Acquisition of Resistance to RAS Inhibition Is Associated with the Upregulation of Macropinocytosis through Both PI3K-Dependent and -Independent Signaling.

Pancreatic ductal adenocarcinoma (PDAC) is a highly KRAS-addicted cancer. Although KRAS-mutant PDAC is susceptible to treatment with direct RAS inhibitors, drug resistance is an ongoing challenge. KRAS-driven PDAC is dependent on nutrient scavenging pathways such as macropinocytosis to fuel increased metabolic demands. Here, we report on the dynamics of macropinocytosis in response to genetic suppression and pharmacologic inhibition of KRAS and the RAS-ERK mitogen-activated protein kinase pathway. We show that macropinocytosis is transiently suppressed following acute genetic suppression of KRAS but is rapidly restored and even enhanced under prolonged RAS pathway inhibition. Furthermore, we demonstrate that human PDAC cell lines that have acquired resistance to RAS inhibitors, as well as cell lines derived from KPC tumors with acquired resistance to RAS inhibitors display markedly elevated macropinocytosis. RAS-inhibitor resistant cell lines also display increased abundance of macropinocytosis-related proteins and enhanced albumin uptake and sensitivity to albumin-bound paclitaxel. Mechanistically, this adaptive phenotype is mediated through both phosphoinositide 3-kinase (PI3K)-dependent and -independent pathways that converge on the activation of RAC1. Our results demonstrate a previously unrecognized link between RAS pathway inhibitor resistance and upregulation of macropinocytosis, underscoring the importance of further elucidating metabolic reprogramming in RASi-R PDAC. SIGNIFICANCE: Macropinocytosis is upregulated following prolonged RAS-inhibitor treatment and in RASi-R models of PDAC via several heterogeneous mechanisms that converge on RAC1 activation and render RASi-R PDAC cell lines more sensitive to nab-paclitaxel.