Exposure to subclinical levels of lipopolysaccharide (LPS) occurs commonly and is seemingly well tolerated. However, recurrent LPS exposure induces cardiac fibrosis over 2 to 3 months in a murine model, not mediated by the renin-angiotensin system. Subclinical LPS induces cardiac fibrosis by unique mechanisms. In a recent study led by researchers from the Veterans Administration San Diego Healthcare System, C57/Bl6 mice, LPS (10 mg/kg) or saline (control) were injected intraperitoneally once a week for 1–4 weeks. During that time, the mice showed no signs of distress, change in activity, appetite, or weight loss. Mice were euthanized after 3 days, 1, 2, or 4 weeks to measure cardiac expression of fibrosis-related genes and potential mediators (measured by QRT-PCR), including micro-RNA (miR) and NADPH oxidase (NOX). Collagen fraction area of the left ventricle was measured with picrosirius red staining and cardiac fibroblasts isolated from adult mouse hearts were incubated with 0, 0.1, 1.0 or 10 ng/ml LPS for 48 hours. Cardiac miRNA expression profiling using LC Sciences’ miRNA microarray service demonstrated decreased miR-29c after 3 and 7 days following LPS, which was confirmed by QRT-PCR. The earliest changes in fibrosis-related genes and mediators that occurred 3 days after LPS were increased cardiac expression of TIMP-1 and NOX-2 (but not of NOX-4). That persisted at 1 and 2 weeks, with additional increases in collagen Iα1, collagen IIIα1, MMP2, MMP9, TIMP1, TIMP2, and periostin. There was no change in TGF-β or connective tissue growth factor. Collagen fraction area of the left ventricle increased after 2 and 4 weeks of LPS and LPS decreased miR-29c and increased NOX-2 in isolated cardiac fibroblasts. Through their work, scientists observed recurrent exposure to subclinical LPS induced cardiac fibrosis after 2–4 weeks. Researchers note that early changes 3 days after LPS were decreased miR-29c and increased NOX2 and TIMP1, which persisted at 1 and 2 weeks, along with widespread activation of fibrosis-related genes. Their work suggests decreased miR-29c and increased NOX2, which induce cardiac fibrosis in other conditions, may uniquely mediate LPS-induced cardiac fibrosis.
Cardiac miRNA expression profiling after LPS.
Cardiac miRNA expression profiling in C57/Bl6 mice injected with i.p. saline (time 0, control), or with hearts harvested 3 or 7 days after LPS (10 mg/kg) (n = 3 mice each time period). Heat map shows miR expression that increased (red), did not change (black) or decreased (green) for three mice each at time 0 (control, columns 1–3), 3 days after LPS (columns 4–6), and 7 days after LPS (columns 7–9), with differences between the three time periods with a p-value of<0.10 (ANOVA).
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Reference
Lew WYW, Bayna E, Dalle Molle E, Contu R, Condorelli G Myocardial Fibrosis Induced by Exposure to Subclinical Lipopolysaccharide Is Associated with Decreased miR-29c and Enhanced NOX2 Expression in Mice. (2014) PLoS ONE 9(9): e107556. [article]