PHOSPHOLIPASE PARTICIPATION IN CANNABINOID-INDUCED RELEASE OF FREE ARACHIDONIC-ACID

The exposure of cells in culture to cannabinoids results in a rapid an d significant mobilization of phospholipid bound arachidonic acid. In vivo, this effect has been observed as a rise in eicosanoid tissue lev els that may account for some of the pharmacological actions of Delta( 9)-tetrahydrocannabinol (THC), the major psychoactive cannabinoid. Flu oroaluminate pretreatment of mouse peritoneal cells potently reduced t he cannabinoid response, while promoting arachidonate release on its o wn, consistent with earlier observations that this effect may be a rec eptor/G-protein-mediated process. Further support for receptor mediati on was the demonstration of saturable, high-affinity cannabinoid bindi ng in these cells. THC potency was reduced in the presence of ethanol, and was accompanied by significant increases in phosphatidylethanol ( PdEt) levels, a unique product of phospholipase D (PLD) activity. THC- dependent arachidonate release was reduced partially in similar amount s by either propranolol or wortmannin, further implicating PLD as a me diator of THC action. A central role for diacylglyceride (DAG), a seco ndary product of PLD metabolism, in this THC-induced process, both as a source of arachidonate and as a stimulator of protein kinase C (PKC) , is supported by the data in this report. Cells exposed to phorbol es ter for 18 hr prior to THC challenge became less responsive, indicatin g a possible role for PKC. The involvement of PKC further suggests par ticipation by phospholipase A(2) (PLA(2)) whose activity may be regula ted by the former. Treatment of cells with interleukin-1 alpha, an age nt known to elevate PLA(2) levels, caused an increase in the THC respo nse, supporting a role for this enzyme in the release reaction. Direct evidence, by immunoblotting, for the activation and phosphorylation o f FLAP by THC was also obtained. In summary, the evidence presented in this report indicates that THC-induced arachidonic acid release occur s through a series of events that are consistent with a receptor-media ted process involving the stimulation of one or more phospholipases.