The HTLV-1 HBZ protein inhibits cyclin D1 expression through interacting with the cellular transcription factor CREB

The role of CREB and MAPK signaling pathways in ATLL patients

BACKGROUND

HTLV-1 is a worldwide distribution retrovirus with 10-20 million infected individuals. ATLL is an Adult T-cell leukaemia lymphoma caused by aggressive T-cell proliferation that is infected by HTLV-1 and is associated with an inferior prognosis. The exact molecular pathogenesis has yet to be fully understood. CREB, a transcription factor, acts as a molecular switch that controls the expression of numerous genes in response to various extracellular signals. Its activation is primarily mediated through phosphorylation by multiple kinases, including MAPKs. MAPKs, a family of serine/threonine kinases, serve as crucial mediators of intracellular signaling cascades.

METHOD AND MATERIAL

This study investigated, 38 HTLV-I-infected individuals, including 18 HTLV-1 asymptomatic carriers (ACs) and 20 ATLL subjects. mRNA was extracted and converted to cDNA from Peripheral blood mononuclear cells (PBMCs), and then the expression of TAX, HBZ, CREB, and MAPK was analyzed by TaqMan qPCR. The genomic HTLV-1 Proviral loads were examined among the study group.

RESULTS

The data analysis showed a significant difference in the mean of CREB expression amongst study groups (ATLL and carriers, (p = 0.002). There is no statistical difference between the MAPK gene expression (p = 0.35). HBZ, TAX, and HTLV-1 proviral load weree significantly higher in ATLL subjects compared to  ACs  (p = 0.002, 0.000, and 0.000), respectively. Moreover, our results, demonstrated a direct positive correlation among HBZ, CREB, and TAX gene expression in ATLL patients (p = 0.001), whilst between the  ACs, TAX gene expression had a positive significant correlation with HBZ and HTLV-1 proviral load (p = 0.007 and p = 0.004, respectively).

CONCLUSION

The present study demonstrated that CREB gene expression was higher in the ATLL group than ACs, while there was no difference for MAPK. Therefore, this pathway may not strongly involve in the activation of CREB. The CREB may be a prognostic factor for the development of HTLV-I-associated diseases and can be used as a monitoring marker for the efficiency of the therapeutic regime and prognosis.

YAP suppresses human T-cell leukemia virus type 1 transcription

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that causes adult T-cell leukemia/lymphoma (ATL). HTLV-1 encodes Tax protein that activates transcription from viral long terminal repeats (LTR). Multiple cofactors are involved in the regulation of HTLV-1 transcription via association with Tax. Yes-associated protein (YAP), which is the key effector of Hippo pathway, is elevated and activated in ATL cells. In this study, we reported that YAP protein suppressed Tax activation of HTLV-1 5' LTR but not 3' LTR. The activation of the 5' LTR by Tax was potentiated when YAP was depleted. Moreover, overexpression of YAP repressed HTLV-1 plus-strand viral gene expression and virion production, whereas compromising YAP by RNA inference augmented the expression of HTLV-1 protein. As mechanisms of YAP-mediated viral transcription inhibition, we found that YAP interacted with Tax, and prevented the association between Tax and p300. It finally led to the inhibition of recruitment of Tax to the Tax-responsive element in the 5' LTR of HTLV-1. Taken together, our results demonstrate the negative regulatory function of YAP in Tax activation of HTLV-1 transcription. It may achieve sufficient transcriptional repression to maintain persistent infection and long-term latency of HTLV-1 in the host cells.

The regulation of cyclin D promoters - review

Cyclins are key regulators of cell cycle progression and survival. Particularly cyclins D (cyclin D1, D2, and D3) act in response to the mitogenic stimulation and are pivotal mediators between proliferative pathways and the nuclear cell cycle machinery. Dysregulation of cyclins expression results in impaired development, abnormal cell growth or tumorigenesis. In this review we summarize current knowledge about regulatory role of the cyclin D promoters, transcriptional factors: regulators, co-activators and adaptor proteins necessary to their activation. We focused on the intracellular signaling pathways vital to cell growth, differentiation and apoptosis including transcription factor families: activator protein 1 (AP1), nuclear factor (NFκB), signal transducer and activator of transcription (STAT), cAMP response element-binding protein (CREB) and Sp/NF-Y, with a special insight into the tissue specific cyclin representation.

