DEPTOR Is a Regulator of Response to mTOR Kinase Inhibitors in Multiple Myeloma

Abstract 2916

Inhibition of the PI3K/mTOR pathway is a promising therapeutic strategy in targeting multiple myeloma (MM) cells in the bone marrow (BM) microenvironment, which abnormally activates PI3K/mTOR signaling cascade mediating proliferation, anti-apoptosis and drug resistance. Exploring the targeting of PI3K/mTOR pathway has led to the development of different therapeutic approaches; however, mTORC1 inhibitors (i.e., temsirolimus and everolimus) have demonstrated only modest activity as single agents. In this regard, several mechanisms underlying rapamycin resistance, including mTOR/S6K1-mediated feedback loops resulting in activation of PI3K/Akt and ERK signaling, have been proposed. Importantly, recent studies have identified mTOR kinase and the mTOR-DEPTOR counter-regulatory cascade as key mediators of mTORC1 and mTORC2 multi-protein complexes, with differential sensitivity to rapamycin. Indeed, targeting DEPTOR/mTORC1/mTORC2 signaling by inhibition of mTOR kinase proved an effective strategy to overcome some of the limitations of TORC1 inhibition in MM cells, evidenced in our studies of the novel dual mTORC1 and mTORC2 selective inhibitor AZD8055. Unlike rapamycin, AZD8055 induced apoptosis and inhibited MM cell growth even when co-cultured with cytokines (i.e., IL-6, IGF1) or BMSCs, presumably through simultaneous suppression of mTORC1 and mTORC2 signaling including the rapamycin-resistant 4E-BP1 (downstream of mTORC1) and Akt as well as NDRG1 (effectors of mTORC2). We examined mRNA and protein level of DEPTOR in MM cell lines treated with AZD8055 versus rapamycin and observed no significant changes. To examine the functional significance of DEPTOR in response to mTOR inhibitors, we utilized lentiviral shRNA to knockdown DEPTOR in OPM1 MM cells. DEPTOR-knockdown cells acquired resistance to AZD8055 treatment, suggesting that DEPTOR is a key modulator of mTORC1/2 signaling. Moreover, DEPTOR knockdown triggered decrease in Akt phosphorylation (Ser473), associated with suppression of Rictor phosphorylation (Thr1135). DEPTOR co-immunoprecipitation with Rictor was also abrogated by both AZD8055 and rapamycin treatment. Taken together, our results indicate the role of DEPTOR, either alone or as an mTOR/Rictor interacting molecule, in mediating the anti-MM activity induced by mTOR kinase inhibitors in MM cells. These data therefore both provide insights into the molecular profiles that may predict sensitivity/resistance to second generation of mTOR inhibitors in MM, and may be useful to select MM patients for mTOR inhibitor therapy.