REGULATION OF TAURINE TRANSPORT IN HUMAN COLON-CARCINOMA CELL-LINES (HT-29 AND CACO-2) BY PROTEIN-KINASE-C

The effect of phorbol 12-myristate 13-acetate (PMA), a phorbol ester known to stimulate protein kinase C, on taurine transport was studied in the human colon carcinoma cell line HT-29. PMA (1 muM) was found to inhibit taurine uptake in confluent monolayers of this cell line by approximately 70% after pretreatment of the cells with the compound for 1 h (IC50 = 42.7 +/- 2.6 nM). The inhibitory effect of PMA on the taurine transporter could be confirmed by using beta-alanine, another substrate for the transporter. Kinetic analysis of taurine uptake indicated that the PMA effect was associated with a decrease in the maximal velocity (954 +/- 26 vs. 676 +/- 28 pmol.10 min-1.mg of protein-1) and an increase in the Michaelis-Menten constant (9.8 +/- 0.5 vs. 13.3 +/- 1.0 muM). The inhibition of taurine uptake could be blocked by cotreatment of the cultures with staurosporine, an inhibitor of protein kinase C. The inactive phorbol ester 4alpha-phorbol 12,13-didecanoate had no effect. Treatment of the cells with PMA did not alter the uptake of leucine and lysine, stimulated the uptake of aspartic acid, and inhibited the uptake of proline. The PMA effect on taurine uptake was not prevented by cycloheximide, actinomycin D, colchicine, or cytochalasin D. Comparison of the taurine transport activity in HT-29 cells with that in Caco-2, another human colon carcinoma cell line, revealed that the latter cell line also expressed the taurine transporter but at a much reduced level (about one-fifth compared with HT-29). However, the inhibitory effect of PMA on the transporter could be demonstrated in the Caco-2 cell line as well. These results suggest that the activity of the taurine transporter in human colon carcinoma cell lines HT-29 and Caco-2 is under the regulatory control of protein kinase C and that the underlying mechanism of this regulation may be phosphorylation of the transporter protein.