The bZIP Proteins of Oncogenic Viruses

Basic leucine zipper (bZIP) transcription factors (TFs) govern diverse cellular processes and cell fate decisions. The hallmark of the leucine zipper domain is the heptad repeat, with leucine residues at every seventh position in the domain. These leucine residues enable homo- and heterodimerization between ZIP domain α-helices, generating coiled-coil structures that stabilize interactions between adjacent DNA-binding domains and target DNA substrates. Several cancer-causing viruses encode viral bZIP TFs, including human T-cell leukemia virus (HTLV), hepatitis C virus (HCV) and the herpesviruses Marek’s disease virus (MDV), Epstein–Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV). Here, we provide a comprehensive review of these viral bZIP TFs and their impact on viral replication, host cell responses and cell fate.

HTLV-1 basic leucine zipper factor downregulates cyclin D1 expression via interactions with NF-κB

Human T cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus. It can cause adult T cell leukemia (ATL) and other diseases. The HTLV-1 basic leucine zipper (bZIP) factor (HBZ), which is encoded by the minus-strand of the provirus, is expressed in all cases of ATL and involved in T cell proliferation. However, the exact mechanism underlying its growth-promoting activity is poorly understood. Herein, we demonstrated that HBZ suppressed cyclin D1 expression by inhibiting the nuclear factor (NF)-κB signaling pathway. Among the potential mechanisms of cyclin D1 inhibition mediated by HBZ, we found that HBZ suppressed cyclin D1 promoter activity. Luciferase assay analysis revealed that HBZ repressed cyclin D1 promoter activity by suppressing NF-κB‑driven transcription mediated by the p65 subunit. Using an immunoprecipitation assay, we found that HBZ could bind to p65, but not p50. Finally, we showed that HBZ selectively interacted with p65 via its AD+bZIP domains. By suppressing cyclin D1 expression, HBZ can alter cell cycle progression of HTLV-1-infected cells, which may be critical for oncogenesis.

The Role of HBZ in HTLV-1-Induced Oncogenesis

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL) and chronic inflammatory diseases. HTLV-1 bZIP factor (HBZ) is transcribed as an antisense transcript of the HTLV-1 provirus. Among the HTLV-1-encoded viral genes, HBZ is the only gene that is constitutively expressed in all ATL cases. Recent studies have demonstrated that HBZ plays an essential role in oncogenesis by regulating viral transcription and modulating multiple host factors, as well as cellular signaling pathways, that contribute to the development and continued growth of cancer. In this article, I summarize the current knowledge of the oncogenic function of HBZ in cell proliferation, apoptosis, T-cell differentiation, immune escape, and HTLV-1 pathogenesis.

Molecular Studies of HTLV-1 Replication: An Update

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus discovered. Studies on HTLV-1 have been instrumental for our understanding of the molecular pathology of virus-induced cancers. HTLV-1 is the etiological agent of an adult T-cell leukemia (ATL) and can lead to a variety of neurological pathologies, including HTLV-1-associated-myelopathy/tropical spastic paraparesis (HAM/TSP). The ability to treat the aggressive ATL subtypes remains inadequate. HTLV-1 replicates by (1) an infectious cycle involving virus budding and infection of new permissive target cells and (2) mitotic division of cells harboring an integrated provirus. Virus replication initiates host antiviral immunity and the checkpoint control of cell proliferation, but HTLV-1 has evolved elegant strategies to counteract these host defense mechanisms to allow for virus persistence. The study of the molecular biology of HTLV-1 replication has provided crucial information for understanding HTLV-1 replication as well as aspects of viral replication that are shared between HTLV-1 and human immunodeficiency virus type 1 (HIV-1). Here in this review, we discuss the various stages of the virus replication cycle—both foundational knowledge as well as current updates of ongoing research that is important for understanding HTLV-1 molecular pathogenesis as well as in developing novel therapeutic strategies.

Modes of Human T Cell Leukemia Virus Type 1 Transmission, Replication and Persistence

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that causes cancer (Adult T cell Leukemia, ATL) and a spectrum of inflammatory diseases (mainly HTLV-associated myelopathy—tropical spastic paraparesis, HAM/TSP). Since virions are particularly unstable, HTLV-1 transmission primarily occurs by transfer of a cell carrying an integrated provirus. After transcription, the viral genomic RNA undergoes reverse transcription and integration into the chromosomal DNA of a cell from the newly infected host. The virus then replicates by either one of two modes: (i) an infectious cycle by virus budding and infection of new targets and (ii) mitotic division of cells harboring an integrated provirus. HTLV-1 replication initiates a series of mechanisms in the host including antiviral immunity and checkpoint control of cell proliferation. HTLV-1 has elaborated strategies to counteract these defense mechanisms allowing continuous persistence in humans.