Tristetraprolin targets Nos2 expression in the colonic epithelium

Tristetraprolin-mediated mRNA destabilization regulates basophil inflammatory responses

BACKGROUND

Basophils, despite being the least common granulocytes, play crucial roles in type 2 immune responses, such as chronic allergic inflammation and protective immunity against parasites. However, the molecular mechanisms regulating basophil activation and inflammatory molecule production remain poorly understood. Therefore, we investigated the role of RNA-binding proteins, specifically tristetraprolin (TTP), in regulating inflammatory molecule production in basophils.

METHODS

Using antigen/IgE-stimulated basophils from wild-type (WT) and TTP-knockout (TTP-KO) mice, we performed bulk RNA sequencing, transcriptome-wide mRNA stability assays, and protein analyses. We also examined mRNA expression and protein production of inflammatory molecules in TTP-KO basophils under stimulation with IL-33 or LPS. Furthermore, we evaluated the in vivo significance of TTP in basophils using basophil-specific TTP-deficient mice and a hapten oxazolone-induced atopic dermatitis model.

RESULTS

TTP expression was upregulated in basophils following stimulation with antigen/IgE, IL-33, or LPS. Under these stimuli, TTP-KO basophils exhibited elevated mRNA expression of inflammatory molecules, such as Il4, Areg, Ccl3, and Cxcl2, compared to WT basophils. Transcriptome-wide mRNA stability assays revealed that TTP deficiency prolonged the mRNA half-life of these inflammatory mediators. Notably, the production of these inflammatory proteins was significantly increased in TTP-KO basophils. Moreover, basophil-specific TTP-deficient mice showed exacerbated oxazolone-induced atopic dermatitis-like skin allergic inflammation.

CONCLUSIONS

TTP is a key regulator of basophil activation, controlling the production of inflammatory mediators through mRNA destabilization. Our in vivo findings demonstrate that the absence of TTP in basophils significantly aggravates allergic skin inflammation, highlighting its potential as a therapeutic target for allergic diseases.

Haploinsufficiency of Cnot3 Aggravates Acid-Induced Acute Lung Injury Likely Through Transcriptional and Post-Transcriptional Upregulation of Pro-Inflammatory Genes

Background

Acute lung injury (ALI) is caused by a variety of illnesses, including aspiration pneumonia and sepsis. The CCR4-NOT complex is a large multimeric protein complex that degrades mRNA through poly(A) tail shortening, whereas it also contributes to regulation of transcription and translation. Cnot3 is a scaffold component of the CCR4-NOT complex and is essential for the integrity of the complex; loss of Cnot3 leads to depletion of whole complex. While the significance of cytokine mRNA degradation in limiting inflammation has been established, the roles of CCR4-NOT complex-mediated in ALI remain elusive.

Methods

The effects of Cnot3 haploinsufficiency in the pathology and cytokine expression were analyzed in the mouse lungs of acid aspiration-induced acute lung injury. The decay rate and transcription activity of cytokine mRNAs under Cnot3 heterozygous deletion were analyzed in lipopolysaccharide (LPS) -stimulated mouse embryonic fibroblasts (MEFs).

Results

Tamoxifen-induced heterozygous deletion of Cnot3 in adult mice (Cnot3 Hetz) did not show body weight loss or any apparent abnormality. Under acid aspiration-induced acute lung injury, Cnot3 Hetz mice exhibited increased pulmonary edema, worse lung pathologies and more severe inflammation compared with wild type mice. mRNA expression of pro-inflammatory genes Il1b and Nos2 were significantly upregulated in the lungs of Cnot3 Hetz mice. Consistently, mRNA expression of Il1b and Nos2 was upregulated in LPS-stimulated Cnot3 Hetz MEFs. Mechanistically, while heterozygous depletion of Cnot3 stabilized both Il1b and Nos2 mRNAs, the nascent pre-mRNA level of Il1b was upregulated in Cnot3 Hetz MEFs, implicating Cnot3-mediated transcriptional repression of Il1b expression in addition to destabilization of Il1b and Nos2 mRNAs. PU.1 (Spi1) was identified as a causative transcription factor to promote Il1b expression under Cnot3 haploinsufficient conditions.

Conclusion

CNOT3 plays a protective role in ALI by suppressing expression of pro-inflammatory genes Il1b and Nos2 through both post-transcriptional and transcriptional mechanisms, including mRNA stability control of Spi1.

