Neutralization of DKK1 rescued OPG expression in osteoblasts grown in the presence of MM sera or primary MM cells. C2C12 cells were treated with BM sera (50% diluted with serum free DMEM medium) from MM patients (n = 8) containing low (L; 2.7 to 8.5 ng/mL) or high (H; 104.5 to 273.5 ng/mL) concentration of DKK1 or recombinant DKK1 (100 ng/mL) as a positive control or normal sera (50% diluted with serum free DMEM medium) for 2 hours. Then rWnt3a or control vehicle was added to the cell culture media, as described above, for 48 hours (A). The cells were treated with rWnt3a or control vehicle for 48 hours after prior treatment with 25% sera from MM patients (n = 21) containing mouse Ig or anti-DKK1 antibody or with control IgG for 2 hours (B). OPG mRNA was determined by qPCR from the RNA (**P < .01, ***P < .001, vs control). Error bars represent SD. C2C12 cells were cocultured with CD138-positive plasma cells (PC) from MM or the lymphoma ST486 cell line (negative control) in the presence or absence of Wnt3a, control IgG or anti-DKK1 antibody for 48 hours. Plasma cells in suspension were removed and harvested. The supernatants were harvested by centrifugation for 10 minutes. C2C12 cells were washed with BPS and homogenized for isolation of RNA. OPG mRNA was determined by qPCR analysis from total RNA isolated from C2C12 cells (C), and OPG protein in cell culture supernatants was measured by ELISA analysis (D).