E-Ring 8-Isoprostanes Are Agonists at EP2- and EP4-Prostanoid Receptors on Human Airway Smooth Muscle Cells and Regulate the Release of Colony-Stimulating Factors by Activating cAMP-Dependent Protein Kinase

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Abstract
8-Isoprostanes are bioactive lipid mediators formed via the nonenzymatic peroxidation of arachidonic acid by free radicals and reactive oxygen species. However, their cognate receptors, biological actions, and signaling pathways are poorly studied. Here, we report the effect of a variety of E- and Fα-ring 8-isoprostanes on the release of granulocyte/macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF) from human airway smooth muscle (HASM) cells stimulated with interleukin-1β (IL-1β). The elaboration of GM-CSF and G-CSF by IL-1β was inhibited and augmented, respectively, in a concentration-dependent manner by 8-iso-prostaglandin (PG) E1 and 8-iso-PGE2, but not by 8-iso-PGF1α, 8-iso-PGF2α, and 8-iso-PGF3α. AH 6809 (6-isopropoxy-9-oxoxanthine-2-carboxylic acid), an EP1-/EP2-/DP-receptor blocking drug, antagonized the inhibitory effect of 8-iso-PGE1 and 8-iso-PGE2 on GM-CSF output with an affinity consistent with an interaction at prostanoid receptors of the EP2-subtype. In contrast, the facilitation by 8-iso-PGE1 and 8-iso-PGE2 of G-CSF release was unaffected by AH 6809 and the selective EP4-receptor antagonist L-161,982 [4′-[3-butyl-5-oxo-1-(2-trifluoromethyl-phenyl)-1,5-dihydro-[1,2,4]triazol-4-ylmethyl]-biphenyl-2-sulfonic acid (3-methyl-thiophene-2-carbonyl)-amide]. However, when used in combination, AH 6809 and L-161,982 displaced 5-fold to the right the 8-iso-PGE and 8-iso-PGE concentration-response curves. The opposing 1effect of E-ring 28-isoprostanes on GM-CSF and G-CSF release was mimicked by 8-bromo-cAMP and abolished in cells infected with an adenovirus vector encoding an inhibitor protein of cAMP-dependent protein kinase (PKA). Together, these data demonstrate that E-ring 8-isoprostanes regulate the secretion of GM-CSF and G-CSF from HASM cells by a cAMP- and PKA-dependent mechanism. Moreover, antagonist studies revealed that 8-iso-PGE1 and 8-iso-PGE2 act solely via EP2 -receptors to inhibit GM-CSF release, whereas both EP2- and EP4-receptor subtypes positively regulate G-CSF output.

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