Up-Regulation of Glioma-Associated Oncogene Homolog 1 Expression by Serum Starvation Promotes Cell Survival in ER-Positive Breast Cancer Cells.

BACKGROUND/AIMS Cancer cells are resistant to ischemia and starvation. Glioma-associated oncogene homolog 1 (Gli1) is a positive transcriptional activator of Hedgehog (Hh) pathway and plays an essential role in the development of cancers, including breast cancer. However, how Gli1 promotes cell survival remains elusive. The main purpose of this study is to investigate the pro-survival effect of Gli1 under serum starvation and its molecular mechanism in ER-positive breast cancer cells. METHODS Gene expression was determined by quantitative real-time PCR (QRT-PCR) and Western blot. The survival of Gli1 stably transfected ER-positive breast cancer cell lines (Gli1-MCF-7 and Gli1-T47D cells) and their untransfected control cells was estimated by WST-8 assay. Microarray analysis was performed to screen downstream Hh/Gli1 target genes in Gli1-overexpressed MCF-7 cells. Transcriptional activities of NF-kappaB were measured by luciferase assays. ChIP analysis was performed to explore whether cIAP2 was a direct target gene of Gli1. RESULTS Serum starvation significantly up-regulated the expression of Gli1 gene through activating PI3K/AKT pathway. Over-expression of Gli1 markedly promoted cell survival under serum starvation. Microarray analysis revealed that 338 genes were differentially expressed in Gli1-MCF-7 cells compared with those in the control cells. Among these genes, cellular inhibitor of apoptosis 2 (cIAP2), coding an anti-apoptosis and pro-survival protein, was significantly up-regulated not only by Hh/Gli1 pathway, but also by serum starvation. However, ChIP assay revealed no binding of Gli1 to cIAP2 promoter at the region of -1792 to -1568bp. Moreover, over-expression of Gli1 resulted in enhanced trans-activation of transcriptional factor NF-κB. Suppression of NF-κB signaling with NF-κB inhibitor Bay11-7082, significantly reduced the expression of cIAP2 and the cell survival under serum starvation. CONCLUSION Serum starvation significantly up-regulated the expression of Gli1, which in turn increased its key target cIAP2 expression and enhanced NF-κB/cIAP2 pathway, resulting in promoting cell survival under serum starvation. These findings may provide new insights into the pro-survival mechanisms of Gli1 in breast cancer.