Calcium ionophore (A-23187) induced peritoneal eicosanoid biosynthesis: a rapid method to evaluate inhibitors of arachidonic acid metabolism in vivo

PPAR-γ activation by Tityus serrulatus venom regulates lipid body formation and lipid mediator production

Tityus serrulatus venom (TsV) consists of numerous peptides with different physiological and pharmacological activities. Studies have shown that scorpion venom increases pro-inflammatory cytokine production, contributing to immunological imbalance, multiple organ dysfunction, and patient death. We have previously demonstrated that TsV is a venom-associated molecular pattern (VAMP) recognized by TLRs inducing intense inflammatory reaction through the production of pro-inflammatory cytokines and arachidonic acid-derived lipid mediators prostaglandin (PG)E2 and leukotriene (LT)B4. Lipid bodies (LBs) are potential sites for eicosanoid production by inflammatory cells. Moreover, recent studies have shown that the peroxisome proliferator-activated receptor gamma (PPAR-γ) is implicated in LB formation and acts as an important modulator of lipid metabolism during inflammation. In this study, we used murine macrophages to evaluate whether the LB formation induced by TsV after TLR recognition correlates with lipid mediator generation by macrophages and if it occurs through PPAR-γ activation. We demonstrate that TsV acts through TLR2 and TLR4 stimulation and PPAR-γ activation to induce LB formation and generation of PGE2 and LTB4. Our data also show that PPAR-γ negatively regulates the pro-inflammatory NF-κB transcription factor. Based on these results, we suggest that during envenomation, LBs constitute functional organelles for lipid mediator production through signaling pathways that depend on cell surface and nuclear receptors. These findings point to the inflammatory mechanisms that might also be triggered during human envenomation by TsV.

Kinetics of prostanoid synthesis by macrophages is regulated by arachidonic acid sources

The dependence of prostanoid synthesis on the nature of free arachidonic acid (AA) appearance was investigated in mouse peritoneal macrophages. AA delivery from intracellular sources to the constitutive prostaglandin (PG)H synthase was provided by action of calcium-ionophore A23187; and from extracellular sources by AA addition to the culture medium. It was found that the kinetics of prostanoid synthesis dramatically depends on the sources of AA. Free AA concentration used for prostanoid synthesis is either a constant or a variable value depending upon the sources. The kinetics of cellular prostanoid synthesis can be regulated by the following processes: (a) the irreversible inactivation of PGH-synthase in the course of the reaction (kin), (b) prostanoid metabolism (kr), and (c) incorporation of exogenous AA into cellular membranes (ka). From our experiments and mathematical calculation these parameters were found to be kin = 0.20 +/- 0.02 min-1, kr = 0.17 +/- 0.03 min-1 in the case of stimulation with A23187, and kin = 0.0156 min-1, kr = 0. 0134 min-1, ka = 0.0025 min-1 in the case of exogenous AA addition. The studies of prostanoid biosynthesis by macrophage microsomes led to independent determination of kin = 0.20 +/- 0.02 min-1. This value perfectly fits the kinetics of the prostanoid cell synthesis under endogenous AA supply but shows a 10-fold decrease in the case of exogenous AA supply. Our study on the kinetics of prostanoid synthesis by mouse peritoneal macrophages clearly demonstrate that AA is able to regulate cellular prostanoid synthesis in the presence of constitutive PGH-synthase only. A regulation mechanism based on the co-operation of the constitutive PGH-synthase isoform and the availability of free AA is proposed and could be confirmed by mathematical modelling.

Role of mast cells in calcium ionophore (A23187)-induced peritoneal inflammation in mice

Several lines of evidence document a critical role for mast cells in immune complex-mediated inflammatory models. However, their role in nonimmune models of acute inflammation is largely unknown. In the present investigation, the role of mast cells was examined in calcium ionophore (A23187)-induced mouse peritoneal inflammation. Intraperitoneal injection of A23187 (20) micrograms/mouse) elicited marked and transient increases in immunoreactive levels of 6-ketoprostaglandin-F2 alpha, leukotrienes B4, C4, D4, E4, and F4. There were no discernible differences in levels of these mediators in male Swiss Webster mice, mast cell-deficient mice (WBB6F1-W/W'), and age-matched controls (WBB6F1-+/+), suggesting a minimal role of mast cells in eicosanoid biosynthesis in this model. However W/W' mice showed smaller increases in levels of myeloperoxidase, a marker for neutrophils, compared to +/+ mice. Both W/W' and +/+ mice have lower constitutive levels of peritoneal N-acetyl-beta-D-glucosaminidase (NAG), a marker for mononuclear cells. Similar to the changes seen in myeloperoxidase, W/W' mice exhibited a blunted NAG response compared to +/+ mice. These results suggest that mast cell products other than eicosanoids may contribute to the changes in cellular trafficking in response to intraperitoneal A23187. These results also suggest that mast cells are required for full expression of inflammatory responses.