Abstract
The goal of this investigation was to define the molecular mechanism underlying physiologic conversion of immune tolerance to resolution of the acute inflammatory response, which is unknown. An example of this knowledge gap and its clinical importance is the broad-based energy deficit and immunometabolic paralysis in blood monocytes from non-survivors of human and mouse sepsis that precludes sepsis resolution. This immunometabolic dysregulation is biomarked by ex vivo endotoxin tolerance to increased glycolysis and TNF-α expression. To investigate how tolerance switches to resolution, we adapted our previously documented models associated with acute inflammatory, immune, and metabolic reprogramming that induces endotoxin tolerance as a model of sepsis in human monocytes. We report here that mitochondrial sirtuin 4 (SIRT4) physiologically breaks tolerance and resolves acute inflammation in human monocytes by coordinately reprogramming of metabolism and bioenergetics. We find that increased SIRT4 mRNA and protein expression during immune tolerance counters the increase in pyruvate dehydrogenase kinase 1 (PDK1) and SIRT1 that promote tolerance by switching glucose-dependent support of immune resistance to fatty acid oxidation support of immune tolerance. By decreasing PDK1, pyruvate dehydrogenase complex reactivation rebalances mitochondrial respiration, and by decreasing SIRT1, SIRT4 represses fatty acid oxidation. The precise mechanism for the mitochondrial SIRT4 nuclear feedback is unclear. Our findings are consistent with a new concept in which mitochondrial SIRT4 directs the axis that controls anabolic and catabolic energy sources.
Highlights
When stressed through TLR4 and other inflammatory and innate immune receptors, monocytes sequentially reprogram their highly conserved life-protection sequence of resistance, tolerance, and resolution [1]
We discovered that mitochondrial pyruvate dehydrogenase kinase 1 (PDK1) persistently deactivates pyruvate dehydrogenase complex (PDC), which is downstream of nuclear epigenetic reprogramming of tolerance by nuclear sirtuin 1 (SIRT1) in mice and human septic monocytes
Bacterial endotoxin [lipopolysaccharide endotoxin (LPS)] was purchased from Sigma (Escherichia coli 0111:B4); d-[5-3H(N)]glucose, d-[6-14C]-glucose, 1-[14C]-palmitic acid and hyamine were obtained from Perkin-Elmer; Mito stress kit was from Seahorse Bioscience; Cell-Tak was from BD company; colorimetric assay kits for pyruvate dehydrogenase (PDH), lactate dehydrogenase (LDH) activities and for pyruvate and lactate levels were from Biovision; gene-specific TaqMan primer/probe sets were from Applied Biosystems; sirtuin 4 (SIRT4) gene-specific siRNA and control siRNA were from Santa Cruz Biotechnology; SIRT4 Flag (Plasmid #13815) [17], control pcDNA3.1+ without SIRT4 sequence, lentiCRISPRv2 vector, lentivirus helper plasmids psPAX2 and pND2.G were from Addgene; gene-specific antibodies were purchased from Gentex company
Summary
When stressed through TLR4 and other inflammatory and innate immune receptors, monocytes sequentially reprogram their highly conserved life-protection sequence of resistance, tolerance, and resolution [1]. SIRT4 Regulates Acute Inflammatory Resolution needs of the effector immune cells that resist inflection; fatty acid oxidation (FAO) with decreased lipogenesis support the catabolic processes associated with tolerance and immune repressor cells [5,6,7]. Despite these gains in understanding how immunity and metabolism are integrated during resistance and tolerance, the reprogramming processes that promote inflammation resolution are unclear. Dominance of catabolic energy over anabolic energy during tolerance deprives immune cells of glucose carbons needed to resist infection
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