Calcyclin-binding protein (CacyBP) is a tumor suppressor in gastric and renal cell carcinoma, but an oncogene in pancreatic cancer. However, the function of CacyBP in breast cancer has not been well elucidated. In this study, we explored the clinical relevance of CacyBP and investigated the relationship between CacyBP and COX-2 in breast cancer. Immunohistochemical analysis in 172 cases of breast tissues showed that the positive rate of CacyBP protein expression in normal breast tissues (NBT) (89.3%) was higher than that in invasive ductal carcinoma (IDC) (56.1%) (P<0.05). RT-PCR and Western blot analysis showed that CacyBP mRNA and protein expression were significantly lower in tumor tissues as compared to those in the corresponding non-tumorous tissues (P<0.05). The expression trend of COX-2 was opposite with CacyBP in breast carcinogenesis. Moreover, the CacyBP expression was significantly negatively associated with the COX expression in the 132 breast cancer samples (correlation coefficient = 0.505, P<0.001). The clinicopathological data analysis in 132 breast cancer samples showed that CacyBP expression was positively correlated with well differentiated samples (P=0.021), low pathologic TNM stage (P=0.009), and no lymphatic metastasis (P=0.027) of patients with breast cancer. Furthermore, reduced CacyBP expression was associated with poor prognosis. Knockdown of CacyBP gene using siRNA enhanced the proliferation and invasion ability of breast cancer cells, which was dependent on COX-2 expression. In conclusion, CacyBP regulation of COX-2 expression may play an important role in human breast carcinogenesis. Restoration of CacyBP gene is a potential therapeutic target of breast cancer.

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