SECRETORY PRODUCTS OF BREAST-CANCER CELLS SPECIFICALLY AFFECT HUMAN OSTEOBLASTIC CELLS - PARTIAL CHARACTERIZATION OF ACTIVE FACTORS

The pathogenesis of tumor-induced osteolysis (TIO) following breast ca ncer metastases in bone remains unclear. We postulated that osteoblast s could be target cells for the secretory products of breast cancer ce lls. We previously showed that serum-free conditioned medium (CM) of t he breast cancer cell line MCF-7 inhibits DNA synthesis by 75% of cont rol values in osteoblast-like cells SaOS-2 and that this effect is onl y in a minor part due to transforming growth factor beta secretion. To establish the specificity of our observations and to look for other b iologically active factors,,ve have tested the effects of medium condi tioned by several cancer and noncancer cell lines (breast, colon, plac enta, or fibrosarcoma) on the proliferation of osteoblast-like cells ( SaOS-2, MG-63), normal human osteoblasts, human fibrosarcoma cells, an d normal human fibroblasts. Culture medium (1:2) of the breast cancer cell lines MCF-7, T-47D, MDA-MB-231, and SK-BR3 inhibited by 25-50% th e proliferation of osteoblast-like cells SaOS-2, MG-63, and normal ost eoblasts as evaluated by the MTT survival test or [H-3]thymidine incor poration. MCF-7 cells completely inhibited the proliferation of normal human osteoblasts in coculture. This inhibitory effect ,vas reversibl e and not due to cytotoxicity. Moreover, the cyclic adenosine monophos phate (cAMP) response to parathyroid hormone (PTH) of osteoblast-like cells SaOS-2 was also increased by 100-240% by the same CM. Such activ ities were, however, not detected in medium from the breast noncancer cell line HBL-100 or in the medium conditioned by non-breast cancer ce ll lines (COLO 320DM, HT-29, JAR, or HT-1080). Medium from the breast cancer cells had no effect on normal human fibroblasts or fibrosarcoma cells (HT-1080), suggesting the specificity of their action on human osteoblasts. After partial purification by ultrafiltration and size-ex clusion chromatography,,ve found that medium of T-47D cells contained at least three nonprostanoid factors of low molecular weights (apparen t MW of 700, 1500, and 4000 D) which affected human osteoblast-like ce lls. These factors were heat stable and could be peptides without disu lfide bonds. In summary, our data show that human breast cancer cells release soluble factors that inhibit osteoblast proliferation and incr ease their cAMP response to PTH, indicating that osteoblasts could be important target cells for breast cancer cells and could be involved i n the process of TIO.