Exploiting pivotal mechanisms behind the senescence-like cell cycle arrest in cancer

RAB27B-regulated exosomes mediate LSC maintenance via resistance to senescence and crosstalk with the microenvironment

The fate of leukaemia stem cells (LSCs) is determined by both their inherent mechanisms and crosstalk with their niches. Although LSCs were confirmed to be eradicated by restarting senescence, the specific key regulators of LSC resistance to senescence and remodelling of the niche to obtain a microenvironment suitable for stemness remain unknown. Here, we found that RAB27B, a gene regulating exosome secretion, was overexpressed in LSCs and associated with the poor prognosis of acute myeloid leukaemia (AML) patients. The increased RAB27B in LSCs prevented their senescence and maintained their stemness in vitro and in vivo. Mechanically, the increased RAB27B expression in LSCs selectively promoted the loading and release of exosomes rich in senescence-inducing proteins by direct combination. Furthermore, RAB27B-regulated LSC-derived exosomes remodelled the niche and induced senescence of mesenchymal stem cells (MSCs) with increased RAB27B expression ex vivo and in vivo. The increased RAB27B in the senescent MSCs conversely promoted LSC maintenance ex vivo and in vivo via selective excretion of exosomes rich in stemness-promoting proteins. Therefore, we identified the specifically increased RAB27B in LSCs and their educated senescent MSCs as a hub molecule for LSC resistance to senescence and maintenance through crosstalk with its niche via selective exosome excretion.

The dual role of the DREAM/G2M pathway in non-tumorigenic immortalization of senescent cells

Anti-aging and tumorigenesis share common genes and pathways, and thus targeting these genes as part of anti-aging interventions carries the risk of tumorigenesis. It is essential to understand the gene signatures that balance tumorigenesis and aging. To achieve this goal, we analyzed RNA-sequencing data from three non-tumorigenic immortalized cell lines that spontaneously escaped from senescence. By single sample gene set enrichment assay (ssGSEA) and GSEA analysis, we found that both cell growth signaling (E2F targets, MYC targets) and tumor surveillance mechanisms (DNA repair, G2M checkpoint, mitotic spindle) were up-regulated in all three cell lines, suggesting that these genes are potential signatures for non-tumorigenic immortalization. Further analysis revealed that the 182 commonly up-regulated genes in these three cell lines overlapped with the DREAM/G2M pathway, which is known to be the upstream regulator of E2F, Myc targets, DNA repair, G2M checkpoint and mitotic spindle pathways in its cell cycle activation or inhibitory form. By western blotting, quantitative PCR and co-immunoprecipitation, we verified that both forms of the DREAM pathway are up-regulated in all three cell lines; this pathway facilitates control of cell cycle progression, supporting a new mechanism for non-tumorigenic immortalization. Thus, we propose that the DREAM/G2M pathway plays important dual roles with respect to preventing tumorigenesis in the process of immortalization. Our data might serve as the basis for the identification of new signature pathways or gene biomarkers for non-tumorigenic immortalization, and may aid in the discovery of new targets for tumor-free anti-aging drug screening.