mTOR kinase-targeted therapy is in the early phase of clinical evaluation in multiple myeloma (MM). Despite promising preclinical results with mTOR inhibitors, resistance to this class of drugs in MM patients may occur due to feedback Akt activation by mTORC1. This led to the development of mTORC1/2 inhibition strategies in the treatment for MM, predicated upon the rationale that mTORC2 inhibitors prevent inhibition of mTORC1 blockade-induced feedback AKT activation by mTORC1 inhibitors. Indeed, our previous studies using a novel dual mTORC1 and mTORC2 selective inhibitor AZD8055 show MM cell growth inhibition via apoptosis, associated with inhibition of mTORC1 and mTORC2 signaling, including rapamycin-resistant 4E-BP1 (downstream of mTORC1) and Akt as well as NDRG1 (an effector of mTORC2). Importantly, AZD8055 also inhibited PI3K/Akt signaling and related MM cell growth induced by cytokines (i.e., IL-6, IGF-1) or co-culture with bone marrow stromal cells (BMSCs). Recent studies, however, reveal that constitutively activated Akt signaling negatively regulates IGF-1 receptor (IGF-1R) at the transcriptional level, independent of mTOR activity. Moreover, AKT-induced IGF-1R down-regulation reduces sensitivity of IRS1 to IGF-1 stimulation. We have also shown that IGF-1R inhibitor triggers significant MM cell toxicity.
In this study, we therefore hypothesized that mTORC2 blockade may upregulate IGF-1R expression and/or activity via Akt modulation in MM cells, and that IGF-1R blockade may enhance the cytotoxic effects of mTOR kinase inhibition in MM cells. We first examined the tyrosine phosphorylation sites (Y1135/1136) in the activation loop of the IGF-1R kinase domain in three MM cell lines (MM.1S, OPM1 and RPMI8226) treated with AZD8055 or rapamycin. AZD8055 induced more pronounced upregulation of p-IGF-1R in MM.1S and OPM1 MM cells than rapamycin at earlier time periods. IGF-1 clearly upregulated Akt phosphorylation in MM cells; however, it had no effect on mTOR phosphorylation (Ser2481). Moreover, AZD8055-treated cells exposed to IGF-1 sustained p-Akt (Ser473) expression, while p-mTOR (Ser2481) remained fully inhibited. These results suggest that IGF-1/IGF-1R signaling may bypass mTORC2/Akt when mediating p-Akt (Ser473) upregulation. Moreover, reactivation of IGF-1 signaling in MM cells in the context of mTOR kinase inhibitors suggests that MM may survive in an IGF-1 R–dependent fashion. We therefore next treated MM.1S, OPM1 and RPMI8226 cells with AZD8055, in the presence or absence of IGF-1. MM.1S and OPM1 MM cells (with higher Akt baseline activity) partially escaped AZD8055 cytotoxicity, while RPMI8226 MM cells (with lower Akt activity) did not. Moreover, the addition of blocking IGF-1R antibody or of IGF-1R inhibitor enhanced AZD8055 cytotoxicity in MM.1S and OPM1 cells.
Our study therefore shows interaction of mTOR/Akt and IGF-1R/Akt pathways in MM tumors with IGF-1-enabled Akt activation. Importantly, they suggest that combination treatment with AZD8055 and IGF-1R inhibitor is a promising strategy to mTOR kinase inhibition in MM with potential IGF-1R/Akt signaling mediated survival.
Hideshima:Acetylon Pharmaceuticals, Inc.: Consultancy. Guichard:AstraZeneca: Employment, Shareholder Other. Anderson:Onyx: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees. Raje:Onyx: Consultancy; Celgene: Consultancy; Millenium: Consultancy; Acetylon: Research Funding; Amgen: Research Funding; Eli-Lilly: Research Funding.
Asterisk with author names denotes non-ASH members.