Effect of tumor promoter 12-O-tetradecanoylphorbol-13-acetate on recovery of methotrexate-, N-(phosphonacetyl)-L-aspartate-, and cadmium-resistant colony-forming mouse and hamster cells.

Abstract

The effect of the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and its nontumor promoting derivative 4-O-methyl-12-O-tetradecanoylphorbol-13-acetate on the frequency of mouse and hamster cells resistant to methotrexate (MTX), N-(phosphonacetyl)-L-aspartate, and cadmium has been examined. TPA alone at concentrations up to 1.0 microgram/ml had no significant effect on the plating efficiency of either mouse or hamster cells. Exposure of 3T3 and 3T6 mouse and V79 and Chinese hamster ovary cells at low density to the 3 compounds in the presence of TPA (0.1 microgram/ml) did not result in any increase in the recovery of resistant colonies. When plated at high density, exposure to drug selection in the presence of TPA resulted in a 3- to 10-fold increase overall in the incidence of MTX-, N-(phosphonacetyl)-L-aspartate-, and cadmium-resistant mouse cells. However, an increase greater than 3-fold was not observed in hamster cells exposed to drug plus TPA under the same conditions. 4-O-Methyl-12-O-tetradecanoylphorbol-13-acetate had no significant effect on the frequency of MTX-resistant cells. Seventy V79 cell clones surviving MTX (200 to 400 nM) alone and 79 surviving MTX plus TPA were isolated and retested for resistance to MTX. None were stable. In contrast, 6 out of 42 mouse colonies isolated from MTX alone and 55 out of 99 isolated from MTX plus TPA showed stable resistance on retesting in MTX. The implications of these results in relation to possible mechanisms of tumor promotion are discussed.