Effects of 12-O-tetradecanoylphorbol-13-acetate and retinoids on intercellular junctional communication measured with a citrulline incorporation assay

Abstract

Inhibition of intercellular junctional communication by 12-O-tetradecanoylphorbol-13-acetate (TPA) and retinoids was investigated using a citrulline incorporation assay. This new assay uses metabolic co-operation between argininosuccinate lyase-deficient human fibroblasts and arginosuccinate synthetase-deficient cells as a measure of junctional communication. Short-term exposure to TPA resulted in virtually complete inhibition of metabolic co-operation when V79 cells were used as the synthetase-deficient type. When synthetase-deficient human fibroblasts were used, inhibition by TPA was only partial. Exposure to high concentrations of TPA for prolonged periods resulted in partial reversal of communication inhibition and a refractory state in which cells were unresponsive to TPA. Retinoic acid and other retinoids also inhibited metabolic co-operation, but did not cause desensitisation of the type seen with TPA after prolonged exposure. Cultures which had been made refractory to TPA remained sensitive to inhibition by retinoic acid and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane, indicating that these latter compounds inhibit junctional communication by a mechanism different from TPA. Simultaneous exposure of cultures to TPA and retinoic acid showed that the inhibitory effects on metabolic co-operation of these compounds were additive. Fluocinolone acetonide did not antagonise the effect of TPA. These results suggest that retinoic acid and fluocinolone acetonide exert their anti-tumor-promoting action by mechanisms which are not mediated by intercellular junctional communication.