ISG15 deficiency features a complex cellular phenotype that responds to treatment with itaconate and derivatives

Abstract

Background Congenital ISG15 deficiency is a rare autoinflammatory disorder that is driven by chronically elevated systemic interferon levels and predominantly affects central nervous system and skin. Methods and results We have developed induced pluripotent stem cell‐derived macrophages and endothelial cells as a model to study the cellular phenotype of ISG15 deficiency and identify novel treatments. ISG15 –/– macrophages exhibited the expected hyperinflammatory responses, but normal phagocytic function. In addition, they displayed a multifaceted pathological phenotype featuring increased apoptosis/pyroptosis, oxidative stress, glycolysis, and acylcarnitine levels, but decreased glutamine uptake, BCAT1 expression, branched chain amino acid catabolism, oxidative phosphorylation, β‐oxidation, and NAD(P)H‐dependent oxidoreductase activity. Furthermore, expression of genes involved in mitochondrial biogenesis and respiratory chain complexes II–V was diminished in ISG15 –/– cells. Defective mitochondrial respiration was restored by transduction with wild‐type ISG15, but only partially by a conjugation‐deficient variant, suggesting that some ISG15 functions in mitochondrial respiration require ISGylation to cellular targets. Treatment with itaconate, dimethyl‐itaconate, 4‐octyl‐itaconate, and the JAK1/2 inhibitor ruxolitinib ameliorated increased inflammation, propensity for cell death, and oxidative stress. Furthermore, the treatments greatly improved mitochondria‐related gene expression, BCAT1 levels, redox balance, and intracellular and extracellular ATP levels. However, efficacy differed among the compounds according to read‐out and cell type, suggesting that their effects on cellular targets are not identical. Indeed, only itaconates increased expression of anti‐oxidant genes NFE2L2, HMOX1 , and GPX7 , and dimethyl‐itaconate improved redox balance the most. Even though itaconate treatments normalized the elevated expression of interferon‐stimulated genes, ISG15 –/– macrophages maintained their reduced susceptibility to influenza virus infection. Conclusions These findings expand the cellular phenotype of human ISG15 deficiency and reveal the importance of ISG15 for regulating oxidative stress, branched chain amino acid metabolism, and mitochondrial function in humans. The results validate ruxolitinib as treatment for ISG15 deficiency and suggest itaconate‐based medications as additional therapeutics for this rare disorder.

ISG15 Is a Novel Regulator of Lipid Metabolism during Vaccinia Virus Infection

“…Among the recent findings, the role of ISG15 as a regulator of cell metabolism stands out, both in mice and humans. In the past 6 years, several studies have demonstrated that ISG15 acts as a modulator of several pathways of cell metabolism and that its absence influences essential metabolic functions, such as OXPHOS and glycolysis ( 16 – 19 , 21 ). These novel functions of ISG15 are closely related to its well-established antiviral activity, as viruses depend on the metabolic features of the host to complete their infectious cycle ( 22 ).…”

“…In addition, it was recently shown that in adipocytes, signaling through the interferon regulatory factor 3 (IRF3)/ISG15 axis results in the inhibition of glycolytic enzymes through ISGylation, strongly reducing oxygen consumption and thermogenic capacity ( 18 ). Of note, Waqas and colleagues ( 21 ) recently demonstrated that macrophages from patients who suffer congenital ISG15 deficiency display metabolic alterations that affect pathways such as mitochondrial respiration, glycolysis, amino acid metabolism, and redox balance, among others, highlighting the relevance of ISG15 as a regulator of cell metabolism also in humans.…”

ISG15 driven cellular responses to virus infection

“…In a separate study with induced pluripotent stem cell-derived macrophages and endothelial cells as models, ISG15 −/− macrophages displayed the expected hyperinflammatory responses, but normal phagocytic function [75]. Their pathology included a range of dysregulated processes such as increased apoptosis/pyroptosis, oxidative stress and glycolysis, but decreased oxidative phosphorylation, β-oxidation, and NAD(P)H-dependent oxidoreductase activity [75].…”

“…ISG15 −/− cells also displayed defective expression of genes involved in mitochondrial biogenesis and respiratory chain complexes II-V, resulting in diminished mitochondrial respiration. This phenotype was rescued upon reconstitution with wild-type ISG15, but only partially by a conjugation-deficient variant, suggesting that at least some of these phenotypes were driven by ISGylation to cellular targets [75]. Pharmacological treatment with itaconate and its derivatives, and the JAK1/2 inhibitor ruxolitinib could also reduce inflammatory responses, cell death, and oxidative stress [75].…”

“…Reduced thermogenic gene expression and oxygen consumption in adipose tissue resulted from ISG15-mediated inhibition of glycolytic enzymes [73]. iPSC-derived ISG15 −/− macrophages display a complex pathological phenotype of increased glycolysis, oxidative stress and apoptosis, but decreased oxidative phosphorylation, β-oxidation and NADH-dependent oxidoreductase activity, underscoring its importance in mitochondrial function [75].…”