The Dual Role of Senescence-Associated Secretory Phenotype (SASP) and Mechanisms of Senescence Escape in Cancer Progression

A major contributor to this paradox is the Senescence-Associated Secretory Phenotype (SASP), which while immunostimulatory in acute contexts, can become chronic and immunosuppressive if senescent cells persist [76]. Persistent SASP factors, such as IL-6, IL-8, MMPs, and TGF-β, can promote a pro-inflammatory and tumor-promoting microenvironment, enhance epithelial-to-mesenchymal transition (EMT), stimulate angiogenesis, and even support the proliferation of nearby non-senescent tumor cells through paracrine signaling [4, 77–79]. SASP secretion is primarily regulated at the transcriptional level by NF-κB and C/EBPβ, both of which are activated downstream of persistent DDR signaling [80]. Such intricate regulatory networks are often explored through biological models . Upon genotoxic stress, sensors such as ATM and ATR phosphorylate CHK2 and γH2AX, which recruit 53BP1 and MDC1 to damage foci, stabilizing the DDR. Chronic DDR leads to the stabilization and nuclear translocation of NF-κB (particularly p65/RelA) via the IKK complex, which phosphorylates and degrades IκBα, an NF-κB inhibitor. Activated NF-κB then drives the expression of SASP genes, including IL-6, IL-8, and MMPs, promoting both inflammation and extracellular matrix (ECM) remodeling [10, 81, 82].

The pro-inflammatory milieu generated by SASP can have tumor-promoting consequences. Chronic secretion of IL-6 and IL-8, for example, activates JAK/STAT3 and MAPK/ERK signaling pathways in surrounding tumor cells, enhancing proliferation, angiogenesis, and EMT [83]. EMT is a process by which epithelial cancer cells acquire mesenchymal features, increasing their motility, invasiveness, and resistance to therapy. SASP-induced TGF-β further promotes EMT and contributes to the creation of a fibrotic and immunosuppressive tumor stroma [84]. SASP also remodels the immune landscape in a context-dependent manner. While early SASP factors such as CCL2 and CXCL10 recruit cytotoxic immune cells (e.g., NK cells, CD8⁺ T cells), prolonged SASP exposure can lead to the recruitment and polarization of immunosuppressive cell types, including M2 macrophages, MDSCs, and regulatory T cells (Tregs). These cells suppress cytotoxic immune responses and contribute to immune evasion [85, 86]. For instance, in prostate cancer models, SASP induced by CDK inhibitors enhanced NK cell recruitment but also polarized macrophages to an M2-like, tumor-supportive phenotype, thereby limiting overall anti-tumor efficacy [5, 87]. Furthermore, SASP-induced MMPs degrade components of the ECM, facilitating tumor cell invasion and metastasis. This proteolytic remodeling also releases ECM-bound growth factors such as VEGF, which enhances angiogenesis and supports tumor growth. Additionally, autocrine SASP signaling can reinforce the senescence state in a feed-forward loop, while paracrine signaling from senescent stromal or tumor cells can induce senescence in adjacent cells, contributing to tissue dysfunction and heterogeneity within the TME [6, 88]. In cases where senescent cells persist due to inefficient immune clearance, the TME becomes increasingly inflamed, fibrotic, and immunosuppressed—an ideal setting for tumor relapse and progression. This has been observed in preclinical models of melanoma and pancreatic cancer, where SASP-related CCL5 secretion was found to recruit T cells only in the presence of co-stimulatory immunotherapy; otherwise, it had minimal tumor-suppressive effects [89]. Such findings are frequently reported in leading journals, including the Journal of Clinical Investigation .