Clinical implications of tristetraprolin (TTP) modulation in the treatment of inflammatory diseases

Abnormal regulation of pro-inflammatory cytokine and chemokine mediators can contribute to the excess inflammation characteristic of many autoimmune diseases, such as rheumatoid arthritis, psoriasis, Crohn's disease, type 1 diabetes, and many others. The tristetraprolin (TTP) family consists of a small group of related RNA-binding proteins that bind to preferred AU-rich binding sites within the 3'-untranslated regions of specific mRNAs to promote mRNA deadenylation and decay. TTP deficient mice develop a severe systemic inflammatory syndrome consisting of arthritis, myeloid hyperplasia, dermatitis, autoimmunity and cachexia, due at least in part to the excess accumulation of proinflammatory chemokine and cytokine mRNAs and their encoded proteins. To investigate the possibility that increased TTP expression or activity might have a beneficial effect on inflammatory diseases, at least two mouse models have been developed that provide proof of principle that increasing TTP activity can promote the decay of pro-inflammatory and other relevant transcripts, and decrease the severity of mouse models of inflammatory disease. Animal studies of this type are summarized here, and we briefly review the prospects for harnessing these insights for the development of TTP-based anti-inflammatory treatments in humans.

Gallium Nanodroplets are Anti-Inflammatory without Interfering with Iron Homeostasis

Gallium (Ga) compounds, as the source of Ga ions (Ga³⁺), have been historically used as anti-inflammatories. Currently, the widely accepted mechanisms of the anti-inflammatory effects for Ga³⁺ are rationalized on the basis of their similarities to ferric ions (Fe³⁺), which permits Ga³⁺ to bind with Fe-binding proteins and subsequently disturbs the Fe homeostasis in the immune cells. Here in contrast to the classic views, our study presents the mechanisms of Ga as anti-inflammatory by delivering Ga nanodroplets (GNDs) into lipopolysaccharide-induced macrophages and exploring the processes. The GNDs show a selective inhibition of nitric oxide (NO) production without affecting the accumulation of pro-inflammatory mediators. This is explained by GNDs disrupting the synthesis of inducible NO synthase in the activated macrophages by upregulating the levels of eIF2α phosphorylation, without interfering with the Fe homeostasis. The Fe³⁺ transferrin receptor-independent endocytosis of GNDs by the cells prompts a fundamentally different mechanism as anti-inflammatories in comparison to that imparted by Ga³⁺. This study reveals the fundamental molecular basis of GND-macrophage interactions, which may provide additional avenues for the use of Ga for anti-inflammatory and future biomedical and pharmaceutical applications.

Beta-hydroxybutyrate dampens adipose progenitors' profibrotic activation through canonical Tgfβ signaling and non-canonical ZFP36-dependent mechanisms

BACKGROUND/PURPOSE

Adipose tissue contains progenitor cells that contribute to beneficial tissue expansion when needed by de novo adipocyte formation (classical white or beige fat cells with thermogenic potential). However, in chronic obesity, they can exhibit an activated pro-fibrotic, extracellular matrix (ECM)-depositing phenotype that highly aggravates obesity-related adipose tissue dysfunction.

METHODS

Given that progenitors' fibrotic activation and fat cell browning appear to be antagonistic cell fates, we have examined the anti-fibrotic potential of pro-browning agents in an obesogenic condition.

RESULTS

In obese mice fed a high fat diet, thermoneutral housing, which induces brown fat cell dormancy, increases the expression of ECM gene programs compared to conventionally raised animals, indicating aggravation of obesity-related tissue fibrosis at thermoneutrality. In a model of primary cultured murine adipose progenitors, we found that exposure to β-hydroxybutyrate selectively reduced Tgfβ-dependent profibrotic responses of ECM genes like Ctgf, Loxl2 and Fn1. This effect is observed in both subcutaneous and visceral-derived adipose progenitors, as well as in 3T3-L1 fibroblasts. In 30 patients with obesity eligible for bariatric surgery, those with higher circulating β-hydroxybutyrate levels have lower subcutaneous adipose tissue fibrotic scores. Mechanistically, β-hydroxybutyrate limits Tgfβ-dependent collagen accumulation and reduces Smad2-3 protein expression and phosphorylation in visceral progenitors. Moreover, β-hydroxybutyrate induces the expression of the ZFP36 gene, encoding a post-transcriptional regulator that promotes the degradation of mRNA by binding to AU-rich sites within 3'UTRs. Importantly, complete ZFP36 deficiency in a mouse embryonic fibroblast line from null mice, or siRNA knock-down in primary progenitors, indicate that ZFP36 is required for β-hydroxybutyrate anti-fibrotic effects.

CONCLUSION

These data unravel the potential of β-hydroxybutyrate to limit adipose tissue matrix deposition, a finding that might exploited in an obesogenic context.

Posttranscriptional regulation of ILC2 homeostatic function via tristetraprolin

Group 2 innate lymphoid cells (ILC2s) are unique in their ability to produce low levels of type 2 cytokines at steady state, and their production capacity is dramatically increased upon stimulation with IL-33. However, it is unknown how constitutive cytokine production is regulated in the steady state. Here, we found that tristetraprolin (TTP/Zfp36), an RNA-binding protein that induces mRNA degradation, was highly expressed in naive ILC2s and was downregulated following IL-33 stimulation. In ILC2s from Zfp36-/- mice, constitutive IL-5 production was elevated owing to the stabilization of its mRNA and resulted in an increased number of eosinophils in the intestine. Luciferase assay demonstrated that TTP directly regulates Il5 mRNA stability, and overexpression of TTP markedly suppressed IL-5 production by ILC2s, even under IL-33 stimulation. Collectively, TTP-mediated posttranscriptional regulation acts as a deterrent of excessive cytokine production in steady-state ILC2s to maintain body homeostasis, and downregulation of TTP may contribute to massive cytokine production under IL-33 stimulation.

Conceptual Advances in Control of Inflammation by the RNA-Binding Protein Tristetraprolin

Regulated changes in mRNA stability are critical drivers of gene expression adaptations to immunological cues. mRNA stability is controlled mainly by RNA-binding proteins (RBPs) which can directly cleave mRNA but more often act as adaptors for the recruitment of the RNA-degradation machinery. One of the most prominent RBPs with regulatory roles in the immune system is tristetraprolin (TTP). TTP targets mainly inflammation-associated mRNAs for degradation and is indispensable for the resolution of inflammation as well as the maintenance of immune homeostasis. Recent advances in the transcriptome-wide knowledge of mRNA expression and decay rates together with TTP binding sites in the target mRNAs revealed important limitations in our understanding of molecular mechanisms of TTP action. Such orthogonal analyses lead to the discovery that TTP binding destabilizes some bound mRNAs but not others in the same cell. Moreover, comparisons of various immune cells indicated that an mRNA can be destabilized by TTP in one cell type while it remains stable in a different cell linage despite the presence of TTP. The action of TTP extends from mRNA destabilization to inhibition of translation in a subset of targets. This article will discuss these unexpected context-dependent functions and their implications for the regulation of immune responses. Attention will be also payed to new insights into the role of TTP in physiology and tissue homeostasis.

Targeting Tristetraprolin Expression or Functional Activity Regulates Inflammatory Response Induced by MSU Crystals

The RNA-binding protein tristetraprolin (TTP) is an anti-inflammatory factor that prompts the mRNA decay of target mRNAs and is involved in inflammatory diseases such as rheumatoid arthritis (RA). TTP is regulated by phosphorylation, and protein phosphatase 2A (PP2A) can dephosphorylate TTP to activate its mRNA-degrading function. Some small molecules can enhance PP2A activation. Short interfering RNA (siRNA) targeting TTP expression or PP2A agonist (Arctigenin) was administered to monosodium urate (MSU) crystal-induced J774A.1 cells, and the expression of inflammatory related genes was detected by RT-PCR and Western blot assays. The effects of Arctigenin in mouse models of acute inflammation induced by MSU crystals, including peritonitis and arthritis, were evaluated. The data indicated that TTP expression levels and endogenous PP2A activity were increased in MSU-crystal treated J774A.1 cells. TTP knockdown exacerbated inflammation-related genes expression and NLRP3 inflammasome activation. However, PP2A agonist treatment (Arctigenin) suppressed MSU crystal-induced inflammation in J774A.1 cells. Arctigenin also relieved mitochondrial reactive oxygen species (mtROS) production and improved lysosomal membrane permeability in MSU crystal-treated J774A.1 cells. Moreover, TTP knockdown reversed the anti-inflammatory and antioxidant effects of Arctigenin. Oral administration of Arctigenin significantly alleviated foot pad swelling, the number of inflammatory cells in peritoneal lavage fluids and the production of IL-1β in the mouse model of inflammation induced by MSU crystals. Collectively, these data imply that targeting TTP expression or functional activity may provide a potential therapeutic strategy for inflammation caused by MSU crystals.

Control of RNA Stability in Immunity

Posttranscriptional control of mRNA regulates various biological processes, including inflammatory and immune responses. RNA-binding proteins (RBPs) bind cis-regulatory elements in the 3' untranslated regions (UTRs) of mRNA and regulate mRNA turnover and translation. In particular, eight RBPs (TTP, AUF1, KSRP, TIA-1/TIAR, Roquin, Regnase, HuR, and Arid5a) have been extensively studied and are key posttranscriptional regulators of inflammation and immune responses. These RBPs sometimes collaboratively or competitively bind the same target mRNA to enhance or dampen regulatory activities. These RBPs can also bind their own 3' UTRs to negatively or positively regulate their expression. Both upstream signaling pathways and microRNA regulation shape the interactions between RBPs and target RNA. Dysregulation of RBPs results in chronic inflammation and autoimmunity. Here, we summarize the functional roles of these eight RBPs in immunity and their associated diseases.

The RNA-binding protein tristetraprolin regulates RALDH2 expression by intestinal dendritic cells and controls local Treg homeostasis

AU-rich element (ARE)-mediated mRNA decay represents a key mechanism to avoid excessive production of inflammatory cytokines. Tristetraprolin (TTP, encoded by Zfp36) is a major ARE-binding protein, since Zfp36^-/- mice develop a complex multiorgan inflammatory syndrome that shares many features with spondyloarthritis. The role of TTP in intestinal homeostasis is not known. Herein, we show that Zfp36^-/- mice do not develop any histological signs of gut pathology. However, they display a clear increase in intestinal inflammatory markers and discrete alterations in microbiota composition. Importantly, oral antibiotic treatment reduced both local and systemic joint and skin inflammation. We further show that absence of overt intestinal pathology is associated with local expansion of regulatory T cells. We demonstrate that this is related to increased vitamin A metabolism by gut dendritic cells, and identify RALDH2 as a direct target of TTP. In conclusion, these data bring insights into the interplay between microbiota-dependent gut and systemic inflammation during immune-mediated disorders, such as spondyloarthritis.

Tristetraprolin destabilizes NOX2 mRNA and protects dopaminergic neurons from oxidative damage in Parkinson's disease

Tristetraprolin (TTP), an RNA-binding protein encoded by the ZFP36 gene, is vital for neural differentiation; however, its involvement in neurodegenerative diseases such as Parkinson's disease (PD) remains unclear. To explore the role of TTP in PD, an in vitro 1-methyl-4-phenylpyridinium (MPP⁺ ) cell model and an in vivo 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) of PD were used. Transfection of small interfering (si)-TTP RNA upregulated pro-oxidative NOX2 expression and ROS formation, downregulated anti-oxidative GSH and SOD activity；si-TTP upregulated pro-apoptotic cleaved-caspase-3 expression, and downregulated antiapoptotic Bcl-2 expression; while overexpression (OE)-TTP lentivirus caused opposite effects. Through database prediction, luciferase experiment, RNA immunoprecipitation (RIP), and mRNA stability analysis, we evaluated the potential binding sites of TTP to 3'-untranslated regions (3'-UTR) of NOX2 mRNA. TTP affected the NOX2 luciferase activity by binding to two sites in the NOX2 3'-UTR. RIP-qPCR confirmed TTP binding to both sites, with a higher affinity for site-2. In addition, TTP reduced the NOX2 mRNA stability. si-NOX2 and antioxidant N-acetyl cysteine (NAC) reversed si-TTP-induced cell apoptosis. In MPTP-treated mice, TTP expression increased and was co-located with dopaminergic neurons. TTP also inhibited NOX2 and decreased the oxidative stress in vivo. In conclusion, TTP protects against dopaminergic oxidative injury by promoting NOX2 mRNA degradation in the MPP⁺ /MPTP model of PD, suggesting that TTP could be a potential therapeutic target for regulating the oxidative stress in PD.

Tristetraprolin Overexpression in Non-hematopoietic Cells Protects Against Acute Lung Injury in Mice

Tristetraprolin (TTP) is a mRNA binding protein that binds to adenylate-uridylate-rich elements within the 3′ untranslated regions of certain transcripts, such as tumor necrosis factor (Tnf) mRNA, and increases their rate of decay. Modulation of TTP expression is implicated in inflammation; however, its role in acute lung inflammation remains unknown. Accordingly, we tested the role of TTP in lipopolysaccharide (LPS)-induced acute lung injury (ALI) in mice. LPS-challenged TTP-knockout (TTP^KO) mice, as well as myeloid cell-specific TTP-deficient (TTP^myeKO) mice, exhibited significant increases in lung injury, although these responses were more robust in the TTP^KO. Mice with systemic overexpression of TTP (TTP^ΔARE) were protected from ALI, as indicated by significantly reduced neutrophilic infiltration, reduced levels of neutrophil chemoattractants, and histological parameters of ALI. Interestingly, while irradiated wild-type (WT) mice reconstituted with TTP^KO hematopoietic progenitor cells (HPCs) showed exaggerated ALI, their reconstitution with the TTP^ΔARE HPCs mitigated ALI. The reconstitution of irradiated TTP^ΔARE mice with HPCs from either WT or TTP^ΔARE donors conferred significant protection against ALI. In contrast, irradiated TTP^ΔARE mice reconstituted with TTP^KO HPCs had exaggerated ALI, but the response was milder as compared to WT recipients that received TTP^KO HPCs. Finally, the reconstitution of irradiated TTP^KO recipient mice with TTP^ΔARE HPCs did not confer any protection to the TTP^KO mice. These data together suggest that non-HPCs-specific overexpression of TTP within the lungs protects against ALI via downregulation of neutrophil chemoattractants and reduction in neutrophilic infiltration.