The many faces of TRAF molecules in immune regulation

TRAF6 as a potential target in advanced breast cancer: a systematic review, meta-analysis, and bioinformatics validation

TRAF6 has emerged as a key regulator of breast cancer (BCa). However, the TRAF family constitutes of seven members that exhibit distinct and overlapping functions. To explore which TRAF represents a potential druggable target for BCa treatment, we searched Medline, Web of Science and Scopus for relevant studies from inception to June 27, 2021. We identified 14 in vitro, 11 in vivo and 4 human articles. A meta-analysis of pharmacological studies showed that in vitro inhibition of TRAF2/4 (mean difference (MD): - 57.49, 95% CI: - 66.95, - 48.02, P < 0.00001) or TRAF6 (standard(Std.)MD: - 4.01, 95% CI: - 5.75, - 2.27, P < 0.00001) is associated with reduction in BCa cell migration. Consistently, inhibition of TRAF2/4 (MD: - 51.08, 95% CI: - 64.23, - 37.94, P < 0.00001) and TRAF6 (Std.MD: - 2.80, 95% CI: - 4.26, - 1.34, P = 0.0002) is associated with reduced BCa cell invasion, whereas TRAF2/4 inhibition (MD: - 40.54, 95% CI: - 52.83, - 28.26, P < 0.00001) is associated with reduced BCa cell adhesion. Interestingly, only inhibition of TRAF6 (MD: - 21.46, 95% CI: - 30.40, - 12.51, P < 0.00001) is associated with reduced cell growth. In animal models of BCa, administration of pharmacological inhibitors of TRAF2/4 (Std.MD: - 3.36, 95% CI: - 4.53, - 2.18, P < 0.00001) or TRAF6 (Std.MD: - 4.15, 95% CI: - 6.06, - 2.24, P < 0.0001) in mice is associated with reduction in tumour burden. In contrast, TRAF6 inhibitors (MD: - 2.42, 95% CI: - 3.70, - 1.14, P = 0.0002) reduced BCa metastasis. In BCa patients, high expression of TRAF6 (Hazard Ratio: 1.01, CI: 1.01, 1.01, P < 0.00001) is associated with poor survival rate. Bioinformatics validation of clinical and pathway and process enrichment analysis in BCa patients confirmed that gain/amplification of TRAF6 is associated with secondary BCa in bone (P = 0.0079), and poor survival rate (P < 0.05). Overall, TRAF6 inhibitors show promise in the treatment of metastatic BCa. However, low study number and scarcity of evidence from animal and human studies may limit the translation of present findings into clinical practice.

TRAF family molecules in T cells: Multiple receptors and functions

The TNFR superfamily of receptors, the major focus of the recent TNFR Superfamily Conference held in June 2019, employ the TNFR-associated factor (TRAF) family of adaptor proteins in key aspects of their signaling pathways. Although many early studies investigated TRAF functions via exogenous overexpression in nonhematopoietic cell lines, it has subsequently become clear that whereas TRAFs share some overlap in function, each also plays unique biologic roles, that can be highly context dependent. This brief review summarizes the current state of knowledge of functions of each of the TRAF molecules that mediate important functions in T lymphocytes: TRAFs 1, 2, 3, 5, and 6. Due to our current appreciation of the contextual nature of TRAF function, our focus is upon findings made specifically in T lymphocytes. Key T cell functions for each TRAF are detailed, as well as future knowledge gaps of interest and importance.

Functional characterization of tumor necrosis factor receptor-associated factor 3 of sea perch (Lateolabrax japonicas) in innate immune

Tumor necrosis factor receptor-associated factor 3 (TRAF3) is a multifunctional regulator implicated in both bacterial defense and antiviral immunity. Here, a TRAF3 gene from the seawater fish sea perch, designated as LjTRAF3, was characterized. The full-length cDNA of LjTRAF3 was 2972 bp including a 5' untranslated region (UTR) of 243 bp, a 3'UTR of 941 bp and a putative open reading frame of 1608 bp encoding a putative protein of 536 amino acid. The deduced LjTRAF3 protein contained a RING finger, two zinc fingers, a coiled-coil, and a meprin and TRAF-C homology domain. Phylogenetic analysis showed that LjTRAF3 shared the closest genetic relationship with Larimichthys crocea TRAF3. Gene expression analyses suggested that LjTRAF3 mRNA was ubiquitously expressed in all the tissues tested, and was up-regulated post red spotted grouper nervous necrosis virus (RGNNV) infection in vivo and in vitro. Reporter gene assay showed that LjTRAF3 significantly activated zebrafish type I interferon (IFN) promoter in vitro. During RGNNV infection, ectopic expression of LjTRAF3 significantly reduced the RNA dependent RNA polymerase transcription of RGNNV, and enhanced the expression of RIG-I-like receptors (RLR), janus kinase-signal transducers and activators of transcription (JAK-STAT) signaling pathway related genes and IFN stimulated genes (ISGs), including ISG15, PKR, VIG and TRIM39. Taken together, our results suggested that LjTRAF3 might trigger the expression of various ISGs to counter RGNNV infection by regulating the RLR-induced IFN and JAK-STAT signaling pathways.

