Dysregulated bioenergetics: a key regulator of joint inflammation

Abstract

ObjectivesThis study examines the relationship between synovial hypoxia and cellular bioenergetics with synovial inflammation.MethodsPrimary rheumatoid arthritis synovial fibroblasts (RASF) were cultured with hypoxia, dimethyloxalylglycine (DMOG) or metabolic intermediates. Mitochondrial respiration, mitochondrial DNA mutations, cell invasion, cytokines, glucose and lactate were quantified using specific functional assays. RASF metabolism was assessed by the XF24-Flux Analyzer. Mitochondrial structural morphology was assessed by transmission electron microscopy (TEM). In vivo synovial tissue oxygen (tpO2 mmHg) was measured in patients with inflammatory arthritis (n=42) at arthroscopy, and markers of glycolysis/oxidative phosphorylation (glyceraldehyde 3-phosphate dehydrogenase (GAPDH), PKM2, GLUT1, ATP) were quantified by immunohistology. A subgroup of patients underwent contiguous MRI and positron emission tomography (PET)/CT imaging. RASF and human dermal microvascular endothelial cells (HMVEC) migration/angiogenesis, transcriptional activation (HIF1α, pSTAT3, Notch1-IC) and cytokines were examined in the presence of glycolytic inhibitor 3-(3-Pyridinyl)-1-(4-pyridinyl)-2-propen-1-one (3PO).ResultsDMOG significantly increased mtDNA mutations, mitochondrial membrane potential, mitochondrial mass, reactive oxygen species and glycolytic RASF activity with concomitant attenuation of mitochondrial respiration and ATP activity (all p<0.01). This was coupled with altered mitochondrial morphology. Hypoxia-induced lactate levels (p<0.01), which in turn induced basic fibroblast growth factor (bFGF) secretion and RASF invasiveness (all p<0.05). In vivo glycolytic markers were inversely associated with synovial tpO2 levels <20 mm Hg, in contrast ATP was significantly reduced (all p<0.05). Decrease in GAPDH and GLUT1 was paralleled by an increase in in vivo tpO2 in tumour necrosis factor alpha inhibitor (TNFi) responders. Novel PET/MRI hybrid imaging demonstrated close association between metabolic activity and inflammation. 3PO significantly inhibited RASF invasion/migration, angiogenic tube formation, secretion of proinflammatory mediators (all p<0.05), and activation of HIF1α, pSTAT3 and Notch-1IC under normoxic and hypoxic conditions.ConclusionsHypoxia alters cellular bioenergetics by inducing mitochondrial dysfunction and promoting a switch to glycolysis, supporting abnormal angiogenesis, cellular invasion and pannus formation.

Glucose-6-Phosphate Isomerase (G6PI) Mediates Hypoxia-Induced Angiogenesis in Rheumatoid Arthritis

“…It has been confirmed that hypoxia presents in the joint microenvironment of RA, and plays a key role in regulating angiogenesis in RA28. Hypoxia alters cellular bioenergetics by inducing mitochondrial dysfunction and promoting a switch to glycolysis, thereby leads to abnormal angiogenesis29. Glycolytic activity is enhanced in the hypoxia microenvironment of synovial tissues in RA30.…”

Lactate at the crossroads of metabolism, inflammation, and autoimmunity

“…In contrast to T cells, synovial RA fibroblasts display an intrinsic ability to employ glycolytic metabolism under metabolic stress. Indeed, glucose deprivation or glycolytic inhibitors impaired their cytokine secretion, proliferation, and migration [63].…”

“…In contrast to T cells, synovial RA fibroblasts display an intrinsic ability to employ glycolytic metabolism under metabolic stress. Indeed, glucose deprivation or glycolytic inhibitors impaired their cytokine secretion, proliferation, and migration [63].Recently a gas chromatography-mass spectrometry study, carried out on sera samples, has highlighted a peculiar metabolic signature of RA patients compared to healthy controls [64]. Specifically, RA patients displayed decreased levels of aas (leucine, phenylalanine, pyroglutamate, serine, isoleucine, methionine, threonine, proline, and valine) and glucose alongside with increased levels of fatty acids such as palmitelaidate, oleate, trans-9-octadecenoate,cis-5,8,11-eicosatrienoate, docosahexaenoate, 2-ketoisocaproateand 3-methyl-2-oxovalerate, and cholesterol.…”

Altered metabolic pathways regulate synovial inflammation in rheumatoid arthritis

“…Evidence for this metabolic switch in RA has been demonstrated by several studies showing an increase in mitochondrial dysfunction in the RA synovium [20], along with increased expression of glucose transporters (GLUTs) [21] and glycolytic enzymes, including hexokinase 2 (HK2), pyruvate kinase M2 (PKM2), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), glucose-6-phosphate isomerase (GPI) and LDH [7,8,[21][22][23][24][25]. Marked accumulation of metabolic intermediates including lactate, glutamine, succinate [26] and itaconate [27] have been demonstrated in the RA joint which, in turn, can activate synovial cells Hexokinase 2 (HK2) converts glucose into glucose 6-phosphate dehydrogenase (G6PD).…”

“…FLS are characterized by increased proliferation, resistance to apoptosis and are potent producers of proinflammatory cytokines and matrix degradation enzymes, resulting in a highly invasive phenotype [71,72]. Previous studies have demonstrated increased mitochondrial dysfunction, coupled by changes in the ultrastructure of mitochondria in RA-FLS in response to inflammatory stimuli, in addition to altered expression of mitochondrial genes associated with apoptosis, redox balance and mitochondrial protein transport [21,23,73]. Metabolic profiling of FLS demonstrated increases in sugar metabolism (glycolysis and PPP) and amino acid metabolism (tyrosine and catecholamine biosynthesis and protein biosynthesis) [74].…”