Since bone metastatic breast cancer is an incurable disease, causing significant morbidity and mortality, an understanding of the underlying molecular mechanisms would be highly valuable. Here, we describe in vitro and in vivo evidences for the importance of serine biosynthesis in the metastasis of breast cancer to bone. We first characterized the bone metastatic propensity of the MDA-MB-231(SA) cell line variant as compared to the parental MDA-MB-231 cells by radiographic and histological observations in the inoculated mice. Genome-wide gene expression profiling of this isogenic cell line pair revealed that all the three genes involved in the l-serine biosynthesis pathway, phosphoglycerate dehydrogenase (PHGDH), phosphoserine aminotransferase 1 (PSAT1), and phosphoserine phosphatase (PSPH) were upregulated in the highly metastatic variant. This pathway is the primary endogenous source for l-serine in mammalian tissues. Consistently, we observed that the proliferation of MDA-MB-231(SA) cells in serine-free conditions was dependent on PSAT1 expression. In addition, we observed that l-serine is essential for the formation of bone resorbing human osteoclasts and may thus contribute to the vicious cycle of osteolytic bone metastasis. High expression of PHGDH and PSAT1 in primary breast cancer was significantly associated with decreased relapse-free and overall survival of patients and malignant phenotypic features of breast cancer. In conclusion, high expression of serine biosynthesis genes in metastatic breast cancer cells and the stimulating effect of l-serine on osteoclastogenesis and cancer cell proliferation indicate a functionally critical role for serine biosynthesis in bone metastatic breast cancer and thereby an opportunity for targeted therapeutic interventions.
Breast cancer Bone metastasis Osteoclast l-serine
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This study was supported by the Academy of Finland Center of Excellence grant (Center of Excellence in Translational Genome-Scale Biology 2006–2011), TIME (Disseminated Tumour Cells as Targets for Inhibiting Metastasis of Epithelial Tumours) project, Drug Discovery Graduate School, NIH grant R01 CA69158 from the National Cancer Institute, and by grants from the Finnish Cancer Organisations, the Sigrid Jusélius Foundation, and the Finnish Cultural Foundation. We thank Barry G. Grubbs and Rami Käkönen for excellent technical assistance in the mouse studies, Pharmatest Services Ltd (http://www.pharmatest.fi/) for technical help and discussions regarding the osteoclast cultures, and John Patrick Mpindi for help in bioinformatic analyses.
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