Inhibition of Glioma Growth in Vivo by Selective Activation of the CB₂ Cannabinoid Receptor

Abstract

The development of new therapeutic strategies is essential for the management of gliomas, one of the most malignant forms of cancer. We have shown previously that the growth of the rat glioma C6 cell line is inhibited by psychoactive cannabinoids (I. Galve-Roperh et al., Nat. Med., 6: 313–319, 2000). These compounds act on the brain and some other organs through the widely expressed CB₁ receptor. By contrast, the other cannabinoid receptor subtype, the CB₂ receptor, shows a much more restricted distribution and is absent from normal brain. Here we show that local administration of the selective CB₂ agonist JWH-133 at 50 μg/day to Rag-2^−/− mice induced a considerable regression of malignant tumors generated by inoculation of C6 glioma cells. The selective involvement of the CB₂ receptor in this action was evidenced by: (a) the prevention by the CB₂ antagonist SR144528 but not the CB₁ antagonist SR141716; (b) the down-regulation of the CB₂ receptor but not the CB₁ receptor in the tumors; and (c) the absence of typical CB₁-mediated psychotropic side effects. Cannabinoid receptor expression was subsequently examined in biopsies from human astrocytomas. A full 70% (26 of 37) of the human astrocytomas analyzed expressed significant levels of cannabinoid receptors. Of interest, the extent of CB₂ receptor expression was directly related with tumor malignancy. In addition, the growth of grade IV human astrocytoma cells in Rag-2^−/− mice was completely blocked by JWH-133 administration at 50 μg/day. Experiments carried out with C6 glioma cells in culture evidenced the internalization of the CB₂ but not the CB₁ receptor upon JWH-133 challenge and showed that selective activation of the CB₂ receptor signaled apoptosis via enhanced ceramide synthesis de novo. These results support a therapeutic approach for the treatment of malignant gliomas devoid of psychotropic side effects.