Characterization of the growth inhibition induced by tumor-promoting phorbol esters and of their receptor binding in A549 human lung carcinoma cells.

Exposure of A549 human lung carcinoma cells to 10(-8) M 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a change in cell morphology and caused the arrest of cell growth. After 4-5 days of exposure to TPA the cells started to proliferate again. However, on removal of the cells from the culture flask and reseeding, the cells had regained their sensitivity towards TPA. Cells which were subcultured in the presence of 10(-8) M TPA for 9 weeks were permanently refractory to the growth-inhibitory properties of TPA. Incubation of A549 cells with [3H]phorbol-12,13-dibutyrate ([ 3H]PDB) showed that the cells possess specific phorbol ester receptors. Exposure of the cells to 10(-7) M PDB preceding the receptor binding assay led rapidly to a decline in the binding of 6 nM [3H]PDB, in case of preincubation for 24 h to 38% of the binding in cells not pre-exposed to PDB. The receptor binding capacity after pretreatment with PDB was only weakly decreased in the cells which were desensitized towards the TPA-induced growth inhibition. Thus the decrease in receptor binding on exposure to phorbol esters does not appear to cause the refractoriness of the cells towards the effect of TPA. It seems more likely that this decrease in binding capacity is part of the events by which phorbol esters cause inhibition of cell growth.