The ubiquitin-proteasome system (UPS) is involved in multiple aspects of cellular processes, such as cell cycle progression, cellular differentiation, and survival (Davis RJ et al., Cancer Cell 26:455-64, 2014; Skaar JR et al., Nat Rev Drug Discov 13:889-903, 2014; Nakayama KI and Nakayama K, Nat Rev Cancer 6:369-81, 2006). F-box and WD repeat domain containing 7 (FBXW7), also known as Sel10, hCDC4 or hAgo, is a member of the F-box protein family, which functions as the substrate recognition component of the SCF E3 ubiquitin ligase. FBXW7 is a critical tumor suppressor and one of the most commonly deregulated ubiquitin-proteasome system proteins in human cancer. FBXW7 controls proteasome-mediated degradation of oncoproteins such as cyclin E, c-Myc, Mcl-1, mTOR, Jun, Notch and AURKA. Consistent with the tumor suppressor role of FBXW7, it is located at chromosome 4q32, a genomic region deleted in more than 30% of all human cancers (Spruck CH et al., Cancer Res 62:4535-9, 2002). Genetic profiles of human cancers based on high-throughput sequencing have revealed that FBXW7 is frequently mutated in human cancers. In addition to genetic mutations, other mechanisms involving microRNA, long non-coding RNA, and specific oncogenic signaling pathways can inactivate FBXW7 functions in cancer cells. In the following sections, we will discuss the regulation of FBXW7, its role in oncogenesis, and the clinical implications and prognostic value of loss of function of FBXW7 in human cancers.

Human T-cell leukemia virus type-I (HTLV-I

The progressive loss of immunological memory during aging correlates with a reduced proliferative capacity and shortened telomeres of T cells. Growing evidence suggests that this phenotype is recapitulated during chronic viral infection. The antigenic volume imposed by persistent and latent viruses exposes the immune system to unique challenges that lead to host T-cell exhaustion, characterized by impaired T-cell functions. These dysfunctional memory T cells lack telomerase, the protein capable of extending and stabilizing chromosome ends, imposing constraints on telomere dynamics. A deleterious consequence of this excessive telomere shortening is the premature induction of replicative senescence of viral-specific CD8+ memory T cells. While senescent cells are unable to expand, they can survive for extended periods of time and are more resistant to apoptotic signals. This review takes a closer look at T-cell exhaustion in chronic viruses known to cause human disease: Epstein–Barr virus (EBV), Hepatitis B/C/D virus (HBV/HCV/HDV), human herpesvirus 8 (HHV-8), human immunodeficiency virus (HIV), human T-cell leukemia virus type I (HTLV-I), human papillomavirus (HPV), herpes simplex virus-1/2 (HSV-1/2), and Varicella–Zoster virus (VZV). Current literature linking T-cell exhaustion with critical telomere lengths and immune senescence are discussed. The concept that enduring antigen stimulation leads to T-cell exhaustion that favors telomere attrition and a cell fate marked by enhanced T-cell senescence appears to be a common endpoint to chronic viral infections.

The antiviral thymidine analog azidothymidine (AZT) is used to treat several virusassociated human cancers. However, to date the mechanism of AZT action remains unclear and thus, reasons for treatment failure are unknown. Adult T-cell leukemia/lymphoma (ATL) is an aggressive malignancy of poor prognosis. Here, we report that enduring AZT treatment of T-cell leukemia virus I-infected cells, in vitro and in vivo in ATL patients, results in inhibition of telomerase activity, progressive telomere shortening, and increased p14 ARF expression. In turn, this elicits stabilization and reactivation of the tumor suppressor p53-dependent transcription, increased expression of the cyclin-dependent kinase inhibitor p21 Waf1 , and accumulation of p27 kip1 , thereby inducing cellular senescence and tumor cell death. While ATL patients carrying a wild-type p53 enter remission following treatment with AZT, those with a mutated p53 did not respond, and patients' disease relapse was associated with the selection of a tumor clone carrying mutated inactive p53.

