Inhibition of Adenylylcyclase Activity in Mouse Cerebellum Membranes upon Hydrolysis of Triacylglycerols by Triacylglycerol Lipase, but Not Phospholipids by Phospholipase A2

Abstract

We previously showed that arachidonic acid and related unsaturated free fatty acids (U-FFAs) inhibit the activity of adenylylcyclase in brain membranes of mice. The level of U-FFAs elevates when the hydrolysis of triacylglycerols (TAGs) and phospholipids is promoted. In this study, we examined whether activation of triacylglycerol lipase (TAG lipase) and phospholipase A2 (PLA2) results in the inhibition of adenylylcyclase activity in cerebellum membranes of mice. Incubation of Intralipos with TAG lipase in the presence of membranes mainly released oleic acid and linoleic acid and caused ≥95% inhibition of adenylylcyclase activity. In contrast, PLA2, though releasing substantial amounts of U-FFAs, increased the enzymatic activity. To account for this difference, we examined how by-products formed in U-FFA release by TAG lipase and PLA2 operated on the arachidonic acid-induced inhibition. Lysophosphatidylcholne and some other lysophospholipids, produced by PLA2, enhanced the adenylylcyclase activity and attenuated the inhibitory effect of arachidonic acid. On the other hand, no such effects were found with by-products of TAG lipase-mediated lipolysis. Rather, monoacylglycerols having U-FFAs, possibly formed by TAG lipase, potentiated the arachidonic acid-induced inhibition of adenylylcyclase. Bovine serum albumin, added into the mixture for the pretreatment of membranes with TAG lipase, prevented the inhibition of adenylylcyclase. These results indicate that by-products formed in U-FFA release have a crucial role for the U-FFA's action on adenylylcyclase and that U-FFAs released from TAG are an inhibitor of adenylylcyclase. It may be that albumin in plasma, and thus FFA-binding proteins within cells, are of importance in protecting adenylylcyclase upon U-FFA release.