Cellular senescence has been perceived as a barrier against carcinogenesis. However, the senescence-associated secretory phenotype (SASP) of senescent cells can promote tumorigenesis. Here, we show senescent tumour cells are frequently present in the front region of collective invasion of papillary thyroid carcinoma (PTC), as well as lymphatic channels and metastatic foci of lymph nodes. In in vitro invasion analysis, senescent tumour cells exhibit high invasion ability as compared with non-senescent tumour cells through SASP expression. Collective invasion in PTC is led by senescent tumour cells characterized by generation of a C-X-C-motif ligand (CXCL)12 chemokine gradient in the front region. Furthermore, senescent cells increase the survival of cancer cells via CXCL12/CXCR4 signalling. An orthotopic xenograft in vivo model also shows higher lymphatic vessels involvement in the group co-transplanted with senescent cells and cancer cells. These findings suggest that senescent cells are actively involved in the collective invasion and metastasis of PTC.

Cutaneous ageing is an important extrinsic process that modifies the pigmentary system. Because cellular senescence is a fundamental ageing mechanism, we examined the role of senescent cells in ageing pigmentation.

Skin aging is induced and sustained by chronological aging and photoaging. Aging skin pigmentation such as mottled pigmentation (senile lentigo) and melasma are typical signs of photoaging. The skin, like other human organs, undergoes cellular senescence, and senescent cells in the skin increase with age. The crosstalk between melanocytes as pigmentary cells and other adjacent types of aged skin cells such as senescent fibroblasts play a role in skin-aging pigmentation. In this review, we provide an overview of cellular senescence during the skin-aging process. The discussion also includes cellular senescence related to skin-aging pigmentation and the therapeutic potential of regulating the senescence process.

The mechanism of senescence-associated cytoplasmic induction of p-Erk1/2 (SA-p-Erk1/2) proteins in human diploid fibroblasts was investigated. p-Erk1/2 proteins were efficiently dephosphorylated in vitro by protein phosphatases 1 and 2A (PP1/2A) and MAPK phosphatase 3 (MKP3). Specific activity of PP1/2A and MKP3 activity significantly decreased during cellular senescence, whereas their protein expression levels did not. To investigate possible mechanism of phosphatase inactivation, we measured reactive oxygen species (ROS) generation by fluorescence-activated cell sorting analysis and found it was much higher in mid-old cells than the young cells. Treating the young cells once with 1 mm H₂O₂ remarkably induced p-Erk1/2 expression; however, it was transient unless repeatedly treated until 72 h. Multiple treatment of the cells with 0.2 mm H₂O₂ significantly duplicated inactivation of PP1/2A; however, thiol-specific reagents could reverse the PP1/2A activities, suggesting the oxidation of cysteine molecule in PP1/2A by the increased ROS. When the cells were pretreated with 10 mm N-acetyl-l-cysteine for 1 h, Erk1/2 activation was completely blocked. To elucidate which cysteine residue and/or metal ion in PP1/2A was modified by H₂O₂, electrospray ionization-tandem mass spectrometry analyses were performed with purified PP1C-α and found Cys⁶²-SO₃H and Cys¹⁰⁵-SO₃H, implicating the tertiary structure perturbation. H₂O₂ inhibited purified PP1C-α activity by both oxidation of Cys residues and metal ion(s), evidenced by dithiothreitol and ascorbate-restoration assay. In summary, SA-p-Erk1/2 was most likely due to the oxidation of PP1/2A, which resulted from the continuous exposure of the cells to vast amounts of ROS generated during cellular senescence by oxidation of Cys⁶² and Cys¹⁰⁵ in PP1C-α and metal ion(s).

Senescent cells in cancer tissue, including senescent fibroblasts and macrophages, have been reported to increase the malignant potency of cancer cells by secreting senescence-associated secretory phenotype (SASP). Otherwise, Senescence of tumor cells has been believed to inhibit tumor growth by halting the massive proliferation and increasing the chances of immune clearance. In particular, senescent tumor cells (STCs) have been thought that they rarely exist in carcinomas because oncogene-induced senescence needs to be overcome for protumorigenic cells to become malignant. However, recent studies have revealed that a considerable number of STCs are present in cancer tissue, even in metastatic sites. In fact, STCs are widely involved in cancer progression by leading to collective invasion and building a cytokine barrier to protect nonsenescent tumor cells from immune attack. Furthermore, therapy-induced STCs can induce tumor progression and recurrence by increasing stemness. However, obscure causative factors and their heterogeneity in various cancers make it difficult to establish the physiological role of STCs. Here, we summarize and review the current knowledge of the pathophysiology and role of STCs. We also outline the current status of therapeutic strategies for directly removing STCs or modulating the SASPs to maximize the positive functions of STCs while suppressing the negative functions.

Cellular senescence can either support or inhibit cancer progression. Here, it is shown that intratumoral infiltration of CD8⁺ T cells is negatively associated with the proportion of senescent tumor cells in colorectal cancer (CRC). Gene expression analysis reveals increased expression of C‐X‐C motif chemokine ligand 12 (CXCL12) and colony stimulating factor 1 (CSF1) in senescent tumor cells. Senescent tumor cells inhibit CD8⁺ T cell infiltration by secreting a high concentration of CXCL12, which induces a loss of CXCR4 in T cells that result in impaired directional migration. CSF1 from senescent tumor cells enhance monocyte differentiation into M2 macrophages, which inhibit CD8⁺ T cell activation. Neutralization of CXCL12/CSF1 increases the effect of anti‐PD1 antibody in allograft tumors. Furthermore, inhibition of CXCL12 from senescent tumor cells enhances T cell infiltration and results in reducing the number and size of tumors in azoxymethane (AOM)/dextran sulfate sodium (DSS)‐induced CRC. These findings suggest senescent tumor cells generate a cytokine barrier protecting nonsenescent tumor cells from immune attack and provide a new target for overcoming the immunotherapy resistance of CRC.