TPA inhibition of gap junctional intercellular communication in Syrian hamster embryo cells through two different pathways.
Exposure of early passage Syrian hamster embryo cells to high TPA concentrations (8 or 16 mum) decreased gap junctional intercellular communication (GJIC) to about 20% of the control during the first hour. Subsequently, the GJIC capacity recovered to 70-80% after 4-10 hr. Exposure to lower TPA concentrations also reduced GJIC, but did not result in 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 induced recurrence of GJIC following high TPA concentrations. The maximal down-regulation of PKC took place at TPA concentrations above 0.16 mum. Addition of 8 mum TPA to cells pretreated with 0.016 and 8 mum for 10 hr reduced GJIC from 80 to 40%, whereas pretreatment with 0.16 or 1.6 mum TPA made the cells refractory to further TPA-induced inhibition. The PKC inhibitor staurosporine suppressed the effect of TPA on GJIC in cells exposed to 0.016 mum TPA, whereas no suppression was observed in cells depleted of PKC. The results indicate that TPA is capable of inhibiting GJIC through two different mechanisms, a staurosporine-sensitive mechanism at low TPA concentrations (EC(50) = 0.18 mum) and a staurosporine-insensitive mechanism at higher concentrations (EC(50) = 7 mum).
Roseng
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Laboratory for Environmental and Occupational Cancer, Institute for Cancer Research, The Norwegian Radium Hospital, N-0310 Oslo 3, Norway.
Rivedal
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