Notch
Prostate
cancer bone metastases are characterized by their ability to induce osteoblastic lesions and local bone formation. It has been
suggested that bone metastatic prostate cancer cells
are osteomimetic and capable of expressing genes and
proteins typically expressed by osteoblasts. The
ability of pre-osteoblasts to differentiate and
express osteoblastic genes depends on several
pathways, including Notch and MAPK. Here we show that Notch1 expression is
increased 4-5 times in C4-2B and MDA PCa2b cells (osteoblastic
skeletal prostate metastatic cancer cell lines) when
compared to non-skeletal metastatic cell lines (LNCaP and DU145). Notch1 ligand,
Dll1, is expressed only in C4-2B cells. Immunohistochemical studies demonstrate that Notch1 is
present in both human clinical samples from prostate cancer bone metastases and
the C4-2B cell line. To determine if prostate cancer bone metastases respond to
osteogenic induction like osteoblasts,
C4-2B cells were cultured in osteogenic media that
promotes mineralization. C4-2B cells mineralize and express HES-1 (a downstream
target of Notch), an effect that is completely inhibited by L-685,458, a Notch
activity inhibitor. Furthermore, osteogenic induction
increases ERK activation, Runx2 expression and nuclear localization,
independent of Notch signaling. Finally, we show that Notch and ERK activation
are essential for Runx2 DNA binding activity and osteocalcin
gene expression in C4-2B cells in response to osteogenic
induction. These studies demonstrate that prostate cancer bone metastatic cell lines acquire osteoblastic
properties through independent activation of ERK and Notch signaling;
presumably both pathways are activated in the bone microenvironment