ABSTRACT
The Wnt pathway is critical for normal development, and mutation of specific components is seen in carcinomas of diverse origins. The role of this pathway in lung tumorigenesis has not been clearly established. Recent studies from our laboratory indicate that combined expression of the combination of Wnt 7a and Frizzled 9 (Fzd 9) in Non-small Cell Lung Cancer (NSCLC) cell lines inhibits transformed growth. We have also shown that increased expression of peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits transformed growth of NSCLC and promotes epithelial differentiation of these cells. The goal of this study was to determine whether the effects of Wnt 7a/Fzd 9 were mediated through PPARgamma. We found that Wnt 7a and Fzd 9 expression led to increased PPARgamma activity. This effect was not mediated by altered expression of the protein. Wnt 7a and Fzd 9 expression resulted in activation of ERK5, which was required for PPARgamma activation in NSCLC. SR 202, a known PPARgamma inhibitor, blocked the increase in PPARgamma activity and restored anchorage-independent growth in NSCLC expressing Wnt 7a and Fzd 9. SR 202 also reversed the increase in E-cadherin expression mediated by Wnt 7a and Fzd 9. These data suggest that ERK5-dependent activation of PPARgamma represents a major effector pathway mediating the anti-tumorigenic effects of Wnt 7a and Fzd 9 in NSCLC.
Subject(s)
Carcinoma, Non-Small-Cell Lung/metabolism , Frizzled Receptors/physiology , Gene Expression Regulation, Neoplastic , Lung Neoplasms/metabolism , Mitogen-Activated Protein Kinase 7/metabolism , PPAR gamma/metabolism , Proto-Oncogene Proteins/physiology , Receptors, G-Protein-Coupled/physiology , Receptors, Neurotransmitter/physiology , Wnt Proteins/physiology , Antineoplastic Agents/pharmacology , Cadherins/biosynthesis , Cell Line, Tumor , Frizzled Receptors/chemistry , Gene Transfer Techniques , Humans , Organophosphorus Compounds/pharmacology , Proto-Oncogene Proteins/metabolism , Receptors, G-Protein-Coupled/chemistry , Receptors, Neurotransmitter/metabolism , Wnt Proteins/metabolismABSTRACT
The Wnt signaling pathway is critical in normal development, and mutation of specific components is frequently observed in carcinomas of diverse origins. However, the potential involvement of this pathway in lung tumorigenesis has not been established. In this study, analysis of multiple Wnt mRNAs in non-small cell lung cancer (NSCLC) cell lines and primary lung tumors revealed markedly decreased Wnt-7a expression compared with normal short-term bronchial epithelial cell lines and normal uninvolved lung tissue. Wnt-7a transfection in NSCLC cell lines reversed cellular transformation, decreased anchorage-independent growth, and induced epithelial differentiation as demonstrated by soft agar and three-dimensional cell culture assays in a subset of the NSCLC cell lines. The action of Wnt-7a correlated with expression of the specific Wnt receptor Frizzled-9 (Fzd-9), and transfection of Fzd-9 into a Wnt-7a-insensitive NSCLC cell line established Wnt-7a sensitivity. Moreover, Wnt-7a was present in Fzd-9 immunoprecipitates, indicating a direct interaction of Wnt-7a and Fzd-9. In NSCLC cells, Wnt-7a and Fzd-9 induced both cadherin and Sprouty-4 expression and stimulated the JNK pathway, but not beta-catenin/T cell factor activity. In addition, transfection of gain-of-function JNK strongly inhibited anchorage-independent growth. Thus, this study demonstrates that Wnt-7a and Fzd-9 signaling through activation of the JNK pathway induces cadherin proteins and the receptor tyrosine kinase inhibitor Sprouty-4 and represents a novel tumor suppressor pathway in lung cancer that is required for maintenance of epithelial differentiation and inhibition of transformed cell growth in a subset of human NSCLCs.
