Rapamycin potentiates the effects of paclitaxel in endometrial cancer cells through inhibition of cell proliferation and induction of apoptosis.

Mammalian target of rapamycin (mTOR) inhibitors modulate signaling pathways involved in cell cycle progression, and recent phase II trials demonstrate activity in patients with endometrial cancer. Our objective was to examine the effects of combination therapy with rapamycin and paclitaxel in endometrial cancer cell lines. Paclitaxel inhibited proliferation in a dose-dependent manner in both cell lines with IC(50) values of 0.1-0.5 nM and 1-5 nM for Ishikawa and ECC-1 cells, respectively. To assess synergy of paclitaxel and rapamycin, the combination index (CI) was calculated by the method of Chou and Talalay. Simultaneous exposure of cells to various doses of paclitaxel in combination with rapamycin (1 nM) resulted in a significant synergistic anti-proliferative effect (CI <1, range 0.131-0.920). Rapamycin alone did not induce apoptosis, but combined treatment with paclitaxel increased apoptosis over that of paclitaxel alone. Treatment with rapamycin and paclitaxel resulted in decreased phosphorylation of S6 and 4E-BP1, two critical downstream targets of the mTOR pathway. Rapamycin decreased hTERT mRNA expression by real-time RT-PCR while paclitaxel alone had no effect on telomerase activity. Paclitaxel increased polymerization and acetylation of tubulin, and rapamycin appeared to enhance this effect. Thus, in conclusion, we demonstrate that rapamycin potentiates the effects of paclitaxel in endometrial cancer cells through inhibition of cell proliferation, induction of apoptosis and potentially increased polymerization and acetylation of tubulin. This suggests that the combination of rapamycin and paclitaxel may be a promising effective targeted therapy for endometrial cancer.