TPA INHIBITION OF GAP JUNCTIONAL INTERCELLULAR COMMUNICATION IN SYRIAN-HAMSTER EMBRYO CELLS THROUGH 2 DIFFERENT PATHWAYS

Exposure of early passage Syrian hamster embryo cells to high TPA conc entrations (8 or 16 mu M) decreased gap junctional intercellular commu nication (GJIC) to about 20% of the control during the first hour. Sub sequently, the GJIC capacity recovered to 70-80% after 4-10 hr. Exposu re to lower TPA concentrations also reduced GJIC, but did not result i n the subsequent rapid enhancement of communication. Treatment of the cells to different TPA concentrations down-regulated protein kinase C (PKC), but this down-regulation did not seem to explain fully the indu ced recurrence of GJIC following high TPA concentrations. The maximal down-regulation of PKC took place at TPA concentrations above 0.16 mu M. Addition of 8 mu M TPA to cells pretreated with 0.016 and 8 mu M fo r 10 hr reduced GJIC from 80 to 40%, whereas pretreatment with 0.16 or 1.6 mu M TPA made the cells refractory to further TPA-induced inhibit ion. The PKC inhibitor staurosporine suppressed the effect of TPA on G JIC in cells exposed to 0.016 mu M TPA, whereas no suppression was obs erved in cells depleted of PKC. The results indicate that TPA is capab le of inhibiting GJIC through two different mechanisms, a staurosporin e-sensitive mechanism at low TPA concentrations (EC(50) = 0.18 mu M) a nd a staurosporine-insensitive mechanism at higher concentrations (EC( 50) = 7 mu M).