Cellular senescence: Between protection and pathologies

Cellular senescence is a stable and irreversible state of proliferative arrest triggered by diverse stressors, inclh3uding DNA damage, oncogenic signaling, oxidative stress, and metabolic imbalance. Once regarded as a culture artifact, senescence is now recognized as a fundamental biological program that governs tissue homeostasis, development, aging, and disease. Based on its origin, senescence can be divided into two principal categories: damage-induced, encompassing replicative, oncogene-induced, and therapy-induced forms, and developmentally programmed, which orchestrates tissue patterning and remodeling during embryogenesis. These processes converge on the activation of p53/p21 and p16/RB tumor suppressor axes, sustained DNA damage response (DDR), and the establishment of the senescence-associated secretory phenotype (SASP). Acute senescence serves beneficial roles in tumor suppression, wound healing, and embryonic morphogenesis by transiently activating SASP-mediated immune clearance. However, persistent senescence becomes detrimental, promoting chronic inflammation, tissue dysfunction, and cancer progression. Within the tumor microenvironment, chronic SASP signaling driven by nuclear factor kB (NF-κB), CCAAT/enhancer-binding protein beta (C/EBPβ), and Signal Transducer and Activator of Transcription 3 (STAT3) fosters epithelial-to-mesenchymal transition (EMT), invasion, and therapy resistance. Therapy-induced senescence (TIS) often leads to polyploidization and the emergence of polyploid giant cancer cells (PGCCs) that can escape arrest, regenerate proliferative progeny, and drive tumor relapses. Thus, senescence represents a biological paradox: a protective, transient process that maintains tissue integrity but, when unresolved, transforms into a driver of aging and malignancy. Understanding the molecular determinants, distinguishing beneficial from pathological senescence is crucial for developing targeted senotherapies.