The cannabinoid receptor inverse agonist AM251 regulates the expression of the EGF receptor and its ligands via destabilization of oestrogen-related receptor α protein

BACKGROUND AND PURPOSE AM251 is an inverse agonist of the cannabinoid 1 receptor (CB1R) that can exert 'off-target' effects in vitro and in CB1R knock-out mice. AM251 is also potent at modulating tumour cell growth, suggesting that growth factor-mediated oncogenic signalling could be regulated by AM251. Since dysregulation of the EGF receptor has been associated with carcinogenesis, we examined AM251 regulation of EGF receptor (EGFR) expression and function. EXPERIMENTAL APPROACH The various biological functions of AM251 were measured in CB1R-negative human cancer cells. Pharmacological and genetic approaches were used to validate the data. KEY RESULTS The mRNA levels for EGFR and its associated ligands, including HB-EGF, were induced several fold in PANC-1 and HCT116 cells in response to AM251. This event was associated with enhanced expression of EGFR on the cell surface with concomitant increase in EGF-induced cellular responses in AM251-treated cells. Exposure to XCT790, a synthetic inverse agonist of the orphan nuclear oestrogen-related receptor alpha (ERR alpha), also induced EGFR and HB-EGF expression to the same extent as AM251, whereas pretreatment with the ERR alpha-selective agonist, biochanin A, blunted AM251 actions. AM251 promoted the degradation of ERR alpha protein without loss of the corresponding mRNA. Knock-down of ERR alpha by siRNA-based approach led to constitutive induction of EGFR and HB-EGF levels, and eliminated the biological responses of AM251 and XCT790. Finally, AM251 displaced diethylstilbestrol prebound to the ligand-binding domain of ERR alpha. CONCLUSIONS AND IMPLICATIONS AM251 up-regulates EGFR expression and signalling via a novel non-CB1R-mediated pathway involving destabilization of ERR alpha protein in selected cancer cell lines.