Although breast cancer is one of the most common malignancies, the molecular mechanisms underlying its development and progression are not fully understood. To identify key molecules involved, we screened publicly available microarray datasets for genes differentially expressed between breast cancers and normal mammary glands. We found that three of the genes predicted in this analysis were differentially expressed among human mammary tissues and cell lines. Of these genes, we focused on the role of the zinc-finger homeobox protein TSHZ2, which is down-regulated in breast cancer cells. We found that TSHZ2 is a nuclear protein harboring a bipartite nuclear localization signal, and we confirmed its function as a C-terminal binding protein (CtBP)-dependent transcriptional repressor. Through comprehensive screening, we identified TSHZ2-suppressing genes such as AEBP1 and CXCR4, which are conversely up-regulated by GLI1, the downstream transcription factor of Hedgehog signaling. We found that GLI1 forms a ternary complex with CtBP2 in the presence of TSHZ2 and that the transcriptional activity of GLI1 is suppressed by TSHZ2 in a CtBP-dependent manner. Indeed, knockdown of TSHZ2 increases the expression of AEBP1 and CXCR4 in TSHZ2-expressing immortalized mammary duct epithelium. Concordantly, immunohistochemical staining of mammary glands revealed that normal duct cells expresses GLI1 in the nucleus along with TSHZ2 and CtBP2, whereas invasive ductal carcinoma cells, which does not express TSHZ2, show the increase in the expression of AEBP1 and CXCR4 and in the cytoplasmic localization of GLI1. Thus, we propose that down-regulation of TSHZ2 is crucial for mammary tumorigenesis via the activation of GLI1.
Keywords: AEBP1; CXCR4; GLI1; TSHZ2; breast cancer.