Human tumor viruses are responsible for one-fifth of all cancers worldwide. These viruses have evolved multiple strategies to evade immune defenses and to persist in the host by establishing a latent infection. Proliferation is necessary for pretumor cells to accumulate genetic alterations and to acquire a transformed phenotype. However, each cell division is associated with a progressive shortening of the telomeres, which can suppress tumor development by initiating senescence and irreversible cell cycle arrest. Therefore, the ability of virus-infected cells to circumvent the senescence program is essential for the long-term survival and proliferation of infected cells and the likelihood of transformation. We review the multiple strategies used by human DNA and RNA tumor viruses to subvert telomerase functions during cellular transformation and carcinogenesis. Epstein-Barr virus, Kaposi sarcoma-associated herpesvirus, human papillomavirus, hepatitis B virus, hepatitis C virus, and human T-cell leukemia virus-1 each can increase transcription of the telomerase reverse transcriptase. Several viruses appear to mediate cis-activation or enhance epigenetic activation of telomerase transcription. Epstein-Barr virus and human papillomavirus have each developed posttranscriptional mechanisms to regulate the telomerase protein. Finally, some tumor virus proteins can also negatively regulate telomerase transcription or activity. It is likely that, as future studies further expose the strategies used by viruses to deregulate telomerase activity and control of telomere length, novel mechanisms will emerge and underscore the importance of increased telomerase activity in sustaining virus-infected cells and its potential in therapeutic targeting.

Human T-cell leukemia virus type I (HTLV-I IntroductionHuman T-cell leukemia virus type I (HTLV-I) transforms human CD4 ϩ T cells in vitro and in vivo causing adult T-cell leukemia/ lymphoma (ATL). 1 ATL has an extremely poor prognosis for survival, characterized by an aggressive proliferation of leukemic cells. 2 The molecular basis for HTLV-I-mediated transformation of T cells is unclear. Tax has been shown to inactivate several tumor suppressors, disrupt cell cycle and DNA repair checkpoints, and stimulate cell growth, while protecting against apoptosis. [3][4][5][6][7] Importantly, the oncogenic activities of Tax have been studied mainly in transgenic models and in rodent fibroblasts. [8][9][10] Although these models provided useful information, they have limitations, because the transformation of human cells in vitro requires more oncogenic events than the transformation of rodent cells. 11 The role of Tax in transformation is based largely on forced overexpression of Tax, which results in cell-cycle abnormalities, faulty mitosis, aneuploidy, and the formation of multinucleated cells. A major caveat to these studies is that they were performed in established cell-culture lines, which are already dysfunctional in key regulatory pathways (tumor suppressor, cell cycle, and DNA repair checkpoints), and therefore are much more susceptible than normal primary cells to Tax-induced alterations. Studying Tax in human primary T cells is preferable, yet, to date, immortalized Tax-expressing human T-cell lines able to proliferate indefinitely in culture have not been established. In one study, the pX region of HTLV-I (which expresses the Tax gene among other viral genes) was found to transform human T cells in the context of a Herpesvirus saimiri vector, 12 although there is no evidence to refute that the herpes genes from the vector synergized or were activated by Tax to promote transformation or that other viral genes were involved. This prompted us to investigate the ability of Tax to immortalize human primary T cells. In the following study, we demonstrate that Tax-induced immortalization of peripheral blood mononuclear cells (PBMCs) is a very rare event. Tax alone enhanced proliferation, but appears insufficient to sustain permanent proliferation. Although most attempts were unsuccessful, we finally obtained an immortalized Tax-expressing CD4 human T-cell line, WT4, which was maintained in continuous culture for more than 4 years. Our results suggest that Tax alone is a poor oncogene that increases the life span and rate of T-cell proliferation in the presence of interleukin-2 (IL-2), thereby increasing the probability for accumulation of genetic defects leading to immortalization. Methods Plasmids and cell linesThe HTLV-I tax gene was cloned into the HRCMV or HRЈ lentiviral vectors ( Figure 1A). Pseudotype virus particles were produced as previously reported. 13 For cell-cycle analysis, tax was cloned into an UBC-IRES-GFP lentiviral vector. The p53 gene of WT4 was sequenced and cloned as previously described. 14 ...

