Concurrent inhibition of PI3K and mTORC1/mTORC2 overcomes resistance to rapamycin induced apoptosis by down-regulation of Mcl-1 in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive form of Non-Hodgkin-Lymphoma (NHL) with an ongoing need for novel treatments. Apart from the translocation t(11;14), which facilitates constitutive transcription of cyclin D1, additional aberrations are frequently observed in MCL, including a recurrent dysregulation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway. mTOR, a key component of this pathway, is pivotal for the assembly of mTOR complex (mTORC) 1 and 2. Temsirolimus an analog of the mTOR inhibitor rapamycin is approved for the treatment of relapsed MCL. Response rates, however, are low and response durations are short. We demonstrate that inhibition of mTORC1 by rapamycin or blocking of mTORC1 and mTORC2 in conjunction with PI3K by NVP-BEZ235 reduces proliferation of MCL cell lines to a similar extent. However, only NVP-BEZ235 is able to sufficiently inhibit the downstream pathway of mTOR and to mediate cell death through activation of the intrinsic apoptosis pathway. Further analysis demonstrated that the anti-apoptotic Bcl-2 family member Mcl-1 plays a central role in regulation of MCL survival. While Mcl-1 protein levels remained unchanged after co-culture with rapamycin, they were down-regulated in NVP-BEZ235 treated cells. Furthermore, inhibition of Mcl-1 by the BH3-only mimetic obatoclax or down-regulation of constitutive Mcl-1, but not of Bcl-2 or Bcl-xL , by siRNA facilitated cell death of MCL cells and enhanced rapamycin's as well as NVP-BEZ235's capacity to induce cell death. Our findings may help to lay the foundation for further improvements in the treatment of MCL