TRAF3 Is Required for NF-κB Pathway Activation Mediated by HTLV Tax Proteins

Abstract

Human T-cell leukemia viruses type 1 (HTLV-1) and type 2 (HTLV-2) share a common genome organization and expression strategy but have distinct pathological properties. HTLV-1 is the etiological agent of Adult T-cell Leukemia (ATL) and of HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP), whereas HTLV-2 does not cause hematological disorders and is only sporadically associated with cases of subacute myelopathy. Both HTLV genomes encode two regulatory proteins that play a pivotal role in pathogenesis: the transactivating Tax-1 and Tax-2 proteins and the antisense proteins HBZ and APH-2, respectively. We recently reported that Tax-1 and Tax-2 form complexes with the TNF-receptor associated factor 3, TRAF3, a negative regulator of the non-canonical NF-κB pathway. The NF-κB pathway is constitutively activated by the Tax proteins, whereas it is inhibited by HBZ and APH-2. The antagonistic effects of Tax and antisense proteins on NF-κB activation have not yet been fully clarified. Here, we investigated the effect of TRAF3 interaction with HTLV regulatory proteins and in particular its consequence on the subcellular distribution of the effector p65/RelA protein. We demonstrated that Tax-1 and Tax-2 efficiency on NF-κB activation is impaired in TRAF3 deficient cells obtained by CRISPR/Cas9 editing. We also found that APH-2 is more effective than HBZ in preventing Tax-dependent NF-κB activation. We further observed that TRAF3 co-localizes with Tax-2 and APH-2 in cytoplasmic complexes together with NF-κB essential modulator NEMO and TAB2, differently from HBZ and TRAF3. These results contribute to untangle the mechanism of NF-κB inhibition by HBZ and APH-2, highlighting the different role of the HTLV-1 and HTLV-2 regulatory proteins in the NF-κB activation.

NF-κB and MicroRNA Deregulation Mediated by HTLV-1 Tax and HBZ

“…The NF-κB transcription factors regulate the expression of more than one hundred genes that participate in cell proliferation, inflammation, and innate immunity [42,43]. HTLV-1 Tax induces constitutive activation of NF-κB, causing aberrant expression of many target genes, including those coding for cytokines/chemokines, inflammatory proteins, adhesion molecules, cell cycle regulators, and apoptosis regulators (representative examples are listed in Table 1) [21,26,[44][45][46]. NF-κB is a family of inducible transcription factors represented by five structurally related proteins, named p50, p52, RelA/p65, RelB and c-Rel, which regulate the expression of target genes by binding to κB promoter/enhancer elements as hetero-or homodimers.…”

“…Tax-1 activates both the canonical and non-canonical NF-κB pathways through mechanisms that involve interactions with NEMO and the IKK complex [26,[72][73][74][75][76][77][78][79][80] and ubiquitination events affecting Tax-1 and pathway components. Indeed, ubiquitination of Tax-1 on C-terminal lysine residues is required for its activation of the NF-κB pathway [81].…”

“…We propose that Tax-1 and Tax-2 may be recruited into the TBK1/IKK complexes as scaffolding-adaptor proteins and thereby enhance expression of IFN-inducible genes [104]. Furthermore, we recently demonstrated that TNF Receptor Associated Factor 3 (TRAF3), an inhibitor of the non-canonical NF-κB pathway also acting in IFN-I signaling, interacts with Tax-1 and Tax-2 and is required for efficient NF-κB activation mediated by Tax [105].…”

“…Tax-1 enhances the expression of genes coded on the plus strand of the proviral genome by recruiting host transcription factors to Tax-responsive elements in the 5 long terminal repeat (LTR) promoter [22,23]. Through its transactivating activity, Tax-1 deregulates several cell signaling pathways, including NF-κB, AP-1, SRF, and CREB, and thus impinges on the mechanisms controlling cell cycle progression, apoptosis, and the DNA damage response [21,[24][25][26]. Both HTLV-1 and HTLV-2 Tax (Tax-1 and Tax-2, respectively) can transform T-cells in vitro [27,28].…”

“…The oncogenic potential of HBZ is linked to its ability to induce T-cell proliferation, inhibit cellular senescence, and contribute to viral persistence [35][36][37]. An interesting property of HBZ is its ability to counteract several functions mediated by Tax-1, including activation of the 5'LTR promoter and stimulation of the NF-κB pathway [26,[31][32][33]38].…”