It has been shown that cycloheximide (CHM), a well-known protein synthesis inhibitor, reduces Cytotoxicity induced by 1-B-D-arabinofuranosylcytosine, nitrogen mustard, or X-irradiation in normal cells, such as small intestine or bone marrow cells. However, the protection by CHM against the Cytotoxicity induced by 1-B-D-arabinofuranosylcytosine was not shown in malignant tumor cells, such as human malignant myeloid progenitors from patients with chronic myelogenous leukemia or human acute nonlymphocytic leukemia cell lines HL-60 and KG-1. Therefore, it was suggested that CHM may be useful in reducing side effects of such an S-phase-specific antitumor drug. This differential protection against normal and malignant cells has been explained in the following way. In normal cells, synthesis of a labile protein is necessary for DNA synthesis to proceed. Inhibition of protein synthesis by CHM causes arrest, prevents DNA synthesis, and therefore diminishes the toxicity of 1-B-D-arabinofuranosylcytosine. In transformed cells, DNA synthesis seems to be independent of such protein synthesis. Consequently CHM can exert differential protection against normal and malignant cells from S-phase specific cytotoxic drugs.
To examine this hypothesis, the protective effect of CHM using the human malignant tumor cell line was studied, KSu osteosarcoma. Furthermore, in order to know whether this protection by CHM is more general or not, the protective effect of CHM against the Cytotoxicity induced by microtubule inhibitors, vincristine and colchicine, or other antitumor drugs, such as doxorubicin, mitomycin C, or cisplatin was examined. In addition, whether such protection is led by the inhibition of protein synthesis or cell cycle arrest by CHM was examined as well.
Effects of Various Doses of Cycloheximide on Vincristine induced Cytotoxicity
KSu cells (2 x 10s cells/2 ml) were inoculated on Day 0. Three h after the inoculation of cells, various doses of CHM and/or 0.5 pg/m\ of vincristine were added. As shown in Fig. 1, although CHM at concentrations of 0.05 jtg/ ml or more inhibited the growth rate of KSu cells in a dosedependent manner on Day 4, it did not decrease the initial cell number (2 x 10s cells/dish) at concentrations of less than 0.5 fig/ml. Vincristine at the concentration of 0.5 fig/ml reduced the cell number to 34% of the initial cell number. In the presence of CHM at concentrations of 0.05 ug/ml or more, the cytotoxicity of 0.5 ug/ml of vincristine was significantly reduced, and cell number on Day 4 recovered to 2-fold at the maximum from the value without CHM (34% to 68% of the initial cell number) (Fig. 1). As shown in Fig. 2A, CHM at the concentration of 0.1 ug/ml protected cells against cytotoxicity induced by 0.5 ug/ml of vincristine from Day 2 to Day 4. Such protective effect was also observed even when drugs were added at the growth phase (Fig. 2B).
Effect of Cycloheximide on Cytotoxicity Induced by Various Antitumor Drugs
Next we examined whether CHM protects cells against cytotoxicity of other antitumor agents. First, we examined the effect of CHM on an antitumor PG, AI2-PGJ2. As shown in Fig. 3A, AI2-PGJ2 at concentrations from 1 ug/ml to 5 ug/ml reduced cell number to 53 to 3% of the control during 4 days, whereas in the presence of 0.1 ug/ml of CHM, cell number was 64 to 14% of the control. As shown in Fig. 3B, 0.1 ug/ml of doxorubicin reduced cell number to 26% of the control during 4 days, whereas in the presence of 0.1 ug/ml of CHM, cell number was 38% of the control (P < 0.01). Further more, as shown in Fig. 3C, 1 ug/ml of mitomycin C reduced cell number to 23% of the control, whereas in the presence of 0.1 ug/ml of CHM, cell number was 33% of the control (P < 0.05). However, 0.1 Mg/ml of CHM did not significantly reduce cytotoxicity induced by cisplatin at concentrations from 0.1 ug/ml to 2 Mg/ml (Fig. 3D). Furthermore, we observed the similar protective effect of 0.1 ug/ml of CHM using a colony formation test. CHM at a concentration of 0.1 ug/ml reduced a colony formation ability loss by various doses of vincristine, colchicine, AI2-PGJ2, doxorubicin, and mitomycin C, while it did not reduce a colony formation ability loss by cisplatin. Thus, the protection by CHM is neither specific to normal cells nor specific against cytotoxicity of l-B-D-rabinofuranosylcytosine or nitrogen mustard, whose main action site is in DNA synthesis.
Sources: Toshiyuki Sakai, Akira Aoike, Nobuyuki Marui, et al.