The Gamma Catenin/CBP Complex Maintains Survivin Transcription In β-Catenin Deficient/Depleted Cancer Cells.

Abstract 1218

Survivin is a member of the inhibitor of apoptosis (IAP) gene family, which plays essential roles in prevention of apoptosis, cell division, and cancer development. Aberrant expression of Survivin is associated with essentially all tumors compared to their normal tissues. A number of signaling pathways are involved in the transcriptional regulation of Survivin. Previously, we demonstrated that Survivin expression is CBP/β-catenin/TCF-dependent and that ICG-001, a specific inhibitor of binding to the N-terminus of CBP, effectively attenuates Survivin expression. Now, using NCI-H28 cells, which harbor a homozygous deletion of β-catenin, we demonstrate that Survivin transcription can similarly be mediated by nuclear γ-catenin. In this study, we confirm that γ-catenin, like β-catenin, accumulates in the nucleus and that it can increase TOPFLASH reporter activity upon Wnt3a stimulation in NCI-H28 cells. In addition, overexpression of γ-catenin increases the expression of the TCF/catenin target genes Survivin and cyclinD1. This effect can be diminished by overexpression of dominant negative TCF or transactivation domain deleted γ-catenin. We demonstrate that g-catenin by binding to TCF family members and specifically recruiting the coactivator CBP drives Survivin transcription particularly in β-catenin-deficient cells indicating that γ-catenin can participate in the canonical Wnt signaling cascade. We also examined the relative expression of γ-catenin and β-catenin in 90 cases of chronic myeloid leukemia (CML) in a published gene expression microarray data base. A statistically significant negative correlation between γ-catenin and β-catenin was found in AP/BC cases (-0.389, P = 0.006). Furthermore, in subsequent independent validation studies by qPCR in 28 CP and BC patients increased g-catenin expression predominated in BC cases and was associated with concomitantly increased Survivin expression. Gene expression was 3- and 6-fold greater in BC patients as compared to CP patients, for γ-catenin and survivin, respectively. Consistent with this observation, nuclear γ-catenin accumulation was evident in this population consistent with a potential transcriptional role. Combined treatment with imatinib mesylate (IM) and ICG-001 significantly inhibited colony formation in sorted CD34⁺ CML progenitors (survivin⁺/γ-catenin^high/β-catenin^low) isolated from one BC and one AP patient resistant to IM. Our data indicates the potential of ICG-001 to block both the CBP/γ-catenin interaction and the CBP/β-catenin interaction and this may have clinical significance in cancers in which γ-catenin plays a significant transcriptional role.