Significance of PTEN Phosphorylation and its Nuclear Function in Lung Cancer

Phosphorylation mediated inactivation of PTEN leads to multiple malignancies with increased severity. However, the consequence of such inactivation on downstream functions of PTEN are poorly understood. Therefore, the objective of my thesis is to ascertain the molecular mechanisms by which PTEN phosphorylation drives lung cancer. PTEN phosphorylation at the C-terminal serine/threonine cluster abrogates its tumor suppressor function. Despite the critical role of the PTEN C-tail in regulating its function, the crystal structure of the C-tail remains unknown. Using bioinformatics and structural analysis, I determined that the PTEN C-tail is an intrinsically disordered region and is a hot spot for post-translational modifications (particularly phosphorylation) and protein-protein interactions. Evolutionary analysis of PTEN and its interacting proteins revealed that the PTEN C-tail has only recently evolved to acquire the ability to engage in a myriad of protein-protein interactions, resulting in its versatile functions. Replacement of the PTEN C-tail serine/threonine residues with alanines generated an artificial mutant, PTEN-4A, which remained “phospho-deficient” and therefore constitutively active. Interestingly, PTEN-4A suppressed cell proliferation and migration to a greater extent than PTEN-WT. PTEN-4A preferentially localized to the nucleus where it suppressed E2F-mediated transcription of cell cycle genes. PTEN physically interacted with the E2F1 protein and at E2F1-binding sites on chromatin, a likely mechanism for its transcriptional function. Further, deletion analysis on various PTEN domains revealed that the C2 domain of PTEN is indispensable for suppression of E2F-related genes. Systematic transcriptional promoter-reporter assays identified disease-associated C2 domain mutations that lose their ability to suppress E2F-mediated transcription, supporting the concept that these mutations are oncogenic in patients. Consistent with my findings, I observed increased level of PTEN phosphorylation and reduced nuclear PTEN levels in lung cancer patient samples. Further, to determine whether the enhanced growth-suppressive properties of PTEN-4A may be due to differential protein-protein interactions, I performed a comparative proteomic profiling of PTEN-WT and PTEN-4A interactomes using the SILAC methodology. Galectin-1 was identified as a candidate protein that binds preferentially to PTEN-WT and inhibits its tumor suppressive function. Taken together, the various tumor suppressive mechanisms of PTEN-4A may be harnessed therapeutically as adjunctive cancer therapy. Use of small molecule inhibitors that hinder PTEN C-tail phosphorylation is a plausible approach to activate PTEN function to reduce tumor burden