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OBJECTIVE: The infl uence of the pregnancy hormone progesterone on macrophage activation is well studied due to the existence of macrophages within the female reproductive tract and uterus during pregnancy. The production of nitric oxide (NO) from L-arginine, occurs via the action of the enzyme nitric oxide synthase-2 (NOS2) and can be induced experimentally by LPS. Progesterone downregulates NO production through binding the glucocorticoid receptor, in order to limit infl ammation. Arginase I also uses L-arginine as a substrate resulting in the production of L-ornithine and urea as a by-product. Induction of arginase I is typically associated with stimulation with the Th2 associated cytokines IL-4 and IL-13. The aim of this work was to investigate the ability of progesterone to modulate arginase gene expression and enzyme activity in murine macrophages. METHOD: Macrophages were grown from the bone marrow of BALB/c mice for 8-10 days. These cells were then exposed to progesterone (62.5µM) for approximately 16 hours prior to stimulation with IL-4 (100U/ml), LPS (200ng/ml) or a combination of both. Expression of argI and nos2 mRNA transcripts were analysed by quantitative real time PCR. NOS2 activity was determined by carrying out the Greiss assay on cell supernatants and arginase activity determined the ability of arginase within cell lysates to produce urea from L-arginine. Statistical signifi cance of differences was calculated by a Mann-Whitney U-test and p<0.05 was accepted as signifi cant. RESULTS: Progesterone downregulates arginase activity in macrophages stimulated with LPS or in combination with IL-4. By comparison with norgestrel, a synthetic progestin that binds only the progesterone receptor, and dexamethasone which binds the glucocorticoid receptor, we demonstrate that progesterone modulates arginase activity through the glucocorticoid receptor. In addition, Real Time PCR analysis revealed that progesterone downregulates IL-4 induced and LPS induced argI gene expression. CONCLUSION: In addition to limiting NO production, progesterone acts to limit arginase I gene expression and enzyme activity, possibly as a means of dampening all forms of macrophage activation.
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