Notch signaling promotes drug resistance in multiple myeloma through the regulation of the CXCR4/SDF1α axis

Multiple myeloma (MM) represents the 13% of the hematologic tumors and its social burden is increasing in western countries due to the progressive aging of the population. Nowadays, MM is still incurable due to the presentation as high-risk form or to the interactions of myeloma cells with the bone marrow (BM) milieu. Indeed, MM cells establish complex interactions with BM stromal cells (BMSCs), which provide a source of pro-survival and growth factors that promotes drug resistance, finally causing patients' relapse. In MM the Notch oncogenic pathway is dysregulated due to the overexpression of the two Notch ligands Jagged1 and Jagged2 by tumor cells. This causes Notch hyperactivation in both the neighboring MM cells and in the normal cells of the BM niche. To test how Jagged1/2 overexpression affects MM cells response to chemotherapeutics, we performed double-silencing of Jagged1/2 (J1/2KD) in myeloma cell lines (HMCLs) and assessed their response to standard-of-care drugs (i.e. bortezomib, melphalan and lenalidomide), both in single culture and in a co-culture system composed of HMCLs and the human BMSC line HS5. Our results showed that J1/2 KD promotes MM cells sensitivity to chemotherapeutics. Moreover, while HS5 are able to protect MM cells from drug-induced apoptosis, J1/2KD revert this effect, decreasing the ability of BMSCs to rescue MM cells. At molecular level, while MM cells are normally able to promote Notch activation in co-cultured BMSCs, this effect is lost after J1/2KD. These variations in Notch activation correlated with HS5 ability to produce one of the key chemokine in MM, SDF1α. This is particularly relevant since our data indicate that SDF1α boosts the expression of the anti-apoptotic genes Bcl2, ABCC1 and Survivin in MM cells, improving their resistance to therapies. Of note, J1/2KD in MM cells causes a decrease in the expression of SDF1α receptor, CXCR4, further potentiating the anti-tumor effect. The improvement in drug-response promoted by J1/2KD can be observed also in ex-vivo co-cultures of cells from MM patients, that further validate the key role of J1/2 in this process. Finally, an in vivo zebrafish xenograft model of MM allowed us to confirm that J1/2KD causes increase in MM cells response to bortezomib in vivo, decreasing tumor burden. These results provide the proof-of-concept for a Jagged1/2-tailored therapy in MM, that may allow to restore the response to standard drugs and to improve patient’s overall survival.