Induction of apoptosis in MDA-231 cells by protein synthesis inhibitors is suppressed by multiple agents

In the present study we investigated the ability of several diverse agents to inhibit MDA-231 cell death induced by two different protein synthesis inhibitors, cycloheximide (CHX) and ricin. Cell death was evaluated by several techniques: trypan blue staining, determination of the released lactic dehydrogenase, transmission electron microscopy, and DNA fragmentation. Results from DNA gel electrophoresis and electron microscopy suggest a mechanism of death by apoptosis which terminates in necrosis. Approximately 60% of cell death was induced either by a continuous exposure to 30 μg/ml CHX for 48 hr or by a 1-hr exposure to 250 pg/ml ricin followed by a subsequent incubation of 48 hr in the absence of the drug. Cell survival, in the protein synthesis-inhibited cells, was enhanced by the following diverse agents: the growth factors EGF (20 ng/ml) and IGF-1 (20 ng/ml), the protein kinase C activator 12-O-tetradecanoyl-phorbol-13-acetate (5 ng/ml), the protein kinase A and activator 8-bromoadenosine 3':5'-cyclic monophosphate (650 μg/ml), the nuclease inhibitor aurintricarboxylic acid (100 μg/ml), and fetal bovine serum (5%). The survival agents that stimulated protein synthesis in the control untreated cells had no effect on the CHX-inhibited protein synthesis, which indicated that new protein synthesis is not required for cell survival. The same survival agents attenuated the continuous decrease in protein synthesis in the ricin-exposed cells; therefore, the involvement of new protein synthesis in the survival mechanism could not be excluded. The protein kinase C inhibitor staurosporine blocked, in a dose-dependent manner, the survival effect of 12-O-tetradecaonyl-phorbol-13-acetate and EGF, but not that of aurintricarboxylic acid or fetal bovine serum, in the protein synthesis-inhibited cells. These results provide evidence for several distinctive pathways, the activation of which inhibits MDA-231 cell death induced by protein synthesis inhibitors. Some of these pathways involved activation of protein kinases, probably protein kinase C.