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Senescent mouse cells fail to overtly regulate the HIRA histone chaperone and do not form robust Senescence Associated Heterochromatin Foci

CR-UK Beatson Labs, Glasgow University, Glasgow, UK. .
Cell Division (Impact Factor: 2.63). 06/2010; 5(1):16. DOI: 10.1186/1747-1028-5-16
Source: PubMed

ABSTRACT Cellular senescence is a permanent growth arrest that occurs in response to cellular stressors, such as telomere shortening or activation of oncogenes. Although the process of senescence growth arrest is somewhat conserved between mouse and human cells, there are some critical differences in the molecular pathways of senescence between these two species. Recent studies in human fibroblasts have defined a cell signaling pathway that is initiated by repression of a specific Wnt ligand, Wnt2. This, in turn, activates a histone chaperone HIRA, and culminates in formation of specialized punctate domains of facultative heterochromatin, called Senescence-Associated Heterochromatin Foci (SAHF), that are enriched in the histone variant, macroH2A. SAHF are thought to repress expression of proliferation-promoting genes, thereby contributing to senescence-associated proliferation arrest. We asked whether this Wnt2-HIRA-SAHF pathway is conserved in mouse fibroblasts.
We show that mouse embryo fibroblasts (MEFs) and mouse skin fibroblasts, do not form robust punctate SAHF in response to an activated Ras oncogene or shortened telomeres. However, senescent MEFs do exhibit elevated levels of macroH2A staining throughout the nucleus as a whole. Consistent with their failure to fully activate the SAHF assembly pathway, the Wnt2-HIRA signaling axis is not overtly regulated between proliferating and senescent mouse cells.
In addition to the previously defined differences between mouse and human cells in the mechanisms and phenotypes associated with senescence, we conclude that senescent mouse and human fibroblasts also differ at the level of chromatin and the signaling pathways used to regulate chromatin. These differences between human and mouse senescence may contribute to the increased propensity of mouse fibroblasts (and perhaps other mouse cell types) to become immortalized and transformed, compared to human cells.

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    • "Average nuclear intensity of the anti-mH2A signal was not enhanced in old brain Purkinje cells; however, granularity was enhanced and signals became concentrated in multiple nuclear foci (Fig. 1E). Whether these foci represent SAHFs is not completely clear because a Wnt-signalling pathway that regulates SAHF formation in human cells has been shown to be absent in mouse fibroblasts (Kennedy et al., 2010). However, accumulation of mH2A at pericentromeric heterochromatin has been shown in multiple tissues from aging mice (Kreiling et al., 2011). "
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    • "Average nuclear intensity of the anti-mH2A signal was not enhanced in old brain Purkinje cells; however, granularity was enhanced and signals became concentrated in multiple nuclear foci (Fig. 1E). Whether these foci represent SAHFs is not completely clear because a Wnt-signalling pathway that regulates SAHF formation in human cells has been shown to be absent in mouse fibroblasts (Kennedy et al., 2010). However, accumulation of mH2A at pericentromeric heterochromatin has been shown in multiple tissues from aging mice (Kreiling et al., 2011). "
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    • "Because ionizing irradiation (IR) and chemical agents such as doxorubicin exhibit cell-killing activity by inducing double-strand breaks (DSBs) and p53-dependent apoptosis, they have been considered therapeutic tools against malignant tumors [1]–[5]. To protect normal cells from injury, tumor cell-specific induction of apoptosis would be one of the most important properties of anti-tumor therapeutics [6], [7]. To regulate the p53-dependent apoptosis caused by DNA-damage, an understanding of upstream activators or regulators of P53 would be vital. "
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