The distribution and metabolism of arachidonate-containing phospholipids in cellular nuclei.

The cell nucleus has been identified as a location to which several arachidonic acid-metabolizing enzymes are located in stimulated cells. However, little information exists describing the distribution of arachidonate-containing phospholipids associated with the nucleus or the control of their composition. In this study, nuclei isolated from human monocyte-like THP-1 cells were found to have a distribution of arachidonyl-phospholipids which is markedly different from that of other cellular membranes. THP-1 nuclei which contained 22% of total cellular arachidonate, showed a near equal distribution of arachidonate in 1-acyl-2-arachidonoyl-glycero-3-phosphocholine, 1-acyl-2-arachidonyl-glycero-3-phosphoethanolamine, 1-acyl-2-arachidonoyl-glycero-3-phosphoinositol and 1-alk-1-enyl-2-arachidonoyl-glycero-3-phosphoethanolamine molecular species. In contrast in non-nuclear membranes, arachidonate was located primarily in 1-alk-1-enyl-2-arachidonoyl-glycero-3-phosphoethanolamine molecular species which accounted for approximately half of the arachidonate in all non-nuclear phospholipids. Isolated nuclei were incapable of initially acylating arachidonic acid into their phospholipids in the absence of cellular cytosol. However, they were capable of efficiently remodelling existing arachidonate between phospholipid classes and subclasses. Isolated nuclei contained 25-30% of the cellular activity of CoA-independent transacylase, the key enzyme responsible for arachidonate-phospholipid remodelling. This enzyme is also critical in the control of arachidonate availability following cell stimulation. Given that the cellular distribution of arachidonate is such that nuclei are enriched in donor substrates for the CoA-independent transacylase reaction, that non-nuclear membranes are enriched in acceptor substrates and that nuclei have the enzymatic machinery to remodel arachidonate efficiently, these results suggest that CoA-independent transacylase may be responsible for the remodelling of arachidonate not only between different phospholipid species within the same organelles but also between different sub-cellular compartments.