Our study provides a detailed characterization of therapy-induced senescence (TIS) in breast cancer, revealing it as a dynamic and reversible cell state. Using optimized in vitro models and multiple molecular approaches – including bulk and single-cell RNA sequencing and surface proteomics – we demonstrated that TIS is not a terminal growth arrest, but rather a transitional state associated with distinct molecular and phenotypic changes.
TIS cells showed resistance to a wide range of chemotherapeutics, including commonly used drugs in breast cancer treatment, yet this resistance was reversible upon exit from senescence, indicating that TIS alone temporarily protects cancer cells from treatment. Despite extensive molecular profiling, no single resistance gene or canonical pathway could explain the observed drug tolerance, suggesting that TIS-associated resistance arises from multifactorial, context-dependent processes. The limited response of TIS cells to senolytic agents further highlights their therapeutic challenge, as our results were inconsistent across different compounds and conditions.
Whereas TIS is often viewed as a beneficial endpoint that halts tumor progression, our results show that TIS cells can re-enter the cell cycle and repopulate cultures, accompanied by transcriptional reversal. Transcriptomic analyses revealed widespread gene expression changes during TIS, characterized by downregulation of proliferation and DNA repair pathways and modulation of immune-related programs. Notably, these changes were reversed in repopulating cells, highlighting the plasticity of this state. Importantly, TIS cells exhibited immune-evasive features, including altered immune signaling and increased expression of immunosuppressive factors, potentially enabling them to escape immune clearance and persist over time, further complicating therapeutic elimination.
In conclusion, this work redefines TIS as a reversible, adaptive resistance phenotype in breast cancer, highlighting the limitations of TIS as a therapeutic endpoint. It also underscores the challenges of targeting TIS cells with current agents and emphasizes the need for novel treatment strategies. Further research will be needed to uncover regulatory mechanisms – such as chromatin remodeling, RNA processing, epigenomic regulation and immune evasion – that sustain the TIS state and enable its reversal, with the goal of preventing relapse and improving long-term treatment outcomes.
