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. ABBREVIATIONS: AKT: V-Akt Murine Thymoma Viral Oncogene; CA: Cancer adjacent; CDK1: Cyclin dependent kinase 1; CENPC-C: Centromere Protein C; ChIP: Chromatin Immunoprecipitation; co-IP: Coimmunoprecipitation; COSMIC: Catalog of Somatic Mutations In Cancer; CREB: cAMP Responsive Element Binding Protein; C-tail: Carboxy terminal tail; E2F1: E2F Transcription Factor 1; ECIS: Electric Cell-substrate Impedance Sensing; EGFR: Epidermal Growth Factor Receptor; GSI: Gamma Secretase Inhibitor; HDAC1: Histone Deacetylase 1; HP1: Heterochromatin protein 1; KAP1/TRIM28: KRAB-Associated Protein 1/ Tripartite Motif Containing 28; MAF1: Repressor of RNA polymerase III transcription MAF1 homolog; MCM2: Minichromosome Maintenance Complex Component 2; miRNA: micro RNA; MTF1: Metal-Regulatory Transcription Factor 1; PARP: Poly(ADP-Ribose) Polymerase; PD-1: Programmed Cell Death 1; PD-L1: Programmed Cell Death 1 Ligand 1; PI3K: Phosphatidylinositol-4,5-Bisphosphate 3-Kinase; PLK: Polo-like Kinase; pPTEN: Phosphorylated PTEN; PTEN: Phosphatase and Tensin Homolog deleted on chromosome ten; PTM: Post Translational Modification; Rad51: RAD51 Recombinase; Rad52: RAD52 Recombinase; RPA1: Replication protein A; SILAC: Stable Isotope Labeling with Amino Acids in Cell Culture; SRF: Serum Response Factor; TKI: Tyrosine Kinase inhbitors; TMA: Tissue Microarray; TOP2A: DNA Topoisomerase 2A