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 F344

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) is an in hibitor of gap junctional intercellular communication (GJIC) of the ra t liver epithelial cell line, WB F-344. We have previously reported th at prolonged treatment of the WB cells with TPA (10 ng/ml) caused a re versal of the inhibition of GJIC that was initially induced (Oh, S.Y., et al. (1988) Carcinogenesis, 9, 135-139). Under this condition, addi tion of fresh TPA did not inhibit GJIC of these cells. In the present investigation we examined whether pulse exposure to TPA delays the ons et of this desensitization response. Cultures were treated for 5 or 15 min with TPA and shifted to normal medium. Intercellular communicatio n was measured at 15 min, 1 h and 6 h after the 5 or 15 min pulse trea tments. Under these pulse treatment conditions, GJIC of the cells was markedly inhibited for up to 4 h and gradually reverted to near contro l levels by 6-8 h. At every sixth hour of pulse treatment the cells we re given an additional pulse treatment (5 or 15 min) and the inhibitor y 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 th eir 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 expos ure to TPA delays the desensitization response normally observed in pr olonged treatment regimens and that this delay is possibly due to main tenance of the TPA activatable pool of protein kinase C under these co nditions.