Background: Head and neck squamous cell carcinoma (HNSCC) is a common cancer worldwide. Emerging evidence indicates that alteration of epigenetics might be a key event in HNSCC progression. Abnormal expression of histone methyltransferase G9a, which contributes to transcriptional repression of tumor suppressors, has been implicated in promoting cancerous malignancies. However, its role in HNSCC has not been previously characterized. In this study, we elucidate the function of G9a and its downstream mechanism in HNSCC.
Methods: We investigated the clinical relevance of G9a in HNSCC using immunohistochemistry (IHC) staining. In vitro cell proliferation and tumorigenesis ability of G9a-manipulated HNSCC cells were examined with MTT assays, clonogenic assays, and soft agar assays. We examined different routes of cell death in HNSCC cells induced by G9a-depletion or enzymatic inhibition by immunoblot, flow cytometry, fluorescent and transmission electron microscopy analysis. Specific targets of G9a were identified by affymetrix microarray and quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Lastly, functions of G9a in vivo were confirmed with a xenograft tumor model.
Results: G9a expression is positively correlated to proliferation marker Ki-67 and to poor prognosis in HNSCC patients. Genetic or pharmacological inhibition of G9a reduced cell proliferation without inducing necrosis or apoptosis. Instead, autophagic cell death was the major consequence, and our investigation of mechanisms suggested it is mediated via the dual specificity phosphatase-4 (DUSP4) dependent ERK inactivation pathway. An orthotopic tumor model further confirmed the growth inhibiting effect and induction of autophagy that followed suppression of G9a.
Conclusions: In this study, we provide evidence that G9a confers the survival advantage of HNSCC. Genetic or pharmacological inhibition of G9a induces autophagic cell death; this finding provides a basis for new therapeutic targets for treating HNSCC.