Transcellular eicosanoid synthesis in cocultures of alveolar epithelial cells and macrophages

We have examined the importance of the relative capacities for arachidonic acid (AA) liberation and oxygenation as determinants of the magnitude of transcellular eicosanoid synthesis in cocultures of [3H]AA-prelabeled alveolar macrophages (AM) and [14C]AA-prelabeled alveolar epithelial cells (AEC). Taking advantage of our previous observation that over time in culture, AEC exhibit increases in AA release (plateau at day 4) that precede increases in cyclooxygenase capacity (maximal at day 7), we studied cocultures of freshly harvested AM together with AEC at culture days 2, 4, and 7. In this model, ionophore stimulation resulted in transcellular eicosanoid synthesis via the bidirectional transfer of free AA, with AM synthesizing their exclusive 5-lipoxygenase products leukotriene B4 and 5-hydroxyeicosatetraenoic from AEC-derived AA, and AEC synthesizing their exclusive cyclooxygenase metabolites prostaglandin E2 and prostacyclin from AM-derived AA. 14C-labeled 5-lipoxygenase products were synthesized by cocultures in the rank order: day 4 > day 2 > day 7, paralleling the relative amounts of unmetabolized AEC-derived [14C]AA. Transcellular flow of [14C]AA to the AM 5-lipoxygenase pathway was also favored when the AEC cyclooxygenase pathway was inhibited with indomethacin. Inhibition of the AM 5-lipoxygenase pathway with the compound MK 886 similarly resulted in the accumulation of unmetabolized AM-derived [3H]AA and greater synthesis of 3H-labeled prostanoids. We conclude that 1) the quantity of unmetabolized AA made available by one cell is an important determinant of the extent to which the fatty acid is utilized for transcellular eicosanoid synthesis by the second cell; 2) the fate of AA derived from a donor cell is influenced by the expression of AA oxygenation pathways in both the donor cell and the recipient cell; and 3) intercellular exchange of AA favors its shunting from an inhibited oxygenation pathway to an uninhibited one.