HTLV-1 bZIP factor enhances TGF-β signaling through p300 coactivator

Abstract

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that is etiologically associated with adult T-cell leukemia. The HTLV-1 bZIP factor (HBZ), which is encoded by the minus strand of the provirus, is involved in both regulation of viral gene transcription and T-cell proliferation. We showed in this report that HBZ interacted with Smad2/3, and enhanced transforming growth factor-␤ (TGF-␤)/Smad transcriptional responses in a p300-dependent manner. The N-terminal LXXLL motif of HBZ was responsible for HBZ-mediated TGF-␤ signaling activation. In a serial immunoprecipitation assay, HBZ, Smad3, and p300 formed a ternary complex, and the association between Smad3 and p300 was markedly enhanced in the presence of HBZ. In addition, HBZ could overcome the repression of the TGF-␤ response by Tax. Finally, HBZ expression resulted in enhanced transcription of Pdgfb, Sox4, Ctgf, Foxp3, Runx1, and Tsc22d1 genes and suppression of the Id2 gene; such effects were similar to those by TGF-␤. In particular, HBZ induced Foxp3 expression in naive T cells through Smad3-dependent TGF-␤ signaling. Our results suggest that HBZ, by enhancing TGF-␤ signaling and Foxp3 expression, enables HTLV-1 to convert infected T cells into regulatory T cells, which is thought to be a critical strategy for virus persistence. (Blood. 2011;118(7):1865-1876) IntroductionHuman T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/ tropical spastic paraparesis (HAM/TSP). 1,2 The unique sequence of HTLV-1 between the env region and 3Ј long terminal repeat, denoted the pX region, encodes regulatory (tax and rex) and accessory (p12, p13, and p30) genes in the plus strand. 3 The pleiotropic actions of Tax are thought to play a central role in the early stage of leukemogenesis. 4 However, approximately 60% of fresh ATL cells lack Tax expression because of genetic and epigenetic changes in the HTLV-1 provirus, suggesting that Tax may not be essential for the maintenance of ATL cells in vivo. 5 The HTLV-1 bZIP factor (HBZ), which is encoded by the complementary strand of the HTLV-1 genome, is expressed in all ATL cases and supports the proliferation of ATL cells. 6,7 In addition, HBZ was found to inhibit Tax-mediated activation of viral transcription from the 5Ј-long terminal repeat by heterodimerizing with c-Jun, CREB2, and to selectively suppress the classic pathway of nuclear factor-B by 2 distinct mechanisms. [8][9][10][11] Thus, HBZ has multifunctional roles in cellular signaling and proliferation. Recently, we reported that nonsense mutations in all HTLV-1 genes except HBZ were generated by APOBEC3G before integration of the provirus in ATL cases and HTLV-1 infected persons, indicating that the HBZ gene is essential for leukemogenesis. 12 Transforming growth factor-␤ (TGF-␤) controls a variety of biologic processes, including cell growth, differentiation, apoptosis, development, and immune homeostasis. 13 The Smad proteins, which are mediators of TGF-␤ signaling, transdu...

Functional Comparison of HBZ and the Related APH-2 Protein Provides Insight into Human T-Cell Leukemia Virus Type 1 Pathogenesis

“…The p65 expression plasmid and B luciferase (luc) reporter plasmid were described previously (42). The TGF-␤-responsive luciferase reporter plasmid 9ϫCAGA-luc was kindly provided by Masao Matsuoka (Kyoto University, Kyoto, Japan) and described previously (43). The IRF-1 expression plasmid and IRF-1 luciferase reporter plasmid (IRF-1-luc) were graciously provided by John Yim (Beckman Research Institute, Duarte, CA) and described previously (44).…”

Protective effect of cytotoxic T lymphocytes targeting HTLV-1 bZIP factor

“…Tax-M22 cannot activate the NF-kB pathway, 28 and HBZ-LL/AA lacks the capacity to activate the transforming growth factor beta/Smad pathway. 29 Infection with rVVTax-M22 and HBZ-LL/AA induced expression of the inserted antigens in RK13 cells (supplemental Figure 1A-B available at the Blood Web site). First, we inoculated 10 7 plaque-forming units of rVV expressing HBZ-LL/AA or Tax-M22 into mice to induce specific immune responses.…”

“…27 In brief, the antigen's gene was inserted into the LC16m8 strain of VV in place of the hemagglutinin gene by homologous recombination with pBMSF7c plasmid in chicken embryonic fibroblasts. An NF-kB transactivation-deficient mutant of HTLV-1 Tax, M22 (T130A/L131S), which still activates the cAMP response element binding protein (CREB)/activating transcription factor (ATF) pathway, 28 and an HBZ-LL/AA (L27A/L28A) mutant, which cannot activate the transforming growth factor beta/Smad pathway, 29 were used as antigens. The rVVs generated were cloned by adsorption with chicken red blood cells on RK13 cells.…”

T memory stem cells are the hierarchical apex of adult T-cell leukemia

“…Although several studies have pointed out close immunophenotypic and functional resemblances between ATL cells and regulatory T cells, [7][8][9] a recent paper noted that such features can be acquired or induced by virus molecules but not inherited from the initially infected T cells. 10 In Tax-transgenic mice, CD117 1 hematopoietic progenitor cells recapitulated T-cell lymphoma. 11 These findings raise the possibility that the initial ATL clones might originate in a clinically disregarded population rather than the major population with conventional phenotypes.…”