Regulation of agonist-induced prostaglandin E₁ versus prostaglandin E₂ production: A mass analysis

Prostaglandin E₁ (PGE₁) and prostaglandin E₂ (PGE₂), derived by enzymatic oxidation of cellular dihomogammalinolenic acid (DHLA) and arachidonic acid (AA), respectively, have diverse and, at times, distinct biological actions. It has been suggested that PGE₁ specifically inhibits a variety of inflammatory processes, and, in light of the potential therapeutic benefit of PGE₁ and its fatty acid precursor in inflammatory disorders, there is growing interest in the biochemical mechanisms which determine the balance between PGE₁ and PGE₂ synthesis. Metabolic studies in this area have been hampered by the difficulties in measuring the extremely small masses of these prostaglandins which are generated in cell culture systems. We studied the regulation of PGE₁ versus PGE₂ synthesis using an essential fatty acid-deficient, PGE-producing, mouse fibrosarcoma cell line, EFD-1. Because EFD-1 cells contain no endogenous AA or DHLA, we were able to replete the cells with AA and DHLA of known specific activities; thus, the mass of both cellular AA and DHLA, and synthesized PGE₁ and PGE₂, could be accurately determined. The major finding of this study is that production of PGE₂ was highly favored over production of PGE₁ due to preferential incorporation of AA versus DHLA into, and release from, the total cellular phospholipid pool. Further, we correlated the selective release of AA versus DHLA from total cellular phospholipids with the selective incorporation of AA versus DHLA into specific phospholipid pools. In addition, we showed that conversion of DHLA to AA by Δ5 desaturase was enhanced by increasing the cellular mass of n - 6 fatty acids and by increasing the cell proliferative activity. Together, these results indicate that the relative abundance of PGE₂ versus PGE₁ in vivo is not merely a function of the relative abundance of AA versus DHLA in tissues, but also relates to markedly different cellular metabolism of these two fatty acids.