Multiple myeloma bone disease (MMBD) is a hallmark feature of multiple myeloma (MM), the second most common hematological malignancy characterized by abnormal proliferation of monoclonal plasma cells (PCs) in the bone marrow (BM). MMBD strikes approximately 80% of MM patients and causes debilitating bone pain, pathologic fractures, vertebral collapse and hypercalcemia, inducing significant patients’ morbidity and mortality [1
The bone marrow microenvironment (BMM) is composed by a mineralized extracellular matrix and cellular components, including osteoclasts (OCs), osteoblasts (OBs), osteocytes (OCYs), immune cells, endothelial cells and stromal cells. Bone remodeling under pathological conditions is characterized by a strong inhibition of OBs activity, which leads to bone loss as OBs are unable to repair the lesions caused by the excessive osteoclastic resorption; the latter process is strongly supported by MM cells, which can exacerbate OCs activity promoting their maturation directly or by physically interacting with other cellular components, such as the BM stromal cells (BMSCs). In turn, cell–cell interactions and soluble factors or matrix-associated growth factors released from the resorbed bone increase MM cell proliferation and prompt tumor progression [2
To effectively trigger BD, cellular components of the BMM produce and/or secrete a number of functional molecules, which collectively contribute to the osteoclastogenic events. In this regard, non-coding RNAs (ncRNAs) have recently emerged as fine regulators of gene expression programs underlying key molecular events featuring bone remodeling in MM.
Herein, we will briefly discuss the signaling pathways implicated in the development of MMBD, and, will then, analyze how they are modulated by manipulation or release of ncRNAs from different BMM cells.
4. Extracellular Vesicle-Associated ncRNAs
It has been widely demonstrated that different ncRNAs species are contained in extracellular vesicles, which are lipoproteic structures heterogeneous in size and content, released by almost all cell types [134
]. In recent years, EVs have gained attention because of the identification of biological molecules as cargo. In fact, if they were initially considered a way of elimination of waste products [136
], current knowledge indicates that EVs represent a cell–cell means of communication.
EVs play a crucial role in the context of MM pathobiology, and specifically in the crosstalk that malignant PCs establish with other cells of the BMM such as endothelial, stromal, MSCs and immune cells [137
]. Such interaction is key both in the progression of the disease and in the onset of pharmacological resistance [143
]. Moreover, growing experimental evidence indicates that EVs released by MM cells alter bone homeostasis and, therefore, contribute to the onset of MMBD [145
We firstly demonstrated that EVs were released from MM cell lines and were also detectable in the serum of MM patients. Such EVs induced the osteoclastic differentiation of murine macrophages as well as of human pre-osteoclasts, enhancing the expression of specific OC differentiation markers, such as CTSK, MMP9 and TRAP. MM-EVs were able to induce a complete osteoclastic differentiation. Notably, pre-osteoclast treated with MM-EVs differentiated in multinuclear and giant OCs, having a strong erosive capability, as evidenced by bone resorption pit assays; these effects were not observed when EVs derived by the metastatic colorectal cancer cell line SW620 were used, demonstrating the MM cell-type specificity of EVs within the BMM [145
Subsequently, many studies have disclosed the molecular mechanisms underlying the EV-dependent osteoclastogenic effect. In 2019, Raimondo et al. showed that the presence of the EGFR ligand, amphiregulin (AREG), partially mediates the EV-mediated OC activation. Authors observed that AREG was specifically enriched in exosome samples, leading to the activation of EGFR in pre-OCs; such effects were abrogated by exosome pre-treatment with anti-AREG neutralizing Ab [146
In parallel, the role of EVs on another crucial cell population involved in bone homeostasis, i.e., OBs, has been investigated. The results of these studies indicate that MM EVs can inhibit the osteogenic differentiation of MSCs, thus contributing to increased osteolysis [146
]. In one of these studies, it emerged that MM EVs carry DKK1 in OBs leading to reduced levels of Runx2, osterix and collagen 1A1 [152
Noteworthy, increasing evidence suggests that the EV-associated ncRNA cargo mediates the profound impact that EVs exert on the gene expression profile of target cells [153
Research has been performed to identify the mechanisms underlying the specific sorting of certain ncRNAs in EVs. For some miRNAs, a small sequence, called hEXO motif, has been identified as recognized by the RNA binding protein SYNCRIP, and found to be responsible for the specific sorting of these miRNAs in vesicles [156
Recent evidence also correlated the osteolytic effect of MM-EVs with its ncRNAs content. A study published by Li et al., showed that the lncRNA RUNX2-AS1 is packaged in MM-EVs and transferred to MSCs, resulting in the transcriptional repression of Runx2 and, thus, prematurely blocking MSCs osteogenic differentiation [147
]. Finally, by analyzing the miRNA repertoire of MM-EVs, several miRNAs involved in the inhibition of osteogenic differentiation [148
] were identified. In a recent study, miR-103a-3p was identified as one of the upregulated miRNAs following the treatment of MSCs with MM-EVs [148
]; in parallel, miR-129-5p, carried by MM-EVs, was found to reduce ALPL levels in MSCs. In addition, the authors also observed that miR-129-5p was more abundant in EVs isolated from MM patients with active BD than in SMM, supporting the notion that this miRNA is key in mediating the EV-dependent MMBD [149
A more in-depth characterization of the ncRNAs contained in these structures will allow the development of new biomarkers for the diagnosis and prognosis of the disease. In parallel, the identification of the mechanisms underlying the exact sorting of ncRNAs, and/or other biomolecules, in EVs, could provide the basis for the definition of new therapeutic targets against MMBD.