Abstract 2337: Defining significance of the novel tGLI1 transcription factor in cancer growth and progression

The goal of this study is to gain a greater understanding of the molecular pathways that drive breast cancer progression and metastasis, the leading cause of breast cancer mortality. In achieving this goal, our research has been concentrated on GLI1 (glioma-associated oncogene homolog 1) zinc-finger transcription factor, the terminal effector of the Hedgehog pathway that is important for many physiological processes, constitutively activated in breast cancer and associated with poor survival of breast cancer patients. While no mutations in the human GLI1 gene have been reported in any cell or tumor type, we recently discovered a novel alternatively spliced truncated variant of GLI1, tGLI1, with an in-frame deletion of 41 codons spanning the entire exon 3 and part of exon 4 of the GLI1 gene (Can Res 17:6790-8, 2009). In the current study, we further discovered that the tGLI1 transcript and protein are highly expressed in the majority of cell lines and specimens of breast cancer we have examined to date, but is undetectable in normal cells or tissues. tGLI1 has retained the functional domains of GLI1 and a similar ability as GLI1 to undergo nuclear import and to activate GLI1-targeted genes. To gain insights into the role of tGLI1 in breast cancer progression, we used MDA-MB-231 cell line (known to be highly invasive and metastatic) to create three isogenic cell lines that stably express control vector, wild-type GLI1 and tGLI1. Functional characterization of these cell lines uncovered that tGLI1, but not wild-type GLI1, has rendered MDA-MB-231 cells significantly more invasive and migratory. This finding is also observed in other breast cancer cells we examined. Colony formation soft agar assays further indicate that tGLI1-expressing MDA-MB-231 cells had a higher propensity to undergo anchorage-independent growth than control cells and those with wild-type GLI1. To identify the molecular mechanisms that underlie tGLI1-associated phenotypes, we investigated the effects of tGLI1 (and wild-type GLI1) on several genes known to regulate cancer cell invasion, migration and growth. The results of these studies showed that tGLI1, but not GLI1, transcriptionally upregulates CD24 (an invasion/metastasis-associated gene) and downregulates PUMA (a proapoptotic gene) in breast cancer cells. In light of these exciting observations, ongoing efforts in our laboratory are to investigate the involvement of CD24 and PUMA (and other genes) in tGLI1-mediated breast cancer growth and progression. Animal studies are also in progress to examine the in vivo impact of tGLI1 on breast tumor invasion and metastasis. In summary, we report in this study that the newly discovered tGLI1 transcription factor is expressed tumor-specifically in breast cancer and associated with increased invasiveness, motility and anchorage-independent growth of breast cancer, and thus could be an important regulator of breast cancer growth and progression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2337. doi:10.1158/1538-7445.AM2011-2337