Bacterial Endotoxin Stimulates Adipose Lipolysis via Toll-Like Receptor 4 and Extracellular Signal-regulated Kinase Pathway

Luxia Zu,Jinhan He,Hongfeng Jiang,Chong Xu,Shenshen Pu,Guoheng Xu

doi:10.1074/jbc.m807852200

Abstract

Bacterial endotoxin/lipopolysaccharide elicits inflammatory responses and also elevates circulating levels of free fatty acids (FFAs) and impairs insulin sensitivity. Serum FFA elevation in acute endotoxemia has long been thought to be due to endotoxin dysregulating lipid disposal and counterregulatory hormones and cytokines. Here, we investigated the direct lipolysis effect of endotoxin in rodents and in isolated primary adipocytes. Endotoxin increases lipolysis in vivo in adipose tissues, elevates circulating FFA level, induces insulin resistance in rats, and directly stimulates chronic lipolysis in vitro in adipocytes. The lipolytic action of endotoxin is mediated via its lipid A moiety and is blocked by anti-endotoxin peptides. Neither adipocytokine secretion nor nuclear factor-kappaB activation is involved in endotoxin-induced lipolysis. Different from catecholamine, endotoxin stimulates lipolysis without elevating cAMP production and activating protein kinase A and protein kinase C. Instead, endotoxin induces phosphorylation of Raf-1, MEK1/2, and ERK1/2. Upon inhibition of ERK1/2 but not JNK and p38 MAPK, endotoxin-stimulated lipolysis ceases. Endotoxin causes perilipin down-regulation and phosphorylation and increases the activity and protein levels of hormone-sensitive lipase and adipose triglyceride lipase but does not induce hormone-sensitive lipase translocation to intracellular lipid droplets. In TLR4 (Toll-like receptor 4)-deficient mice and adipocytes, endotoxin fails to increase in vivo and in vitro lipolysis. These findings suggest that endotoxin stimulates lipolysis via TLR4 and ERK1/2 signaling in adipocytes. The lipolytic action of endotoxin liberates FFA efflux from adipocytes to the bloodstream, which is a possible basis for systemic FFA elevation and insulin resistance in endotoxemia or Gram-negative bacterial infection.

Highlights

Endotoxin/lipopolysaccharide (LPS)4 is a membrane component of Gram-negative bacteria that consists of three parts: a core polysaccharide, the repeating O-antigen structures, and lipid A
Catecholamines and cytokines are increased in the circulation in endotoxemia [3,4,5,6], they seem not to be the only lipolytic stimulators that account for elevated level of serum free fatty acids (FFAs)
In an effort to understand the underlying mechanism of LPS-induced FFA elevation, this study was designed to investigate the in vivo and in vitro lipolytic actions of LPS in primary adipocytes and adipose tissues in rodents

Summary

EXPERIMENTAL PROCEDURES

Materials—LPS from Escherichia coli serotype O55:B5 and O127:B8, detoxified LPS from E. coli O127: B8, diphosphoryl lipid A derived from E. coli F583 (Rd mutant), and phenol red-free Dulbecco’s modified Eagle’s medium were from Sigma. The increased glycerol release was attributed to the LPS originally present in the conditioned medium, because this effect was abolished by the preaddition of the anti-endotoxin neutralizer BNEP, which indicates that adipocytokine production in the media was not sufficient to induce lipolysis (B). We determined whether LPS directly stimulates lipolysis ex vivo in adipose tissues and in primary adipocytes isolated from normal rats. In the conditioned medium with LPS, glycerol release was elevated by 44%, but this effect was completely abolished when the original LPS in the conditioned medium was neutralized by BNEP (Fig. 3B), which suggests that adipowith three different species of LPS: from the Salmonella min- cytokine production of LPS-stimulated adipocytes was not sufnesota rough mutant strain 595 and from E. coli serotype ficient to induce detectable lipolysis. PKC activator, 1.5 ␮M phorbol myristate acetate (Fig. 4G), as a positive control)

Lipolysis Stimulation by LPS and

DISCUSSION