ISR Addiction: Co-opting a Stress Sentinel to Fuel Tumor Resilience and Therapy Resistance

The integrated stress response (ISR), a conserved cellular sentinel, is pathologically commandeered in cancer to sustain survival under chronic duress. Here, we posit that tumors evolve a state of "ISR Addiction", a profound, non-oncogenic dependency on persistent ISR signaling orchestrated through sustained eIF2α phosphorylation and ATF4-driven transcription. We delineate a three-stage continuum—utilization, dependence, and fixation—tracing how this addiction emerges under microenvironmental pressure and becomes entrenched during therapy. This dependency actively fuels metastasis and acts as a central hub for resistance to chemotherapy, targeted agents, and immunotherapy. Expanding this view, we embed ISR addiction within a multilayered "Ecosystem of Tumor Addictions", where bidirectional crosstalk with redox addiction (maintaining biochemical balance) and neural addiction (processing systemic signals) collectively creates a buffered resilience network. Crucially, this ecosystem reveals a therapeutically exploitable fragility: the addicted state is maintained in a precarious "Goldilocks" balance ("Goldilocks" refers to a "just right" condition—not too much, not too little—borrowed from the children’s tale Goldilocks and the Three Bears, where the protagonist finds the porridge that is neither too hot nor too cold, but perfectly suited), susceptible to either "enforced withdrawal" (blocking pro-survival ATF4 outputs) or "toxic overload" (hyperactivating the pro-apoptotic CHOP arm). We propose that rational combinations targeting this addiction network, particularly with standard-of-care or immunomodulatory agents, offer a transformative strategy to dismantle the resilient core of advanced malignancies.