Investigation of the Subcellular Localization-Dependent Anti- or Pro-Tumor Functions of Maspin in Human Lung Adenocarcinoma Cell Line

Background: Mammary serine protease inhibitor (maspin) is well known as a tumor suppressor gene in several types of cancers and its nuclear localization is essential for its tumor-suppressive function. We previously reported that the cytoplasmic-only localization of maspin is significantly correlated with unfavorable prognosis in patients with lung adenocarcinoma (LUAD). To clarify whether maspin in LUAD acts as a tumor promoter or suppressor, we examined the subcellular localization-dependent biological functions of maspin in human LUAD cell lines. Methods: The expression levels and subcellular localization of maspin were investigated by performing immunoblotting and immunofluorescence in human LUAD cell lines (PC-9, A549, NCI-H23, RERF-LCKJ) and human bronchial epithelial cell line (BEAS-2B). We then established stable cell lines overexpressing maspin (A549-maspin and RERF-LC-KJ-maspin) and investigated their subcellular localization. Cell invasion assays of these cell lines were performed to examine their invasiveness. Moreover, the mRNA expression levels between epithelial cell markers (E-cadherin) and mesenchymal cell markers (N-cadherin and vimentin) were compared. Results: The expression of maspin in PC-9 cells was comparable to that in BEAS-2B cells, whereas its expression in A549, NCI-H23, and RERF-LC-KJ cells was decreased. The cell invasion capability of A549-maspin cells showing pancellular expression was significantly decreased compared with that of A549-control cells. By contrast, the cell invasion capability of RERF-LC-KJmaspin cells showing cytoplasmic-only expression was significantly increased compared with that of RERFLC-KJ-control cells. The mRNA expression levels of N-cadherin, but not E-cadherin and vimentin, in A549-maspin cells was significantly downregulated compared with that in A549-control cells. No significant differences in these markers were observed between RERFLC-KJ-maspin and RERF-LC-KJ-control cells. Conclusion: The invasive capability of LUAD cells is regulated by the intracellular localization of maspin. Clarification of the molecular mechanism underlying the subcellular localization-dependent function of maspin will promote a deeper understanding of LUAD development and progression.