Cellular senescence in cancer: Friend or fraud?

Cellular senescence, a stress-induced state of stable cell cycle arrest accompanied by a senescence-associated secretory phenotype (SASP), plays a paradoxical role in cancer biology. On the one hand, senescent cells function as a barrier to tumor initiation by activating the DNA damage response (DDR) and tumor suppressor pathways such as p53/p21 and p16^INK4a-retinoblastoma (RB). On the other hand, their long-term persistence promotes chronic inflammation, immune evasion, and tissue remodeling via sustained SASP, ultimately facilitating tumor progression, metastasis, and therapeutic resistance. This review elucidates the hallmarks of cellular senescence, explores its dual roles and mechanistic underpinnings in tumor suppression and promotion, highlighting the key molecular circuits governing the senescence phenotype, such as telomere dynamics, autophagy-lysosome function, and immunosurveillance. We further examine targeted therapeutic approaches, such as senolytics and senomorphics, and their integration into sequential induction and clearance regimens. These interventions aim to leverage the transient SASP to enhance immune recognition while minimizing the pro-tumorigenic effects associated with persistent SASP. Despite these advances, challenges such as tissue specificity, off-target effects, biomarker inconsistency, and cellular heterogeneity remain major hurdles to clinical translation. To transcend the traditional static and binary perspective of senescence, we introduce a dynamic plasticity model that conceptualizes senescence as a context-dependent and dynamically regulated program, potentially reversible and modulated by molecular switches, temporal patterns of SASP, and microenvironmental factors. Targeting these plasticity checkpoints holds promise for optimizing "one-two punch" combination regimens and expanding immunotherapeutic windows, thereby offering a novel paradigm for improving outcomes in aging-related cancers.