MYC dephosphorylation by the PP1/PNUTS phosphatase complex regulates chromatin binding and protein stability

Area of Science:

• Oncology
• Molecular Biology
• Biochemistry

Background:

• The c-MYC (MYC) oncoprotein is frequently deregulated in over 50% of human cancers.
• Regulatory mechanisms governing MYC stability and function remain incompletely understood.
• Identifying MYC interactors is crucial for understanding its role in cancer.

Purpose of the Study:

• To investigate the MYC interactome and identify novel regulatory partners.
• To elucidate the role of identified interactors in MYC stability and function.
• To explore the therapeutic potential of targeting MYC-regulatory pathways.

Main Methods:

• BioID proximity labeling followed by mass spectrometry (MS) to identify MYC interactors.
• Co-immunoprecipitation and immunofluorescence to validate MYC-PNUTS interaction.
• RNA interference (RNAi) and pharmacological inhibition to study PP1 function.
• Western blotting and proteasomal degradation assays to assess MYC protein levels.
• Chromatin immunoprecipitation (ChIP) to evaluate MYC chromatin binding.

Main Results:

• Protein phosphatase 1 (PP1) and its regulatory subunit PNUTS were identified as novel MYC interactors.
• Endogenous MYC and PNUTS interact and co-occupy MYC target gene promoters.
• Inhibition of PP1 leads to MYC hyperphosphorylation and subsequent proteasomal degradation via the SCF^FBXW7 pathway.
• PP1/PNUTS specifically stabilizes chromatin-bound MYC in proliferating cells.
• Hyperphosphorylated MYC retains interaction with MAX but exhibits compromised chromatin binding.

Conclusions:

• The PP1/PNUTS complex plays a critical role in stabilizing chromatin-bound MYC.
• Targeting the PP1/PNUTS interaction or PP1 activity could represent a novel therapeutic strategy for MYC-driven cancers.
• Understanding MYC regulation by phosphatases offers new avenues for cancer therapy development.