CB1 Receptor Agonist ACEA Resists ER Stress-Mediated Apoptosis <i>via</i> CB1R-Independent Mechanism

Cannabinoid receptor type 1 (CB1R) plays a key role in neuronal homeostasis, synaptic plasticity, and neuroprotection. CB1R antagonists typically protect against CB1R agonists-induced neurotoxicity. However, we previously found that the CB1R antagonists rimonabant and its analog AM251 can also be neurotoxic: under serum-free conditions, these compounds induce apoptosis in human neuroblastoma SH-SY5Y cells through mitochondrial damage and endoplasmic reticulum (ER) stress. To elucidate the mechanisms of this neurotoxicity, we examined the effects of CB1R agonists. We co-treated SH-SY5Y cells with rimonabant or AM251 in combination with either the CB1R agonist arachidonyl 2-chloroethylamide (ACEA) or WIN 55212-2 mesylate (WIN). ACEA, but not WIN, protected cells from rimonabant- and AM251-induced apoptosis. While ACEA had only a limited effect on mitochondrial damage, it significantly reduced phosphorylation of the eukaryotic initiation factor 2 alpha (eIF2α), a key marker of ER stress. Given that ACEA also functions as an agonist of transient receptor potential vanilloid 1 (TRPV1), we investigated its role in ACEA-mediated neuroprotection. The TRPV1 antagonist capsazepine blocked ACEA's protective effects, suggesting that ACEA acts through TRPV1 rather than CB1R. ACEA also prevented apoptosis induced by camptothecin, a well-established apoptosis inducer, through a similar capsazepine-sensitive mechanism, demonstrating its broader protective effects against apoptosis. These findings indicate that rimonabant and AM251 induce neurotoxicity independently of CB1R under serum-free conditions and that ER stress is likely to be a key target of CB1R-independent neuroprotection by ACEA. Our study highlights the complexity of CB1R ligand-associated neurotoxicity and neuroprotection.