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Introduction
Ornithine decarboxylase antizyme 1 (OAZ1) is a member of the ornithine decarboxylase (ODC) antizyme family, which targets ODC for ubiquitin-independent proteasome degradation, thereby inhibiting the synthesis of polyamines and cell proliferation (1). OAZ1 can also inhibit cell growth through ODC-independent mechanisms, including the inhibition of cellular uptake of polyamines by inactivating the polyamine uptake transporter, promoting cyclin D1 degradation and preventing ubiquitin-independent msp1 degradation (2–5). Studies of cancer have also proved that OAZ1 has tumor suppressor activities and it can affect the apoptosis and proliferation of multiple tumor cell lines (6–8).
Several investigators have studied the involvement of OAZ1 in tongue squamous cell carcinoma (TSCC). In p53-knockout mice, overexpression of the OAZ1 protein inhibited the production of keratinocyte proliferation marker, keratin 6 (K6), whereas it upregulated the expression of keratinocyte differentiation marker, loricrin (LOR) (9). In hamster malignant oral keratinocytes, ectopic expression of OAZ1 induced epithelial differentiation with the overexpression of involucrin (IVL) (10). In several human oral cancer cell lines, the expression level of the OAZ1 gene was downregulated (11,12). A previous study has shown that OAZ1 induces the conversion of the human tongue squamous cancer cell line, UM1, to the less metastatic type, UM2, with the hypomethylation of genome DNA and histone H3 lysine 9 dimethylation (12). However, the overall effects of OAZ1 on cellular proliferation and differentiation in human oral cancer cells and the underlying mechanism remain to be studied.
In the present study, the lentiviral vector containing the OAZ1 gene was constructed and transfected into the human tongue cancer cell line, SCC15, to evaluate the effects of OAZ1 expression on proliferation and differentiation of the cells. The results showed that the stable expression of OAZ1 in SCC15 inhibited the cell proliferation rate and induced G0/G1 arrest. OAZ1 expression also induced the formation of epithelial islands with elevation of several differentiation marker genes (K10, FLG and LOR). OAZ1 was also found to inhibit Smad nuclear interacting protein 1 (SNIP1) and silencing of SNIP1 increased the expression of LOR in SCC15 cells. The results of the study proved that OAZ1 can simultaneously inhibit proliferation and induce differentiation of oral cancer cells in humans.
Materials and methods
Cell culture and transfection