Podia in Multiple Myeloma (MM) Cells Promote Adhesion with Bone Marrow (BM) Fibroblastic Stromal Cells

Abstract 626

The physical contact between MM cells and the BM microenvironment can lead to cell adhesion mediated drug resistance. Although some components of adhesive structures in MM cells have been identified very little is known about their exact organisation and dynamics. We have observed formation of proteoglycan CD138 and F-actin containing membrane extensions in MM cells in in vitro cultures of BM aspirates as well as in sections of BM trephines of MM patients. MM membrane extensions elongated on the surface of BM stromal cells or interconnected MM cells forming seemingly cellular networks. Morphologically, MM membrane extensions appear similar to other structures in haematopoietic cells such as nanotubes formed by T-cells involved in cell communication and podia in CD34+ cells and leukemia cell lines whose function remains unknown. Dexamethasone (a drug commonly used clinically against MM) increased the percentage of MM cells displaying membrane extensions and the length of these structures. This correlated with enhanced expression of surface CXCR4, increased adhesion of MM cells on fibroblastic stromal cells and protection of MM cells against Dexamethasone-mediated apoptosis. Treatment with Bortezomib at clinically achievable doses decreased CXCR4 levels in MM cells and did not induce podia formation/extension and correlated with inhibition of MM cell adhesion and induction of MM cell apoptosis in a dose dependent manner. Blocking CXCR4 signalling with Plerixafor (AMD3100) inhibited the rate of formation and length of MM membrane extensions and correlated with sensitisation of MM cells to Dexamethasone. Similar inhibitory results were obtained using Dexamethasone in combination with the c-Abl/Src kinase inhibitor Dasatinib. Neither Dasatinib nor Plerixafor at doses achievable in patients directly induced apoptosis in MM or BM stromal cells. We conclude that membrane extensions/podia formed by MM cells are involved in the interaction with BM stromal cells, require CXCR4 and Src and/or c-Abl activity and could be involved in resistance to treatment with Dexamethasone.