Blockade of Notch Signaling by GSI Induces Apoptosis in Human Breast Cancer Cells through Various Molecular Mechanisms Involving Survivin and BH3-Only Members of the Bcl-2 Family.

The Notch pathway is multifunctional and controls key nodes in cell signaling. In most cases, its deregulation has oncogenic effects, and aberrant activation of Notch signaling has been evidenced in breast cancer (Stylianou S, CR 2006). In particular, a functional Notch pathway appears to be of critical importance in the maintenance of basal breast cancer (Lee CW, CR 2008, BCR 2008). Moreover, because Notch signaling is repressed by both ER (Rizzo P, CR 2008) and HER2 (Osipo C, Oncogene 2008) oncogenic pathways, its reactivation during tamoxifen or trastuzumab treatment may restrain the efficiency of these inhibitors. Thus, Notch signaling inhibitors, such as competitive inhibitors of the g-secretase, GSI, that prevent proteolytic processing of the Notch receptor and the activation of its intracellular domain, are being actively investigated for the treatment of breast cancer, even though the mechanisms that might underlie their effects are still poorly understood.We first observed that GSI XII (z-Ile-Leu-CHO) was more effective in inducing apoptosis, as a single agent, on ER-/PR-/Her2- breast cancer cell lines than GSI X (z-Leu-Leu-Nle-CHO). This effect was associated with decreased expression of the anti-apoptotic proteins Survivin and XIAP and increased expression of, depending on cell lines, either NOXA or PUMA, two pro-apoptotic BH3-only members of the Bcl2 family.In the opposite, ER+ breast cancer cell lines were more sensitive to induction of apoptosis by GSI X, but this was manifest only in combination with Tamoxifen. Interestingly, Tamoxifen treatment of these cells induced both GSI X-sensitive Notch transcriptional activity and Survivin expression. Moreover, Survivin knock-down by RNA interference increased Tamoxifen sensitivity. Thus, Survivin might be an important actor of survival signal triggered by Notch (re)activation during ER pathway inhibition. Our results corroborate previous data showing the interest to use GSI in breast cancer but underline also the need to unravel the sub-type and context dependent molecular events involved in their biological activity. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2160.