Decreased nuclear expression and increased cytoplasmic expression of ING5 may be linked to tumorigenesis and progression in human head and neck squamous cell carcinoma

This study aimed to assess the protein level of inhibitor of growth gene 5 (ING5) in head and neck squamous cell carcinoma (HNSCC) and to explore its roles in tumorigenesis and cancer progression. ING5 expression was assessed in 172 cases of HNSCC by immunohistochemistry using tissue microarray, and in 3 oral SCC cell lines by immunohistochemistry and Western blot. Expression of ING5 was compared with clinicopathological variables, TUNEL assay staining, and the expression of several tumorigenic markers. In addition, double immunofluorescence labeling was performed in order to analyze the colocalization of ING5 with p300 and p21. ING5 expression was primarily observed in the nuclei, but was also occasionally found in the cytoplasm of both SCC cell lines and tissue samples of HNSCC. Nuclear expression of ING5 in HNSCC was significantly lower than that of non-cancerous epithelium, and was positively correlated with a well-differentiated status. In contrast, cytoplasmic expression of ING5 was significantly increased in HNSCC, and was inversely correlated with a well-differentiated status and nuclear ING5 expression. In addition, nuclear expression of ING5 was positively correlated with p21 and p300 expression, and with the apoptotic index. In contrast, cytoplasmic expression of ING5 was negatively correlated with the expression of p300, p21, and PCNA. Although no statistical association was found between the expression of nuclear ING5 and mutant p53 in HNSCC, patients with high expression of nuclear ING5 tended to have converse prognoses when grouped according to mutant p53 expression. Our results suggest that a decrease in nuclear ING5 localization and cytoplasmic translocation are involved in tumorigenesis and tumor differentiation in HNSCC. Nuclear ING5 may modulate the transactivation of target genes, and may promote apoptosis and cell cycle arrest by interacting with the p300 and p21 proteins. ING5 may function as a tumor suppressor gene or oncogene tightly linked with p53 status, and may play an important role in the prognosis of HNSCC patients. Therefore, we propose that ING5 represents a novel potential molecular therapeutic target for HNSCC.