Gambogic acid (GA) has been approved by the Chinese Food and Drug Administration for the treatment of
lung cancer in clinical trials. However, whether GA has chemosensitizing properties when combined with other
chemotherapy agents in the treatment of lung cancer is not known. Here we investigated the effects of GA combined with
adriamycin (ADM), a common chemotherapy agent, in regard to their activities and the possible mechanisms against lung
cancer in vitro and in vivo. Cell viability results showed that sequential GA-ADM treatment was synergistic, while the
reverse sequence and simultaneous treatments were antagonistic or additive, in lung cancer cells and ADM resistant cells,
but not in normal cells. The combined use of GA and ADM synergistically displayed apoptosis-inducing activities in lung
cancer cells. Moreover, GA in combination with ADM could promote PARP cleavage, enhance caspases activation and
decrease the expression of anti-apoptotic proteins in lung cancer cells. The combined use of GA and ADM decreased the
expression of P-glycoprotein and increased the accumulation of ADM in lung cancer cells. Furthermore, it was found that,
prior to ADM treatment, GA could inhibit NF-κB signaling pathways, which have been validated to confer ADM
resistance. The critical role of NF-κB was further confirmed by using PDTC, a NF-κB inhibitor, which significantly
increased apoptosis induction by the combination of GA and ADM and inhibited ADM-induced ABCB1 upregulation.
Importantly, our results indicated that the combination of GA and ADM exerted enhanced anti-tumor effects on A549
xenograft models through inhibiting NF-κB and P-glycoprotein, and attenuated ADM-induced cardiotoxicity.
Collectively, these findings indicate that GA sensitizes lung cancer cells to ADM in vitro and in vivo, providing a
rationale for the combined use of GA and ADM in lung cancer chemotherapy.