EPIDERMAL GROWTH-FACTOR AND INSULIN-LIKE GROWTH-FACTOR-I PROTECT MDA-231 CELLS FROM DEATH INDUCED BY ACTINOMYCIN-D - THE INVOLVEMENT OF GROWTH-FACTORS IN DRUG-RESISTANCE

In the present study, we investigated the ability of epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), and insulin to pro tect the human breast cancer cell line MDA-231 from death induced by t he antitumor drug actinomycin D (ACT-D). ACT-D is an inhibitor of RNA and protein synthesis, and its cytotoxicity may result due to continuo us depletion in some vital protein molecules. Cell death was induced i n the MDA-231 cells by either continuous exposure to a low dose of ACT -D (0.2 mu g/ml), or by a short-time exposure to a high dose of ACT-D (2 mu g/ml) and further culturing in the absence of the drug. Cell dea th was evaluated by the trypan blue dye exclusion test, the release of lactic dehydrogenase into the culture medium, and the depletion in th e cellular ATP content. EGF and IGF-1, each at an optimal concentratio n of 20 ng/ml, enhanced substantially survival of cells exposed either to a low or a high dose of ACT-D. The combination of EGF (10 ng/ml) a nd IGF-1 (10 ng/ml) had an additive survival effect, which proposes th at each of the growth factors enhanced survival by a distinct pathway. Insulin up to 40 ng/ml had no effect on cell survival. Pretreatment o f the cells for 1 to 5 h with EGF and IGF-1 protected cells from the c ytotoxic effect of ACT-D. Exposure of the cells to 2 mu g/ml of ACT-D for 1 h resulted in a drastic inhibition in uridine incorporation and only in a slight inhibition in leucine incorporation. Further incubati on in the absence of ACT-D resulted in a continuous decrease in uridin e and in leucine incorporation, either in the absence or presence of t he growth factors. However, EGF and IGF-1, but not insulin, attenuated significantly this continuous decrease. We assume that EGF and IGF-1 protect cell viability by a mechanism that maintains a critical level of some vital protein molecule above the critical level at which cells die. Our finding that EGF and IGF-1 induced resistance to ACT-D sugge sts that growth factors may be involved in the mechanism of drug resis tance.