The stimulation of ketogenesis by cannabinoids in cultured astrocytes defines carnitine palmitoyltransferase I as a new ceramide-activated enzyme

The effects of cannabinoids on ketogenesis in primary cultures of rat astro cytes were studied. Delta 9-Tetrahydrocannabinol (THC), the major active co mponent of marijuana, produced a malonyl-CoA-independent stimulation of car nitine palmitoyltransferase I (CPT-I) and ketogenesis from [C-14]palmitate. The THC-induced stimulation of ketogenesis was mimicked by the synthetic c annabinoid HU-210 and was prevented by pertussis toxin and the CB1 cannabin oid receptor antagonist SR141716. Experiments performed with different cell ular modulators indicated that the THC-induced stimulation of ketogenesis w as independent of cyclic AMP, Ca2+, protein kinase C, and mitogen-activated protein kinase (MAPK). The possible involvement of ceramide in the activat ion of ketogenesis by cannabinoids was subsequently studied. THC produced a CB1 receptor-dependent stimulation of sphingomyelin breakdown that was con comitant to an elevation of intracellular ceramide levels. Addition of exog enous sphingomyelinase to the astrocyte culture medium led to a MAPK-indepe ndent activation of ketogenesis that was quantitatively similar and not add itive to that exerted by THC. Furthermore, ceramide activated CPT-I in astr ocyte mitochondria. Results thus indicate that cannabinoids stimulate ketog enesis in astrocytes by a mechanism that may rely on CB1 receptor activatio n, sphingomyelin hydrolysis, and ceramide-mediated activation of CPT-I.