Although Tumor-Inducing Senescence (TIS) was initially regarded as a stable and irreversible growth arrest, accumulating evidence reveals that senescent tumor cells can escape this state, re-enter the cell cycle, and contribute to tumor relapse, metastasis, and therapy resistance. This phenomenon—termed senescence escape or reversal—is driven by a complex interplay of epigenetic changes, cell cycle reactivation, immune evasion, and metabolic reprogramming, often occurring under conditions of chronic stress, hypoxia, or incomplete clearance of senescent cells [4, 90, 91]. At the heart of senescence maintenance are the CDK inhibitors p16INK4a and p21Cip1/Waf1, which suppress the activity of CDK4/6 and CDK2, respectively. This suppression keeps the Rb in a hypophosphorylated, active state, preventing E2F-driven transcription and S-phase entry. In senescence escape, downregulation or loss-of-function mutations in CDKN2A or CDKN1A genes can relieve this checkpoint, allowing CDK activity to resume. This leads to Rb hyperphosphorylation, E2F activation, and re-entry into the cell cycle, particularly in tumor cells under selective pressure or with accumulated genomic alterations [92]. Epigenetic deregulation is another major driver of senescence reversal. In stable senescence, SAHF help enforce cell cycle arrest by silencing proliferation-related genes via histone modifications such as H3K9me3 and H3K27me3. Enzymes like EZH2 (a methyltransferase within the PRC2 complex) maintain H3K27me3 marks on pro-growth genes. However, epigenetic plasticity, including loss of heterochromatin marks or upregulation of histone demethylases (e.g., KDM6A/UTX, KDM4), can destabilize these chromatin structures. This leads to derepression of E2F target genes, re-expression of cyclins, and reversal of the senescent phenotype [93, 94].

A critical facilitator of senescence escape is the AKT/mTOR signaling pathway, which regulates cell growth, survival, and metabolism. In senescent cells, mTORC1 activity is typically suppressed to conserve energy. However, reactivation of mTORC1—for instance, via PI3K activation, PTEN loss, or increased nutrient signaling—can promote biosynthesis and proliferation, undermining senescence maintenance. mTORC1 also suppresses autophagy, which is essential for sustaining the senescent state by removing damaged organelles and limiting ROS. Loss of autophagic flux in this context supports mitochondrial dysfunction reversal, increased ATP production, and cell cycle re-entry [6, 95, 96]. In parallel, senescence escape is tightly linked to mitochondrial dynamics and metabolic rewiring. While senescent cells exhibit increased mitochondrial mass and elevated ROS, escape is associated with mitochondrial clearance, reduced ROS levels, and a shift toward glycolytic metabolism (Warburg effect), facilitating renewed proliferation. This metabolic switch is often driven by c-Myc, HIF-1α, and PGC-1α, which reprogram cellular energy use and redox balance to a proliferative state [97, 98]. Importantly, immune evasion also contributes to the persistence and eventual escape of senescent tumor cells. When SASP signaling becomes chronic, it not only promotes inflammation and tissue remodeling but also attracts immunosuppressive cell types, including M2 macrophages and Tregs. These immune cells dampen cytotoxic responses, allowing senescent cells to persist and accumulate further mutations that facilitate escape. Moreover, senescent cells often upregulate PD-L1, allowing them to engage in immune checkpoint interactions and evade T cell-mediated clearance [99]. In some contexts, senescence escape leads to the emergence of stem-like, therapy-resistant tumor cells. These cells display markers of cancer stemness (e.g., ALDH1, CD44high/CD24low) and exhibit high tumor-initiating potential. They are thought to originate from a subpopulation of senescent cells that undergo transcriptional reprogramming through Wnt, Notch, or YAP/TAZ pathways. This escape-reprogramming phenomenon has been observed following genotoxic stress and is associated with increased tumor heterogeneity and metastatic potential [100–102].

Comparison of Stable Senescence and Senescence Escape Mechanisms

This senescence reversal has been observed in cancer models following extended exposure to genotoxic stress or hypoxia and is associated with epigenetic deregulation and downregulation of CDK inhibitors [92]. These “escaped” cells not only regain proliferative capacity but may also exhibit stem-like features and enhanced metastatic potential. The tumor-promoting potential of senescence is further supported by studies in immune-suppressed or immune-excluded tumors, where the lack of proper immune clearance of senescent cells allows them to accumulate and exert deleterious effects.