Aberrant activation of Wnt/β-catenin signaling plays a central role in the pathogenesis of a wide variety of malignancies and is typically caused by mutations in core Wnt pathway components driving constitutive, ligand-independent signaling. In multiple myelomas (MMs), however, these pathway intrinsic mutations are rare despite the fact that most tumors display aberrant Wnt pathway activity. Recent studies indicate that this activation is caused by genetic and epigenetic lesions of Wnt regulatory components, sensitizing MM cells to autocrine Wnt ligands and paracrine Wnts emanating from the bone marrow niche. These include deletion of the tumor suppressor CYLD, promotor methylation of the Wnt antagonists WIF1, DKK1, DKK3, and sFRP1, sFRP2, sFRP4, sFRP5, as well as overexpression of the co-transcriptional activator BCL9 and the R-spondin receptor LGR4. Furthermore, Wnt activity in MM is strongly promoted by interaction of both Wnts and R-spondins with syndecan-1 (CD138) on the MM cell-surface. Functionally, aberrant canonical Wnt signaling plays a dual role in the pathogenesis of MM: (I) it mediates proliferation, migration, and drug resistance of MM cells; (II) MM cells secrete Wnt antagonists that contribute to the development of osteolytic lesions by impairing osteoblast differentiation. As discussed in this review, these insights into the causes and consequences of aberrant Wnt signaling in MM will help to guide the development of targeting strategies. Importantly, since Wnt signaling in MM cells is largely ligand dependent, it can be targeted by drugs/antibodies that act upstream in the pathway, interfering with Wnt secretion, sequestering Wnts, or blocking Wnt (co)receptors.

The online version of this article has a Supplementary Appendix. BackgroundMultiple myeloma is a hematologic malignancy characterized by a clonal expansion of malignant plasma cells in the bone marrow, which is accompanied by the development of osteolytic lesions and/or diffuse osteopenia. The intricate bi-directional interaction with the bone marrow microenvironment plays a critical role in sustaining the growth and survival of myeloma cells during tumor progression. Identification and functional analysis of the (adhesion) molecules involved in this interaction will provide important insights into the pathogenesis of multiple myeloma. Design and MethodsMultiple myeloma cell lines and patients' samples were analyzed for expression of the adhesion molecule N-cadherin by immunoblotting, flow cytometry, immunofluorescence microscopy, immunohistochemistry and expression microarray. In addition, by means of blocking antibodies and inducible RNA interference we studied the functional consequence of N-cadherin expression for the myeloma cells, by analysis of adhesion, migration and growth, and for the bone marrow microenvironment, by analysis of osteogenic differentiation. ResultsThe malignant plasma cells in approximately half of the multiple myeloma patients, belonging to specific genetic subgroups, aberrantly expressed the homophilic adhesion molecule N-cadherin. N-cadherin-mediated cell-substrate or homotypic cell-cell adhesion did not contribute to myeloma cell growth in vitro. However, N-cadherin directly mediated the bone marrow localization/retention of myeloma cells in vivo, and facilitated a close interaction between myeloma cells and N-cadherin-positive osteoblasts. Furthermore, this N-cadherin-mediated interaction contributed to the ability of myeloma cells to inhibit osteoblastogenesis. ConclusionsTaken together, our data show that myeloma cells frequently display aberrant expression of Ncadherin and that N-cadherin mediates the interaction of myeloma cells with the bone marrow microenvironment, in particular the osteoblasts. This N-cadherin-mediated interaction inhibits osteoblast differentiation and may play an important role in the pathogenesis of myeloma bone disease.Key words: N-cadherin, multiple myeloma, osteoblast differentiation.Citation: Groen RWJ, de Rooij MFM, Kocemba KA, Reijmers RM, de Haan-Kramer A, Overdijk MB, Aalders L, Rozemuller H, Martens ACM, Bergsagel PL, Kersten MJ, Pals ST, and Spaargaren M. N-cadherin-mediated Haematologica 2011;96(11):1653-1661. doi:10.3324/haematol.2010 This is an open-access paper. interaction with multiple myeloma cells inhibits osteoblast differentiation. N-cadherin-mediated interaction with multiple myeloma cells inhibits osteoblast differentiation

The unrestrained growth of tumor cells is generally attributed to mutations in essential growth control genes, but tumor cells are also affected by, or even addicted to, signals from the microenvironment. As therapeutic targets, these extrinsic signals may be equally significant as mutated oncogenes. In multiple myeloma (MM), a plasma cell malignancy, most tumors display hallmarks of active Wnt signaling but lack activating Wnt-pathway mutations, suggesting activation by autocrine Wnt ligands and/or paracrine Wnts emanating from the bone marrow (BM) niche. Here, we report a pivotal role for the R-spondin/leucine-rich repeat-containing G protein-coupled receptor 4 (LGR4) axis in driving aberrant Wnt/ β-catenin signaling in MM. We show that LGR4 is expressed by MM plasma cells, but not by normal plasma cells or B cells. This aberrant LGR4 expression is driven by IL-6/STAT3 signaling and allows MM cells to hijack R-spondins produced by (pre)osteoblasts in the BM niche, resulting in Wnt (co)receptor stabilization and a dramatically increased sensitivity to auto-and paracrine Wnts. Our study identifies aberrant R-spondin/LGR4 signaling with consequent deregulation of Wnt (co)receptor turnover as a driver of oncogenic Wnt/ β-catenin signaling in MM cells. These results advocate targeting of the LGR4/R-spondin interaction as a therapeutic strategy in MM.M ultiple myeloma (MM) in most patients is an incurable hematologic malignancy characterized by the accumulation of clonal plasma cells in the bone marrow (BM). Despite the wide variety of underlying structural and numerical genomic abnormalities (1-3), virtually all MMs are highly dependent on a protective BM microenvironment, or "niche," for growth and survival (4). Understanding the complex reciprocal interaction between MM cells and the BM microenvironment is critical for the development of new targeted therapies.In MM, aberrant activation of the canonical Wnt pathway drives proliferation and is associated with disease progression, dissemination, and drug resistance (5-9). Because MMs with hallmarks of active Wnt signaling do not harbor mutations that typically underlie constitutive Wnt pathway activation, this oncogenic Wnt pathway activity was proposed to involve autocrine and/or paracrine Wnt ligands (6,8,10). Wnts are lipid-modified glycoproteins that function as typical niche factors because they are relatively unstable and insoluble due to their hydrophobic nature, which constrains long-range signaling (11). Binding of a Wnt ligand to its receptor Frizzled (Fzd) initiates a signaling cascade that ultimately results in stabilization and nuclear translocation of the Wnt effector β-catenin. In cooperation with TCF/LEF family transcription factors this orchestrates a transcriptional program (12, 13), comprising targets such as MYC and CCND1 (Cyclin D1) that play crucial roles in the pathogenesis of MM (14-16). Aberrant Wnt signaling in cancer typically results from mutations in APC, β-catenin (CTNNB1), or AXIN that drive constitutive, ligand-independent pathway ac...