Lung cancer is the leading cause of cancer-related deaths worldwide. 85% of lung cancer is non-small cell lung cancer (NSCLC), with adenocarcinoma (AD) and squamous cell carcinoma (SCC) as the two major pathological subtypes . Lung SCC is closely associated with tobacco smoking, and it accounts 35% of NSCLC, causing an estimated 400,000 deaths per year worldwide . While recent improvements in targeted therapies such as the EGFR tyrosine kinase inhibitors (TKI), bevacizumab and ALK inhibitors have significantly benefited patients with AD, the effectiveness of these treatments are unfortunately disappointing for lung SCC . Lung SCC patients suffer from poor prognosis with significant rates of reoccurrence and metastasis, largely due to the differences in genetic profiles . Recent studies identified potentially actionable genetic abnormalities in lung SCC, such as phosphoinositide 3-kinase (PIK3CA) amplification, fibroblast growth factor receptor 1 (FGFR1) amplification, and discoidin domain receptor 2 (DDR2) mutation. However, significant efforts are still needed to help in the investigation of the biological characteristics of lung SCC in order to decipher and the mechanism underlying the invasion and metastasis of lung SCC.
Epithelial–mesenchymal transition (EMT) was originally characterized during embryonic development. The concept that EMT being a critical event in the invasion, progression and metastasis of epithelial cancers is well established [5, 6]. The molecular basis of EMT involves multiple changes in expression, distribution, and/or function of proteins, i.e. E-cadherin, and the process of EMT is regulated by many molecular events including multiple signaling pathways in various cancers . Furthermore, acquisition of the features of the EMT has been associated with poor prognosis and chemo-resistance, which may allow for recurrence and metastasis to occur after treatment with a standard chemotherapeutic treatment [7–10]. The mechanistic study of EMT regulation could contribute to our understanding of recurrence and metastasis in cancer.
Activation of Hedgehog (Hh) signaling has been implicated in tumorigenesis and metastasis in various cancer types [11–23]. Hh signaling is orchestrated by two trans-membrane receptors, Patched (Ptch) and Smoothened (Smo). In the canonical Hh pathway, in the absence of the Hh ligand, Ptch inhibits Smo, causing cleavage of Gli to the N-terminal repressor form. Once Hh binds to Ptch, the inhibitory effect on Smo is released, causing active full-length Gli to transport into the nucleus and activate transcription of Hh target genes in a context- and cell-type specific manner. Moreover, several studies have revealed "non-canonical Gli activation" in many cancer cell types by which Gli is activated independent of Hh/Smo regulation [12, 14]. It needs to be elucidated if the canonical Hh pathway or the non-canonical Gli activation is involved in lung SCC, and if Gli activation contributes to the regulation of metastasis.
Studies of EMT regulation by Hh pathway have recently emerged in literature; data, however, is rare and controversial. While Alexaki et al.  and Inaguma et al.  suggested that Gli-factor facilitates cancer cell migration and invasion through E-Cadherin in melanoma and pancreatic cancers, Joost et al.  proposed that inhibition of Gli promoted EMT in pancreatic cancers. Our study intends to extend the research to lung SCC to help us better understand the regulation of EMT by Hh signaling.
We reported the activation of Hh signaling in two cohorts of patient samples, and revealed the reverse association between Gli1 expression and the expression of EMT markers. Inhibition of the Shh/Gli pathway suppressed migration and up-regulated E-Cadherin expression in lung SCC cells. Stimulation of the pathway increased migration and down-regulated E-Cadherin expression in lung SCC cells.