Sustained Oncogenic Signaling in the Cytostatic State Enables Targeting of Nonproliferating Persistent Cancer Cells.

Many advanced therapeutics possess cytostatic properties that suppress cancer cell growth without directly inducing death. Treatment-induced cytostatic cancer cells can persist and constitute a reservoir from which recurrent growth and resistant clones can develop. Current management approaches primarily comprise maintenance and monitoring because strategies for targeting nonproliferating cancer cells have been elusive. Here, we used targeted therapy paradigms and engineered cytostatic states to explore therapeutic opportunities for depleting treatment-mediated cytostatic cancer cells. Sustained oncogenic AKT signaling was common, while nonessential, in treatment-mediated cytostatic cancer cells harboring PI3K-pathway mutations, which are associated with cancer recurrence. Engineering oncogenic signals in quiescent mammary organotypic models showed that sustained, aberrant activation of AKT sensitized cytostatic epithelial cells to proteasome inhibition. Mechanistically, sustained AKT signaling altered cytostatic state homeostasis and promoted an oxidative and proteotoxic environment, which imposed an increased proteasome dependency for maintaining cell viability. Under cytostatic conditions, inhibition of the proteasome selectively induced apoptosis in the population with aberrant AKT activation compared with normal cells. Therapeutically exploiting this AKT-driven proteasome vulnerability was effective in depleting treatment-mediated cytostatic cancer cells independent of breast cancer subtype, epithelial origin, and cytostatic agent. Moreover, transient targeting during cytostatic treatment conditions was sufficient to reduce recurrent tumor growth in spheroid and mouse models. This work identified an AKT-driven proteasome-vulnerability that enables depletion of persistent cytostatic cancer cells harboring PTEN-PI3K pathway mutations, revealing a viable strategy for targeting nonproliferating persistent cancer cell populations before drug resistance emerges. SIGNIFICANCE: This study finds that sustained oncogenic signaling in therapy-induced cytostatic cancer cells confers targetable vulnerabilities to deplete persistent cancer cell populations and reduce cancer recurrence.