Gambogic acid binds to human CB2 receptors and exhibits potential partial agonist activity

Abstract

Gambogic acid (GA) induces apoptosis in glioma cells with a capacity to cross the blood brain barrier, thereby representing a potential glioblastoma therapy. Cannabinoid type‐2 receptors (CB2Rs) are upregulated in glioblastomas and their activation also results in impaired tumor growth and cell death. Therefore, it is possible that CB2 receptor activation might contribute to the pro‐apoptotic effects of GA. Employing receptor binding studies in CHO‐hCB2 cells, we report that GA binds to CB2Rs with a Ki of 3.6 ± 0.3 μM. Furthermore, GA dose‐dependently inhibits the activity of adenylyl cyclase (AC) in CHO‐hCB2 cells with an IC50 of 677 ± 274 nM and an Imax of 49 ± 5.3 %. Interestingly, GA also reduces intracellular cAMP levels in wild‐type CHO cells, but with an Imax of only 24 ± 3 %. In CHO‐hCB2 cells treated with pertussis toxin, GA still inhibits AC activity, but with a reduced Imax similar to that observed in CHO‐WT cells. Finally, a concentration (100 nM) of the CB2 antagonist AM630 that is predicted to fully occupy CB2Rs, but does not alter cAMP levels alone, reduces AC inhibition produced by GA to a level observed in CHO‐WT cells. These data suggest that GA produces AC inhibition in CHO‐hCB2 cells via both CB2 and non‐CB2 receptor mediated mechanisms. Therefore, it is concluded that GA binds to CB2Rs with low micromolar affinity and potential partial agonist activity at CB2 receptors might contribute to its anti‐cancer effects.