Lineage plasticity of the integrated stress response is a hallmark of cancer evolution.
The link between the "stress phenotype"-a well-established hallmark of cancer-and its role in tumor progression and intratumor heterogeneity remains poorly defined. The integrated stress response (ISR) is a key adaptive pathway that enables tumor survival under oncogenic stress. While ISR has been implicated in promoting tumor growth, its precise role in driving tumor evolution and heterogeneity has not been elucidated. In this study, using a genetically engineered mouse models, we demonstrate that ISR activation--indicated by elevated levels of phosphorylated eIF2 (p-eIF2) and ATF4--is essential for the emergence of dedifferentiated, therapy-resistant cell states. ISR, through the coordinated actions of ATF4 and MYC, facilitates the development of tumor cell populations characterized by high plasticity, stemness, and an epithelial-mesenchymal transition (EMT)-prone phenotype. This process is driven by ISR-mediated expression of genes that maintain mitochondrial integrity and function, critical for sustaining tumor progression. Importantly, genetic, or pharmacological inhibition of the p-eIF2-ATF4 signaling axis leads to mitochondrial dysfunction and significantly impairs tumor growth in mouse models of lung adenocarcinoma (LUAD). Moreover, ISR-driven dedifferentiation is associated with poor prognosis and therapy resistance in advanced human LUAD, underscoring ISR inhibition as a promising therapeutic strategy to disrupt tumor evolution and counteract disease progression.