Abstract
Melphalan is a cytotoxic chemotherapy used to treat patients with multiple myeloma (MM). Bone resorption by osteoclasts, by remodeling the bone surface, can reactivate dormant MM cells held in the endosteal niche to promote tumor development. Dormant MM cells can be reactivated after melphalan treatment; however, it is unclear whether melphalan treatment increases osteoclast formation to modify the endosteal niche.Melphalan treatment of mice for 14 days decreased bone volume and the endosteal bone surface, and this was associated with increases in osteoclast numbers. Bone marrow cells (BMC) from melphalan-treated mice formed more osteoclasts than BMCs from vehicle-treated mice, suggesting that osteoclast progenitors were increased. Melphalan also increased osteoclast formation in BMCs and RAW264.7 cells in vitro, which was prevented with the cell stress response (CSR) inhibitor KNK437. Melphalan also increased expression of the osteoclast regulator the microphthalmia-associated transcription factor (MITF), but not nuclear factor of activated T cells 1 (NFATc1). Melphalan increased expression of MITF-dependent cell fusion factors, dendritic cell-specific transmembrane protein (Dc-stamp) and osteoclast-stimulatory transmembrane protein (Oc-stamp) and increased cell fusion. Expression of osteoclast stimulator receptor activator of NFκB ligand (RANKL) was unaffected by melphalan treatment.These data suggest that melphalan stimulates osteoclast formation by increasing osteoclast progenitor recruitment and differentiation in a CSR-dependent manner. Melphalan-induced osteoclast formation is associated with bone loss and reduced endosteal bone surface. As well as affecting bone structure this may contribute to dormant tumor cell activation, which has implications for how melphalan is used to treat patients with MM.
Highlights
The alkylating drug melphalan in combination with corticosteroids has been the first-line therapy for patients with multiple myeloma (MM), an incurable plasma cell cancer [1], for many decades
We found that melphalan treatment causes rapid bone loss in mice and strongly enhances osteoclast formation through several mechanisms
In this study we have found strong evidence that melphalan therapy modifies the bone microenvironment by stimulating the formation of osteoclasts
Summary
The alkylating drug melphalan in combination with corticosteroids has been the first-line therapy for patients with multiple myeloma (MM), an incurable plasma cell cancer [1], for many decades. Whilst novel targeted agents such as thalidomide, bortezomib and lenalidomide have been introduced, melphalan remains a commonly used therapy for non-transplant patients and is used in high-dose therapy associated with autologous stem cell transplant [1]. These treatments improve survival, patients typically undergo relapse and the disease progresses. Recent studies have shown that in MM cells of the osteoblast lineage play an important role in the endosteal niche and have the capacity to hold MM in a dormant state for a prolonged period [5, 6]. Osteoclasts play a role in both controlling the re-activation of dormant MM cells and in mediating the development of osteolytic MM bone disease
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