Identification of breast cancer cell line-derived paracrine factors that stimulate osteoclast activity

Metastatic breast cancer causes destruction of significant amounts of bone, and, although bone is the most likely site of breast cancer metastasis, li ttle is understood about interactions between tumor cells and bone-resorbin g osteoclasts. We have investigated the paracrine factors produced by breas t cancer cells that are involved in increasing osteoclast activity. We have determined by immunoassay that the human breast cancer cell line MDA MB 23 1 (231) cultured in serum-free medium secretes transforming growth factors type beta (TGF-beta) 1 and 2, macrophage colony-stimulating factor (M-CSF), granulocyte macrophage colony-stimulating factor (GM-CSF), interleukin (IL ) -1 and -6, tumor necrosis factor alpha (TNF-alpha), insulin-like growth f actor II (IGF II), and parathyroid hormone-related peptide. To determine wh ich of these are involved in increased bone destruction, we have fractionat ed serum-free 231-conditioned media and measured these fractions for effect s on osteoclast resorption activity using multiple activity assays, The pat tern of responses was complex. Several fractions stimulated osteoclast reso rption either by increasing the number of osteoclasts binding to the bone o r by elevating the resorption activity of the individual osteoclasts. Other fractions inhibited osteoclast activity. Analysis of active fractions for the factors identified in the 231-conditioned medium revealed that the pres ence of TNF-alpha and IGF-II was restricted to separate fractions that stim ulated osteoclast resorption activity. The fractions that inhibited osteocl ast resorption activity contained M-CSF, IL-6, TGF-beta 2, and GM-CSF, No T GF-beta 1 or IL-1 was detected in any of the active fractions. Our data sup port the hypothesis that breast cancer cells modulate osteoclast activity u sing multiple regulatory factors that increase both the number of mature os teoclasts attached to the bone and the bone resorption activity of these in dividual osteoclasts. Once it is understood how metastatic breast cancer el evates osteoclast-mediated bone loss, effective therapies to slow the progr ession and/or prevent this bone loss will become possible.