Signal Transduction Utilization of Liquid Chromatography Mass Spectrometry Analyses to Identify LKB1–APC Interaction in Modulating Wnt/b-Catenin Pathway of Lung Cancer Cells

STK11/LKB1, a serine/threonine protein kinase and tumor suppressor, is a key upstream kinase of adenine monophosphate-activated protein kinase, which is a kinase involved in controlling cell polarity and maintaining cellular energy homeostasis. LKB1 is mutated in a significant number of Peutz–Jeghers syndrome (PJS) cases and sporadic cancers, and is most frequently mutated in lung adenocarcinomas; however, little is known about how LKB1 is involved in lung cancer progression. In this study, immunoprecipitation-HPLC tandem mass spectrom-etry (IP-LC-MS/MS) was performed to identify novel proteins interacting with LKB1 in lung cancer. Interestingly, many LKB1-interacting proteins acquired from the LC-MS/MS approach were mapped, usingMetaCore pathway analysis, to the cystic fibrosis transmembrane conductance regulator activation pathway. Moreover, it was determined that LKB1 directly interacts with APC, and this LKB1–APC interaction was further confirmed by reverse immunoprecipitation assays, but GSK3b was dispensable for the association of LKB1 and APC. Importantly, LKB1 binds to APC to suppress the Wnt/b-catenin signaling pathway, which is known to be involved in cell proliferation and migration. Subsequent analysis of the downstream targets of the Wnt/TCF pathway led to the identification of several Wnt-regulated genes, such as CD44, COX-2, survivin, and c-Myc, whose expression levels are downregulated by LKB1. In summary, these results demonstrate that LKB1 regulates the Wnt pathway through a direct interaction with APC to suppress the tumorigenic/metastatic potential of lung tumors. Implications: LKB1 status influences the molecular circuitry (Wnt/b-catenin pathway), cellular biology, and may serve as a potential therapeutic node in genetically defined subsets of lung cancer.Mol Cancer Res; 12(4); 622–35. 2014 AACR