Direct contact of multiple myeloma (MM) cells with the bone marrow (BM) stroma promotes cell survival leading to cell adhesion mediated drug resistance (CAM-DR). Dexamethasone is a conventional anti-MM drug that effectively induces MM cell death at presentation and in relapsed patients but the signalling pathways involved in its mechanisms of action are not completely understood. Resistance of MM to Dexamethasone may result from genetic changes in MM cells or through the contact of MM cells with the BM microenvironment. It has been shown that CAM-DR blocks the effect of Dexamethasone by the binding of MM cells to the BM stroma. Our data indicate a key role of the binding of MM CXCR4 receptor on stromal cell derived SDF1-a in CAM-DR against Dexamethasone. Dexamethasone induced upregulation of CXCR4 in the MM1.S cell line (sensitive to Dexamethasone) whilst, as expected, having no effect on the MM1.R cell line (resistant to Dexamethasone by expression of a mutated form of the glucocorticoid receptor). Bortezomib induced down regulation of CXCR4 in both MM1.S and MM1.R cell lines in a dose dependent manner that correlated with decreased adhesion on BM stromal cells and increased sensitisation of MM cells in the presence of the BM stroma. The Wiskott Aldrich Syndrome Protein (WASP) is an adaptor protein that regulates actin polymerization and organization of cell adhesion molecules of the integrin family in haematopoietic cells. WASP is involved in the signalling pathway downstream of CXCR4 in various leukocytes and we hypothesised that blocking this pathway would prevent MM cell adhesion on stromal cells and sensitise them to treatment with Dexamethasone. We found that downregulation of WASP in the Dexamethasone-sensitive cell line MM1.S using shRNA reduced the adhesion to the BM stroma. However, it also rendered MM1.S cells resistant to treatment with Dexamethasone but not with Bortezomib, Doxorubicin or Melphalan independently of the presence of BM stromal cells. Similarly, downregulation of WASP in MM1.R cells did not affect their sensitivity to Bortezomib, Doxorubicin or Melphalan. Dexamethasone has been shown to induce changes in actin dynamics or in levels or activity of actin and/or adhesion related proteins. Western blot analysis showed both Dexamethasone and Bortezomib to cause modulation of WASP expression, inducing partial downregulation and upregulation, respectively. Downregulation of WASP expression in MM1.S cells using shRNA or treatment with Dexamethasone or Bortezomib did not affect the expression of other proteins involved in WASP-mediated F-actin remodelling or cell adhesion such as WIP, Arp 2/3 or vinculin. We conclude that WASP is involved in the signalling pathways that promote cell-adhesion of MM cells on BM stroma but it is also a vital component of the signalling pathway of Dexamethasone's mechanism of action. These data indicate that while blocking WASP signalling could theoretically have potential therapeutic benefits to prevent CAM-DR to Dexamethasone, it would inhibit the action of Dexamethasone rendering cells resistant to treatment with this drug. Our study suggest a possible paradox of a dual pro-apoptotic and pro-survival effect of certain therapies targeting pathways involved in the interaction of MM cells with the microenvironment. Detailed studies of the intricate connexion between the intracellular pathways involved in the process of adhesion and drug induced cell death through modulation of actin dynamics may lead to improved therapeutic strategies to overcome drug resistance against Dexamethasone and perhaps other drugs.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.