Advanced prostate carcinoma is often associated with bone metastases. The mechanisms responsible for the organ-specific metastasis of this form of cancer are not known. In our quest to identify factors that mediate the bidirectional interactions between prostate cancer cells and the bone microenvironment, we demonstrate here that p45-sErbB3 stimulates bone to secrete osteonectin, which subsequently enhances the invasiveness of bone-tropic prostate cancer cells. Although mouse calvaria was used in this study, we consider that our observations are likely to be translatable to human bone. Mouse model has been widely used in addressing both developmental and disease-related aspects of human bone biology and most of the known human mutations that cause abnormal bone density, remodeling and mineralization have resulted in similar phenotypes in mice. Both mouse and human osteoblasts originate from the mesenchyme and respond similarly to various stimuli in vitro. In addition, human osteonectin peptide sequence is 92% identical to mouse osteonectin. Thus, our study warrants further investigation of the correlation of p45-sErbB3 and osteonectin in clinical specimens of the bone metastasis of prostate cancer.
Osteonectin is a major noncollagenous glycoprotein originally identified in bovine bone (14
). Osteonectin is important in bone formation; osteonectin-null mice exhibit profound osteopenia due to decreases in the numbers and surface areas of osteoblasts and osteoclasts (16
). Osteonectin may also be involved in the progression of prostate cancer to bone-metastatic disease (17
). Jacob et al. (12
) demonstrated that osteonectin promoted the invasiveness of bone-metastasizing cancer cells including PC-3. Similarly, De et al. (13
) reported that osteonectin increases prostate cancer cell invasiveness by interacting with the cell-surface receptors αv
. The functional activity of αv
integrins is higher on the bone-metastasizing C4-2B cells than on the lymph node-metastasizing LNCaP cells, despite the similar expression levels of these two integrins in C4-2B and LNCaP. In the study reported here, we showed that p45-sErbB3 stimulates osteonectin secretion from bone, which increased the invasiveness of prostate cancer cells PC3 and C4-2B. These observations are consistent with the conclusion that osteonectin is an osteoblastic factor that could selectively interact with the bone-tropic cancer cells and enhance their invasiveness.
The mechanism by which osteonectin increased the invasiveness of PC-3 cells and the C4-2B cells in our study is not clear. Jacob et al. (12
) showed that treating PC-3 cells with osteonectin increased the activity of matrix metalloprotease (MMP) -2 in PC-3 cells and that this increased activity may lead to increased tumor cell invasiveness. MMPs are a group of proteinases that degrade collagens, elastin, and other components of the extracellular matrix. To see whether p45-sErbB3 would increase the expression of MMPs by PC-3 cells, we examined MMP-2 and MMP-9 activity in medium from PC-3 cells that were treated or not treated with conditioned medium from calvaria treated with p45-sErbB3. In contrast to the report by Jacob et al. (12
), we did not detect significant differences in MMP-2 or MMP-9 activity in these PC-3 cells by zymography (data not shown). Whether other MMPs or matrix degrading enzymes are involved in the invasion activity seen in our study is not known.
Other bone extracellular matrix proteins, such as osteopontin and bone sialoprotein, have also been reported to contribute to the survival and growth of prostate cancer cells in bone. Osteopontin assists in cell adhesion, migration, and survival (18
). Overexpression of osteopontin in LNCaP and PC-3 cells has been shown to increase the invasion of those prostate cancer cells (19
). Bone sialoprotein facilitates the attachment of cancer cells to the bone and enhances the metastatic potential of cancer cells (20
). In our study, we found that osteonectin stimulated the invasiveness of C4-2B cells, but neutralizing antibodies against osteonectin could not inhibit the invasiveness of C4-2B cells in response to conditioned medium from p45-sErbB3–treated calvaria (data not shown), suggesting that other invasion-enhancing factors are present in the conditioned medium. Whether biglycan, type I collagen, or both are responsible for differences in prostate cancer cell invasiveness awaits further investigation.