Sclerostin is expressed by osteocytes but not myeloma cells. (A) Gene expression profiling using DNA microarray analysis of memory B cells (MBCs [n = 5]), in vitro–generated PPCs (n = 5), normal BMPCs (n = 10), malignant plasma cells from patients with newly diagnosed MM (n = 630) and relapsed/refractory MM (RMM [n = 82]), as well as human myeloma cell lines (HMCLs [n = 54]). Gray dots represent the absence of expression, and black dots represent the presence of expression according to the presence-absence calls with negative probesets (PANP) algorithm. (B) RNA-seq analysis of DKK1 and SOST of MBCs (n = 4), PPCs (n = 4), BMPCs (n = 10), MM cells (n = 263), and HMCLs (n = 19). DKK1 (204602_at) is expressed by the majority of malignant plasma cell samples from previously untreated and relapsed myeloma patients. In contrast, SOST (223869_at) expression is absent in all normal and malignant plasma cells. (C) Immunohistochemical staining for CD138 (top panel) and sclerostin (bottom panel) counter-stained with hematoxylin in bone marrow of naive mice and mice bearing 5TGM1-eGFP myeloma cells. Slides were scanned on Scanscope CS2 (Aperio) up to original magnification ×40, and images were captured by using Aperio Imagescope at digital magnification ×20. Scale bar represents 50 μm. (D) Sclerostin protein was measured in media conditioned by MM1.S, OPM2 human myeloma cells lines, 5TGM1 murine myeloma cells, and primary osteocytes (n = 4; data are mean ± 1 standard error of the mean [SEM]; ****P < .0001). (E) Density plot highlighting the osteocyte-enriched expression of secreted Wnt signaling antagonists (n = 6). (F) Tumor burden significantly alters the expression of the secreted Wnt signaling pathway antagonists Sost, Dkk1, and Frzb (n = 6; data are mean ± 1 standard deviation [SD]; multiple comparison–adjusted P value *P < .05; **P < .01; ****P < .0001). (G) Immunohistochemical staining for sclerostin in osteocytes from naive and 5TGM1-bearing mice; arrow heads denote positive-stain osteocytes with positive-stained canaliculi (inset). Slides were scanned on Scanscope CS2 (Aperio) up to original magnification ×40, and images were captured by using Aperio Imagescope at digital magnification ×40. Scale bar represents 50 µm.