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Nuclear-cytoplasmic shuttling of APC regulates beta-catenin subcellular localization and turnover
1Westmead Institute for Cancer Research, University of Sydney, Westmead Millenium Institute, Westmead NSW 2145, Australia. beric_henderson@wmi.usyd.edu.au
Abstract:
Mutational inactivation of the APC gene is a key early event in the development of familial adenomatous polyposis and colon cancer. APC suppresses tumour progression by promoting degradation of the oncogenic transcriptional activator beta-catenin. APC gene mutations can lead to abnormally high levels of beta-catenin in the nucleus, and the consequent activation of transforming genes. Here, we show that APC is a nuclear-cytoplasmic shuttling protein, and that it can function as a beta-catenin chaperone. APC contains two active nuclear export sequences (NES) at the amino terminus, and mutagenesis of these conserved motifs blocks nuclear export dependent on the CRM1 export receptor. Treatment of cells with the CRM1-specific export inhibitor leptomycin B shifts APC from cytoplasm to nucleus. beta-catenin localization is also regulated by CRM1, but in an APC-dependent manner. Transient expression of wild-type APC in SW480 (APCmut/mut) colon cancer cells enhances nuclear export and degradation of beta-catenin, and these effects can be blocked by mutagenesis of the APC NES. These findings suggest that wild-type APC controls the nuclear accumulation of beta-catenin by a combination of nuclear export and cytoplasmic degradation.
Insights
The Adenomatous Polyposis Coli (APC) protein acts as a chaperone for beta-catenin, regulating its levels in the nucleus. Wild-type APC promotes beta-catenin export and degradation, preventing colon cancer development.
Area of Science:
- Molecular Biology
- Cancer Research
- Cell Biology
Background:
- Mutations in the Adenomatous Polyposis Coli (APC) gene are critical in familial adenomatous polyposis and colon cancer.
- APC normally suppresses tumor progression by targeting the oncogenic transcriptional activator beta-catenin for degradation.
- APC gene mutations result in elevated nuclear beta-catenin levels, activating oncogenes.
Purpose of the Study:
- To investigate the role of APC in regulating beta-catenin localization and degradation.
- To determine if APC functions as a nuclear-cytoplasmic shuttling protein.
- To elucidate the mechanism by which APC controls nuclear beta-catenin accumulation.
Main Methods:
- Utilized mutagenesis of APC's nuclear export sequences (NES).
- Employed CRM1 export receptor inhibition using leptomycin B.
- Assessed beta-catenin localization and degradation in colon cancer cells (SW480) with wild-type APC expression.
Main Results:
- Demonstrated that APC is a nuclear-cytoplasmic shuttling protein.
- Identified two active NES motifs in APC, crucial for CRM1-dependent nuclear export.
- Showed that wild-type APC enhances nuclear export and degradation of beta-catenin in a CRM1-dependent and APC NES-dependent manner.
Conclusions:
- Wild-type APC functions as a beta-catenin chaperone, controlling its nuclear accumulation.
- APC regulates beta-catenin levels through a combination of nuclear export and cytoplasmic degradation pathways.
- These findings provide insights into APC's tumor-suppressive function and potential therapeutic targets in colon cancer.