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Published on: September 3, 2011
Protein Kinase C Quality Control by Phosphatase PHLPP1 Unveils Loss-of-Function Mechanism in Cancer
Timothy R Baffi1, An-Angela N Van1, Wei Zhao2
1Department of Pharmacology, University of California at San Diego, La Jolla, CA 92093, USA; Biomedical Sciences Graduate Program, University of California at San Diego, La Jolla, CA 92093, USA.
Abstract:
Protein kinase C (PKC) isozymes function as tumor suppressors in increasing contexts. In contrast to oncogenic kinases, whose function is acutely regulated by transient phosphorylation, PKC is constitutively phosphorylated following biosynthesis to yield a stable, autoinhibited enzyme that is reversibly activated by second messengers. Here, we report that the phosphatase PHLPP1 opposes PKC phosphorylation during maturation, leading to the degradation of aberrantly active species that do not become autoinhibited. Cancer-associated hotspot mutations in the pseudosubstrate of PKCβ that impair autoinhibition result in dephosphorylated and unstable enzymes. Protein-level analysis reveals that PKCα is fully phosphorylated at the PHLPP site in over 5,000 patient tumors, with higher PKC levels correlating (1) inversely with PHLPP1 levels and (2) positively with improved survival in pancreatic adenocarcinoma. Thus, PHLPP1 provides a proofreading step that maintains the fidelity of PKC autoinhibition and reveals a prominent loss-of-function mechanism in cancer by suppressing the steady-state levels of PKC.
Insights
The phosphatase PHLPP1 ensures Protein Kinase C (PKC) stability by preventing aberrant activation. Loss of PHLPP1 function in cancer leads to decreased PKC levels and promotes tumor suppression.
Area of Science:
- Biochemistry
- Molecular Biology
- Cancer Research
Background:
- Protein Kinase C (PKC) isozymes act as tumor suppressors.
- PKC requires constitutive phosphorylation for stable, autoinhibited function, unlike oncogenic kinases regulated by transient phosphorylation.
Purpose of the Study:
- To investigate the role of the phosphatase PHLPP1 in regulating PKC phosphorylation and stability.
- To identify mechanisms of PKC dysfunction in cancer.
Main Methods:
- Analysis of protein phosphorylation and stability.
- Investigation of cancer-associated mutations in PKC pseudosubstrate.
- Protein-level analysis in patient tumors.
Main Results:
- PHLPP1 opposes PKC phosphorylation during maturation, targeting unstable, aberrantly active species for degradation.
- Cancer-associated mutations impairing PKCβ autoinhibition lead to dephosphorylated, unstable enzymes.
- PKCα is fully phosphorylated at the PHLPP1 site in over 5,000 tumors, with higher PKC levels inversely correlating with PHLPP1 levels and positively with improved survival in pancreatic adenocarcinoma.
Conclusions:
- PHLPP1 acts as a crucial proofreading mechanism for PKC autoinhibition fidelity.
- Loss of PHLPP1 function represents a significant cancer-associated loss-of-function mechanism by reducing PKC steady-state levels.
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