DKK1 promotes MM cell malignancy. (A) Heat map shows selected up- and downregulated cytokines of the WT and BR MM.1S cells. (B) Kyoto Encyclopedia of Genes and Genomes analysis highlights the upregulated or downregulated signaling pathways in the BR MM.1S cells. (C) Representative protein levels of DKK1 in WT and BR MM cells. (D) DKK1 levels in WT and BR MM.1S and OPM-2 cells measured by ELISA (n = 12). (E) Quantification of apoptotic MM cells treated with increasing dosage of BTZ (0-10 nM) with or without 100 ng/mL recombinant human DKK1 for 48 hours (n = 3). (F) Cleavage of PARP as the apoptotic marker in MM cells treated with or without rhDKK1 (100 ng/mL), combined with increasing dosage of BTZ and CFZ (G) for 24 hours (n = 3). (H) DKK1 levels in bone marrow plasma cell (BMPC) from patients with CR (n = 12) and RR patients RR (n = 12) measured by ELISA. (I) Quantification of apoptosis in CD138⁺ cells from 6 MM patients treated with BTZ (5 nM) with or without 100 ng/mL rhDKK1 for 12 hours.