Rapamycin inhibits doxorubicin-induced NF-kappaB/Rel nuclear activity and enhances apoptosis in melanoma.

Inhibition of NF-kB/Rel sensitizes many tumor cells to death-inducing stimuli including chemotherapeutic agents and there are findings that disruption of NF-kB may be of therapeutic interest in melanoma. We found that rapamycin sensitized a human melanoma cell line, estabilished by a patient, to the cytolitic effect of doxorubicin. Doxorubicin is a striking NF-kB/Rel-inducer, thus we investigated if rapamycin interfered with the pathway of NF-kB/Rel activation, i.e. IkBa-phosphorylation, –degradation and NF-kB/Rel nuclear translocation, and found that the macrolide agent caused a block of IKK kinase activity responsible for reduced nuclear translocation of the transcription factors. The western blotting analysis of Bcl-2 and c-IAP1 showed increased levels of these anti-apoptotic proteins in cells incubated with doxorubicin, in accord to NF-kB/Rel activation, while rapamycin clearly downmodulated them, in line with the pro-apoptotic ability. The effect of the macrolide on NF-kB/Rel induction appeared to be independent of the block of the PI3k/Akt pathway, because it could not be reproduced by the PI3k inhibitor wortmannin. Very recently the immunophilin FKBP51 has been shown to be essential for the function of IKK kinase. We found high levels of expression of FKBP51 in melanoma cells, moreover we confirmed the involvement of this immunophilin in the control of IKK activity. Indeed apparently IkBa could not be degraded when FKBP51 was downmodulated by siRNAs. These findings provide a possible mechanism of NF-kB downmodulation by rapamycin, since the macrolide agent specifically inhibits FKBP51 isomerase activity. In conclusion our study demonstrated that rapamycin blocked NF-kB/Rel activation in a way independent of PI3k/Akt inhibition suggesting that the macrolide agent could sinergize with NF-kB-inducer anti cancer-drugs also in PTEN positive tumors.