Opposing Functions of Maspin Are Regulated by Its Subcellular Localization in Lung Squamous Cell Carcinoma Cells

Mammary serine protease inhibitor (maspin) is a tumor suppressor protein downregulated during carcinogenesis and cancer progression; cytoplasmic-only maspin expression is an independent, unfavorable prognostic indicator in patients with lung squamous cell carcinoma (LUSC). We hypothesized that the cytoplasmic-only localization of maspin has tumor-promoting functions in LUSC. The subcellular localization of maspin and the invasive capability of LUSC cell lines were investigated using RNA sequencing (RNA-seq), Western blotting, and siRNA transfection. Maspin mRNA and protein expression were suppressed in LK-2 and RERF-LC-AI cells. Cell invasion significantly increased in response to siRNA-mediated maspin knockdown in KNS-62 cells expressing both nuclear and cytoplasmic maspin. In LK-2 cells, both nuclear and cytoplasmic maspin were re-expressed, and cell invasion and migration were significantly decreased. In contrast, re-expressed maspin in RERF-LC-AI cells was detected only in the cytoplasm (cytMaspin), and cell invasion and migration were significantly promoted. RNA-seq and downstream analyses revealed that increased cytMaspin expression downregulated the genes associated with cell adhesion and activated PYK2 and SRC, which play important roles in cancer progression. Our study demonstrates a novel biological function of cytMaspin in enhancing the invasive capabilities of LUSC cells. Understanding cytoplasm-to-nuclear maspin translocation dysregulation may develop novel therapeutic approaches to improve the prognosis of patients with LUSC.