Synergistic antitumoral effect of the immunomodulator AS101 + paclitaxel (Taxol): Association with ras-dependent signal transduction pathways

Optimal doses of Taxol combined with the immunomodulator AS101, previously shown to have antitumoral effects, administered to B16 melanoma-bearing mice, decreased tumor volume and resulted in over 60% cure. Taxol + AS101 directly inhibited clonogenicity of B16 melanoma cells in a synergistic dose-dependent manner. We suggest that this results from both the reduced Taxol-induced BM toxicity and the induction of differential signal transduction pathways which lead to apoptosis of tumor cells. Taxol + AS101 synergistically activated c-raf-1 and the MAPK ERK1 and ERK2. This activation was essential for the synergistic induction of p21 waf protein. Cell cycle analysis of B16 cells treated with both compounds revealed an increased accumulation of the cells in G2M, although AS101 alone produced a significant G1 arrest. These activities were ras-dependent, AS101 + Taxol induced a significant synergistic phosphorylation (inactivation) of the antiapoptotic protein Bcl-2. Whereas phosphorylation of Bcl-2 by Taxol was only raf-dependent, the synergistic effect of both compounds together was both ras-, raf- and MAPK-dependent. No effect of the combined treatment on the Bax protein expression was observed. We suggest that AS101 renders more cells susceptible to Bcl-2 phosphorylation by Taxol; possibly by increasing the accumulation of Taxol-induced cells in G2M: Exposure of B16 cells to clinically achievable concentration of Taxol + AS101 increased the rate of apoptosis of treated cells. Apoptosis induced by AS101 alone was both raf- and MAPK-dependent, while that induced by Taxol was only raf-dependent.