The Role of Senescent Macrophages in KRAS-Driven Lung Cancer:Insights from Novel p16-FDR Mouse Models and Therapeutic Implications

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Senescent cells are cells that have stopped dividing but have not undergone apoptosis. These cells secrete various pro-inflammatory and pro-tumor factors, collectively known as the senescence-associated secretory phenotype (SASP). In the tumor microenvironment, senescent cells may promote tumor initiation and progression by secreting SASP. One of the most common mutations in non-small cell lung cancer (NSCLC) is the KRAS mutation, which is associated with poor prognosis and treatment outcomes. However, the specific mechanisms underlying these outcomes remain unclear. Scott et al. utilized mouse models and human samples, employing techniques such as single-cell RNA sequencing, immunofluorescence staining, and pharmacological interventions to investigate the role of senescent macrophages in KRAS-driven lung cancer and to determine whether the elimination of these cells could mitigate tumor initiation and progression.

Firstly, the researchers developed a new mouse model called p16-FDR. This model was created by inserting an optimized flipase (FLPo) and a diphtheria toxin receptor (DTR)/mCherry fusion protein-encoding FDR cassette into the Cdkn2a (p16) locus, providing a novel tool for studying p16INK4a-expressing cells in vivo, enabling the labeling and clearance of senescent cells. Secondly, they induced KRAS mutations to establish a lung cancer model and used fluorescence labeling techniques to track and analyze senescent macrophages in the tumor microenvironment. The results showed that p16FDR/+ mouse embryonic fibroblasts (MEFs) exhibited significant markers of senescence by the sixth generation. Subsequent single-cell RNA sequencing of KRAS-driven lung cancer mouse models revealed that senescent macrophages within tumors expressed high levels of SASP factors such as IL-10, CCL2, and CCL7. Following the clearance of senescent cells using diphtheria toxin (DT) and ABT-737, the tumor burden in KRAS-driven lung cancer mice significantly decreased, with tumor volume reduced by approximately 50% and survival extended by about 30%. Finally, immunohistochemistry and flow cytometry analyses of different cell types and marker expressions in the tumor microenvironment revealed that, after clearing senescent cells, the proportion of regulatory T cells decreased, while CD8+ T cells increased, indicating enhanced immune surveillance. Additionally, the analysis of human lung cancer samples for the presence and characteristics of senescent macrophages yielded results consistent with the mouse model, suggesting the potential clinical application of this strategy.

This study systematically reveals for the first time the pro-tumor role of senescent macrophages in KRAS-driven lung cancer and demonstrates that clearing these cells can significantly inhibit tumor initiation and progression. Senescent macrophages create a microenvironment conducive to tumor growth and immune evasion by secreting various SASP factors.

Previous studies lacked precise methods to label and clear senescent cells, making it difficult to pinpoint their specific roles in the tumor microenvironment. The p16-FDR model, by specifically marking and clearing p16INK4a-expressing cells, achieved precise intervention of senescent cells. Moreover, this study not only validated the effects of clearing senescent cells in mouse models but also confirmed these findings in human samples, ensuring the broad applicability and potential clinical value of the results. In summary, these new methods not only theoretically reveal the important role of senescent cells in the tumor microenvironment but also provide new therapeutic targets and strategies for clinical application.

However, the method of clearing senescent cells using DT and ABT-737 may lack specificity, as these drugs could potentially affect non-senescent cells as well. Consequently, the observed anti-tumor effects might be partially attributed to non-specific cellular damage. Future studies need to develop more specific clearance methods and conduct detailed mechanistic investigations to elucidate the precise effects of these drugs. Additionally, the experiments primarily focused on the early stages of tumor development, lacking studies on long-term effects and late-stage tumors. This limitation hampers a comprehensive understanding of the role of senescent cells throughout the entire tumor progression. Furthermore, the study did not delve deeply into specific immune mechanisms. For instance, after clearing senescent cells, which immune cells or pathways play critical roles? These questions require further experiments and validation.

Addressing these issues in future research on senescent macrophages and KRAS-driven lung cancer will not only deepen our understanding of tumor biology but also advance the development of new technologies, potentially revolutionizing cancer treatment. This progress has the potential to lead to more effective and safer anti-cancer therapies, ultimately benefiting a greater number of patients.

Reference：

Haston, Scott, et al. "Clearance of senescent macrophages ameliorates tumorigenesis in KRAS-driven lung cancer." Cancer Cell 41.7 (2023): 1242-1260.