Rapamycin controls multiple signalling pathways involved in cancer cell survival

Suppression of apoptosis by survival signals is considered a hallmark of malignant transformation and resistance to anti-cancer therapy. The phosphoinositide-3 kinase (PI3k)/Akt pathway and NF-kB transcription factors are potent mediators of tumour cell survival. The carbocyclic lactone-lactam antibiotic rapamycin, a widely used immunosuppressant, inhibits the oncogenic transformation of human cells induced by PI3k or Akt by blocking the downstream mTOR kinase. However, inhibition of the PI3k/Akt/mTOR cascade may not be the only mechanism whereby rapamycin exerts anticancer effects. We previously demonstrated that rapamycin inhibits NF-kB by acting on FKBP51, a large immunophilin whose isomerase activity is essential for the functioning of the IKK kinase complex. This suggested that rapamycin may be effective also against neoplasias that express the tumour suppressor PTEN, which, by reducing cellular levels of phosphatidyl-inositol triphosphate, antagonizes the action of PI3k. To address this issue, we over-expressed PTEN in a human melanoma cell line characterized by high phospho-Akt and phospho-mTOR levels, and examined the effect of rapamycin on the apoptotic response to the NF-kB inducer doxorubicin versus cisplatin, which does not activate NF-kB. Rapamycin increased both cisplatin- and doxorubicin-induced apoptosis. Transient transfection of PTEN remarkably decreased phospho-mTOR levels and increased sensitivity to cisplatin’s cytotoxic effect. Under these conditions, rapamycin failed to enhance cisplatin-induced apoptosis. This finding supports the notion that inhibition of a survival pathway increases the efficacy of cytotoxic drugs, and suggests that the pro-apoptotic effect of the rapamycin-cisplatin association requires activated mTOR. Rapamycin retained the capacity to enhance doxorubicin-induced apoptosis in cells over-expressing PTEN, which confirms our earlier observation that inhibition of the PI3k/Akt/mTOR pathway is not involved in the effect exerted by the rapamycin-doxorubicin association. These findings indicate that constitutive activation of mTOR is sufficient but not necessary for rapamycin’s anti-cancer effect. Finally, we show that a decrease in FKBP51 expression levels, obtained with the small interfering RNA technique in the leukemic cell line Jurkat, increased doxorubicin-induced apoptosis, suggesting that this rapamycin ligand is involved in resistance to chemotherapy-induced apoptosis. In conclusion, rapamycin affects more than one signalling survival pathway and more than one target. Our data may impact on the synthesis of rapamycin derivatives. Thus far, rapamycin derivatives used in clinical trials have been tested for their mTOR-inhibiting effect. Our study opens the door to a novel class of anti-cancer drugs that specifically target immunophilins.