The Role of PI3K Signaling in Cell Trafficking of Multiple Myeloma
Authors: Azab Feda , Azab Abdel Kareem , Maiso Patricia , Quang Phong , Ngo Hai T , Sacco Antonio , Liu Yang , Zhang Yong , Reagan Michaela , Roccaro Aldo M , Ghobrial Irene M. 
Published: October 2019

Abstract
Abstract 1804

Introduction:
Phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway is a fundamental pathway for the regulation of cell proliferation, survival, adhesion, migration, and metabolism in a variety of physiological and pathological processes. The pathway and its downstream effectors are frequently activated in patients with acute leukemias, chronic leukemias, various types of lymphomas, and multiple myeloma (MM). In this study we investigated the expression of the different PI3K catalytic subunits in MM and effect of three different PI3K inhibitors on the interaction of MM cells with the bone marrow (BM) microenvironment.

Methods and Results:
First we characterized the baseline expression of the different PI3K-p110-alpha, beta, gamma and delta catalytic subunits in MM cell lines (MM1s, OPM1, OPM2, H929, RPMI, INA6, U266, and U266LR7) by immnunobloting. PI3K-p110-alpha was highly expressed in MM1s and RPMI8226; PI3K-p110-beta was highly expressed in all cell lines; PI3K-p110-gamma was highly expressed in OPM1 and OPM2; and PI3K-p110-delta was highly expressed in MM1s and INA6. Furthermore, we investigated BKM120, a novel pan-PI3K inhibitor (Novartis, Swizerland). MM cells (MM1s, H929, OPM1, and OPM2) were treated with increasing concentrations of BKM120 (0, 100, 250, 500 and 1000 nM) for 4 hrs, labeled with Calcein-AM, and applied to fibronectin adhesion plate, or to 96-well plate pre-coated with stroma. Cells were incubated for 1 hr at 37C, non-adherent cells were washed and MM adhesion was measured by fluorescence-reader. BKM120 reduced the adhesion of all MM cell lines to fibronectin and stromal cells in a dose-dependent manner. Mechanistically, BKM120 decreased the activation of adhesion-related signaling in MM cells induced by co-culture with stroma including pFAK, pSRC pCoffilin and pMyosin light chain, as detected by immunoblotting. Moreover, BKM120 inhibited the downstream signaling of PI3K: p-Akt, p-P70S6, and p-S6R and regulated the survival of MM cells with or without co-culture with Bone Marrow stroma (IC50- 1uM 48hrs) and caused cell cycle arrest, as detected by PI staining and analyzed by flow cytometry, and decreased the expression of cyclin D1, p-Rb and increased the expression of P27 and P21, as detected by immunoblotting. Furthermore, we compared the activity of BKM120 to other PI3K inhibitors NVP-BEZ-235, a dual PI3K/mTOR inhibitor (Selleck, Houston, TX); and CAL101, a potent PI3K-p110-delta inhibitor (Selleck, Houston, TX); we examined the effect of different PI3K inhibitors (BKM120 500nM, CAL101 500nM or NVP-BEZ-235 100nM) on adhesion of MM1s cells to fibronectin, and found that the three inhibitors decreased the adhesion. Similarly, down-regulation of the expression of the PI3K catalytic subunits in MM1s, using siRNA decreased the adhesion of MM cells to fibronectin. In addition, we tested the effect of inhibition of PI3K on chemotaxis of MM cells. MM1s cells treated with BKM120 500nM, CAL101 500nM or NVP-BEZ-235 100nM for 4hrs, or with knockdown of PI3K by siRNA were applied to the upper chamber of a Boyden-migration assay, and allowed to migrate towards media with or without SDF1 30nM or conditioned media from MM stroma in the lower chamber for 4hrs. Interestingly, BKM120, NVP-BEZ-235 and the PI3K knock down increased significantly the chemotaxis of MM cells towards SDF1 and BM stromal, while CAL-101 had no effect on chemotaxis. These results were in accord with the effect of the drugs on the surface expression of CXCR4; as both BKM120 and NVP-BEZ-235, but not CAL101, increased the expression of CXCR4 in MM cells.

Conclusion:
we characterized the basal expression of the different PI3K catalytic subunits in MM cells lines, and showed that BKM120 inhibited PI3K signaling including proliferation and cell cycle in MM cells. BKM120 inhibited MM adhesion; an effect which was observed in NVP-BEZ-235 and CAL101, while only BKM120 and NVP-BEZ-235 increased the chemotaxis and CXCR4 surface expression of MM cells. These findings suggest that BKM120 can be used for regulation of MM cell trafficking in vivo by disrupting adhesion of MM cells to the BM and inducting of mobilization, leading to increased sensitivity to therapeutic agents.

Disclosures:
Roccaro: Roche:. Ghobrial:Novartis: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Research Funding; Noxxon: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding.