Circulating Metabolomic Signature in Generalized Pustular Psoriasis Blunts Monocyte Hyperinflammation by Triggering Amino Acid Response

Generalized pustular psoriasis (GPP), the most grievous variant of psoriasis, is featured by dysregulated systemic inflammatory response. The cellular and molecular basis of GPP is poorly understood. Blood monocytes are key players of host defense and producers of inflammatory cytokines including IL-1β. How the immune response of monocytes is affected by metabolic internal environment in GPP remains unclear. Here, we performed a metabolomic and functional investigation of GPP serum and monocytes. We demonstrated a significant increase in IL-1β production from GPP monocytes. In GPP circulation, serum amyloid A (SAA), an acute-phase reactant, was dramatically increased, which induced the release of IL-1β from monocytes in a NLRP3-dependent manner. Using metabolomic analysis, we showed that GPP serum exhibited an amino acid starvation signature, with glycine, histidine, asparagine, methionine, threonine, lysine, valine, isoleucine, tryptophan, tyrosine, alanine, proline, taurine and cystathionine being markedly downregulated. In functional assay, under amino acid starvation condition, SAA-stimulated mature IL-1β secretion was suppressed. Mechanistically, at post-transcriptional level, amino acid starvation inhibited the SAA-mediated reactive oxygen species (ROS) formation and NLRP3 inflammasome activation. Moreover, the immune-modulatory effect of amino acid starvation was blocked by silencing general control nonderepressible 2 kinase (GCN2), suggesting the involvement of amino acid response (AAR) pathway. Collectively, our results suggested that decreased serum amino acids in GPP blunted the innate immune response in blood monocytes through AAR pathway, serving as a feedback mechanism preventing excessive inflammation in GPP.

BTK inhibition sensitizes Acute Lymphoblastic Leukemia to asparaginase by suppressing the Amino Acid Response pathway

Asparaginase (ASNase) therapy has been a mainstay of Acute Lymphoblastic Leukemia (ALL) protocols for decades and shows promise in the treatment of a variety of other cancers. To improve the efficacy of ASNase treatment, we employed a CRISPR/Cas9-based screen to identify actionable signaling intermediates that improve the response to ASNase. Both genetic inactivation of Bruton's Tyrosine Kinase (BTK) and pharmacological inhibition by the BTK inhibitor ibrutinib strongly synergize with ASNase by inhibiting the amino acid response pathway, a mechanism involving c-Myc mediated suppression of GCN2 activity. This synthetic lethal interaction was observed in 90% of patient derived xenografts, irrespective of the genomic subtype. Moreover, ibrutinib substantially improved ASNase treatment response in a murine PDX model. Hence, ibrutinib may be used to enhance the clinical efficacy of ASNase in ALL.

Aminoacyl-tRNA synthetase inhibition activates a pathway that branches from the canonical amino acid response in mammalian cells

Signaling pathways that sense amino acid abundance are integral to tissue homeostasis and cellular defense. Our laboratory has previously shown that halofuginone (HF) inhibits the prolyl-tRNA synthetase catalytic activity of glutamyl-prolyl-tRNA synthetase (EPRS), thereby activating the amino acid response (AAR). We now show that HF treatment selectively inhibits inflammatory responses in diverse cell types and that these therapeutic benefits occur in cells that lack GCN2, the signature effector of the AAR. Depletion of arginine, histidine, or lysine from cultured fibroblast-like synoviocytes recapitulates key aspects of HF treatment, without utilizing GCN2 or mammalian target of rapamycin complex 1 pathway signaling. Like HF, the threonyl-tRNA synthetase inhibitor borrelidin suppresses the induction of tissue remodeling and inflammatory mediators in cytokine-stimulated fibroblast-like synoviocytes without GCN2, but both aminoacyl-tRNA synthetase (aaRS) inhibitors are sensitive to the removal of GCN1. GCN1, an upstream component of the AAR pathway, binds to ribosomes and is required for GCN2 activation. These observations indicate that aaRS inhibitors, like HF, can modulate inflammatory response without the AAR/GCN2 signaling cassette, and that GCN1 has a role that is distinct from its activation of GCN2. We propose that GCN1 participates in a previously unrecognized amino acid sensor pathway that branches from the canonical AAR.

The LINC00961 transcript and its encoded micropeptide, small regulatory polypeptide of amino acid response, regulate endothelial cell function

Aims Long non-coding RNAs (lncRNAs) play functional roles in physiology and disease, yet understanding of their contribution to endothelial cell (EC) function is incomplete. We identified lncRNAs regulated during EC differentiation and investigated the role of LINC00961 and its encoded micropeptide, small regulatory polypeptide of amino acid response (SPAAR), in EC function. Methods and results Deep sequencing of human embryonic stem cell differentiation to ECs was combined with Encyclopedia of DNA Elements (ENCODE) RNA-seq data from vascular cells, identifying 278 endothelial enriched genes, including 6 lncRNAs. Expression of LINC00961, first annotated as an lncRNA but reassigned as a protein-coding gene for the SPAAR micropeptide, was increased during the differentiation and was EC enriched. LINC00961 transcript depletion significantly reduced EC adhesion, tube formation, migration, proliferation, and barrier integrity in primary ECs. Overexpression of the SPAAR open reading frame increased tubule formation; however, overexpression of the full-length transcript did not, despite production of SPAAR. Furthermore, overexpression of an ATG mutant of the full-length transcript reduced network formation, suggesting a bona fide non-coding RNA function of the transcript with opposing effects to SPAAR. As the LINC00961 locus is conserved in mouse, we generated an LINC00961 locus knockout (KO) mouse that underwent hind limb ischaemia (HLI) to investigate the angiogenic role of this locus in vivo. In agreement with in vitro data, KO animals had a reduced capillary density in the ischaemic adductor muscle after 7 days. Finally, to characterize LINC00961 and SPAAR independent functions in ECs, we performed pull-downs of both molecules and identified protein-binding partners. LINC00961 RNA binds the G-actin sequestering protein thymosin beta-4x (Tβ4) and Tβ4 depletion phenocopied the overexpression of the ATG mutant. SPAAR binding partners included the actin-binding protein, SYNE1. Conclusion The LINC00961 locus regulates EC function in vitro and in vivo. The gene produces two molecules with opposing effects on angiogenesis: SPAAR and LINC00961.

Angiogenesis Induced By Aminoacyl-tRNA Synthetase Deficiency Is Dependent on Amino Acid Response (AAR) but Not Unfolded Protein Response (UPR) Pathways

Stress-induced angiogenesis enormously contributes to both normal development and pathogenesis of various diseases including cancer. Among many stress response pathways implicated in regulation of angiogenesis, the amino acid response (AAR) and the unfolded protein response (UPR) pathways are closely interconnected, as they converge on the common target, eIF2α, which is a key regulator of protein translation. Two kinases, namely Gcn2 (Eif2ak4) and Perk (Eif2ak3), are responsible for transducing signals from AAR and UPR, respectively, to phosphorylation of eIF2α. Even though numerous studies have been performed, this close interconnection between AAR and UPR makes it difficult to clearly distinguish different contributions of these two pathways in regulation of angiogenesis. In this study, we generated a zebrafish angiogenic model harboring a loss-of-function mutation of the threonyl-tRNA synthetase (tars) gene. Tars belongs to a family of evolutionarily conserved enzymes, aminoacyl-tRNA synthetases (aaRSs), which control the first step of protein translation through coupling specific amino acids with their cognate tRNAs. Deficiencies of several aaRSs in zebrafish have been shown to cause increased branching of blood vessels, and this angiogenic phenotype has roughly been explained by activation of AAR and UPR; however, it is unclear whether both AAR and UPR are required and to what extent they contribute to this process. To address this issue, we first performed RNA-seq analyses of Tars-mutated and control zebrafish embryos, as well as those with knockdown of either Gcn2 or Perk in both genotypes. We found that the AAR target genes are dramatically activated in the Tars-mutants, whereas the genes associated with the three UPR sub-pathways (i.e., Perk-, Ire1- and Atf6-mediated pathways) remain inactive, except for very few genes (e.g., Atf3, Atf4, Asns and Igfbp1) that are shared in both AAR and UPR, thus suggesting activation of AAR, but not UPR, in the Tars-mutants. In support of this notion, knockdown of the AAR-associated kinase Gcn2 in the Tars-mutants largely represses the activated genes, while the Perk knockdown shows very little effect. Nonetheless, in contrast to the apparently dispensable role of Perk in Tars-mutants, knockdown of Perk in control embryos leads to specific gene expression alterations, suggesting that Perk effectively functions in homeostatic states (i.e., controls), but, in the stress condition (i.e., Tars-mutants), its function is largely overwhelmed by activation of the Gcn2-mediated AAR. To validate these observations, we investigated the angiogenic phenotypes of the zebrafish models upon genetic and pharmacological interference with the AAR and UPR pathways. A transgenic zebrafish line, Tg(flk1:EGFP), was crossed with the Tars-mutants to visualize angiogenesis in vivo. We observed increased branching of blood vessels in the Tars-mutants, which is rescued by tars mRNA but not an enzymatically dead version. Importantly, knockdown of Gcn2 in the Tars-mutants rescues this phenotype. In contrast, knockdown of Perk, or knockdown of two other known eIF2α kinases, Hri (Eif2ak1) or Pkr (Eif2ak2), shows no effect. Furthermore, knockdown of either one of two major factors downstream to eIF2α, namely Atf4 and Vegfα, or inhibition of Vegf receptor with the drug SU5416, also rescue the phenotype. Thus, these results confirm that AAR, but not UPR, is required for the Tars-deficiency-induced angiogenesis. Taken together, this study demonstrates that, despite being closely interconnected and even sharing a common downstream target, the Gcn2-mediated AAR and the Perk-mediated UPR can be activated independently in different conditions and differentially regulate cellular functions such as angiogenesis. This notion reflects the specificity and efficiency of multiple stress response pathways that are evolved integrally to benefit the organism by ensuring sensing and responding precisely to different types of stresses. This study also provides an example of combining systematic gene expression profiling and phenotypic validations to distinguish activities of such interconnected pathways. Further clarification of the mechanisms shall advance our understanding of how the organisms respond to diverse stresses and how the abnormalities in these regulatory machineries cause cellular stress-related diseases such as cancer, diabetes and immune disorders. Disclosures No relevant conflicts of interest to declare.

Inhibition of GCN2 sensitizes ASNS-low cancer cells to asparaginase by disrupting the amino acid response

General control nonderepressible 2 (GCN2) plays a major role in the cellular response to amino acid limitation. Although maintenance of amino acid homeostasis is critical for tumor growth, the contribution of GCN2 to cancer cell survival and proliferation is poorly understood. In this study, we generated GCN2 inhibitors and demonstrated that inhibition of GCN2 sensitizes cancer cells with low basal-level expression of asparagine synthetase (ASNS) to the antileukemic agent l-asparaginase (ASNase) in vitro and in vivo. We first tested acute lymphoblastic leukemia (ALL) cells and showed that treatment with GCN2 inhibitors rendered ALL cells sensitive to ASNase by preventing the induction of ASNS, resulting in reduced levels of de novo protein synthesis. Comprehensive gene-expression profiling revealed that combined treatment with ASNase and GCN2 inhibitors induced the stress-activated MAPK pathway, thereby triggering apoptosis. By using cell-panel analyses, we also showed that acute myelogenous leukemia and pancreatic cancer cells were highly sensitive to the combined treatment. Notably, basal ASNS expression at protein levels was significantly correlated with sensitivity to combined treatment. These results provide mechanistic insights into the role of GCN2 in the amino acid response and a rationale for further investigation of GCN2 inhibitors for the treatment of cancer.