Cellular Transformation by HTLV1
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Cellular Transformation by HTLV1

HTLV1 (Human T-Lymphotropic Virus-1) is the etiological agent for ATL (Adult T-Cell Leukemia) as well as for TSP (Tropical Spastic Paraparesis) and HAM (HTLV1 Associate Myelopathy). Involvement of HTLV1 in ATL is dependent on the virally-encoded transcriptional activator Tax (Transactivator X). Its ability to modulate the expression and function of many cellular genes is reasoned to be a major contributory mechanism explaining HTLV1 mediated transformation of cells. HTLV1 carries no viral oncogene. The oncogenic potential of the virus is linked to the regulatory gene Tax. In activating cellular gene expression, Tax impinges upon several cellular signal-transduction pathways, including those for CREB (cAMP Response Element-Binding Protein), NF-KappaB (Nuclear Factor-KappaB), etc. Tax’s transcriptional potential is linked to IKK (I-KappaB Kinase) Complex and MAP3Ks (Mitogen-Activated Protein Kinase Kinase Kinases) (Ref.1 & 2). Tax protein acts as a transcriptional transactivator which functionally inactivates tumor suppressor proteins such as p53, CDKN2A (Cyclin Dependent Kinase Inhibitor-2A) and inhibit cellular DNA repair. The HTLV1 Tax also interacts with a series of cellular proteins characterized by the presence of a PDZ domain. One of them, Tax1BP3 (Tax1 (Human T-cell Leukemia Virus Type-1) Binding Protein-3), binds to the C-terminal end of the HTLV1 Tax protein. HTLV1 Tax inhibits p53-mediated Tax1BP2 (Tax1 (Human T-cell Leukemia Virus Type-1) Binding Protein-2) protein degradation. However, the effects of the Tax oncoprotein on cell cycle progression remain unclear (Ref.3,4 & 5).

Tax influences the cellular capacity to correct damaged DNA. Tax through interactions with inhibitors of CDKs (Cyclin-Dependent Kinases) and G1 Cyclins, deregulates the cell cycle control and removes the corrective pauses. Indirectly, accelerating uncontrolled cell cycle progression. Tax is also considered to directly affect the machinery for DNA repair. Thus it represses the expression of Pol-Beta (Polymerase DNA-Beta), an enzyme important for base-excision repair and the MAD1L1 (Mitotic Arrest-Deficient-1 Yeast Homolog-Like-1) protein, a factor likely critical for surveillance against aneuploidies (Ref.6 & 7). Attachment receptor for HTLV1 is the cellular glucose transporter protein, GLUT1 (Glucose Transporter-1). The viral RT (Reverse Transcriptase) has strong affinity to ssRNA. Upon removal of CA (Capsid) and Env (Envelope) glycoprotein, RT synthesizes the first strand of DNA that is referred to as (-) DNA from viral ssRNA, plus-sense. The (-) strand is extended to the end of the viral RNA, which is degraded by RNAaseH (Ribonuclease-H). The new (+) strand is then synthesized from the (-) strand. Reverse transcription thus leads to the formation of dsDNA; RNAaseH destroys the remaining RNA. These viruses penetrate via fusion, uncoat to core and then the Provirus is made through random integration. The Proviral ORFs (Open Reading Frames), pX-III and pX-IV encode for regulatory proteins like Tax through alternative splicing of mRNAs (Ref.8).

