Human breast-cancer cells stimulate the fusion, migration and resorptive activity of osteoclasts in bone explants

A central event in bone resorption is the recruitment of osteoclasts to fut ure resorption sites, Breast-cancer cells invariably metastasise to the ske leton and induce extensive bone destruction by osteoclasts, However, our un derstanding of the mechanisms by which cancer cells interact with osteoclas ts remains unclear. Consequently, we compared the effects of conditioned me dium (CM) from 2 human breast-cancer cell lines, MB-MDA-231 and MCF-7, with those of a normal human breast epithelial cell line, HME, on osteoclastic fusion, resorptive activity and migration from the periosteum to the develo ping marrow cavity of fetal mouse metatarsals in culture. Osteoclastic reso rptive activity was assessed by pre-labelling 17-day-old fetal metatarsal e xplants with Ca-45, whilst fusion and migration were monitored by histomorp hometry and osteoclasts were identified by their tartrate-resistant acid ph osphatase activity. CM from TPA-stimulated breast-cancer cell lines produce d a significant increase in osteoclastic resorptive activity, whilst the no rmal breast cell line produced a minimal increase, The breast-cancer cell l ines also stimulated osteoclastic fusion and migration in the metatarsal ex plants, but the normal breast cell line was without effect. The stimulatory effect of CM from MDA-MB-231 cells on osteoclastic fusion, but not migrati on, was partially inhibited by preventing prostaglandin and leukotriene syn thesis by cells within the bone explants, In contrast, a synthetic matrix m etalloproteinase (MMP) inhibitor, but not a cysteine proteinase inhibitor, prevented the migration of osteoclasts to the calcified centre of the metat arsal explants in response to CM from MDA-MB-231 cells. MDA-MB-231 cells al so induced an increase in the expression of MMP-9 by migrating osteoclasts. Fractionation of the TPA-stimulated breast cancer cell CM established that the resorptive activity was associated with factors of m.w. >3 kDa. We det ermined by immune-assay that human breast-cancer cells secrete parathyroid hormone-related protein (PTH-rP), tumour necrosis factor-alpha (TNF-alpha) and interleukins (ILs) 6 and 11. Neutralizing experiments with human antibo dies to these cytokines established that PTH-rP and TNF-a production by MDA -MB-231 cells were responsible for mediating their effects on osteoclastic migration and ultimately bone resorption in the metatarsal explants, (C) 20 01 Wiley-Liss, Inc.