Hepatic cannabinoid-1 receptors mediate diet-induced insulin resistance by increasing de novo synthesis of long-chain ceramides.

UNLABELLED: Obesity is associated with increased activity of two lipid signaling systems (endocannabinoids [ECs] and ceramides), with both being implicated in insulin resistance. Cannabinoid-1 receptor (CB1 R) antagonists reverse obesity and insulin resistance, but have psychiatric side effects. Here we analyzed the role of ceramide in CB1 R-mediated insulin resistance in C57Bl6/J mice with high-fat diet-induced obesity (DIO), using JD5037, a peripherally restricted CB1 R inverse agonist. Chronic JD5037 treatment of DIO mice reduced body weight and steatosis and improved glucose tolerance and insulin sensitivity. Peripheral CB1 R blockade also attenuated the diet-induced increase in C14:0, C16:0, C18:0, and C20:0 ceramide species with either C16 or C18 sphingosine-base in the liver. Decreased ceramide levels reflected their reduced de novo synthesis, due to inhibition of the activity of serine-palmitoyl transferase (SPT) and the expression of its SPTLC3 catalytic subunit, as well as reduced ceramide synthase (CerS) activity related to reduced expression of CerS1 and CerS6. JD5037 treatment also increased ceramide degradation due to increased expression of ceramidases. In primary cultured mouse hepatocytes and HepG2 cells, the EC anandamide increased ceramide synthesis in an eIF2α-dependent manner, and inhibited insulin-induced akt phosphorylation by increased serine phosphorylation of IRS1 and increased expression of the serine/threonine phosphatase Phlpp1. These effects were abrogated by JD5037 or the SPT inhibitor myriocin. Chronic treatment of DIO mice with myriocin or JD5037 similarly reversed hepatic insulin resistance, as verified using a euglycemic/hyperinsulinemic clamp.
CONCLUSION: ECs induce CB1 R-mediated, endoplasmic reticulum stress-dependent synthesis of specific ceramide subspecies in the liver, which plays a key role in obesity-related hepatic insulin resistance.