Cigarette smoking is known to be the leading cause of chronic obstructive pulmonary disease (COPD). However, the detailed mechanisms have not been elucidated. Platelet-activating factor (PAF), a potent inflammatory mediator, is involved in the pathogenesis of various respiratory diseases, such as bronchial asthma or COPD. We focused on lysophospholipid acyltransferase 9 (LPLAT9), a biosynthetic enzyme of PAF, in the pathogenesis of COPD. LPLAT9 gene expression was observed in excised COPD lungs and single-cell RNA sequencing data of alveolar macrophage (AM). LPLAT9 was predominant and upregulated in AM, particularly monocyte-derived AM, in patients with COPD. To identify the function of LPLAT9/PAF in AM on the pathogenesis of COPD, we exposed cigarette smoke (CS) to systemic LPLAT9 knockout (LPALT9-/-) mice. CS increased the number of AM, especially monocyte-derived fraction, which secreted matrix metalloprotease 12 (MMP12). Also, CS augmented LPLAT9 phosphorylation/activation on macrophage and, subsequently, PAF synthesis in mice lung. LPLAT9-/- mice lung reduced PAF production after CS exposure. Intratracheal PAF administration accumulated AM by increasing monocyte chemoattractant protein-1 (MCP1). After CS exposure, AM accumulation and subsequent pulmonary emphysema, a primary pathologic change of COPD, were reduced in LPALT9-/- mice than in LPLAT9+/+ mice. Notably, these phenotypes got worsened again by LPLAT9+/+ bone marrow transplantation in LPALT9-/- mice. Thus, CS-induced LPLAT9 activation in monocyte-derived AM aggravated pulmonary emphysema via PAF-induced further AM accumulation. These results suggest that PAF synthesized by LPLAT9 has an important role in the pathogenesis of COPD.
Keywords: chronic obstructive pulmonary disease; cigarette smoke; lysophospholipid acyltransferase 9; monocyte-derived macrophage; platelet-activating factor.