Reduced 5-lipoxygenase metabolism of arachidonic acid in macrophages rrom 1,25-dihydroxyvitamin D3-deficient rats.

The peripheral blood monocyte (PBM) migrates into tissues and differentiates into mature tissue macrophages. Previous investigations from our laboratory have demonstrated that PBM have reduced 5-lipoxygenase (5-LO) metabolism of arachidonic acid (AA) and 5-LO activating protein (FLAP) expression as compared to differentiated alveolar macrophages (AM). Moreover, PBM differentiated with 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) displayed increased leukotriene synthesis and a parallel increase in FLAP expression. In the present study, we sought to examine the physiological role of 1,25-(OH)2D3 in the regulation of eicosanoid metabolism in terminally differentiated alveolar and peritoneal macrophages (PM), utilizing a well characterized rat model of vitamin D3-deficiency. AM from vitamin D3-deficient rats demonstrated reduced 5-LO metabolism of AA and a parallel reduction in FLAP expression compared to control rats. Similarly, PM from vitamin D3-deficient rats demonstrated reduced 5-LO metabolism of AA. The effect of vitamin D3 was specific for the 5-LO pathway, not affecting total release of AA or its metabolism via 12-lipoxygenase or cyclooxoygenase (COX) pathways in macrophages. Furthermore, it did not affect COX protein expression in macrophages or type II alveolar epithelial cells. In control animals, 1,25-(OH)2D3 concentrations were greater in bronchoalveolar lavage fluid (2.6-fold) and peritoneal lavage fluid (1.6-fold) than in serum, which may account for the greater FLAP expression in AM and PM than in PBM. These observations suggest that 1,25-(OH)2D3 plays a physiological role in upregulating the 5-LO pathway in tissue macrophages in vivo.