Effects of 6-thioguanine on communication competence and factors affecting reversibility of phorbol ester inhibition in V79 cells measured by the metabolic cooperation assay and by dye coupling.

One assay used to study the role of intracellular communication in tumor promotion examines chemically-induced inhibition of metabolic cooperation between 6-thioguanine (6-TG) sensitive (HGPRT+) V79 Chinese hamster cells in co-culture with 6-TG resistant (HGPRT-) cells. In the present study metabolic cooperation and Lucifer Yellow dye coupling were applied to detect intercellular communication. HGPRT+ cells were still communication competent when pre-incubated with 6-TG for up to 48 h before HGPRT- cells were plated. In both assays the phorbol esters 12-O-tetradecanoylphorbol-13-acetate (TPA) and phorbol-12,13-dibutyrate (PDBu) inhibited intercellular communication completely at 1 ng/ml. Using 6-TG metabolic cooperation, extensive washing of cells failed to reverse the inhibition caused by 100 ng TPA/ml after exposure for as little as 1 h. However, the effect of PDBu was completely eliminated if the drug was removed within 24 h. Complete blockade of dye coupling by 1, 10 or 100 ng TPA/ml was reversible after washing only at 1 ng/ml. Cells cultured in the presence of these TPA concentrations displayed significant reversal of dye coupling inhibition after 40 h. The results indicate that the onset of communication competence impairment between V79 cells by 6-TG is much slower than hitherto implied. Failure of intercellular communication to recover even from short exposure to 100 ng TPA/ml appears to be attributable to its lipophilic nature. Not enough TPA can be removed by washing to overcome its potent inhibitory action. These observations indicate that the physicochemical properties of a chemical must be taken into consideration before making judgements about the duration of inhibition.