DKK1 maintains MM cell immatureness by downregulating CD138. (A) Quantitative PCR (qPCR) shows expressions of SDC1 (CD138) of MM.1S and OPM-2 cells treated with increasing dosage of rhDKK1 for 48 hours. (B) Representative protein level of CD138 in MM.1S and OPM-2 cells treated with rhDKK1 for 48 hours. (C) qPCR shows expressions of SDC1 of CD138⁺ plasma cells from 6 myeloma patients treated with rhDKK1 for 12 hours. (D) Representative protein levels of CD138 in MM cells treated with different dosage of rhDKK1 in presence of Wnt3a (200 ng/mL) or (E) WntC59 (100 nM) for 48 hours. (F) Schematic illustration of LEF1 binding sites, sequences, and positions on the SDC1 promoter. (G) Luciferase assay of a 1100-bp hSDC1-promoter in HEK293T cells transfected with increasing amount of TCF1 plasmid. (H) Western blotting shows CD138 protein level in MM.1S cells infected with increasing lentiviral carrying overexpression of LEF1 for 72 hr. (I) The luciferase assay of a 1100 bp hSDC1-promoter in HEK293T cells transfected with increasing amount of pCSV-TCF1 plasmid. (J) Representative western blotting showing the knockdown effects of short hairpin RNA (shRNA) targeting coding sequencing of LRP6 gene (shLRP6) compared with the NT-Ctrl in MM.1S and OPM-2 cells infected for 72 hours. (K) Representative of CD138 protein level in MM cells stably expressing shRNA targeting LRP6 or non-target control (NT Ctrl), and subsequently treated with rhDKK1 (100 ng/mL) for 48 hours. (L) Western blotting showing the cleavage of PARP to detect effect of rhDKK1, or (M) DKK1 neutralizing antibodies (1 μg/mL) on BTZ-induced apoptosis in MM cell with shLRP6 knockdown.