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Nucleophosmin interacts with and inhibits the catalytic function of eukaryotic initiation factor 2 kinase PKR
Qishen Pang1, Tracy A Christianson, Tara Koretsky
1OHSU Cancer Institute, Schools of Medicine and Dentistry, Oregon Health Sciences University, Portland, OR 97201, USA.
Abstract:
In normal cells the protein kinase PKR effects apoptosis in response to various extra and intracellular cues and can also function to suppress the neoplastic phenotype. Because most neoplastic cells are resistant to certain apoptotic cues, we reasoned that an early molecular event in carcinogenesis or leukemogenesis might be the inactivation of PKR by expression or activation of intracellular PKR inhibitors. Seeking novel PKR-modulating proteins we report here that nucleophosmin (NPM), a protein frequently overexpressed in a variety of human malignancies, binds to PKR, and inhibits its activation. Co-immunoprecipitation and in vitro binding experiments showed that NPM associated with PKR. Kinase assays demonstrated that recombinant NPM inhibited PKR activation in a dose-dependent manner. In addition, purified recombinant NPM was phosphorylated by activated PKR. Most importantly, overexpression of NPM suppressed PKR activity, enhanced protein synthesis, and inhibited apoptosis. Lymphoblasts from patients with Fanconi anemia (FA) expressed low levels of NPM, which correlated with high ground-state activation of PKR and cellular hypersensitivity to apoptotic cues, but enforced expression of NPM in these mutant cells reduced aberrant apoptotic responses. Inhibition of PKR by NPM may be one mechanism by which neoplastic clones evolve in sporadic malignancies and in neoplastic cells arising in the context of the cancer predisposition syndrome, Fanconi anemia.
Insights
Nucleophosmin (NPM) inhibits the protein kinase PKR, a key regulator of apoptosis and cancer suppression. This finding reveals a potential mechanism for cancer development, particularly in Fanconi anemia.
Area of Science:
- Molecular Biology
- Cancer Research
- Cellular Signaling
Background:
- The protein kinase PKR induces apoptosis and suppresses neoplastic phenotypes in normal cells.
- Neoplastic cells often exhibit resistance to apoptosis, suggesting potential molecular mechanisms of inactivation.
- Identifying regulators of PKR is crucial for understanding carcinogenesis and leukemogenesis.
Purpose of the Study:
- To identify novel proteins that modulate protein kinase PKR (PKR) activity.
- To investigate the role of nucleophosmin (NPM) in regulating PKR activation and function.
- To explore the implications of NPM-PKR interaction in cancer development and Fanconi anemia.
Main Methods:
- Co-immunoprecipitation and in vitro binding assays to confirm NPM-PKR association.
- Kinase assays using recombinant NPM to assess its inhibitory effect on PKR activation.
- Overexpression studies in cells to evaluate the functional consequences of NPM on PKR activity, protein synthesis, and apoptosis.
- Analysis of NPM levels and PKR activation in lymphoblasts from Fanconi anemia patients.
Main Results:
- Nucleophosmin (NPM) directly binds to and inhibits the activation of protein kinase PKR (PKR).
- Recombinant NPM demonstrated dose-dependent inhibition of PKR activation and was phosphorylated by activated PKR.
- Overexpression of NPM suppressed PKR activity, enhanced protein synthesis, and inhibited apoptosis.
- Low NPM levels in Fanconi anemia lymphoblasts correlated with high PKR activation and increased apoptosis sensitivity; NPM restoration reduced these aberrant responses.
Conclusions:
- Nucleophosmin (NPM) acts as an inhibitor of protein kinase PKR (PKR).
- NPM-mediated inhibition of PKR may contribute to the evolution of neoplastic clones in sporadic malignancies and Fanconi anemia.
- Targeting the NPM-PKR interaction could offer therapeutic strategies for cancers associated with PKR dysregulation.