Doctor of Philosophy
Nichols, Anthony C.
The phosphoinositide 3-kinase (PI3K)/Akt/mechanistic target of rapamycin (mTOR) signalling pathway is aberrantly activated in most head and neck squamous cell carcinomas (HNSCCs). PI3K signalling drives cellular proliferation, protein synthesis and cell survival. Although numerous targeted agents are available to inhibit PI3K signalling, results have been variable and factors influencing response to PI3K inhibition (e.g. genomic aberrations, pathway interconnectivity, acquired drug resistance) are not well defined. In this thesis, we employ a multifaceted approach to elucidate the molecular underpinnings of biomarkers of response and mediators of resistance to PI3K inhibition in HNSCC. We began by combining a large panel of HNSCC cell lines with a clinical trial of patient-derived xenograft (PDX) models to characterize biomarkers of response. In doing so, we discovered hotspot mutations in the PI3K-encoding PIK3CA gene to only correlate with treatment efficacy in vitro. In vivo, PI3K inhibition was broadly-active, though not clinically-effective as a single agent, pointing to its potential in neoadjuvant settings. Activating HRAS mutations were identified in models non-responsive to PI3K inhibition, indicative of innate resistance due to constitutively-active HRAS signalling. We identified persistent mTOR complex 1 (mTORC1) signalling in mutant HRAS cells and uncovered ERK-TSC2 signalling contributing to growth and survival despite PI3K inhibition. We also characterized acquired resistance to PI3K inhibition following prolonged drug treatment and identified upregulation of the receptor tyrosine kinases AXL and TYRO3, as well as activation of MAPK signalling in drug-resistant models. Targeting either AXL, TYRO3, or P90RSK re-sensitized cells to PI3K inhibition and underscored the involvement of these signalling effectors in drug resistance. Finally, upon observing a pattern of upregulation of Akt (Ser473) phosphorylation following PI3K inhibition throughout our studies, we focused on mTORC2 as a mediator of response to PI3K inhibition. We generated a genomic knockout model of mTORC2 by targeting its obligate co-factor RICTOR and found that loss of mTORC2 improved sensitivity of HNSCC tumour cells to PI3K inhibition and other therapies. Collectively, this work defines several key effectors and considerations for targeted PI3K inhibition and provides a mechanistic basis to aid the design of combination therapies and the stratification of HNSCC patients for PI3K inhibitor therapy.
Ruicci, Kara M., "Elucidating mechanisms of innate and acquired resistance to PI3Ka inhibition in head and neck squamous cell carcinoma" (2018). Electronic Thesis and Dissertation Repository. 5788.