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...