The CB2 cannabinoid receptor signals apoptosis via ceramide-dependent activation of the mitochondrial intrinsic pathway

Δ9-Tetrahydrocannabinol and other cannabinoids exert pro-apoptotic actions in tumor cells via the CB2 cannabinoid receptor. However, the molecular mechanism involved in this effect has remained elusive. Here we used the human leukemia cell line Jurkat—that expresses CB2 as the unique CB receptor—to investigate this mechanism. Our results show that incubation with the selective CB2 antagonist SR144528 abrogated the pro-apoptotic effect of Δ9-tetrahydrocannabinol. Cannabinoid treatment led to a CB2 receptor-dependent stimulation of ceramide biosynthesis and inhibition of this pathway prevented Δ9-tetrahydrocannabinol-induced mitochondrial hypopolarization and cytochrome c release, indicating that ceramide acts at a pre-mitochondrial level. Inhibition of ceramide synthesis de novo also prevented caspase activation and apoptosis. Caspase 8 activation—an event typically related with the extrinsic apoptotic pathway—was also evident in this model. However, activation of this protease was post-mitochondrial since (i) a pan-caspase inhibitor as well as a selective caspase 8 inhibitor were unable to prevent Δ9-tetrahydrocannabinol-induced loss of mitochondrial-membrane transmembrane potential, and (ii) cannabinoid-induced caspase 8 activation was not observed in Bcl-xL over-expressing cells. In summary, results presented here show that CB2 receptor activation signals apoptosis via a ceramide-dependent stimulation of the mitochondrial intrinsic pathway.