7-fold, respectively. However, at >200 microM MUF, MUF-GA secretion and induction was smaller, probably due to inhibition of intracellular UGT activity. MRP3 protein was localized to the basolateral surface of Caco-2 cells but was not induced by TBHQ or BNF. The results suggest that MUF-GA is mostly secreted basolaterally in Caco-2 cell monolayers. Treatment with TBHQ or BNF significantly enhanced MUF-GA formation in the intact cell.