The expression of inflammatory mediators such as interleukin-6 (IL-6) and IL-8 reinforces growth arrest in cells that have undergone oncogene-induced senescence.
The secretion of cytokines and chemokines by senescent cells has been associated with the potential to promote tumorigenesis. However, two papers published in Cell indicate that the expression of inflammatory mediators can help to maintain growth arrest.
CORBIS
Jesús Gil and colleagues instigated a short hairpin RNA inhibition screen to look for genes whose knockdown can extend the proliferative lifespan of near-senescent primary human fibroblasts. Among the genes that were identified was the chemokine-encoding CXCR2 (also known as IL8RB). Knockdown of CXCR2 was shown to extend the replicative lifespan of several primary cells as well as fibroblasts that had undergone oncogene-induced senescence (OIS) owing to the expression of the kinase MEK1. Levels of expression of CXCR2 were shown to increase as cells underwent either replicative senescence or OIS, so Gil and colleagues asked whether similar alterations were evident in chemokines that bind CXCR2. Antibody arrays, ELISA and quantitative reverse transcription PCR showed that expression of CXCR2 ligands, including interleukin 8 (IL8), were increased in cells undergoing OIS. Importantly, the authors also found that increased staining for CXCR2 is evident in samples of human prostate intraepithelial neoplasia, which are associated with high levels of senescent cells, and CXCR2 staining levels decreased in more advanced disease. What induces expression of these ligands? By using several chemical inhibitors and analysing transcription factor binding sites within the promoters of these ligands, the authors established the involvement of both NFB and C/EBP. Knockdown of either of these transcription factors suppressed expression of IL8, and both factors bind the IL8 promoter.
Daniel Peeper and colleagues also found that C/EBP is a crucial factor in OIS. They used an unbiased genome-wide expression microarray analysis in human fibroblasts expressing oncogenic BRAF to identify mRNAs that had increased expression during OIS and reduced expression on bypass of OIS. Several cytokine and chemokine mRNAs followed this pattern, and further analyses limited this to 24 genes that included IL6. IL6 seems to be upregulated in response to OIS independent of cell type, and is required to maintain OIS. Interestingly, this function seems to be restricted to OIS, as exogenous IL6 arrested cultured human fibroblasts only in the context of oncogenic stress. Importantly, the senescent effect of IL6 is mediated in an autocrine manner. Paracrine IL6 is known to have tumour-promoting effects, and media from OIS cells induced proliferation of B9 hybridoma cells in culture, which was inhibited by IL6 antibodies. C/EBP is known to regulate IL6 transcription and knockdown of this transcription factor prevented expression of IL6 (and IL8) and prevented OIS. Further microarray expression analyses indicated that IL6 expression maintains an active inflammatory network, which includes IL8, and that this requires cooperation between IL6 and C/EBP. Are these findings replicated in clinical samples? In 18 out of 20 human colon adenomas, IL8 staining specifically co-localized with arrested (INK4A-positive, Ki67-negative) epithelial cells.
Given the association of inflammatory mediators with both pro- and anti-tumorigenic effects, further analyses are needed to understand the function of these proteins in a cell-autonomous context as well as a tumour-microenvironmental one.
Nicola McCarthy
References
Acosta, J. C. et al. Chemokine signaling via the CXCR2 receptor reinforces senescence. Cell133, 1006–1018 (2008) | Article | PubMed |
Kuilman, T. et al. Oncogene induced senescence relayed by an interleukin-dependent inflammatory network. Cell133, 1019–1031 (2008) | Article | PubMed |
B and C/EBP
. Knockdown of either of these transcription factors suppressed expression of IL8, and both factors bind the IL8 promoter.