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p16Ink4a and p21Cip1/Waf1 act as tumour suppressors through induction of cellular senescence. However, senescence-independent roles of these CDK inhibitors are not well understood. Here, we report an unexpected function of p16Ink4 and p21Cip1/Waf1, namely, tumour promotion through chemotaxis. In monocytic myeloid-derived suppressor cells (Mo-MDSCs), p16Ink4 and p21Cip1/Waf1 are highly expressed and stimulate CX3CR1 chemokine receptor expression by preventing CDK-mediated phosphorylation and inactivation of SMAD3. Thus, deletion of p16 Ink4 and p21 Cip1/Waf1 reduces CX3CR1 expression, thereby inhibiting Mo-MDSC accumulation in tumours expressing CX3CL1 and suppressing the tumour progression in mice. Notably, blockade of the CX3CL1/CX3CR1 axis suppresses tumour growth, whereas inactivation of CDKs elicits the opposite effect. These findings reveal an unexpected function of p16 Ink4a and p21 Waf1/Cip1 and indicate that regulation of Mo-MDSCs chemotaxis is a valuable potential strategy for control of tumour development.

Cellular senescence prevents the proliferation of cells at risk for neoplastic transformation. However, the altered secretome of senescent cells can promote the growth of the surrounding cancer cells. Although extracellular vesicles (EVs) have emerged as new players in intercellular communication, their role in the function of senescent cell secretome has been largely unexplored. Here, we show that exosome-like small EVs (sEVs) are important mediators of the pro-tumorigenic function of senescent cells. sEV-associated EphA2 secreted from senescent cells binds to ephrin-A1, that is, highly expressed in several types of cancer cells and promotes cell proliferation through EphA2/ephrin-A1 reverse signalling. sEV sorting of EphA2 is increased in senescent cells because of its enhanced phosphorylation resulting from oxidative inactivation of PTP1B phosphatase. Our results demonstrate a novel mechanism of reactive oxygen species (ROS)-regulated cargo sorting into sEVs, which is critical for the potentially deleterious growth-promoting effect of the senescent cell secretome., Although senescent cell secretome can promote the growth of surrounding cancer cells, the role of extracellular vesicles in this process has not been well understood. Here the authors show that ROS increase the sorting of EphA2 into extracellular vesicles in senescent cells, which promotes proliferation of cancer cells.

Emerging evidence is revealing that exosomes contribute to many aspects of physiology and disease through intercellular communication. However, the biological roles of exosome secretion in exosome-secreting cells have remained largely unexplored. Here we show that exosome secretion plays a crucial role in maintaining cellular homeostasis in exosome-secreting cells. The inhibition of exosome secretion results in the accumulation of nuclear DNA in the cytoplasm, thereby causing the activation of cytoplasmic DNA sensing machinery. This event provokes the innate immune response, leading to reactive oxygen species (ROS)-dependent DNA damage response and thus induce senescence-like cell-cycle arrest or apoptosis in normal human cells. These results, in conjunction with observations that exosomes contain various lengths of chromosomal DNA fragments, indicate that exosome secretion maintains cellular homeostasis by removing harmful cytoplasmic DNA from cells. Together, these findings enhance our understanding of exosome biology, and provide valuable new insights into the control of cellular homeostasis.

Chromatin ubiquitylation flanking DNA double-strand breaks (DSBs), mediated by RNF8 and RNF168 ubiquitin ligases, orchestrates a two-branch pathway, recruiting repair factors 53BP1 or the RAP80-BRCA1 complex. We report that human demethylase JMJD1C regulates the RAP80-BRCA1 branch of this DNA-damage response (DDR) pathway. JMJD1C was stabilized by interaction with RNF8, was recruited to DSBs, and was required for local ubiquitylations and recruitment of RAP80-BRCA1 but not 53BP1. JMJD1C bound to RNF8 and MDC1, and demethylated MDC1 at Lys45, thereby promoting MDC1-RNF8 interaction, RNF8-dependent MDC1 ubiquitylation and recruitment of RAP80-BRCA1 to polyubiquitylated MDC1. Furthermore, JMJD1C restricted formation of RAD51 repair foci, and JMJD1C depletion caused resistance to ionizing radiation and PARP inhibitors, phenotypes relevant to aberrant loss of JMJD1C in subsets of breast carcinomas. These findings identify JMJD1C as a DDR component, with implications for genome-integrity maintenance, tumorigenesis and cancer treatment.