Tax initiates the immortalization and transformation of human thymocytes, cord blood lymphocytes and fibroblast cells. Tax potently stimulates the expression of its cognate viral LTR (Long Terminal Repeat) that is U3-R-U5 (Unique 3-Repeat-Unique 5), as well as the promoters of several cellular genes. Tax has the ability to act through four discrete cellular signaling pathways: CREB, NF-KappaB, AP-1 (Activator Protein-1) and SRF (Serum Response Factor). The HTLV1 Tax protein is predominantly a viral nuclear antigen with a well-defined NLS (Nuclear Localization Signal) occurring in its N-terminal 48 residues. Tax protein co-localizes primarily in nuclear bodies with RNA Polymerase-II , splicing complexes and specific transcription factors (Ref.5 & 9). A small amount of Tax protein resides in the cytoplasm of mammalian cells. Thus, in modulating gene expression, Tax is envisioned to have promoter-poximal (i.e. nuclear) as well as promoter-distal (i.e. cytoplasmic) effects. Tax by itself cannot associate directly with DNA. However Tax dysregulates the expression of several cellular IE (Immediately Early) genes. The Tax-mediated dysregulation of gene expression contributes importantly to its abrogation of normal cellular metabolism. An early clue as to how HTLV1 affects cellular IE genes emerged from the finding that the DNA-binding SRF recruits the Tax protein to cellular promoters such as those for c-Fos (Cellular Oncogene Fos). The CarG-box-tethered SRF binds Tax, thereby bringing the viral oncoprotein to the promoter. The C-terminal activation domain of Tax then makes a contact directly with TATA-box bound TBP (TATA Box-Binding Protein)-protein resulting in enhanced transcription. The HTLV1 LTR contains three imperfectly conserved 21 bp TxRE (Tax-Responsive) sequences, each of which contains a core CREB binding sites flanked by 5’G- and 3’C-rich residues. Binding of Tax to CREB enhances CREB-CREB homodimerization and heightens resulting association to DNA. Contact with DNA results in proper folding of the Tax protein, which leads to a functional presentation of its C-terminal activation domain. Correctly folded Tax protein is known to recruit through its amino acid residues 81-95 the transcriptional co-activators, CBP (CREB-Binding Protein) and p300. The p300-CBP associated factor, PCAF (p300/CBP-Associated Factor) also binds to the C-terminal activation domain of Tax (Ref.1).

The biology of ATL cells is characterized by increased expression of genes coding for lymphokines and lymphokine receptors. Expression of these genes is in part regulated by the NF-KappaB family of transcription factors. Tax generally activates cellular transcription through a NF-KappaB-dependent pathway in a manner distinct from its activation of CREB. The most frequently observed NF-KappaB form is that of a p65-p50 dimer, which is ambiently retained in the cytoplasm by I-KappaB (Inhibitor of Kappa Light Chain Gene Enhancer in B-Cells) molecules (Ref.9). In the commonly accepted paradigm, I-KappaB-Alpha and I-KappaB-Beta play major roles in sequestering p65-p50 dimer in the cytoplasm. Phosphorylated I-KappaB-Alpha and I-KappaB-Beta subunits are then targeted by Ubiquitin ligase component protein, Beta-TRCP (Beta-Transducin Repeat-Containing Protein); this interaction leads to the Ubiquination and proteosomal degradation of the I-KappaBs. Upon removal of I-KappaBs , NF-KappaB-molecules are freed to migrate from the cytoplasm into the nucleus. Within the nucleus, NF-KappaBs bind promoter upstream DNA-motifs and activate the transcription of a diverse subset of target genes. Because of Tax’s predominant nuclear localization in cells, an initial mechanistic explanation for Tax-NF-KappaB interplay invokes events within the nucleus (Ref.1).

