Review: Synovial Cell Metabolism and Chronic Inflammation in Rheumatoid Arthritis

Abstract

Metabolomic studies of body fluids show that immune-mediated inflammatory diseases such as rheumatoid arthritis (RA) are associated with metabolic disruption. This is likely to reflect the increased bioenergetic and biosynthetic demands of sustained inflammation and changes in nutrient and oxygen availability in damaged tissue. The synovial membrane lining layer is the principal site of inflammation in RA. Here, the resident cells are fibroblast-like synoviocytes (FLS) and synovial tissue macrophages, which are transformed toward overproduction of enzymes that degrade cartilage and bone and cytokines that promote immune cell infiltration. Recent studies have shown metabolic changes in both FLS and macrophages from RA patients, and these may be therapeutically targetable. However, because the origins and subset-specific functions of synoviocytes are poorly understood, and the signaling modules that control metabolic deviation in RA synovial cells are yet to be explored, significant additional research is needed to translate these findings to clinical application. Furthermore, in many inflamed tissues, different cell types can forge metabolic collaborations through solute carriers in their membranes to meet a high demand for energy or biomolecules. Such relationships are likely to exist in the synovium and have not been studied. Finally, it is not yet known whether metabolic change is a consequence of disease or whether primary changes to cellular metabolism might underlie or contribute to the pathogenesis of early-stage disease. In this review article, we collate what is known about metabolism in synovial tissue cells and highlight future directions of research in this area.

Figure 1. Similarities between the rheumatoid arthritis synovium and the solid tumor microenvironment

In both tissues, fibroblasts and macrophages (MØ) reside in close proximity and within an oxygen and nutrient deprived, cytokine rich environment. Here they sustain mitochondrial damage and take on a chronically activated phenotype supported by increased glycolytic metabolism. In RA, the fibroblasts themselves are proliferative, invasive and migratory while in cancer fibroblasts support proliferation, invasion and metastasis of tumor cells. Adaptive immune cell-fibroblast interactions differ in tumor and RA microenvironments. Activated T (green) and B (blue) cells are present in the synovium in RA but are suppressed in the tumor microenvironment. Little is known about fibroblast or macrophage metabolism in early RA and the metabolic changes which take place during the transition from health to disease. (ROS, reactive oxygen species).

Figure 2. Major pathways important in synoviocyte metabolism

FLS and monocyte derived macrophages are heavily reliant upon glucose metabolism and regulate glucose transporter member 1 (Glut1) in response to inflammatory and stress stimuli. This fuels adenosine triphosphate (ATP) production in conditions of high energetic demand. Glucose is utilized in the pentose phosphate pathway to synthesize building blocks for nucleic acids, and to generate NADPH to control redox status and support lipid synthesis. Alternatively glucose is metabolized via glycolysis to pyruvate which is either transported into the mitochondria to contribute to tricarboxylic acid (TCA) cycle flux or is converted to lactate in the cytoplasm and removed from the cell via monocarboxylate transporter 4 (MCT4). TCA cycle flux contributes to ATP production via oxidative and substrate level phosphorylation. When matrix citrate levels rise, citrate is transported to the cytoplasm and yields acetyl-CoA, the starting material for synthesis of fatty acids, cholesterol and lipids. Some such lipids are exported from the cell as bioactive metabolites such as sphingosine 1 phosphate (S1P), free fatty acids (FFA), phospholipids and eicosanoids. Acetyl-CoA as well as succinate generated from the TCA cycle can be utilized in production chromatin modifying enzymes (CME) and cofactors. Choline is taken up via choline transporter-like (CTL) 1/2 is an important substrate in FLS biology. Choline can be converted to betaine which is used in production CME and cofactors or converted to glycine for use in protein synthesis. Alternatively, choline is phosphorylated to phosphocholine and utilized in membrane phospholipid and bioactive lipid synthesis. A number of signalling molecules have been identified which control the described metabolic pathways but have sparsely been explored in FLS. AKT, protein kinase B; AMPK, 5′ adenosine monophosphate-activated protein kinase; G6P, glucose-6-phosphate; HIF-1α, hypoxia-inducible factor 1α; Myc, Myc proto-oncogene protein; p53, cellular tumor antigen p53; PI3K, phosphatidylinositol 4,5-bisphosphate 3-kinase; R5P, ribose-5-phosphate; S1P, sphingosine-1-phosphate; SREBP, sterol regulatory element-binding protein; mTOR, mechanistic target of rapamycin; PPARγ, peroxisome proliferator-activated receptor gamma; NF-κB, nuclear factor κB pathway; uPFK, 2phosphofructokinase 2.

Figure 3. Mitochondrial dynamics; the cycle of fusion and fission

Mitochondrial morphology changes dynamically in response to stress and changing energetic demand. This is under the control of signalling molecules including sirtuin 1 (SIRT1), signal transducer and activator of transcription 6 (STAT6), nuclear respiratory factors 1 and 2 (NRF1/2) and peroxisome proliferator-activated receptor gamma coactivators 1α and β (PGC-1α/β). Mitochondria fuse to make tubular networks under the control of mitofusins (MFN) 1 and 2 and optic atrophy (OPA1), a mechanism which is thought to increase ATP production by oxidative phosphorylation, protect mitochondrial DNA from damage in the presence of elevated reactive oxygen species (ROS) and leads to mitochondrial biogenesis and increased mitochondrial mass. Mitochondrial fission occurs under the control of dynamin related protein 1 (DRP1) and mitochondrial fission 1 (FIS1) and produces increased numbers of punctate mitochondria. Fission usually corresponds with reduced oxidative phosphorylation and increased aerobic glycolysis and can predispose to mitochondrial-selective autophagy (mitophagy) to regulate mitochondrial mass or remove damaged organelles. The box shows a number of mitochondrial observations made in FLS cultured from RA patients both in a resting state and after stimulation with proinflammatory cytokines, alluding to possible but as yet uninvestigated changes in mitochondrial dynamics.

Figure 4. The ‘reverse Warburg’ effect in cancer and rheumatoid arthritis?

In epithelial tumours, the reactive oxygen species (ROS) produced by metabolically active cancer cells causes mitochondria-selective autophagy (mitophagy) and activate hypoxia inducible factor 1α (HIF1α) in local cancer-associated fibroblasts (CAF). As a result the CAF upregulate aerobic glycolysis, producing copious lactate which is expelled from the cell via monocarboxylate transporter 4 (MCT4) and taken up by the cancer cell via MCT1. Lactate, pyruvate and other metabolic intermediates such as amino acids and ketone bodies can feed the mitochondrial TCA cycle in cancer cells or indeed local endothelium to increase ATP and biomolecule synthesis and drive pathogenic proliferation, invasion and metastasis. MCT 1 and 4 can be blocked in vitro using the small molecule inhibitor α-cyano-4-hydroxycinnamic acid (4CIN) and in vivo using AZD3965, which is in early phase clinical trials for treatment of small cell lung cancer. In RA it is known that mitochondrial damage, HIF1α activation and upregulation of MCT4 can be induced in late stage disease FLS by the pathogenic microenvironment but the metabolic relationships between these cells and other cells within the joint have yet to be elucidated.