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A novel human homologue of Escherichia coli, yeast and plant 1-acylglycerol-3-phosphate acyltransferase has been isolated from U937 cell cDNA. Expression of the cloned sequence in 1-acylglycerol-3-phosphate acyltransferase-deficient E. coli resulted in increased incorporation of oleic acid into cellular phospholipids. Membranes made from COS7 cells transfected with the cDNA exhibited higher acyltransferase activity towards a range of donor fatty acyl-CoAs and lysophosphatidic acid. Northern-blot analysis of the cDNA sequence indicated high levels of expression in immune cells and epithelium. Rapid amplification of cDNA ends revealed differentially expressed splice variants, which suggests regulation of the enzyme by alternative splicing. This cDNA therefore represents the first described sequence of a mammalian gene homologous to non-mammalian lysophosphatidic acid acyltransferases.

Mononuclear phagocytes play a pivotal role in the progression of septic shock by producing tumor necrosis factor‐α (TNF‐α) and other inflammatory mediators in response to lipopolysaccharide (LPS) from Gram‐negative bacteria. Our previous studies have shown monocyte and macrophage activation correlate with changes in membrane phospholipid composition, mediated by acyltransferases. Interferon‐γ (IFN‐γ), which activates and primes these cells for enhanced inflammatory responses to LPS, was found to selectively activate lysophosphatidylcholine acyltransferase (LPCAT) (P < 0.05) but not lysophosphatidic acid acyltransferase (LPAAT) activity. When used to prime the human monocytic cell line MonoMac 6, the production of TNF‐α and interleukin‐6 (IL‐6) was approximately five times greater in cells primed with IFN‐γ than unprimed cells. Two LPCAT inhibitors SK&F 98625 (diethyl 7‐(3,4,5‐triphenyl‐2‐oxo2,3‐dihydro‐imidazole‐1‐yl)heptane phosphonate) and YM 50201 (3‐hydroxyethyl 5,3′‐thiophenyl pyridine) strongly inhibited (up to 90%) TNF‐α and IL‐6 production in response to LPS in both unprimed MonoMac‐6 cells and in cells primed with IFN‐γ. In similar experiments, these inhibitors also substantially decreased the response of both primed and unprimed peripheral blood mononuclear cells to LPS. Sequence‐based amplification methods showed that SK&F 98625 inhibited TNF‐α production by decreasing TNF‐α mRNA levels in MonoMac‐6 cells. Taken together, the data from these studies suggest that LPCAT is a key enzyme in both the pathways of activation (priming) and the inflammatory response to LPS in monocytes.

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