Differential metabolism of dihomo-γ-linolenic acid and arachidonic acid by cyclo-oxygenase-1 and cyclo-oxygenase-2: implications for cellular synthesis of prostaglandin E1 and prostaglandin E2

Abstract

Prostaglandin (PG) E1 has been shown to possess anti-inflammatory properties and to modulate vascular reactivity. These activities are sometimes distinct from those of PGE2, suggesting that endogenously produced PGE1 may have some beneficial therapeutic effects compared with PGE2. Increasing the endogenous formation of PGE1 requires optimization of two separate processes, namely, enrichment of cellular lipids with dihomo-γ-linolenic acid (20:3 n—6; DGLA) and effective cyclo-oxygenase-dependent oxygenation of substrate DGLA relative to arachidonic acid (AA; 20:4 n—6). DGLA and AA had similar affinities (Km values) and maximal reaction rates (Vmax) for cyclo-oxygenase-2 (COX-2), whereas AA was metabolized preferentially by cyclo-oxygenase-1 (COX-1). To overcome the kinetic preference of COX-1 for AA, CP-24879, a mixed Δ5/Δ6 desaturase inhibitor, was used to enhance preferential accumulation of DGLA over AA in cells cultured in the presence of precursor γ-linolenic acid (18:3 n—6). This protocol was tested in two cell lines and both yielded a DGLA/AA ratio of approx. 2.8 in the total cellular lipids. From the enzyme kinetic data, it was calculated that this ratio should offset the preference of COX-1 for AA over DGLA. PGE1 synthesis in the DGLA-enriched cells was increased concurrent with a decline in PGE2 formation. Nevertheless, PGE1 synthesis was still substantially lower than that of PGE2. It appears that employing a dietary or a combined dietary/pharmacological paradigm to augment the cellular ratio of DGLA/AA is not an effective route to enhance endogenous synthesis of PGE1 over PGE2, at least in cells/tissues where COX-1 predominates over COX-2.