Role of IL-6 and PTHrP in Rankl expression and osteoclast formation. A: mRNA expression of IL-6 and PTHrP in various cancer cell lines. B and C: Effects of anti-human PTHrP antibody and anti-mouse IL-6 (mIL-6) receptor antibody on Rankl mRNA expression induced by BHY-CM (B) and Ca9-CM (C) in ST2 cells. ST2 cells were cultured for 2 days in the presence or absence of BHY or Ca9-CM, and the expression of Rankl was determined by using mouse specific primers. *P < 0.05, **P < 0.01, significantly different from the value for BHY-CM(+) and antibody(−) in B, and *P < 0.05, significantly different from the value for Ca9-CM(+) and antibody (−) in C. D: Effects of anti-human PTHrP and anti-mouse IL-6 receptor antibodies on formation of tartrate-resistant acid phosphatase (TRAP)-positive osteoclast after stimulation with BHY-CM in cocultured ST2 and bone marrow cells. *P < 0.01, **P < 0.005, significantly different from the value of BHY-CM treated cells without any antibodies. In each experiment, the cells were treated with 10 μg/ml of each antibody. MNCs, multinucleated cells.