The Hedgehog (Hh) signaling pathway is critical for embryonic developmental processes and its deregulation is implicated in a wide variety of tumor types. However, the role of the Hh signaling pathway in the initiation and growth of non-small cell lung cancer is largely unknown. The purpose of this study is to investigate the role of GLI1, a major Hh pathway transcription factor, in lung squamous cell carcinoma (SCC) and to test the therapeutic potential of targeting GLI1.

GLI1 expression in human SCC cell lines was evaluated by quantitative PCR and Western Blot. siRNA and shRNA of GLI1 in these cell lines were utilized in vitro and in vivo to test the requirement of GLI1 in tumor growth. Small molecule modulators of GLI1 were tested for their therapeutic potential.

We have demonstrated that GLI1 has a critical role in SCC progression. GLI1 expression was significantly elevated in lung SCC compared to normal lung and lung adenocarcinoma patient specimens in several human genomic databases. Importantly, overexpression of GLI1 was correlated with poor overall survival in lung cancer patients. siRNA-mediated knock down of GLI1 in SCC cell lines decreased the expression of GLI1 target genes and caused a significant reduction in colony formation. Stable knock down of GLI1 in SCC cell lines caused a significant reduction in growth of xenograft tumors indicating the critical role of GLI1 in lung SCC progression. Inhibition or activation of SMO, an upstream component of Hh pathway, did not change GLI1 expression level in SCC cell lines. However, inhibition of PI3K/mTOR and MAPK signaling pathways down regulated GLI1 expression. These results suggested that GLI1 expression is dependent on PI3K/mTOR and MAPK pathway activity rather than Hh ligand. PI3K/mTOR inhibitor, or arsenic trioxide (ATO), a direct inhibitor of GLI proteins, significantly reduced GLI1 expression, proliferation, and clonogenicity in SCC cell lines. We have also observed a significant growth reduction in SCC xenografted tumors treated with combination of PI3K inhibitor and ATO.

Our findings suggest that GLI1 is essential for lung SCC tumor progression. Furthermore, GLI1 expression in SCC is independent of Hh pathway ligand action and dependent on MAPK and PI3K pathway activity. Direct inhibition of GLI1 by repurposing ATO in combination with a PI3K inhibitor may represent a novel therapeutic strategy for lung SCC.

Citation Format: Sahba Kasiri, Chunli Shao, Baozhi Chen, Alexandra Wilson, Paul Yenerall, Brenda Timmons, patrick dospoy, Suzie Hight, Luc Girard, Hui Tian, Carmen Behrens, Ignacio Wistuba, Adi Gazdar, James Kim. Non-classical activation of GLI1 as a therapeutic target for squamous cell lung cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3017.