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

TL;DR

Maspin's location in lung cancer cells determines whether it suppresses or promotes tumor growth, with cytoplasmic maspin increasing cancer spread.

Contribution

The study reveals that cytoplasmic maspin promotes invasion in lung squamous cell carcinoma by activating PYK2 and SRC.

Findings

Nuclear maspin reduces cell invasion and migration in LK-2 cells.

Cytoplasmic maspin increases invasion and migration in RERF-LC-AI cells.

CytMaspin downregulates cell adhesion genes and activates PYK2 and SRC.

Abstract

Mammary serine protease inhibitor (maspin) is a tumor suppressor protein, and its nuclear localization is essential for its tumor suppressive activity. We previously reported that cytoplasmic-only maspin expression is an independent unfavorable prognostic indicator in patients with lung squamous cell carcinoma (LUSC). Taken together, we hypothesized that maspin has opposing roles in LUSC depending on its subcellular localization. Maspin was re-expressed in both the nucleus and cytoplasm of LK-2 cells, resulting in significantly decreased cell invasion and migration. In contrast, re-expressed maspin in RERF-LC-AI cells was detected only in the cytoplasm (cytMaspin) and significantly promoted cell invasion and migration. Increased cytMaspin expression downregulates the genes relevant to cell adhesion and activates PYK2 and SRC. This study suggests that the cytoplasm-to-nuclear translocation of maspin is dysregulated in RERF-LC-AI cells, and cytMaspin enhances the invasive potential by activating PYK2 and SRC in LUSC.

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.

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