Targeting Y-box binding protein-1 (YB-1) in Her-2 over-expressing breast cancer cells induces apoptosis via the signal transducer and activator or transcription-3 (STAT3) pathway and suppresses tumor growth.

A14 Her-2 is over-expressed in 20-30% of breast carcinomas, where it is associated with poor outcome and very high rates of relapse. Unfortunately, inhibiting Her-2 itself has been met with limited success in many cases. Perhaps this is because additional molecular targets lay farther downstream of Her-2. The Y-box binding protein-1 (YB-1) is a transcription/translation factor co-expressed in primary breast tumors harboring amplified Her-2. It also induces Her2 itself along with its dimerization partner EGFR by directly binding to their promoters. Therefore we addressed whether tumors cells that over-express Her-2 are dependent upon YB-1 for sustained growth. Small interfering RNAs targeting YB-1 induced apoptosis in BT474-1 and Au565 breast cancer cells known to have Her-2 amplifications. To address the underlying mechanism for YB-1 mediated survival the potential role for STAT3 was pursued. We determined that inhibition of YB-1 decreased P-STAT3S727 and therefore MCL-1. This was accompanied by decreased P-mTORS2448, total mTOR, and P-ERK1/2T202/Y204. Alternatively, constitutively active STAT3 rescued YB-1 induced apoptosis correlative with increased MCL-1. Furthering the role of STAT3 in these cells, we demonstrate that knocking it down recapitulated the induction of apoptosis. Finally, targeting YB-1 with two different siRNA9s remarkably suppressed tumor cell growth in soft agar by >90% and delayed tumorigenesis in nude mice. We therefore conclude that Her-2 over-expressing breast cancer cells dependent upon YB-1 for survival suggesting a new therapeutic target.