Dysregulation of Lipid Metabolism in Macrophages Is Responsible for Severe Endotoxin Tolerance in FcgRIIB-Deficient Lupus Mice
Thiranut Jaroonwitchawan, Peerapat Visitchanakun, Phi Cong Dang, Patcharee Ritprajak, Tanapat Palaga, Asada Leelahavanichkul
Abstract
The defect in FcgRIIB is commonly found in patients with lupus, especially in Asia. Because i) the extreme LPS-tolerance enhances sepsis-susceptibility in FcgRIIB-/- lupus mice and ii) LPS-tolerant macrophages demonstrates cellular energy depletion, then lipid might be an important alternative source for cell energy in LPS-tolerance and is explored. Hence, LPS-tolerance was induced by twice LPS administration in macrophages and in mice. Accordingly, LPS-tolerant FcgRIIB-/- macrophages demonstrated the lesser mitochondrial DNA (mtDNA) with the more severe ATP depletion together with the lower cytokine production in comparison with LPS-tolerant WT cells. With the lower energy status of LPS-tolerance in FcgRIIB-/- macrophages compared with WT, there was the more prominent accumulation of intracellular lipid droplets by oil red O staining with the higher phosphatidylethanolamine (PE) phospholipid in composition as determined by mass-spectrometry-based lipidomic analysis. In parallel, the expression of phosphatidylethanolamine N-methyltransferase (pemt), an enzyme responsible for PE metabolism, was lower and the abundance of phosphorylated AMP-activated protein kinase (AMPK-p), a molecule of ATP-restoration process, was higher in LPS-tolerant FcgRIIB-/- macrophages compared with WT. Interestingly, the induction of the proper intracellular lipid component attenuated LPS tolerance as compound C, an AMPK inhibitor, attenuated LPS-tolerance either in FcgRIIB-/- macrophages or in FcgRIIB-/- mice. Taken together, the intensely decreased cytokine production after 2nd LPS stimulation (LPS-tolerance) in FcgRIIB-/- macrophages was possibly due to the impact of an immense cytokine synthesis from the 1st dose of LPS including; i) the using-up of PEMT, an enzyme of the phospholipid synthesis pathway during the cytokine production, and ii) the induction of AMPK in response against the profound ATP-depletion. Hence, the manipulation of AMPK/PEMT axis provides a novel therapeutic candidate for treatment of severe LPS-tolerance in lupus.