Notch signaling and ERK activation are important for the osteomimetic properties of prostate cancer bone metastatic cell lines

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 preosteoblasts 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 PCa 2b cells (osteoblastic skeletal prostate metastatic cancer cell lines) when compared with nonskeletal 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 whether prostate cancer bone metastases respond to osteogenic induction similar to osteoblasts, C4-2B cells were cultured in osteogenic medium 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.