The Hedgehog signaling pathway is one of the most dysregulated pathways in human
cancers. The
glioma-associated oncogene homolog 1 (GLI1)
transcription factor is the terminal effector of the Hedgehog pathway, frequently activated in human
breast cancer and an emerging target of
breast cancer therapy. While somatic mutations in the human GLI1 gene have never been reported in any cell or
tumor type, we recently uncovered the existence of a novel alternatively spliced, truncated GLI1 (tGLI1) that has an in-frame deletion of 41
codons spanning the entire exon 3 and part of exon 4 of the GLI1 gene. Using
glioblastoma models, we showed that tGLI1 has gained the ability to promote
glioblastoma migration and invasion via its gain-of-function transcriptional activity. However, the pathological impact of tGLI1 on
breast cancer remains undefined. Here, we report that tGLI1 is frequently expressed in human
breast cancer cell lines and primary specimens we have examined to date, but is undetectable in normal breast tissues. We found for the first time that tGLI1, but not GLI1, binds to and enhances the human
vascular endothelial growth factor-A (
VEGF-A) gene promoter, leading to its upregulation. Consequently, tGLI1-expressing MDA-MB-231
breast cancer cells secret higher levels of
VEGF-A and contain a higher propensity, than the isogenic cells with control vector and GLI1, to stimulate in vitro angiogenesis of human vascular endothelial cells. We further showed that tGLI1 has gained the ability to enhance the motility and invasiveness of
breast cancer cells in a proliferation-independent manner and that this functional gain is associated with increased expression of migration/invasion-associated genes, CD24, MMP-2 and MMP-9. tGLI1 has also acquired the property to facilitate anchorage-independent growth of
breast cancer cells. Collectively, our results define tGLI1 as a gain-of-function GLI1
transcription factor and a novel mediator of the behavior of clinically more aggressive
breast cancer.