The focus of this study was to gain insight into the role(s) of
osteonectin in the preferential
metastasis of
breast cancer cells to bone.
Osteonectin was isolated from
conditioned media of several cell lines including
breast cancer (MDA-MB-435, MDA-MB-468), osteoblasts (hFOB1.19), non-neoplastic breast epithelial (hTERT-HME1), and vascular endothelial cells isolated from a bone biopsy (HBME-1). Chemical/physical properties of
osteonectin from these five sources was analyzed to determine if unique configurations of
osteonectin exist and therefore identify a chemotactic
isoform.
Osteonectin from all sources had a molecular weight of approximately 46 kDa, N-linked glycosylation, and undetectable phosphorylated serines,
sialic acids and O-linked
oligosaccharides. The
cDNA for
osteonectin from the
breast cancer, osteoblast, and breast epithelial cell lines was identical, while the vascular endothelial cell
cDNA contained point mutations that resulted in eight amino acid substitutions. Bone-derived
osteonectin was then analyzed to assess its influence on
breast cancer cell motility and migration. Although
osteonectin increased undirected MDA-MB-231 cell motility, it did not chemoattract the same
breast cancer cell line. However, the
breast cancer cells did migrate toward the known
chemoattractant vitronectin and to bone extracts derived from wild-type and
osteonectin-null mice. Migration to
vitronectin was enhanced when
osteonectin was also present. We concluded that
osteonectin was not a
chemotactic factor. However, through its anti-adhesive properties,
osteonectin induced undirected
breast cancer cell motility, and may have enhanced chemoattraction to
vitronectin.