Inhibition of mTOR complex 2 restrains tumor angiogenesis in multiple myeloma.

Aurelia Lamanuzzi,Domenico Ribatti,Vanessa Desantis,Marcella Prete,Angelo Vacca,Vito Racanelli,Beatrice Nico,Antonio Giovanni Solimando,Roberto Ria,Francesco Dammacco,Patrizia Leone,Maria Antonia Frassanito,Ilaria Saltarella,Paolo Ditonno

doi:10.18632/oncotarget.25003

Abstract

The mammalian Target of Rapamycin (mTOR) is an intracellular serine/threonine kinase that mediates intracellular metabolism, cell survival and actin rearrangement. mTOR is made of two independent complexes, mTORC1 and mTORC2, activated by the scaffold proteins RAPTOR and RICTOR, respectively. The activation of mTORC1 triggers protein synthesis and autophagy inhibition, while mTORC2 activation promotes progression, survival, actin reorganization, and drug resistance through AKT hyper-phosphorylation on Ser473. Due to the mTOR pivotal role in the survival of tumor cells, we evaluated its activation in endothelial cells (ECs) from 20 patients with monoclonal gammopathy of undetermined significance (MGUS) and 47 patients with multiple myeloma (MM), and its involvement in angiogenesis. MM-ECs showed a significantly higher expression of mTOR and RICTOR than MGUS-ECs. These data were supported by the higher activation of mTORC2 downstream effectors, suggesting a major role of mTORC2 in the angiogenic switch to MM. Specific inhibition of mTOR activity through siRNA targeting RICTOR and dual mTOR inhibitor PP242 reduced the MM-ECs angiogenic functions, including cell migration, chemotaxis, adhesion, invasion, in vitro angiogenesis on Matrigel®, and cytoskeleton reorganization. In addition, PP242 treatment showed anti-angiogenic effects in vivo in the Chick Chorioallantoic Membrane (CAM) and Matrigel® plug assays. PP242 exhibited a synergistic effect with lenalidomide and bortezomib, suggesting that mTOR inhibition can enhance the anti-angiogenic effect of these drugs. Data to be shown indicate that mTORC2 is involved in MM angiogenesis, and suggest that the dual mTOR inhibitor PP242 may be useful for the anti-angiogenic management of MM patients.

Highlights

Multiple myeloma (MM) is a hematologic malignancy characterized by the monoclonal expansion and accumulation of immunoglobulin-secreting plasma cells (PCs) within the bone marrow (BM) [1]
To establish whether mammalian Target of Rapamycin (mTOR) is more activated in MM-endothelial cells (ECs) than in monoclonal gammopathy of undetermined significance (MGUS)-ECs, we examined mTOR mRNA and protein levels in both cells
When the expression of RAPTOR and RICTOR was assessed, a significant increase of RICTOR was detected in MM-ECs (+120%), whereas RAPTOR resulted more expressed in MGUS-ECs (+80%)

Summary

Introduction

Multiple myeloma (MM) is a hematologic malignancy characterized by the monoclonal expansion and accumulation of immunoglobulin-secreting plasma cells (PCs) within the bone marrow (BM) [1]. In the BM milieu, a correlation is established between microvessel area and the rate of proliferating PCs: angiogenesis is enhanced in patients with active MM compared to those with non-active MM or with monoclonal gammopathy of undetermined significance (MGUS) [3], and MM PCs themselves trigger angiogenesis that in its turn sustains disease progression. As emphasized by Hose et al [4], at variance from memory B cells, normal BMPCs express a surplus of proangiogenic over antiangiogenic genes, resulting in induction of a basal level of in vitro angiogenesis. In MM patients, proangiogenic genes are aberrantly expressed and, antiangiogenic genes are down-regulated by MMCs, explaining the presence of BM angiogenesis to a variable extent in all of them. The detection of new drugs, capable of simultaneously targeting tumor cells and angiogenesis, is eagerly awaited

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