Formation of diacyl and alkylacyl glycerophosphocholine in rabbit alveolar macrophages

Phospholipase A2-mediated superoxide production of murine peritoneal macrophages induced by chrysotile stimulation

In the present study, we investigated how chrysotile-stimulated macrophages generate superoxide using murine peritoneal macrophages, with special attention to the modulatory role of phospholipase A(2) (PLA(2)). We examined the effects of the following inhibitors and antagonists for signaling molecules on the superoxide anion (O(2)(-)) production of chrysotile-stimulated macrophages: p-bromophenacyl bromide (pBPB) and mepacrine for PLA(2); islet-activating protein (IAP) for G-protein; H-7 for protein kinase C (PKC); AA861 for 5-lipoxygenase (5-LO); indomethacin for cyclo-oxygenase (COX); ETYA for both 5-LO and COX; hexanolamine PAF for platelet-activating factor (PAF). The PLA(2) and PKC inhibitors effectively inhibited the chrysotile-induced superoxide anion production of macrophages, but not the G-protein inhibitor, the 5-LO and COX inhibitors, and the PAF antagonist. We also examined the effects of the PLA(2) inhibitors on macrophages stimulated by phorbol 12-myristate 13-acetate (PMA) which directly activates PKC. The two structurally different PLA(2) inhibitors showed differential effects on the PMA-induced superoxide generation: pBPB inhibited it but mepacrine did not. These results suggested that (1) PLA(2) and PKC modulate the chrysotile-induced O(2) production, and (2) two different kinds of PLA(2) work upstream and downstream of PKC, but (3) G-protein, 5-LO and COX metabolites, and PAF have no modulatory role in the reaction.

Agonist-stimulated glycerophospholipid acyl turnover in alveolar macrophages

The inflammatory compounds beta-glucan, a particulate agonist, and tannin, a soluble agonist, are present in cotton dust and, when inhaled, cause massive arachidonic acid release from alveolar macrophages. Earlier work had shown that these agonists exhibit different effects on arachidonate liberation and release, and that only tannin inhibits the uptake and incorporation of exogenous arachidonic acid, suggesting inhibition of reacylation. Here we have used the time-dependent incorporation of 18O from H218O-containing media into glycerophospholipid acyl groups as an indicator of acyl turnover in resting and agonist-treated rabbit alveolar macrophages. Highest turnover rates were seen in phosphatidylinositol ( approximately 30% per hour) and in choline phospholipids (10-20% per hour). Both beta-glucan and tannin stimulated acyl turnover, especially arachidonic acid turnover, in these and other lipid classes by a factor of 2 or more. We conclude that neither agonist promotes arachidonic acid accumulation in and release from alveolar macrophages by inhibiting reacylation.

Sources for brain arachidonic acid uptake and turnover in glycerophospholipids

Brain arachidonic acid comes from linoleic acid and arachidonic acid in the blood. Part of the brain arachidonic acid is elongated to adrenic acid, 22:4 (n = 6), especially in higher animals. With labeled arachidonic acid injected into cerebral ventricles of mice, the highest specific radioactivity was in triacylglycerols. The highest labeling in PtdCho and PtdIns was found at 15 to 60 minutes. Labeling of PtdEtn was much less. The molecular species with 16:0 and 18:1 were labeled better than those with 18:0. Adrenic acid was preferred by alkylacyl-GroPEtn. The highest level of labeling by arachidonic acid was found in the choline plasmalogens and the alkylacyl-GroPCho at 24 hours after injection. The PtdCho arachidonic acid turned over several times within 24 hours. Part of this turnover probably represents the transfer of labeled arachidonic acid to unlabeled ether-linked choline glycerophospholipids, including 1-alkyl-2-lyso-GroPCho, also known as lyso platelet activating factor. The energy-independent transfer of arachidonic acid from PtdCho to ether-linked choline glycerophospholipids may follow removal of their arachidonic acid by phospholipase A2 due to receptor activation. The lack of pulse labeling of ether-linked choline glycerophospholipids complicates the study of their function.