SDF‐1α stiffens myeloma bone marrow mesenchymal stromal cells through the activation of RhoA‐ROCK‐Myosin II

Multiple myeloma (MM) is a malignant disorder of postgerminal center B-cells.1 MM is generally characterized by the clonal expansion of neoplastic plasma cells in the bone marrow (BM), presence of monoclonal proteins in blood and/or urine and organ dysfunction such as kidney failure, bone density loss and subsequent bone fractures, spinal compressions with severe pain, etc.1,2 Despite recent advances in cancer treatments, MM remains an incurable disease owing to its proclivity for recurrence, which is believed to be caused by minimal residual disease or existence of a myeloma cancer stem cell (mCSC) niche in the BM.1,3 BM constitutes a suitable niche for mCSCs, favoring their self-renewal, differentiation and development of drug resistance through direct and indirect communications with various cell types present in the BM microenvironment.4–7 Among them, BM mesenchymal stromal cells (BMSCs) have been extensively studied in the context of MM disease progression and resistance to chemotherapeutics. Several studies had shown that BMSCs communicate with mCSCs through direct cell–cell interactions and paracrine signaling.4,8–13 In addition, as with other cancer-associated stromal cells in a number of cancer types, genetic and biochemical abnormalities in BMSCs have also been reported.5–7,9,12,14,15 However, distinctions between MM BMSCs (mBMSCs) and normal BMSCs (nBMSCs) are not well defined with respect to biomechanics. To partially address this question, our previous study reported marked differences in cell stiffness observed between mBMSCs and nBMSCs, although the underlying mechanism leading to these differences and how they influenced mBMSCs to promote mCSC pathophysiologic functions remained largely unknown.15 Stromal cell-derived factor-1 (SDF-1α) or C-X-C motif chemokine 12 (CXCL12) is a well-studied member of the chemokine family that specifically activates C-X-C motif chemokine receptor 4 (CXCR4).16 One of the major functions of SDF-1α is to promote chemotaxis of cancer cells.16 Particularly, SDF-1α is known to promote homing of MM cells to the BM,17,18 and to facilitate cell–cell interactions between mCSCs and BMSCs, leading to enhanced mCSC survival and proliferation.19,20 It is, however, still unknown how SDF-1α affects the biophysical properties of BMSCs and regulates their interaction with mCSCs. Thus, the aims of our study were three-pronged: (i) to investigate if the differences in cell stiffness (defined in terms of cells’ Young’s modulus) are constitutive, (ii) how the cell stiffness contributes to the cancer microenvironment and (iii) how SDF-1α affects the mechanical properties of BMSCs. Herein, we present the first experimental evidence that SDF-1α can increase Young’s modulus in BMSCs by activating the G(i./o)-PI3K-RhoA-ROCK-Myosin II signaling pathway. Moreover, mBMSCs express a constitutively elevated, as compared to nBMSCs, level of activated myosin (MYL2). Finally, our data indicate that the activated myosin influences the contractile potential of the cells, which regulates cell–matrix and cell–cell interactions between mCSCs and mBMSCs.