Endothelin signaling in osteoblastic metastases: molecular mechanisms and biomarkers

A86 Osteoblastic metastases are common in advanced prostate cancer and some cases of breast cancer. It has been shown that uncontrolled bone formation at the site of metastasis is caused by a number of growth factors, in particular endothelin-1 (ET-1). Secretion of ET-1 by prostate cancer cells is a major factor in formation of osteoblastic metastases. A highly specific antagonist of the ETa receptor, ABT-627, has been developed by Abbott for the treatment of metastatic prostate cancer.To elucidate the molecular mechanism of ET1-mediated bone formation, we established an in vitro osteoblast model system that was responsive to ET-1 treatment. A genomic screen of the ET-1-treated osteoblasts was followed by pathway analysis of the resulting gene expression signature. This analysis revealed three major themes: osteoblastic differentiation, survival, and invasion. We also observed coordinated induction of genes that constitute the calcineurin/NFAT pathway. Follow-up experiments demonstrated that ET-1 induces calcineurin activity in osteoblasts, followed by nuclear translocation of NFATc1 and NFAT-mediated transcription.An independent set of experiments demonstrated that ET-1 protects osteoblasts from apoptosis induced by several known apoptosis inducers with different mechanisms of action (such as actinomycin D and TNF-alpha). This finding, combined with the previous observations of only weak mitogenic effects of ET-1 on osteoblasts, suggests that suppression of apoptosis is the main mechanism whereby ET-1 promotes osteoblastic metastasis in prostate cancer.The ET-1 signature in osteoblasts contained several genes coding for secreted proteins previously implicated in invasion and metastasis. Their secretion was confirmed by ELISA. These proteins are currently being explored as biomarkers for osteoblastic metastasis and PD biomarkers for ABT-627.Our results suggest that ET-1 stimulates bone growth at the metastatic sites by activating the calcineurin/NFAT pathway in osteoblasts and suppressing apoptosis in osteoblasts. These findings provide the foundation for further drug discovery efforts in metastatic prostate cancer. Additionally, we identified several candidate biomarkers that can be used to monitor the progression of osteoblastic metastases and the efficacy of antimetastatic therapies.