AMPK-independent inhibition of human macrophage ER stress response by AICAR.

Marcel Boß,Sahil Gupta,Bernhard Brüne,Yvette Newbatt,Ian Collins,Dmitry Namgaladze

doi:10.1038/srep32111

Abstract

Obesity-associated insulin resistance is driven by inflammatory processes in response to metabolic overload. Obesity-associated inflammation can be recapitulated in cell culture by exposing macrophages to saturated fatty acids (SFA), and endoplasmic reticulum (ER) stress responses essentially contribute to pro-inflammatory signalling. AMP-activated protein kinase (AMPK) is a central metabolic regulator with established anti-inflammatory actions. Whether pharmacological AMPK activation suppresses SFA-induced inflammation in a human system is unclear. In a setting of hypoxia-potentiated inflammation induced by SFA palmitate, we found that the AMP-mimetic AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) potently suppressed upregulation of ER stress marker mRNAs and pro-inflammatory cytokines. Furthermore, AICAR inhibited macrophage ER stress responses triggered by ER-stressors thapsigargin or tunicamycin. Surprisingly, AICAR acted independent of AMPK or AICAR conversion to 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranosyl monophosphate (ZMP) while requiring intracellular uptake via the equilibrative nucleoside transporter (ENT) ENT1 or the concentrative nucleoside transporter (CNT) CNT3. AICAR did not affect the initiation of the ER stress response, but inhibited the expression of major ER stress transcriptional effectors. Furthermore, AICAR inhibited autophosphorylation of the ER stress sensor inositol-requiring enzyme 1α (IRE1α), while activating its endoribonuclease activity in vitro. Our results suggest that AMPK-independent inhibition of ER stress responses contributes to anti-inflammatory and anti-diabetic effects of AICAR.

Highlights

The importance of metabolism in regulating immunity and inflammation is widely recognized[1,2]
Of all AMPK activators only AICAR consistently inhibited palmitate-induced inositol-requiring enzyme 1 (IRE1) and cJun phosphorylation (Fig. 1B). mRNA expression of endoplasmic reticulum (ER) stress-responsive genes Grp[78] and CHOP as well as inflammatory cytokines IL-1β, TNFα, and IL-6 confirmed that AICAR was the most potent inhibitor (Fig. 1C)
Our data indicate that AMPK activators suppress SFAtriggered inflammatory responses to a variable degree, suggesting that AICAR may block palmitate-induced ER stress and inflammation through mechanisms unrelated to AMPK activation

Summary

Introduction

The importance of metabolism in regulating immunity and inflammation is widely recognized[1,2]. SFA-induced inflammation was explained by activation of toll-like receptors[10,11] as well as induction of intracellular stress signalling cascades, in particular, c-Jun N-terminal kinase (JNK)[12] and endoplasmic reticulum (ER) stress response[13]. On the other hand, increasing fatty acid mitochondrial oxidation, by diverting the fatty acid flux towards catabolism, attenuates ER stress and inflammation in palmitate-treated macrophages[17]. AMPK is a well-known activator of fatty acid oxidation (FAO) and its absence in macrophages increased palmitate-induced inflammatory responses, partly through diminished FAO18. An alternative mechanism how AMPK attenuates inflammation in palmitate-treated macrophages suggestively involves activation of the protein deacetylase (Sirt1)[19]. We report that AICAR profoundly inhibits SFA-induced ER stress and inflammatory responses to palmitate in human macrophages. AICAR inhibited mRNA and protein induction of the major transcriptional effectors of the ER stress response, without interfering with initiation of ER stress

Methods

Results

Conclusion