BACKGROUND: Cellular senescence can be a functional barrier to carcinogenesis. 
We hypothesized that inflammation modulates carcinogenesis through senescence 
and DNA damage response (DDR). We examined the association between senescence 
and DDR with macrophage levels in inflammatory bowel disease (IBD). In vitro 
experiments tested the ability of macrophages to induce senescence in primary 
cells. Inflammation modulating microRNAs were identified in senescence colon 
tissue for further investigation.
METHODOLOGY/PRINCIPAL FINDINGS: Quantitative immunohistochemistry identified 
protein expression by colon cell type. Increased cellular senescence (HP1γ; P = 
0.01) or DDR (γH2A.X; P = 0.031, phospho-Chk2, P = 0.014) was associated with 
high macrophage infiltration in UC. Co-culture with macrophages (ANA-1) induced 
senescence in >80% of primary cells (fibroblasts MRC5, WI38), illustrating that 
macrophages induce senescence. Interestingly, macrophage-induced senescence was 
partly dependent on nitric oxide synthase, and clinically relevant NO• levels 
alone induced senescence. NO• induced DDR in vitro, as detected by 
immunofluorescence. In contrast to UC, we noted in Crohn's disease (CD) that 
senescence (HP1γ; P<0.001) and DDR (γH2A.X; P<0.05, phospho-Chk2; P<0.001) were 
higher, and macrophages were not associated with senescence. We hypothesize that 
nitric oxide may modulate senescence in CD; epithelial cells of CD had higher 
levels of NOS2 expression than in UC (P = 0.001). Microarrays and 
quantitative-PCR identified miR-21 expression associated with macrophage 
infiltration and NOS2 expression.
CONCLUSIONS: Senescence was observed in IBD with senescence-associated 
β-galactosidase and HP1γ. Macrophages were associated with senescence and DDR in 
UC, and in vitro experiments with primary human cells showed that macrophages 
induce senescence, partly through NO•, and that NO• can induce DDR associated 
with senescence. Future experiments will investigate the role of NO• and miR-21 
in senescence. This is the first study to implicate macrophages and nitrosative 
stress in a direct effect on senescence and DDR, which is relevant to many 
diseases of inflammation, cancer, and aging.