Tax makes a contact with I-KappaB-molecules through similar ankyrin-motifs, thereby dissociating I-KappaBs from cytoplasmically sequestered NF-KappaBs. This is the contact-dependent dissociation mechanism through which Tax targets I-KappaBs for proteosomal degradation. Besides the dissociation of I-KappaB from NF-KappaB, another general mechanism of NF-KappaB activation describes site-specific phosphorylation of I-KappaB-Alpha, followed by its ubiquitination and degradation (Ref.1). Biochemically, over-expressed Tax protein is present within the large intracellular IKK-Alpha (Inhibitor of Kappa Light Polypeptide Gene Enhancer in B-Cells Kinase of Alpha), IKK-Beta (Inhibitor of Kappa Light Chain Gene Enhancer in B-Cells Kinase of Beta) and IKK-Gamma (Inhibitor of Kappa Light Chain Gene Enhancer in B-Cells Kinase of Gamma) complex. It is IKK-Gamma that binds Tax directly. Thus, IKK-Gamma functionally adapts the Tax oncoprotein into the large IKK-Alpha/IKK-Beta/IKK-Gamma complex. Tax activation of NF-KappaB occurs at a point down-stream of the small G-Proteins and the TNFR (Tumor Necrosis Factor Receptor) interacting factors. At this juncture, Tax bridges the IKK-complex with a MAP3K. MAP3K, as recruited by Tax, then phosphorylates IKK-Alpha/Beta leading to a cascade of events which releases NF-KappaB for nuclear migration. Thus Tax activates the IKK and JNK (c-Jun Kinase) pathways in the cytoplasm. The JNK pathway leads to the activation of c-Jun and c-Fos. Tax activation of NF-KappaB contributes to cellular transformation (Ref.2).

Further signaling by Ifn-Gamma (Interferon-Gamma) stimulates anti-viral responses and tumor suppression through the heterodimeric Ifn-GammaR (Ifn-Gamma Receptor). Signaling is initiated by binding of Ifn-Gamma to its receptor, activating the receptor-associated JAK2 (Janus Kinase-2) tyrosine kinase to phosphorylate STAT (Signal Transducer and Activator of Transcription) transcription factors that activate Ifn (Interferon) responsive genes. Molecular chaperones that modulate or alter protein folding interact with different components of the Ifn Signaling Pathway. One chaperone that modulates Ifn signaling is DNAJA3 (DNAJ (HSP40) Homolog Subfamily-A Member-3) and a co-chaperone for the HSPA8 (Heat Shock-70 KD Protein-8)/HSP70 (Heat-Shock Protein 70-KD), another molecular chaperone. DNAJA3 binds to JAK2 and interacts with the Ifn-GammaR1 and Ifn-GammaR2. JAK2 and Ifn-GammaR2 associate with DNAJA3 and HSPA8/HSP70 to form a complex. DNAJA3 recruits HSPA8/HSP70 to the receptor complex. DNAJA3 acts as a co-chaperone causing a conformational change in HSPA8/HSP70 that allows it to interact with JAK2. The interaction of HSPA8/HSP70 with JAK2 then inhibits the kinase activity of JAK2 (Ref.10). DNAJA3 and HSPA8/HSP70 interact with other signaling proteins as well. One of this is Tax, a protein encoded by the HTLV1 virus that binds to DNAJA3. DNAJA3 represses NF-KappaB activation by blocking the phosphorylation and inactivation of I-KappaBs by the IKKs. One of the actions of Ifn (Interferons) is to induce apoptosis of infected target cells, in part through a mitochondrial dependent mechanism. An interaction between Ifn signaling and HSPA8/HSP70 alter the Mitochondrial Apoptosis Pathway, playing a role in Ifn-mediated apoptosis of infected or transformed cells. The HTLV1 Tax protein that interacts with HSPA8/HSP70 blocks mitochondrial induced apoptosis, providing a protection against Ifn-mediated cellular defenses. The biological activities of Ifn-Gamma mediated through the Ifn-GammaR are capable of activating the JAK/STAT Pathway. DNAJA3 acts as a negative modulator of the JAK/STAT pathway (Ref.11).

HTLV1 induces a rather weak growth transformation of T-cells in the laboratory but, is probably never sufficiently strong to induce T-cell leukaemia on its own. The virus naturally infects CD4+ T lymphocytes and can be transmitted between close contacts through blood transfer or from mother to infant through cells in breast milk. In most cases the infection is harmless. HTLV1 does not contain an oncogene and it is believed that the malignant change is the result of interruption and deregulation of host DNA (Ref.1).