Decreasing CB receptor signaling in Kupffer cells improves insulin sensitivity in obese mice.

Objective: Obesity-induced accumulation of ectopic fat in the liver is thought to contribute to the development of insulin resistance, and increased activity of hepatic CBR has been shown to promote both processes. However, lipid accumulation in liver can be experimentally dissociated from insulin resistance under certain conditions, suggesting the involvement of additional mechanisms. Obesity is also associated with pro-inflammatory changes which, in turn, can promote insulin resistance. Kupffer cells (KCs), the liver's resident macrophages, are the major source of pro-inflammatory cytokines in the liver, such as TNF-α, which has been shown to inhibit insulin signaling in multiple cell types, including hepatocytes. Here, we sought to identify the role of CBR in KCs in obesity-induced hepatic insulin resistance.

Methods: We used intravenously administered β-D-glucan-encapsulated siRNA to knock-down CBR gene expression selectively in KCs.

Results: We demonstrate that a robust knock-down of the expression of Cnr1, the gene encoding CBR, results in improved glucose tolerance and insulin sensitivity in diet-induced obese mice, without affecting hepatic lipid content or body weight. Moreover, Cnr1 knock-down in KCs was associated with a shift from pro-inflammatory M1 to anti-inflammatory M2 cytokine profile and improved insulin signaling as reflected by increased insulin-induced Akt phosphorylation.

Conclusion: These findings suggest that CBR expressed in KCs plays a critical role in obesity-related hepatic insulin resistance via a pro-inflammatory mechanism.