Phospholipase A2, an in vivo immunomodulator

Macrophage signaling and respiratory burst

Macrophages are key defenders of the lung and play an essential role in mediating the inflammatory response. Critical to this is the activation of the NADPH oxidase. Through receptor-mediated interactions, extracellular stimuli activate pathways that signal for the phosphorylation and assembly of the NADPH oxidase. Once the NADPH oxidase is activated, it produces superoxide and H2O2 in a process known as the respiratory burst. The involvement of O2.- and H2O2 in the antimicrobicidal function of macrophages has been assumed for many years, but it is now clear that the H2O2 produced by the respiratory burst functions as a second messenger and activates major signaling pathways in the alveolar macrophage. Both the nuclear factor-kappaB and activator protein-1 transcription factors are activated by H2O2 produced by the respiratory burst, and, since these control the inducible expression of genes whose products are part of the inflammatory response, this may be a critical link between the respiratory burst and other inflammatory responses. The c-Jun N-terminal kinase (JNK) and extracellular-regulated kinase (ERK) pathways, two members of the mitogen-activated protein kinase family, are also activated by the respiratory burst. JNK is activated by both exogenous and endogenously produced H2O2. Studies with ERK have shown that specific agonists of the respiratory burst, but not bolus H2O2, can activate this pathway. The ERK pathway also modulates the expression of genes via phosphorylation of the transcription factor Elk-1 that controls the production of the c-Fos transcription factor. Although an understanding of the mechanism of redox signaling is in its infancy, it is becoming clear that the reactive oxygen species produced by the respiratory burst have a profound effect on intracellular signaling pathways and ultimately in modulating gene expression.

Demonstration of membrane-associated phospholipase A2 in cultivated heart muscle cells by immunogold-technique in surface replicas

Recently we produced mAb's to phospholipase A2 (PIA2) from bee venom for different purposes (allergen-identification and -standardization). Obviously one of these monoclonal antibodies "5D5" binds to a conserved (cross reactive) epitope of membrane PIA2 of intact cells. In this study we demonstrate binding of 5D5 to membrane associated phospholipase A2 of rat heart muscle cells by means of two-step immunogold technique in combination with replica technique.

Enhanced metabolism of arachidonic acid by macrophages from nonobese diabetic (NOD) mice

The inbred nonobese diabetic (NOD) mouse spontaneously develops an autoimmune diabetes, which is now recognized as an experimental model for human type I insulin-dependent diabetes mellitus (IDDM). The autoimmune reaction, specifically directed against pancreatic beta cells (insulitis), involves both macrophages and T lymphocytes. The study of the production of cyclooxygenase and lipoxygenase derivatives of arachidonic acid metabolism shows that in some conditions, and in particular in the presence of zymosan A, macrophages from NOD mice produced significantly more 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and leukotriene C4 (LTC4) than macrophages from age- and sex-matched C57BL/6 mice. Moreover, zymosan A-stimulated macrophages from NOD females produced significantly more LTC4 than did macrophages from NOD males. These results may be of interest, given the bidirectional relationship between the various cytokines involved in the destruction of beta cells of the islets of Langerhans and different eicosanoids.

Increased phagocytic activity of peripheral blood monocytes after intravenous injection of phospholipase A2 to monkeys

One of the expressions of the activity of phagocytic cells such as monocytes or macrophages is a burst of increased oxidative activity on stimulation. The free oxygen radicals liberated, mainly O2- and H2O2, lead to chemoluminescence, which is thus a measure of activation. Chemoluminescence also depends on arachidonic acid metabolism, and this depends on phospholipase A2 (PLA2). We modified monocyte activity in monkeys by injecting them i.v. with this enzyme and observed that 30 min after injection, the phagocytic activity of peripheral blood monocytes and the chemoluminescence they emitted was greater than that of controls. We suggest that PLA2 may act as an in vivo immunomodulator in mammals.

Bestatin, an aminopeptidase inhibitor with a multi-pharmacological function

Bestatin, the dipeptide N-((2S, 3R)-3-amino-2-hydroxy-4-phenyl butanyl) L-leucine is a leucine aminopeptidase and aminopeptidase-B inhibitor. It exerts a direct stimulating effect on lymphocytes (and monocytes) via its fixation on cell surface leucine-aminopeptidase, and an indirect effect on monocytes (and lymphocytes) via aminopeptidase B inhibition of tuftsin catabolism. Thus it is an immuno-modifier as shown by the Japanese and also our groups: a) immuno-modifier, especially stimulator, in normal young mice; b) immunorestorator and spontaneous tumor preventive agent in aged mice; c) immunorestorator in the elderly and in cancer patients and HIV infected subjects. It stimulates granulocytopoiesis and thrombocytopoiesis in vitro, and can restore them in vivo in myelo-hypoplastic man. The anti-aminopeptidase action of Bestatin also protects enkephalins against their catabolism, which encouraged us, with good preliminary results, to study its analgesic action and search for a preventive effect on "desaddicted" toxicomaniac relapses.