Purpose: Malignant ascites of epithelial ovarian cancer (EOC) helps identify prognostic biomarkers or mechanisms of tumor progression. Vitamin D–binding protein (DBP) was revealed to be upregulated in EOC ascites in our previous proteomic study. Here, we examined the role of DBP in EOC. Experimental Design: We analyzed ascites, serum, and tissue samples of patients with newly diagnosed EOC to determine the prognostic effects of DBP. We verified DBP function using orthotopic animal models and DBP regulation in ovarian cancer cell lines. Results: Elevated ascitic DBP was significantly associated with poor response to chemotherapy, short progression-free interval, increased cancer progression, and death. Ascitic DBP overexpression was an independent unfavorable biomarker for progression-free survival; DBP overexpression in cancerous tissue was significantly related to chemoresistance. In vivo and in vitro investigations demonstrated an important role for DBP in ovarian cancer progression. Orthotopic model mice inoculated with DBP knockdown ovarian cancer cells displayed a significant reduction in tumor formation, malignant cell number, ascitic DBP levels, invasiveness, and metastasis, and increased survival compared with controls. In presence of vitamin D receptor (VDR), DBP promoted cell aggression (invasion and doubling time) via activation of the insulin-like growth factor-1/insulin-like growth factor–binding protein-2/Akt axis, and induced suppression of vitamin D–responsive genes. A NF-kB p65-binding site in the VDR promoter was identified as a major determinant of DBP-dependent VDR promoter activation. Conclusions: This study highlights the importance of DBP in ovarian tumor progression and the potential application of DBP as a therapeutic target for EOC.
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