Roles of TRAF3 in T cells: many surprises

Tumor necrosis factor receptor (TNFR)-associated factor 3 (TRAF3) is broadly involved in different receptor-mediated signaling pathways. Considerable progress was made recently in understanding the role of TRAF3 in T cell biology. Here we review these new findings about how TRAF3 participates in T cell development and function. The different roles of TRAF3 in distinct immune cells are also compared. That TRAF3 is required for T cell effector functions, and invariant Natural Killer T cell function and development, was unexpected. Another surprising finding is that TRAF3 normally restrains regulatory T cell development. It is now clear that TRAF3 regulates signaling to T cells not only through costimulatory members of the TNFR superfamily, but also through the T cell receptor complex, and cytokine receptors. The diverse roles it plays support the multifaceted nature of this molecule. How TRAF3 mediates integration of different signaling cascades is an important topic for future study.

Targeting TRAF3 signaling protects against hepatic ischemia/reperfusions injury

BACKGROUND & AIMS

The hallmarks of hepatic ischemia/reperfusion (I/R) injury, a common clinical problem that occurs during liver surgical procedures, include severe cell death and inflammatory responses that contribute to early graft failure and a higher incidence of organ rejection. Unfortunately, effective therapeutic strategies are limited. Tumor necrosis factor receptor (TNFR)-associated factor (TRAF) 3 transduces apoptosis and/or inflammation-related signaling pathways to regulate cell survival and cytokine production. However, the role of TRAF3 in hepatic I/R-induced liver damage remains unknown.

METHODS

Hepatocyte- or myeloid cell-specific TRAF3 knockdown or transgenic mice were subjected to an I/R model in vivo, and in vitro experiments were performed by treating primary hepatocytes from these mice with hypoxia/reoxygenation stimulation. The function of TRAF3 in I/R-induced liver damage and the potential underlying mechanisms were investigated through various phenotypic analyses and biological approaches.

RESULTS

Hepatocyte-specific, but not myeloid cell-specific, TRAF3 deficiency reduced cell death, inflammatory cell infiltration, and cytokine production in both in vivo and in vitro hepatic I/R models, whereas hepatic TRAF3 overexpression resulted in the opposite effects. Mechanistically, TRAF3 directly binds to TAK1, which enhances the activation of the downstream NF-κB and JNK pathways. Importantly, inhibition of TAK1 almost completely reversed the TRAF3 overexpression-mediated exacerbation of I/R injury.

CONCLUSIONS

TRAF3 is a novel hepatic I/R mediator that promotes liver damage and inflammation via TAK1-dependent activation of the JNK and NF-κB pathways. Inhibition of hepatic TRAF3 may represent a promising approach to protect the liver against I/R injury-related diseases.

The adaptor TRAF3 restrains the lineage determination of thymic regulatory T cells by modulating signaling via the receptor for IL-2

The number of Foxp3+ regulatory T cells (Treg cells) must be tightly controlled for efficient suppression of autoimmunity with no impairment of normal immune responses. Here we found that the adaptor TRAF3 was intrinsically required for restraining the lineage determination of thymic Treg cells. T cell-specific deficiency in TRAF3 resulted in a two- to threefold greater frequency of Treg cells, due to the more efficient transition of precursors of Treg cells into Foxp3+ Treg cells. TRAF3 dampened interleukin 2 (IL-2) signaling by facilitating recruitment of the tyrosine phosphatase TCPTP to the IL-2 receptor complex, which resulted in dephosphorylation of the signaling molecules Jak1 and Jak3 and negative regulation of signaling via Jak and the transcription factor STAT5. Our results identify a role for TRAF3 as an important negative regulator of signaling via the IL-2 receptor that affects the development of Treg cells.

TRAF3 regulates homeostasis of CD8+ central memory T cells

Our laboratory reported previously that TNF receptor associated factor 3 (TRAF3) is a positive regulator of TCR signaling and T cell function. In the current study, we present new findings that reveal differential roles for TRAF3 in the regulation of CD4⁺ and CD8⁺ T cells. In response to TCR stimulation in vitro, TRAF3 has greater impact in CD4⁺ T cells than in CD8⁺ T cells. However, T cell-specific TRAF3 deficient mice (CD4^Cre TRAF3^fl°^x/fl°^x; T-TRAF3^−/−) have a greater number of CD4⁺CD44^hi effector/memory T cells than littermate control (LMC) mice, possibly due to an inefficient suppressive effect of TRAF3 deficient Foxp3⁺ regulatory T cells. In contrast, CD8⁺CD44^hiCD62L^hi central memory (Tcm) cells are markedly reduced in T-TRAF3^−/− mice in comparison to LMC mice, although CD8⁺CD44^hiCD62L^l°^w effector memory T (Tem) cells and naïve T cells (CD8⁺CD44^l°^wCD62L^hi) do not show significant differences in number. Importantly, TRAF3-deficient Tcm cells exhibit defective homeostasis due to impaired IL-15 signaling. These results indicate that the involvement of TRAF3 in IL-15 mediated signaling to T cells plays a previously unappreciated and critical role in CD8⁺ Tcm cell regulation and maintenance.