PTEN Physically Interacts with and Regulates E2F1-mediated Transcription in Lung Cancer

PTEN phosphorylation at its C-terminal (C-tail) serine/threonine cluster negatively regulates its tumor suppressor function. However, the consequence of such inhibition and its downstream effects in driving lung cancer remain unexplored. Herein, we ascertain the molecular mechanisms by which phosphorylation compromises PTEN function, contributing to lung cancer. Replacement of the serine/threonine residues with alanine generated PTEN-4A, a phosphorylation-deficient PTEN mutant, which suppressed lung cancer cell proliferation and migration. PTEN-4A preferentially localized to the nucleus where it suppressed E2F1-mediated transcription of cell cycle genes. PTEN-4A physically interacted with the transcription factor E2F1 and associated with chromatin at gene promoters with E2F1 DNA-binding sites, a likely mechanism for its transcriptional suppression function. Deletion analysis revealed that the C2 domain of PTEN was indispensable for suppression of E2F1-mediated transcription. Further, we uncovered cancer-associated C2 domain mutant proteins that had lost their ability to suppress E2F1-mediated transcription, supporting the concept that these mutations are oncogenic in patients. Consistent with these findings, we observed increased PTEN phosphorylation and reduced nuclear PTEN levels in lung cancer patient samples establishing phosphorylation as a bona fide inactivation mechanism for PTEN in lung cancer. Thus, use of small molecule inhibitors that hinder PTEN phosphorylation is a plausible approach to activate PTEN function in the treatment of lung cancer. AbbreviationsAKT V-Akt Murine Thymoma Viral OncogeneCA Cancer adjacentCDK1 Cyclin dependent kinase 1CENPC-C Centromere Protein CChIP Chromatin Immunoprecipitationco-IP Co-immunoprecipitationCOSMIC Catalog of Somatic Mutations In CancerCREB cAMP Responsive Element Binding ProteinC-tail Carboxy terminal tailE2F1 E2F Transcription Factor 1ECIS Electric Cell-substrate Impedance SensingEGFR Epidermal Growth Factor ReceptorGSI Gamma Secretase InhibitorHDAC1 Histone Deacetylase 1HP1 Heterochromatin protein 1KAP1/TRIM28 KRAB-Associated Protein 1/Tripartite Motif Containing 28MAF1 Repressor of RNA polymerase III transcription MAF1 homologMCM2 Minichromosome Maintenance Complex Component 2miRNA micro RNAMTF1 Metal-Regulatory Transcription Factor 1PARP Poly(ADP-Ribose) PolymerasePD-1 Programmed Cell Death 1PD-L1 Programmed Cell Death 1 Ligand 1PI3K Phosphatidylinositol-4,5-Bisphosphate 3-KinasePLK Polo-like KinasepPTEN Phosphorylated PTENPTEN Phosphatase and Tensin Homolog deleted on chromosome tenPTM Post Translational ModificationRad51 RAD51 RecombinaseRad52 RAD52 RecombinaseRPA1 Replication protein ASILAC Stable Isotope Labeling with Amino Acids in Cell CultureSRF Serum Response FactorTKI Tyrosine Kinase inhbitorsTMA Tissue MicroarrayTOP2A DNA Topoisomerase 2A.