Pulse treatment with the tumor promoter TPA delays the onset of desensitization response and prolongs the inhibitory effect on gap junctional intercellular communication of a rat liver epithelial cell line WB F-344

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) is an inhibitor of gap junctional intercellular communication (GJIC) of the rat liver epithelial cell line, WB F-344. We have previously reported that prolonged treatment of the WB cells with TPA (10 ng/ml) caused a reversal of the inhibition of GJIC that was initially induced (Oh, S.Y., et al. (1988) Carcinogenesis, 9, 135–139). Under this condition, addition of fresh TPA did not inhibit GJIC of these cells. In the present investigation we examined whether pulse exposure to TPA delays the onset of this desensitization response. Cultures were treated for 5 or 15 min with TPA and shifted to normal medium. Intercellular communication was measured at 15 min, 1 h and 6 h after the 5 or 15 min pulse treatments. Under these pulse treatment conditions, GJIC of the cells was markedly inhibited for up to 4 h and gradually reverted to near control levels by 6–8 h. At every sixth hour of pulse treatment the cells were given an additional pulse treatment (5 or 15 min) and the inhibitory effect of TPA on the GJIC of the cells was assayed 15 min after each such treatment. The results clearly showed that, when the cells were treated with 10 ng/ml TPA for 5 or 15 min every 6 h they maintained their sensitivity to the inhibitory effect of TPA on GJIC. This response to TPA was sustained for a considerably longer time when the duration of the pulse treatment was 5 min. Our data suggested that pulse exposure to TPA delays the desensitization response normally observed in prolonged treatment regimens and that this delay is possibly due to maintenance of the TPA activatable pool of protein kinase C under these conditions.