An Interaction between the Human T Cell Leukemia Virus Type 1 Basic Leucine Zipper Factor (HBZ) and the KIX Domain of p300/CBP Contributes to the Down-regulation of Tax-dependent Viral Transcription by HBZ

Abstract

Activation of human T cell leukemia virus type 1 (HTLV-1) transcription is established through the formation of protein complexes on the viral promoter that are essentially composed of the cellular basic leucine zipper (bZIP) transcription factor cAMP-response element-binding protein (CREB (or certain other members of the ATF/CREB family), the HTLV-1-encoded transactivator Tax, and the pleiotropic cellular coactivators p300/CBP. HTLV-1 bZIP factor (HBZ) is a protein encoded by HTLV-1 that contains a bZIP domain and functions to repress HTLV-1 transcription. HBZ has been shown to repress viral transcription by dimerizing with CREB, which occurs specifically through the bZIP domain in each protein, and preventing CREB from binding to the DNA. However, we previously found that HBZ causes only partial removal of CREB from a chromosomally integrated viral promoter, and more importantly, an HBZ mutant lacking the COOH-terminal bZIP domain retains the ability to repress viral transcription. These results suggest that an additional mechanism contributes to HBZ-mediated repression of HTLV-1 transcription. In this study, we show that HBZ binds directly to the p300 and CBP coactivators. Two LXXLL-like motifs located within the NH 2 -terminal region of HBZ are important for this interaction and specifically mediate binding to the KIX domain of p300/CBP. We provide evidence that this interaction interferes with the ability of Tax to bind p300/CBP and thereby inhibits the association of the coactivators with the viral promoter. Our findings demonstrate that HBZ utilizes a bipartite mechanism to repress viral transcription.Human T-cell leukemia virus type 1 (HTLV-1) 5 is a retrovirus that is the causative agent of adult T-cell leukemia and a neurodegenerative disorder termed tropical spastic paraparesis/HTLV-1-associated myelopathy (1, 2). Following its integration, the provirus utilizes the cellular RNA polymerase II transcription machinery for replication of the viral genome and expression of viral genes. These processes are dependent on the viral transactivator Tax, which is essential for strong activation of HTLV-1 transcription. Tax alone lacks DNA binding activity and is therefore recruited to the viral promoter as part of a complex with the cellular transcription factor CREB or other members of the activating transcription factor/cyclic AMP-responsive element (CRE)-binding protein (ATF/CREB) family (3-6). These proteins carry a basic leucine zipper (bZIP) domain that stimulates protein dimerization and subsequent DNA binding. Dimer formation is specifically mediated through the leucine zipper (ZIP) domain, whereas DNA binding involves the basic region of each binding partner directly contacting the DNA. The Tax-CREB complexes associate with three Tax-responsive elements called viral CREs (vCREs) within the viral promoter that encompasses the U3 region of the 5Ј-long terminal repeat of the provirus. Each vCRE contains a central sequence similar to that of a cellular CRE that is recognized by CREB and flanking GC-rich s...

Effects of valproate on Tax and HBZ expression in HTLV-1 and HAM/TSP T lymphocytes

“…HBZ is a potent suppressor of Tax-mediated virus-gene transcription by interacting with activating transcription factor/cAMP responsive element-binding protein (CREB) and CREB-binding protien/p300 on the 5Ј-LTR promoter. 24,34,35 The HBZ role in the tightly regulated pattern of HTLV-1-gene expression also is suggested by HBZ repression of Gag-p19 synthesis in stable virus-producing cell clones. 36 Kinetic study of gene expressions in cells transiently transfected with the HTLV-1-provirus plasmid and in newly infected PBMCs showed that Gag/Pol, Tax/Rex, and Env mRNA are detected first and at their highest levels, whereas HBZ transcription was significantly lower and peaked later.…”

Mechanisms of HTLV-1 persistence and transformation

“…The recently identified HBZ (HTLV-1 bZIP) factor acts as a negative regulator of TAX-mediated viral transactivation by heterodimerising with CREB, CREB2 and p300/CBP (Clerc et al, 2008). HBZ also modulates AP1 pathway by interacting with c-Jun, JunB and JunD.…”

Leucémies T induites par HTLV-1

“…Alors que HBZ est incapable de s'homodimériser, il peut former in vivo des dimères avec des membres de la famille ATF/CREB et Jun (Figure 2) [9,[17][18][19][20][21]. En outre, grâce à son domaine d'activation, HBZ interagit aussi avec les cofacteurs transcriptionnels CBP/p300 [22]. Il est frappant de constater que HBZ cible les facteurs nucléaires qui sont impliqués dans le contrôle de la transcription virale Tax-dépendante (Figure 1).…”

“…Il est frappant de constater que HBZ cible les facteurs nucléaires qui sont impliqués dans le contrôle de la transcription virale Tax-dépendante (Figure 1). En fait, nous avons pu démontrer que HBZ régule négativement la transcription des autres gènes viraux en inhibant l'association de Tax avec CREB sur le promoteur du LTR 5' et en empêchant le recrutement de CBP/p300 sur ce même promoteur (Figure 3) [21,22]. …”

“…C'est par sa capacité à former un hétérodimère avec JunD que HBZ activerait hTERT [18] : JunD interagirait avec le facteur de transcription Sp1 permettant à HBZ d'être recrutée sur les sites Sp1 présents sur le promoteur proximal de hTERT [30]. Le recrutement ultérieur de CBP/p300 par le domaine activateur de HBZ [22,30] stimulerait la transcription cellulaire (Figure 4). Cette étude démontrait pour la première fois le rôle direct de la protéine HBZ dans la régulation d'un gène impliqué dans le développement d'un processus tumoral.…”