Inhibitory effect of tumor promoting phorbol esters and mezerein on human mixed lymphocyte reaction

Augmentation of mixed lymphocyte response. Stimulatory activity by phorbol ester

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces a variety of phenotypic changes on normal and malignant cells. In chronic and acute lymphocytic leukemia we have observed a marked augmentation in mixed lymphocyte reaction stimulatory capacity (MLRs) following pretreatment of leukemia blasts with TPA. We have now characterized the effects of TPA on MLRs utilizing the non-T, non-B lymphoblastic leukemia-cell line, HOON. Following pretreatment with TPA (optimal concentration, 1.6 X 10(-10) M), but not with other phorbol esters, there was a marked increase in MLRs, particularly at lower stimulator-cell concentrations. This effect was maximal following a 44-hr preincubation period and the enhancement in MLRs was not accompanied by changes in levels of Ia expression. This cell line provides a model for determining the molecular basis for the TPA-induced augmentation of stimulatory capacity in the mixed lymphocyte reaction.

Phorbol 12,13-dibutyrate (P(Bu)2)-treated human blood mononuclear cells bind to each other

Treatment of human blood mononuclear cells with nanomolar concentrations of phorbol 12,13-dibutyrate (P(Bu)2) induced their aggregation. The phenomenon was seen within a few minutes and reached its maximum manifestation after 20 min. At this time, 20-30% of unfractionated and nylon wool-passed mononuclear leukocytes were in the aggregates. The influence of pretreatment with 2-deoxyglucose, NaN3, EDTA, cyclohexamide, or incubation at 4 degrees C on the phenomenon indicated that it is energy and temperature dependent, requires the presence of extracellular divalent cations, and is independent of protein synthesis. Nonaggregating 2-deoxyglucose-and NaN3-pretreated cells could still bind [3H]P(Bu)2 which rules out the possibility that the phorbol ester molecule acts as a bridge between the aggregated cells. Seventeen percent of the P(Bu)2-treated T-cell population bound untreated autologous and allogeneic cells. The binding property has a certain species specificity because only 4% of the cells interacted with mouse lymphocytes. At the ultrastructural level, the intercellular binding showed broad areas of surface contact (both between lymphocytes and lymphocyte-monocyte) and "trapping" by surface processes was not seen. Aggregated cells did not show cytopathogenic changes.

Tumor promoters cause a rapid and reversible inhibition of the formation and maintenance of electrical cell coupling in culture

The effect of tumor promoters on electrical coupling between human FL cells was investigated with a microelectrode technique. When a low concentration (100 ng/ml) of 12-O-tetradecanoylphorbol 13-acetate (TPA) was added to culture medium, only 6% of the cells showed electrical coupling after 5 hr, whereas in control medium more than 90% of the cells were coupled. In the presence of TPA, cell coupling remained suppressed for at least another 19 hr. When TPA was washed out from the culture medium, the cells commenced electrical coupling: 90% of the cells were coupled within 4 hr of the removal of TPA, a rate very similar to that of nontreated control cells. Therefore, TPA-mediated inhibition of cell coupling is reversible. When TPA was added to a culture in which more than 90% of the cells had already established electrical coupling, the percentage of coupled cells decreased to 6% within 8 hr, indicating that TPA can also diminish already established cell coupling. Inhibition of cell coupling also was achieved with other mouse skin tumor promoters--phorbol 12,13-didecanoate, ingenol dibenzoate, and mezerein--whereas nonpromoting derivatives--phorbol and 4 alpha-phorbol 12,13-didecanoate-showed no effect. None of these compounds changed the membrane potential, membrane resistance, or growth rate of FL cells. Thus, it appears that TPA and structurally-related tumor promoters specifically disturb the formation or function, or both, of cell-cell junctions, without significantly affecting the general properties of the surface membrane or the growth of FL cells.