THE TRANSCRIPTION FACTOR Y-BOX BINDING PROTEIN-1 (YB-1) INDUCES EXPRESSION OF THE PIK3CA ONCOGENE LEADING TO INCREASED INVASION OFBASAL-LIKE BREAST CARCINOMA CELLS

Abstract

Background: Basal-like breast carcinoma (BLBC) is the mostaggressive subtype of breast cancer. 73% of BLBC over-express YB-1, anoncogenic transcription/translation factor. PIK3CA, which codes for the p110? catalytic subunit ofphosphatidylinositol-3-kinase (PI3K), is another oncogene. The PI3K signalling pathway is fundamental in the regulation of many cellular functions and isoften deregulated in cancer. Despite its importance, the knowledge on the transcriptional regulation of PIK3CA is limited. Indeed, we have recently published the first report on the PIK3CA promoter. Methods and Results: A genome-wide chromatin immunoprecipitation on chip (ChIP-on-chip) analysis of a BLBC cell-line(SUM149) suggested binding of YB-1 to the PIK3CA promoter. This binding was verified using traditional chromatin immunoprecipitation (ChIP). Furthermore, electrophoretic mobility shift assay (EMSA) using oligonucleotides with eitherwild-type or mutated YB-1 responsive elements mapped YB-1 binding to three sites on the PIK3CA promoter. Silencing YB-1 in BLBC cell-lines (SUM149, HCC1937, andMDA-MB-231) decreased, while over-expression of YB-1 increased the PIK3CA promoter activity, transcript, and protein levels. Interestingly, array comparative genomic hybridization(aCGH) and quantitative PCR demonstrated PIK3CA copy number gains in HCC1937 andMDA-MB-231 cells. Although PIK3CA amplifications are overall uncommon (9%) in breast cancer, we demonstrated here that low level gains in PIK3CA copy number are present in 30%of primary BLBC cases. Furthermore, it has previously been demonstrated that mutations of PIK3CA are the most common genetic aberration (27%) found in breast cancer. These mutations lead to constitutive activation of p110? and are highly oncogenic. Over-expression of YB-1in MCF-7 cells, which harbour an activating PIK3CA mutation, increased PIK3CA transcript and protein levels. Furthermore, induction of PIK3CA by YB-1 leads to increased levels of urokinase plasminogenactivator (uPA) and invasion. Conclusions: Our data demonstrates that YB-1 binds to the PIK3CA promoter and induces itsexpression whether the gene is wild-type or amplified. Moreover, since YB-1induces expression of the active mutant p110?, then therapeutic inhibition of YB-1 may lead to decreased p110? and interference with the constitutively activated PI3K pathway in cancers. In addition, the YB-1/PIK3CA/uPA network provides information regarding the possible therapeutic targets for prevention of breast cancer invasion and metastasis. A.A. is supported by a Child and Family–CIHR–UBC MD/PhD Studentship Award.