ACTIVITIES OF ENZYMES INVOLVED IN THE METABOLISM OF PLATELET-ACTIVATING-FACTOR IN NEURAL CELL-CULTURES DURING PROLIFERATION AND DIFFERENTIATION

Platelet-Activating Factor (PAF) is a potent lipid mediator involved i n physiological and pathological events in the nervous tissue where it can be synthesized by two distinct pathways. The last reaction of the de novo pathway utilizes CDPcholine and alkylacetylglycerol and is ca talyzed by a specific phosphocholinetransferase (PAF-PCT) whereas the remodelling pathway ends with the reaction catalyzed by lyso-PAF acety ltransferase (lyso-PAF AcT) utilizing lyso-PAF, a product of phospholi pase A, activity, and acetyl-CoA. The levels of PAF in the nervous tis sue are also regulated by PAF acetylhydrolase that inactivates this me diator. We have studied the activities of these enzymes during cell pr oliferation and differentiation in bo experimental models: 1) neuronal and glial primary cell cultures from chick embryo and 2) LA-N-1 neuro blastoma cells induced to differentiate by retinoic acid (RA). in undi fferentiated neuronal cells from 8-days chick embryos the activity of PAF-PCT was much higher than that of lyso-PAF AcT but it decreased dur ing the period of cellular proliferation up to the arrest of mitosis ( day 1-3). During this period no significant changes of lyso-PAF AcT ac tivity was observed. Both enzyme activities increased during the perio d of neuronal maturation and the formation of cellular contacts and sy naptic-like junctions. The activity of PAF acetylhydrolase was unchang ed during the development of the neuronal cultures. PAF-PCT activity d id not change during the development of chick embryo glial cultures bu t lyso-PAF AcT activity increased up to the 12th day. RA treatment of LA-N-1 cell culture in proliferation decreased PAF-PCT activity and ha d no significant effect on lyso-PAF AcT and PAF acetylhydrolase indica ting that the synthesis of PAF by the enzyme catalyzing the last step of the de novo pathway is inhibited when the LA-N-1 cells are induced to differentiate. These data suggest that: 1) in chick embryo primary cultures, both pathways are potentially able to contribute to PAF synt hesis during development of neuronal cells particularly when they form synaptic-like junctions whereas, during development of glial cells, o nly the remodelling pathway might be particularly active on synthesizi ng PAF; 2) in LA-N-1 neuroblastoma cells PAF-synthesizing enzymes coex ist and, when cells start to differentiate the contribution of the de novo pathway to PAF biosynthesis might be reduced.