CD44-downregulation in multiple myeloma inhibits cytoskeleton rearrangement through actin depolymerization

It is well known that multiple myeloma (MM) cells are highly dependent on the bone marrow (BM) microenvironment. However, the complex interactions and signaling pathways between MM cells and BM stromal cells remain inadequately defined. In this study, we employed an in vitro coculture model to investigate these interactions. We found that coculturing MM cells with BM-derived HS5 stromal cells stimulated the secretion of hyaluronic acid (HA) and interleukin-6 (IL-6), and significantly increased the expression of CD44 and F-actin stress fibers polymerization in MM cells. Among the three hyaluronan synthase (HAS) isoforms, HAS3 mRNA expression was most significantly elevated in MM cells following coculture with HS5. This coculture also resulted in upregulation of HAS3 and IL-6 mRNA in MM cells. Notably, MM cells in direct contact with HS5 cells exhibited higher proliferative capacity compared to those not in contact with the stromal cells. Additionally, coculturing MM cells with HS5 led to the formation of membrane protrusions in MM cells, with CD44 enrichment observed at these polarized regions. Further analysis revealed that Rac1 co-localizes with CD44 on MM cells within the coculture system, suggesting that Rac1 signaling plays a critical role in CD44-mediated cytoskeletal rearrangements. Importantly, silencing CD44 expression in MM cells reduced F-actin polymerization, as well as impaired MM cell migration and adhesion to HS5. Our findings highlight the involvement of the HA/CD44/F-actin pathway in MM-BM migration and adhesion, suggesting that CD44 may serve as a novel therapeutic target to disrupt the MM-BM microenvironment.