Abstract
Breast cancer cells frequently metastasize to the skeleton, where they induce OCL formation and activity, resulting in extensive bone destruction. However, the mechanisms by which breast cancer cells mediate increased osteolysis remain unclear. To elucidate this point, we investigated how 3 human breast cancer cell lines, MDA‐MB‐231, MDA‐MB‐435 and MCF‐7, induce OCL formation using a murine osteoblast–spleen cell coculture system and compared their effects with a human colorectal cancer cell line, HCT‐15; a human lung cancer cell line, HT‐1080; and a normal human breast cell line, HME. The breast cancer cell lines supported OCL formation only when osteoblasts were present in spleen cell cocultures, whilst the non‐breast cancer cell lines and the normal breast cell line, HME, had no effect. Fractionation of BCCM by ultrafiltration established that osteoclastogenic activity was associated with factors having m.w. >3 kDa. Breast cancer cell lines produced primarily PTHrP, with lesser amounts of IL‐6, IL‐11 and TNF‐α. The effect of BCCM on OCL formation in osteoblast–spleen cell cocultures was partially prevented by a neutralising antibody to human PTHrP and completely prevented by a neutralising antibody to either murine IL‐11 or the murine IL‐11 receptor; neutralising antibodies to human IL‐6, IL‐11 or TNF‐α were without effect. BCCM or human PTHrP induced an increase in murine osteoblast IL‐11 mRNA and protein production, effects that were prevented in the presence of a neutralising antibody to human PTHrP. The osteoclastogenic activity of IL‐11 was mediated by enhancing osteoblast production of PGE~2~ effects, which were abrogated by an inhibitor of cyclooxygenase. PGE~2~ apparently enhanced OCL formation by downregulating GM‐CSF production by spleen cells since recombinant murine GM‐CSF inhibited OCL formation and a neutralising antibody to murine GM‐CSF blocked these inhibitory effects. We conclude that breast cancer cells induce OCL formation by stimulating osteoblastic production of IL‐11. The subsequent release of PGE~2~ followed by inhibition of GM‐CSF production by cells within the bone microenvironment plays an important part in mediating the effects of breast cancer cells on OCL formation and their resorptive activity. © 2004 Wiley‐Liss, Inc.