Abstract A047: Mechanistic dissection and inhibitor targeting of nutrient scavenging in pancreatic cancer

Abstract

Mutational activation of the KRAS oncogene occurs in over 90% of pancreatic ductal adenocarcinoma (PDAC) patients, and plays a crucial role in tumor initiation, progression, and tumor maintenance. Historically considered undruggable, there has recently been a surge in efforts to develop and improve drugs that target RAS. RAS inhibitors are well-tolerated, with promising clinical efficacy against RAS-driven PDAC; however, adaptive and acquired resistance develops upon inhibition of RAS alone. Multiple genomic- and signaling-based compensatory mechanisms have been proposed and account for approximately 50% of the cases in which resistance emerges. We determined that inhibition of the RAS pathway increased autophagy, a process of "self-eating" whereby cancer cells degrade defective organelles and recycle macromolecules for energy. Our studies established the provocative concept that concurrent treatment with an autophagy stimulating small molecule (e.g., RAS inhibitors) enhanced the anti-tumor activity of the autophagy inhibitors chloroquine/ hydroxychloroquine (CQ/HCQ). Moving forward, we propose studies to facilitate and improve the clinical advancement of this therapeutic strategy, as well as further elucidate the metabolic changes associated with acute RAS inhibition and RAS inhibitor resistance. The only FDA-approved autophagy inhibitor, HCQ, has poor potency and bioavailability. Thus, we evaluated other therapeutic strategies to block autophagy in PDAC. The Unc-51-like autophagy activating serine/threonine kinases 1 and 2 (ULK1/2) are essential for the initiation of autophagosome biogenesis, making ULK1/2 an attractive potential anti-autophagy therapeutic target. However, we found that inhibition of ULK1/2 antagonizes RAS inhibition in multiple PDAC models. Ongoing studies are focused on delineating the mechanism underlying the antagonism between RAS and ULK inhibition. We recently discovered that autophagy upregulation associated with acute RAS pathway inhibition is transient and that the nutrient scavenging process, macropinocytosis, is upregulated following prolonged RAS inhibition and in RASi-resistant cells. RASi resistant cells also display increased abundance of macropinocytosis-related proteins. Mechanistically, this adaptive phenotype is mediated through PI3K-, FAK-, and TEAD-associated signaling dependencies that support macropinocytic uptake in subsets of RASi-resistant cells, revealing that this process can be enhanced through multiple compensatory routes that likely converge on cytoskeletal remodeling and RAC1-dependent macropinosome formation. Ongoing analyses are focused on determining whether upregulated macropinocytosis is a driver of RASi-resistance and to what extent autophagy contributes to the viability of RASi-resistant PDAC models. Overall, we aim to elucidate how autophagy and macropinocytosis mediate adaptation to acute RAS inhibition and promote RAS inhibitor resistance with the goal of discovering targets and delineating mechanisms to inhibit these tumor promoting processes.