Detection of cytoplasmic and nuclear localization of adenomatous polyposis coli (APC) protein in cells

Identifying novel protein complexes in cancer cells using epitope-tagging of endogenous human genes and affinity-purification mass spectrometry

Affinity-purification mass spectrometry (AP-MS) is the preeminent technique for identification of eukaryotic protein complexes in vivo. AP-MS workflows typically express epitope-tagged bait proteins, immunopurify, and then identify associated protein complexes using mass spectrometry. However, challenges of existing strategies include the construction of expression vectors for large open reading frames and the possibility that overexpression of bait proteins may result in expression of nonphysiological levels of the bait protein with concomitant perturbation of endogenous protein complexes. To address these issues, we use human cell lines with epitope-tagged endogenous genes as AP-MS substrates to develop a platform that we call "knock-in AP-MS", thereby avoiding the challenges of expression vector construction and ensuring that expression of tagged proteins is driven by endogenous regulatory mechanisms. Using three different bait genes (MRE11A, DNMT1 and APC), we show that cell lines expressing epitope-tagged endogenous genes make good substrates for sensitive and reproducible identification of protein interactions using AP-MS. In particular, we identify novel interactors of the important oncoprotein Adenomatous Polyposis Coli (APC), including an interaction with Flightless-1 homologue (FLII) that is enriched in nuclear fractions.

Nuclear translocation of β-catenin is essential for glioma cell survival

Identification of molecular pathways that are essential for cancer cell survival is vital for understanding the underlying biology, as well as to design effective cancer therapeutics. β-catenin, a multifunctional oncogenic protein, participates in cell development. Its multifaceted functions primarily lie to the subcellular distribution. The present study demonstrated that β-catenin accumulated in the nucleus to a greater extent in high-grade gliomas compared with low-grade gliomas. In addition, nuclear localization correlated with a worse prognosis for patients, as determined by immunohistochemical analysis of 74 glioma samples. Nuclear expression of β-catenin was down-regulated in LN229 and U87 glioma cells by a small molecule inhibitor of β-catenin/TCF4 signaling, demonstrating strongly inhibited β-catenin/TCF4 transcriptional activity and STAT3 luciferase activity, as well as decreased mRNA and protein levels of nuclear β-catenin, TCF4, EGFR, AKT1, AKT2 and STAT3. Furthermore, repressed nuclear translocation of β-catenin resulted in inhibition of proliferation and invasiveness, and also induced apoptosis of glioma cells. Similar results were also observed in vivo; intratumoral injection of such small molecule inhibitor downregulated expression of nuclear β-catenin, TCF4, and components of the EGFR pathway, and also delayed tumor growth in nude mice harboring subcutaneous U87 xenografts. Results from the present study provided evidence that nuclear accumulation of β-catenin participated in malignant progression of gliomas and implicated poor prognosis, highlighting it as a potential therapeutic target for gliomas.