Phospholipase A2 in experimental allergic bronchitis: a lesson from mouse and rat models.

BACKGROUND: Phospholipases A2 (PLA2) hydrolyzes phospholipids, initiating the production of inflammatory lipid mediators. We have previously shown that in rats, sPLA2 and cPLA2 play opposing roles in the pathophysiology of ovalbumin (OVA)-induced experimental allergic bronchitis (OVA-EAB), an asthma model: Upon disease induction sPLA2 expression and production of the broncho-constricting CysLTs are elevated, whereas cPLA2 expression and the broncho-dilating PGE2 production are suppressed. These were reversed upon disease amelioration by treatment with an sPLA2 inhibitor. However, studies in mice reported the involvement of both sPLA2 and cPLA2 in EAB induction. OBJECTIVES: To examine the relevance of mouse and rat models to understanding asthma pathophysiology. METHODS: OVA-EAB was induced in mice using the same methodology applied in rats. Disease and biochemical markers in mice were compared with those in rats. RESULTS: As in rats, EAB in mice was associated with increased mRNA of sPLA2, specifically sPLA2gX, in the lungs, and production of the broncho-constricting eicosanoids CysLTs, PGD2 and TBX2 in bronchoalveolar lavage (BAL). In contrast, EAB in mice was associated also with elevated cPLA2 mRNA and PGE2 production. Yet, treatment with an sPLA2 inhibitor ameliorated the EAB concomitantly with reverting the expression of both cPLA2 and sPLA2, and eicosanoid production. CONCLUSIONS: In both mice and rats sPLA2 is pivotal in OVA-induced EAB. Yet, amelioration of asthma markers in mouse models, and human tissues, was observed also upon cPLA2 inhibition. It is plausible that airway conditions, involving multiple cell types and organs, require the combined action of more than one, essential, PLA2s.

MMP production in human fibrosarcoma cells and their invasiveness are regulated by group IB secretory phospholipase A2 receptor-mediated activation of cytosolic phospholipase A2.

Matrix metalloproteinases (MMPs), especially MMP-2 and MMP-9, are expressed in most colonic, gastric, and ovarian carcinomas, and they play a key role in their invasiveness. Previous studies have shown the involvement of arachidonic acid (AA)-derived metabolites in the regulation of MMP expression and cancer dissemination, thus suggesting a role for phospholipase A2, the AA producing enzymes, in these processes. The present study was undertaken to explore the role of phospholipases in MMP production and tumor cell invasiveness. Human fibrosarcoma cells were found to express and secrete type IB, IIA and V sPLA2. The cells were found also to express the M-type sPLA2 receptor. Treatment with an extracellular sPLA2 inhibitor inhibited tumor cell's invasiveness concomitantly with MMP-2/9 production. Correspondingly, adding an exogenous sPLA2-IB (but not IIA) resulted in significant elevation of MMP-2/9 secretion from the fibrosarcoma cells. Time-course determination of AA and oleic acid release by HT-1080 cells suggested that cPLA2 is activated subsequently to sPLA2 action. Accordingly, using Western blot analysis it was found that sPLA2-IB induced cPLA2 phosphorylation, a requirement for its activation, by a receptor-mediated activity, rather than its lipolytic activity. At the same time, sPLA2-IIA did not induce either MMPs secretion or cPLA2 phosphorylation. The results of this study show for the first time that MMP-2/9 production by human fibrosarcoma HT-1080 cells and their invasiveness is regulated by sPLA2-IB acting as a receptor ligand to activate cPLA2, which in turn provides the AA for production of eicosanoids required for MMP expression.

Negative feedback between secretory and cytosolic phospholipase A2 and their opposing roles in ovalbumin-induced bronchoconstriction in rats.

Phospholipase A2 (PLA2) hydrolyzes cell membrane phospholipids (PL) to produce arachidonic acid and lyso-PL. The PLA2 enzymes include the secretory (sPLA2) and cytosolic (cPLA2) isoforms, which are assumed to act synergistically in production of eicosanoids that are involved in inflammatory processes. However, growing evidence raises the possibility that in airways and asthma-related inflammatory cells (eosinophils, basophils), the production of the bronchoconstrictor cysteinyl leukotrienes (CysLT) is linked exclusively to sPLA2, whereas the bronchodilator prostaglandin PGE2 is produced by cPLA2. It has been further reported that the capacity of airway epithelial cells to produce CysLT is inversely proportional to PGE2 production. This seems to suggest that sPLA2 and cPLA2 play opposing roles in asthma pathophysiology and the possibility of a negative feedback between the two isoenzymes. To test this hypothesis, we examined the effect of a cell-impermeable extracellular sPLA2 inhibitor on bronchoconstriction and PLA2 expression in rats with ovalbumin (OVA)-induced asthma. It was found that OVA-induced bronchoconstriction was associated with elevation of lung sPLA2 expression and CysLT production, concomitantly with suppression of cPLA2 expression and PGE2 production. These were reversed by treatment with the sPLA2 inhibitor, resulting in amelioration of bronchoconstriction and reduced CysLT production and sPLA2 expression, concomitantly with enhanced PGE2 production and cPLA2 expression. This study demonstrates, for the first time in vivo, a negative feedback between sPLA2 and cPLA2 and assigns opposing roles for these enzymes in asthma pathophysiology: sPLA2 activation induces production of the bronchoconstrictor CysLT and suppresses cPLA2 expression and the subsequent production of the bronchodilator PGE2.