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Multidrug resistance (MDR) is one of the prime reasons for the failure of cancer chemotherapy, which continues to be a great challenge to be solved. In this work, α-tocopherol succinate (α-TOS) and doxorubicin (DOX)-based self-delivery nanomedicine (designated as α-TD) is prepared to combat drug resistance for cancer synergistic chemotherapy. Carrier-free α-TD possesses a fairly high drug loading rate and improves the cellular uptake via the endocytosis pathway. More importantly, the apoptotic inducer α-TOS could elevate the reactive oxygen species (ROS) generation, disrupt mitochondrial function and reduce adenosine 5'-triphosphate (ATP) production, which facilitate the intracellular drug retention while decreasing its efflux. As a result, α-TD achieves a considerable synergistic chemotherapeutic effect against drug resistant cancer cells. Moreover, it also exhibits a preferable inhibitory effect on tumor growth with a low system toxicity in vivo. This synergistic drug self-delivery strategy would open a new window for developing carrier-free nanomedicine for overcoming drug resistance in cancer therapy.

The proteasome inhibitor, bortezomib, has been demonstrated to sensitize tumor cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis. Natural killer (NK) cells represent potent antitumor effector cells. They also express TRAIL. Therefore, we investigated whether bortezomib could sensitize tumor cells to NK cell-mediated killing, and have the same effect in human prostate cancer cell lines (LNCaP and DU145). We found that bortezomib strongly inhibits proliferation in both cell lines. Furthermore, compared with LNCaP cells, DU145 cells are more sensitive to bortezomib-induced apoptosis. However, bortezomib is unable to sensitize these two cell lines to NK cell-mediated killing in short-term assays. In long-term assays, we found that killing mediated by activated NK cells following bortezomib treatment leads to greater antitumor effects than either treatment alone. In addition, treatment with bortezomib causes these cells to upregulate apoptosis-related mRNA as well as death receptors and downregulate the major histocompatibility class (MHC)-I molecule on the cell surface of DU145 cells. In contrast, LNCaP cells are not sensitized by this treatment. Death receptors and the MHC-I molecule did not change in this cell line. These data suggest that bortezomib can be used to sensitize prostate cancer cells to NK cell-mediated killing and improve current cancer therapies. This therapeutic strategy may be more effective in patients with androgen-insensitive prostate cancer.