Archives

  • 2018-07
  • 2019-04
  • 2019-05
  • 2019-06
  • 2019-07
  • In vitro studies suggest that

    2019-07-11

    In vitro studies suggest that cPLA2α does not show clear preference for the headgroup present at the sn-3 position, although a slight preference for zwitterionic phospholipids such as PC and PE has recently been pointed out [24]. In this regard, mass spectrometry analyses of glycerophospholipid hydrolysis during macrophage activation by phagocytic stimuli have unveiled links between the hydrolysis of either AA-containing PC or -PE and the formation of specific eicosanoid molecules [57]. During macrophage phagocytosis, the bulk of AA lost from membranes appears to originate pgds inhibitor from PC and PI, with seemingly little or no contribution from PE [57]. However, use of an inhibitor of CoA-IT that blocks the transfer of AA from PC to PE results in highly significant losses of AA from PE, indicating that during stimulation conditions AA is hydrolyzed from PE and the pools are rapidly replenished with AA from PC via transacylation reactions mediated by CoA-IT. Importantly, under these inhibitory conditions, the production of lipoxygenase metabolites increases, suggesting that the process of AA transfer between phospholipid classes does regulate the production of specific eicosanoids [57]. This opens the intriguing question of whether the synthesis of different eicosanoids is regulated by the action of cPLA2α on different phospholipid pools, and whether the different accessibility of cPLA2α to such pools can modify the course of the inflammatory response. In studies on the cPLA2α-regulated eicosanoid response of bone marrow-derived mast cells, it was found that maturation of these pgds inhibitor is associated with phospholipid remodeling regulated in part by cPLA2α, leading to a general decrease in AA-containing PC and PE. During the process of mast cell maturation in co-culture with fibroblasts, the AA released from mast cells by cPLA2α is transferred to adjacent fibroblasts for the synthesis of anti-allergic prostaglandin E2 [58]. The wide range of biological roles attributed to cPLA2α has been extended by recent work implicating this enzyme in the selective production of lipid mediators during macrophage polarization to either M1 (pro-inflammatory) or M2 (anti-inflammatory) states. A recent study investigated the synthesis of lipid mediators released by macrophages treated with GM-CSF or M-CSF, which polarize the cells to M1 and M2 states, respectively [59]. While under unstimulated conditions both types of macrophages generate pro-resolving lipid mediators in a similar manner, upon cell stimulation with bacteria, M2 macrophages mobilize larger amounts of AA via increased cPLA2α activation, and generate more leukotriene C4, resembling in this regard the M2-like cells in lung allergy [59]. These differences may be due, at least in part, to differences in the localization/compartmentalization of AA-metabolizing enzymes, which determine substrate accessibility. Consistent with these results, more recent work has also highlighted the different lipid mediator signatures of macrophages responding to bacteria, depending on whether they have been polarized to M1 or M2 [60]. Still, much work remains to be carried out to clarify the compartmentalized regulation of enzymes participating in the synthesis of these lipid mediators, including cPLA2α. Previous studies demonstrated that cPLA2α translocates to the phagosomal membrane during macrophage phagocytosis through mechanisms that, in humans, involve phosphorylation of the enzyme by c-Jun N-terminal kinases and membrane association via the cationic cluster of four lysine residues present at the catalytic domain of the enzyme (Fig. 2) [50,52,[61], [62], [63], [64], [65]]. More recently, it has been shown that the N-terminal C2 domain of cPLA2α is sufficient to support FcR-mediated phagocytosis [66]. Importantly, overexpression of a cPLA2α mutant without a complete catalytic domain, and therefore without enzymatic activity, rescues FcR-mediated phagocytosis in cells from Pla2g4a−/− mice [66]. Based on these data, the proposal was made that the C2 domain of cPLA2α induces perturbation of the membrane phospholipid packing, potentially generating membrane bending that is necessary for phagosome formation in a manner independent of its enzymatic activity [66]. These studies are important in that they provide the first example of a biological function associated to cPLA2α that does not depend on enzyme activity. It is also worth noting in this regard another recent study implicating cPLA2α in the regulation of the tumor suppressor gene SIRT2 via mechanisms likely independent of the hydrolytic activity of the enzyme [67].