Background Many signaling pathways activated under inflammatory and hypoxic conditions have profound effects on intracellular metabolism to support cell growth and survival. The recent findings of cancer-specific metabolic changes, including increased glucose and glutamine consumption, has provided new therapeutic targets for consideration. The microenvironment in inflamed joints in RA is also characterized by hypoxia and low concentration of nutrients, and fibroblast-like synoviocytes from RA patients (RA-FLS) is known to have several tumor-like characteristics. However, the role of glucose and glutamine metabolism in the aberrant proliferation of these cells is unclear.
Objectives We evaluated the role of these metabolic pathways in RA-FLS proliferation and in autoimmune arthritis in SKG mice.
Methods The expression of glycolysis- or glutaminolysis-related enzymes was evaluated by real-time PCR and Western blotting, and the intracellular metabolites were evaluated by metabolomic analyses. The effects of glucose or glutamine on RA-FLS cell growth were investigated using glucose- or glutamine-free medium. Glutaminase 1 (GLS1) siRNA and the GLS1 inhibitor compound 968 were used to inhibit GLS1 in RA-FLS. Arthritis was induced in SKG mice by zymosan A injection. Compound 968 was used to study the effect of GLS1 inhibition on Zymosan A-injected SKG mice. Ki-67-positive cells were analyzed by immunohistochemistry.
Results GLS1 expression was increased in RA-FLS, and metabolomic analyses revealed that glutamine and glutamate consumption were increased in RA-FLS. RA-FLS proliferation was reduced under glutamine-deprived, but not glucose-deprived conditions. Cell growth of RA-FLS was inhibited by GLS1 siRNA transfection or GLS1 inhibitor treatment. Silencing of GLS1 in RA-FLS did not affect IL-6 or MMP-3 production in supernatants. GLS1 expression in RA-FLS was not affected by pro-inflammatory cytokine stimulation. Compound 968 ameliorated the autoimmune arthritis and decreased the number of Ki-67-positive synovial cells in SKG mice.
Conclusions Our findings suggested that glutamine metabolism plays an important role in regulating RA-FLS proliferation, without being affected by pro-inflammatory cytokine stimulation or affecting cytokine production, and may be a novel therapeutic target for RA.
Disclosure of Interest None declared