Breast cancer cells release factors that induce apoptosis in human bone marrow stromal cells

Breast cancer is associated frequently with skeletal metastases, which caus e significant morbidity. The main mechanism is an increase in osteoclast-me diated bone resorption. We postulated that osteoblasts could be other essen tial target cells and previously showed that conditioned medium (CM) of bre ast cancer cells (BCCs) inhibits the proliferation of osteoblast-like cells .]In this study, we investigated the effects of BCC-secreted products on os teoprogenitor cells using a clonal fetal human bone marrow stromal preosteo blastic cell line (FHSO-6) that expresses alkaline phosphatase (ALP) activi ty, type I collagen (COLI), and increased osteocalcin (OC) and osteopontin under treatment with dexamethasone (Dex), 1,25-dihydroxyvitamin D [1,25(OH) (2)D], or recombinant human bone morphogenetic protein 2 (rhBMP-2). Treatme nt with MCF-7 CM inhibited FHSO-6 cell survival in a dose-dependent and irr eversible manner. Morphological investigation indicated that MCF-7 CM incre ased both apoptotic and necrotic cell number. MCF-7 CM increased caspases a ctivity and a broad inhibitor of caspase activity (benzyloxycarbonyl-Val-Al a-Asp(OMe)-fluoromethyI ketone [z-VAD-fmk]) partly reversed the CM-induced inhibition of FHSO-6 cell survival. Western blot analyses revealed an incre ased bax/bcl-2 ratio in MCF-7 CM-treated FHSO-6 cells. MCF-7 cells exhibit FasLigand as membrane-bound protein and as a soluble cytokine in the CM. De privation of MCF-7 CM from active FasLigand by saturation with a soluble Fa s molecule suppressed the induction of FHSO-6 apoptosis, whereas fibroblast CM, which did not contain FasLigand, only weakly modified FHSO-6 cell surv ival because of increased cell necrosis. These data indicate that FasLigand secreted by BCCs induces apoptosis and necrosis of human preosteoblastic s tromal cells through caspase cascade modulated by the bax and bcl-2 protein level. The induction of apoptosis in human bone marrow stromal cells by BC Cs may contribute to the inappropriately low osteoblast reaction and bone f ormation during tumor-induced osteolysis in bone metastases.