Objective The World Health Organization predicts that cardiovascular disease will be the number one cause of death by the year 2020. Cardiac fibrosis, a major finding in end-stage heart disease, contributes to impaired function and may lead to the development of arrhythmias. The mechanisms through which cardiac fibrosis develop are incompletely understood, and consequently there are no specific therapies for fibrosis. Mice with macrophage-targeted overexpression of urokinase plasminogen activator (SR-uPA+/o mice) develop macrophage accumulation and subsequent cardiac fibrosis. Arginase, an immunoregulatory enzyme produced by macrophages in response to stimulation by TH2 cytokines, promotes fibroblast proliferation and collagen production. Therefore, we hypothesized that SR-uPA+/o macrophages secrete increased levels of arginase protein, which could then act as a proximal mediator of the development of fibrosis.
Methods Arginase activity in macrophage and heart lysates from SR-uPA+/o and SR-uPAo/o mice was determined by addition of l-arginine followed by reaction with α-isonitrosopropiophenone, producing a magenta product detected by spectroscopy at 562 nm. Arginase 1 was detected in macrophages by Western blot and normalized to beta-actin.
Results SR-uPA+/o peritoneal macrophages showed increased arginase activity as compared to SR-uPAo/o macrophages (14.5 [11.8 to 17.4] vs 5.1 [4.3 to 7.8] mmol urea formed/mg protein/min, p = .029, n = 4). SR-uPA+/o heart lysates also showed more arginase activity than uPAo/o lysates (0.003 [0.000-0.018] vs 0.000 [0.000-0.001] mmol urea formed/mg protein/min, p = .049, n = 11). No significant difference was found in arginase protein production between SR-uPA+/o and SR-uPAo/o peritoneal macrophages (0.480 ± 0.272 vs 0.744 ± 0.197 normalized densitometry units, p = .246, n = 3).
Conclusions Both SR-uPA+/o macrophages and hearts showed increased arginase activity without a corresponding increase in arginase protein production. Increased activity without changes in production could be due to changes in other factors influencing arginase availability. It will be important to determine if increased arginase activity in our model influences cardiac fibroblast collagen production.
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