Although clinical outcomes for patients with locally advanced head and neck squamous cell carcinoma (HNSCC) have improved significantly with multimodality treatment approaches, that is not the case for patients with recurrence and/or distant metastasis. Therefore, understanding the biologic processes of treatment resistance and metastasis is critical for successful intervention. Furthermore, identification of novel therapeutic targets for addressing metastasis and development of clinically effective targeted agents to improve survival remains a difficult challenge.
Hedgehog (Hh) signaling is one of the key master regulators of both invertebrate and vertebrate development.1–4
Constitutive Hh signal activation due to mutations or activating deregulation is implicated in numerous neoplastic or hyperplastic conditions. For instance, constitutive activation of the Hh pathway has been shown to play a critical role in tumorigenesis in malignant medulloblastoma, basal cell carcinoma of the skin, and breast, urogenital, GI, pancreatic, and lung cancers.5–14
Although significant expression of proteins involved in Hh signaling has been reported,15
the oncogenic role of protein expression has not been examined in HNSCC.
There are three members of the Hh family of extracellular signaling molecules—sonic hedgehog (SHH), Indian hedgehog, and desert hedgehog—that activate a membrane receptor complex (Appendix Fig A1
, online only). Binding of Hh to a transmembrane receptor, patched 1 (PTCH1), releases its inhibition of smoothened (SMO), a distant cousin of the 7-transmembrane G-protein coupled receptor family.16
Activation of SMO by Hh results in activation of glioma-associated oncogene family zinc finger 1 (GLI1) along with GLI2 and GLI3, which are thought to mediate most of the cellular effects.1
In the absence of Hh, GLI transcription factors are phosphorylated, ubiquitinated, and degraded. Although GLI1 and GLI2 generally function as transcriptional activators, the partially degraded GLI3 can function as a transcriptional repressor.17–21
Hh signaling is known to be induced in bronchial epithelial cells exposed to cigarette smoke; thus, it contributes to cell proliferation, anchorage-independent growth, and tumor formation in nude mice.22
In addition, GLI can be activated by noncanonical pathways, including RAS-MEK, AKT, and TGF-beta in the absence of Hh ligands,23–26
and epidermal growth factor (EGF) signaling has been shown to modulate Hh signaling in keratinocytes.27
Furthermore, Hh signaling has been shown to induce epithelial-to-mesenchymal transition (EMT) by inhibition of WNT/beta-catenin (CTNNB1) signaling and by upregulating secreted frizzled-related protein 1 (SFRP1). EMT-related genes were one of the three most significant gene sets that were enriched in high-risk patients with HNSCC in our previous study.28,29
In recent studies, a novel SMO inhibitor, GDC-0449, induced response and disease stabilization in patients with advanced basal cell carcinoma and medulloblastoma who frequently harbor inactivating mutations of PTCH1
or less common activating mutations of SMO
As expected, overexpression of GLI1
mRNA, indicating activation of the Hh pathway, was associated with clinical benefits. The oncogenic significance and clinical development of Hh pathway inhibitors were also summarized by Low et al32
in a recent comprehensive review.
To assess a potential oncogenic role of Hh signaling in HNSCC, we examined the nuclear expression of GLI1 in patients treated with radiation therapy in the Radiation Therapy Oncology Group (RTOG) 9003 clinical trial. We also assessed the correlation of GLI1 expression with CTNNB1 to determine the inhibitory effect of GLI1 on WNT signaling and correlation with EGF receptor (EGFR) expression to examine a potential noncanonical activation of GLI1 through the EGFR pathway. Additionally, these protein expression levels were correlated with clinical outcomes, including time to metastasis (TTM), time to disease progression (TDP), and overall survival (OS), to evaluate their roles as prognostic biomarkers.