DIVERGENT RESPONSES IN EPIDERMAL BASAL CELLS EXPOSED TO THE TUMOR PROMOTER 12-O-TETRADECANOYLPHORBOL-13-ACETATE

Abstract

Mouse epidermal basal cells can be selectively cultivated in medium with 0.02-0.09 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 h of exposure. This activity increase paralleled morphological differentiation in .apprx. 50% of the basal cell population, and differentiating cells sloughed from the culture dish within 24-48 h 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 2nd 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 2nd 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 4-10 days, transglutaminase inducibility was restored in basal cells to either TPA or 1.2 mM Ca2+ as inducers. Heterogeneity apparently exists within the epidermal cell population and 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.