Modulation of the proteostasis network promotes tumor resistance to oncogenic KRAS inhibitors

Despite substantial advances in targeting mutant KRAS, tumor resistance to KRAS inhibitors (KRASi) remains a major barrier to progress. Here, we report proteostasis reprogramming as a key convergence point of multiple KRASi-resistance mechanisms. Inactivation of oncogenic KRAS down-regulated both the heat shock response and the inositol-requiring enzyme 1α (IRE1α) branch of the unfolded protein response, causing severe proteostasis disturbances. However, IRE1α was selectively reactivated in an ER stress–independent manner in acquired KRASi-resistant tumors, restoring proteostasis. Oncogenic KRAS promoted IRE1α protein stability through extracellular signal–regulated kinase (ERK)–dependent phosphorylation of IRE1α, leading to IRE1α disassociation from 3-hydroxy-3-methylglutaryl reductase degradation (HRD1) E3-ligase. In KRASi-resistant tumors, both reactivated ERK and hyperactivated AKT restored IRE1α phosphorylation and stability. Suppression of IRE1α overcame resistance to KRASi. This study reveals a druggable mechanism that leads to proteostasis reprogramming and facilitates KRASi resistance. Description Editor’s summary Most cancers depend on a balanced proteostasis network to maintain oncogenic growth, and therapeutic insults often disrupt proteostasis. However, how residual drug-tolerant cells overcome imbalances in the proteostasis network to survive targeted therapy is poorly understood. Lv et al. discovered a mechanism that rewires the proteostasis network to escape oncogenic KRAS addiction and promote resistance to KRAS inhibitors (KRASi). Inactivation of mutant KRAS, a key oncogenic driver for many human cancers, shuts down major proteostasis regulatory pathways, causing severe protein aggregations. However, multiple resistance mechanisms converge to selectively reactivate an ancient stress sensor, IRE1α, to reestablish proteostasis in the KRASi-resistant cells. Targeting IRE1α could collapse the rewired proteostasis network and overcome resistance to KRASi therapy. —Stella M. Hurtley and Priscilla N. Kelly Endoplasmic reticulum stress–independent phosphorylation of IRE1α is a convergence point of multiple KRAS inhibitor resistance mechanisms. INTRODUCTION KRAS is one of the most frequently mutated genes in human cancer. Despite advances in the development of inhibitors that directly target mutant KRAS and the approval of KRASG12C inhibitors sotorasib and adagrasib for the treatment of KRASG12C-mutant non–small cell lung cancer (NSCLC) patients, multiple lines of clinical and preclinical evidence demonstrate that adaptive resistance to KRAS inhibitors (KRASi) is rapid and almost inevitable. The heterogeneous resistance mechanisms in patients and dose-limiting toxicity associated with targeting multiple KRASi resistance pathways—such as receptor tyrosine kinases (RTKs), extracellular signal–regulated kinase (ERK), and AKT–remain a major barrier to progress. RATIONALE Most cancers require a balanced protein homeostasis (proteostasis) network to maintain oncogenic growth. Therapeutic insults often disrupt proteostasis and induce proteotoxic stresses. Residual drug-tolerant cells must overcome imbalances in the proteostasis network to maintain survival. How a proteostasis network is orchestrated by driver oncogenes and the proteostasis reprogramming mechanisms that bypass oncogene addiction and allow for acquired resistance to targeted therapies remain largely unknown. In this study, we investigated the regulation of proteostasis by oncogenic KRAS and the rewiring of proteostasis network underlying the acquired resistance to KRAS inhibition. RESULTS We show that oncogenic KRAS is critical for protein quality control in cancer cells. Genetic or pharmacological inhibition of oncogenic KRAS rapidly inactivated both cytosolic and endoplasmic reticulum (ER) protein quality control machinery, two essential components of the proteostasis network, through inhibition of the master regulators heat shock factor 1 (HSF1) and inositol-requiring enzyme 1α (IRE1α). However, residue cancer cells that survive KRASi directly reactivated IRE1α through an ER stress–independent phosphorylation mechanism that reestablished proteostasis and sustained acquired resistance to KRAS inhibition. We identified four oncogenic signaling–regulated phosphorylation sites in IRE1α (Ser525, Ser529, Ser549, and Thr973) that are distinct from IRE1α autophosphorylation sites but are required for enhanced protein stability. The phosphorylation of IRE1α at these sites prevents IRE1α binding with the SEL1L/HRD1 E3 ligase complex, thus impairing the ubiquitination-dependent degradation of IRE1α and stabilizing the protein. These sites are the convergence points of multiple resistance mechanisms in KRASi-resistant tumors. RTK-mediated reactivation of ERK and hyperactivation of AKT sustained the unconventional phosphorylation of IRE1α in the KRASi-resistant tumors, which consequently restored its protein stability and reestablished proteostasis. Genetic or pharmacological suppression of IRE1α collapsed the rewired proteostasis network and overcame resistance to KRAS–MAPK (mitogen-activated protein kinase) inhibitors. CONCLUSION This study reveals the direct cross-talk between oncogenic signaling and the protein quality control machinery and uncovers the mechanisms that account for the proteostasis rewiring in response to KRAS inhibition. Multiple resistance mechanisms converge on IRE1α through ER stress–independent phosphorylation to restore proteostasis and promote KRASi-resistant tumor growth. Targeting this key convergence point represents an effective therapeutic strategy to overcome KRASi resistance. Proteostasis reprogramming upon KRAS inhibition. Inhibition of oncogenic KRAS inactivates both cytosolic and ER protein quality control machinery by inhibiting HSF1 and IRE1α. Residual cells that survive KRASi directly restore IRE1α phosphorylation through receptor tyrosine kinase–mediated reactivation of ERK and hyperactivation of AKT, preventing IRE1α from SEL1L/HRD1–mediated ubiquitination and degradation. Multiple heterogeneous resistance pathways converge on IRE1α to reestablish proteostasis and promote resistance to KRASi.