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APC-mediated downregulation of beta-catenin activity involves nuclear sequestration and nuclear export
K L Neufeld1, F Zhang, B R Cullen
1Department of Oncological Sciences, Huntsman Cancer Institute, University of Utah, Salt Lake City 84112, USA. kristi.neufeld@genetics.utah.edu
Abstract:
Mutational inactivation of adenomatous polyposis coli (APC) initiates most colon carcinomas. APC functions include targeting cytoplasmic beta-catenin, a Wnt pathway mediator, for proteolysis. Although APC shuttles between cytoplasm and nucleus, the role of nuclear APC protein, particularly with respect to nuclear beta-catenin levels and activity, remains unclear. Here, we demonstrate that APC lacking functional nuclear localization signals (NLSs) or nuclear export signals (NESs) does not effectively downregulate nuclear beta-catenin levels; neither does wild-type APC when nuclear export is blocked. While APC bearing mutated NLSs could not downregulate beta-catenin-mediated transcriptional activation, APC lacking NESs remained active. Consistent with the hypothesis that nuclear APC lacking NESs can inhibit beta-catenin function by sequestration, we show that endogenous APC and beta-catenin proteins interact within the nucleus. These data demonstrate that nuclear APC binding to beta-catenin, and then inducing its nuclear export, plays a critical role in the control of nuclear beta-catenin levels and activity.
Insights
Nuclear adenomatous polyposis coli (APC) protein controls colon cancer by regulating beta-catenin. Nuclear APC binds beta-catenin, promoting its export and reducing cancer-promoting activity.
Area of Science:
- Molecular biology
- Cell biology
- Cancer research
Background:
- Adenomatous polyposis coli (APC) mutations initiate most colon carcinomas.
- APC regulates cytoplasmic beta-catenin, a key Wnt pathway mediator, targeting it for degradation.
- The function of nuclear APC, especially concerning nuclear beta-catenin, is not fully understood.
Purpose of the Study:
- To investigate the role of nuclear APC in regulating nuclear beta-catenin levels and activity.
- To determine how nuclear localization signals (NLSs) and nuclear export signals (NESs) of APC affect beta-catenin.
- To elucidate the mechanism by which nuclear APC influences beta-catenin-mediated transcription.
Main Methods:
- Utilizing APC mutants lacking functional NLSs or NESs.
- Assessing the impact of blocked nuclear export on wild-type APC function.
- Measuring beta-catenin levels and transcriptional activity.
- Investigating the interaction between endogenous APC and beta-catenin in the nucleus.
Main Results:
- APC mutants lacking NLSs or NESs failed to downregulate nuclear beta-catenin.
- Wild-type APC could not reduce nuclear beta-catenin when its nuclear export was inhibited.
- APC mutants lacking NLSs could not inhibit beta-catenin transcriptional activation.
- APC lacking NESs retained the ability to inhibit beta-catenin activity, suggesting nuclear sequestration.
Conclusions:
- Nuclear APC binding to beta-catenin is crucial for controlling nuclear beta-catenin levels.
- The nuclear export of beta-catenin, facilitated by APC, is a critical regulatory step.
- Nuclear APC plays a significant role in suppressing colon carcinogenesis by modulating the Wnt pathway.