Divergent responses in epidermal basal cells exposed to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate.

Mouse epidermal basal cells can be selectively cultivated in medium with 0.02 to 0.09 mM Ca2+ and can be induced to differentiate by medium containing 1.2 mM Ca2+. Basal cell cultures were studied to determine if all cells in this population responded identically to the skin tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Studies on the induction of the enzyme epidermal transglutaminase by TPA demonstrated a 2- to 4-fold increase in activity within 12 hr of exposure. This activity increase paralleled morphological differentiation in approximately 50% of the basal cell population, and differentiating cells sloughed from the culture dish within 24 to 48 hr as transglutaminase activity returned to basal levels. The cells which remained were resistant to induced differentiation by 1.2 mM Ca2+ medium, in that they failed to demonstrate increased transglutaminase activity or decreased thymidine incorporation, both characteristics of control basal cells induced to differentiate by 1.2 mM Ca2+. Cells remaining after a single exposure to TPA did not respond to a second exposure with an induction of transglutaminase if the interval between exposures was 4 days. TPA-pretreated cells did not undergo a transient decrease in thymidine incorporation (characteristic of control cells) when exposed to TPA a second time but instead were directly stimulated to proliferate by the phorbol ester, indicating that such cells were not refractory to the promoter. When the treatment-free interval after TPA was extended from 4 to 10 days, transglutaminase inducibility was restored in basal cells to either TPA or 1.2 mM Ca2+ as inducers. These results indicate that heterogeneity exits within the epidermal cell population and that exposure to phorbol esters induces differentiation in some cells, while stimulating proliferation in others. Such heterogeneous responses would cause a selective redistribution of the epidermal cell population and could lead to clonal expansion of initiated cells.