Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Medicine

Major Professor

Vrushank Davé, Ph.D.

Committee Member

Meera Nanjundan, Ph.D.

Committee Member

Yu Chen, Ph.D.

Committee Member

Subhra Mohapatra, Ph.D.

Committee Member

Patricia Kruk, Ph.D.

Keywords

Phosphatase and Tensin Homolog, γ-AAPeptides, Allosteric activation, PTEN restoration therapy, PTEN protein aggregation

Abstract

PTEN, a dual protein and lipid phosphatase, regulates a myriad of cellular functions including PI3K pathway signaling, cell migration, proliferation, invasion and apoptosis. PTEN mutations often lead to multiple malignancies, including prostate, breast, endometrial, skin and brain cancers, associated with hyperactive PI3K signaling. PTEN mutations have also been associated with a variety of other diseases, classified as PTEN Hamartoma Tumor Syndromes (PHTS). In addition, compromised function or reduced expression of PTEN due to non-genomic mechanisms are associated with many types of hyperproliferative diseases, such as restenosis and neoplastic diseases, including melanoma, lung, breast, prostate and colon cancers. Although PI3K pathway inhibitors are the first line of targeted therapy in these cancers, off-target effects, toxicities, and chemoresistance persists in the clinic, warranting development of alternative therapeutic paradigms. Consequently, restoration of PTEN expression and/or function has increasingly become an attractive option. Cell-based and pre-clinical studies have demonstrated that reintroduction of PTEN attenuates oncogenic signaling and tumorigenic potential. Unfortunately, restoration methods involve expression of a gene, mRNA or protein as therapy, which are difficult to implement in the clinic. My thesis attempts to fill this therapy gap by developing a novel small-molecule compound that can easily be administered to patients. I have identified small molecules that enhance endogenous PTEN activity and reduce oncogenic potential of cancer cells. This approach has engendered a novel avenue for PTEN restoration therapy that will likely serve as an adjunctive treatment for patients with PTEN-related diseases.

To begin to define molecules that can enhance endogenous PTEN activity, I first identified a likely binding hotspot for small molecules on PTEN between its two core domains, and determined that targeting of this hotspot via an α-helical peptide can modulate PTEN lipid phosphatase activity. Furthermore, in vitro biochemical assays determined a lead compound to enhance lipid phosphatase activity. This compound suppressed the canonical PI3K signaling cascade and inhibited cell proliferation, migration and cell cycle progression of A549 lung cancer cells. Computational docking and molecular dynamics simulations revealed that binding of this small molecule can induce conformational changes in the active site that enhance binding affinity and orientation of PTEN substrate, PIP3, in the catalytic pocket. Thus, I have identified and discerned the likely mechanism of the first known small molecule that directly binds and enhances PTEN enzymatic activity in vitro and asserts its biological functions in cancer cells.

Mutations which compromise PTEN phosphatase activity have been well-studied; however, little is known about other clinically relevant PTEN mutations which alter its structure and function. Indeed, PTEN has overlapping clinical significance to tumor suppressor p53 and both molecules form multimers in vivo. Since select p53 mutant proteins are inactivated via an aggregation mechanism, I hypothesized that likewise, select clinically relevant PTEN mutations may aggregate and inactivate PTEN function, driving disease pathogenesis. To this end, I curated 1,523 known PTEN mutations in cancers and PHTS that were derived from various patient databases and assessed their potential to alter chemical properties, secondary protein structure, and aggregation propensity. I identified >250 mutations that increase the aggregation propensity of PTEN wild-type protein and determined that β-sheet structure and increased hydrophobicity greatly enhanced the potential of PTEN to become aggregation-prone. Validation of aggregation of these mutants in vivo will aid in the development of small molecule-based therapies that can target specific aggregation-prone PTEN regions, re-activating endogenous PTEN. In summary, my work implicates small molecule-based therapy that enhances endogenous PTEN activity as a potentially beneficial alternative or adjunctive therapy for patients suffering from diseases associated with hyperactive PI3K signaling, either due to PI3K pathway aberrations or compromised PTEN function.

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