Insulin increases expression of cannabinoid receptor 1 by suppressing lysosomal degradation via ERK signaling pathway

Overactivation of the endocannabinoid (EC) system is an important pathogenic factor in the development of obesity. The inactivation of the EC system through administration of cannabinoid receptor 1 (CB1R) antagonist or CB1R knockout can ameliorate obesity-associated metabolic disorders such as insulin resistance and compensatory hyperinsulinemia and hepatic steatosis. However, the specific mechanisms by which hyperinsulinemia drives the progression of metabolic dysfunction-associated steatotic liver disease (MASLD) through CB1R remains poorly understood. The purpose of this study was to investigate the mechanism by which insulin regulates CB1R expression in hepatocytes. We studied the time and dose effects of insulin on CB1R expression in AML12 hepatocytes. Using specific inhibitors that act on insulin signaling pathways, we clarified the involvement of downstream signaling molecules in insulin-regulated CB1R expression. We also studied the effects of insulin on CB1R mRNA expression using real-time RT-PCR assays. In an in vivo study, we administered a hyperinsulinemic-euglycemic infusion to rats for 8 h, then measured CB1R expression in their hepatocytes. The insulin significantly increased the CB1R expression in AML12 and primary hepatocytes, and also in the rats' livers. Pretreatment with ERK inhibitors abolished this insulin-stimulated CB1R expression. Insulin suppressed the protein degradation of CB1R by inhibiting the lysosomal degradation pathway. Additionally, the activation of CB1R with selective agonists enhanced its lipogenic effects on insulin-pretreated hepatocytes. In conclusion, insulin increases hepatic CB1R expression by activating the ERK-dependent pathway and suppressing lysosomal degradation. This insulin-mediated CB1R stabilization provides a novel mechanistic link between hyperinsulinemia and the exacerbation of MASLD/MASH, suggesting that the hepatic insulin-ERK-CB1R axis may be a potential therapeutic target for metabolic liver diseases.