Here, we report that freshly isolated unstimulated adult T-cell leukemia (ATL) cells present high telomerase activity compared to asymptomatic carriers or normal donors. In spite of this high telomerase activity, ATL cells retained shorter telomeres compared to those of uninfected cells isolated from the same patients. Because the safeguarding of telomere length is critical to the unlimited proliferation of tumor cells, we investigated the underlying mechanism for short telomere maintenance in ATL cells. Transcriptional and posttranscriptional expression of telomere-binding proteins TRF1, TRF2, TIN2 and POT1, known to regulate telomere homeostasis and protection, were evaluated. We found that TRF1 and TRF2 are overexpressed in in vivo patient's samples from ATL but not asymptomatic carriers, while levels of POT1 expression did not specifically increase in ATL. To gain insights into the regulation of TRF genes in HTLV-I infected cells, we investigated the expression of TIN2, a regulator of these genes, and found an increase in TIN2 expression in ATL patients. Together our results underscore the importance of telomerase and telomere length regulating factors as novel markers for ATL disease progression and as potential therapeutic targets for the treatment of HTLV-I-associated malignancies. ' 2006 Wiley-Liss, Inc.Key words: HTLV-I; ATL; telomerase; telomere; TRF1; leukemia Human T-cell leukemia virus type 1 (HTLV-I) is the etiological agent of adult T-cell leukemia/lymphoma (ATL), an aggressive and fatal lymphoproliferative disorder.1,2 The poor prognosis of ATL patients is associated with the resistance of neoplastic cells to the conventional combination of high-dose chemotherapy and radiotherapy. 3 The mechanism by which HTLV-I engenders ATL is not clear, but the long latency period of several decades, preceding the disease, suggests that it relies upon long term survival and proliferation of virus-infected cells. [4][5][6] In fact, previous studies have shown that proliferation of ATL cells occurs mainly by replication of infected cells, leading to oligoclonal or monoclonal expansion.4-6 HTLV-I has evolved regulatory mechanisms to usurp cell cycle checkpoints 7-12 and apoptosis regulators, [13][14][15][16][17] and infected cells may escape from immune defenses, using a combinatorial effect of the regulatory proteins p30 and p12 that reduce viral expression and downregulate major histocompatibility complex expression, respectively. 18,19The DNA polymerase is unable to replicate telomeric structures, and thus each cellular division results in a progressive shortening of the telomere length. While telomere shortening may be associated with increased chromosomal instability and may facilitate carcinogenesis, it can also lead to irreversible senescence. Although in most cancer cells avoidance of telomere shortening beyond a critical size requires reactivation of telomerase expression, the sole expression of telomerase may not be sufficient for telomere elongation because of the ''open or closed'' nature of the telo...

Human T-cell leukemia virus type 1 (HTLV-I) is associated with adult T-cell leukemia (ATL), an aggressive lymphoproliferative disease with a dismal prognosis. We have previously described the presence of Notch1 activating mutations and constitutive Notch1 signaling in patients with acute ATL. In this study, we report a high frequency of F-box and WD repeat domain containing 7 (FBXW7)/hCDC4 mutations within the WD40 substrate-binding domain in 8 of 32 acute ATL patients (25%). Functionally, ATL FBXW7 mutants lost their ability to interact with intracellular Notch (NICD), resulting in increased protein stability and constitutive Notch1 signaling. Consistent with the loss-of-function found in ATL patients, expression of WT FBXW7 in several patient-derived ATL lines demonstrated strong tumor-suppressor activity characterized by reduced proliferation of ATL cells. Remarkably, two FBXW7 mutants, D510E and D527G, demonstrated oncogenic activity when expressed in the presence of HTLV-I Tax, mutated p53 R276H, or c-Myc F138C found in human cancers. Transforming activity was further demonstrated by the ability of the FBXW7 D510E mutant to provide IL-2–independent growth of Tax-immortalized human T cells and increase the tumor formation in a xenograft mouse model of ATL. This study suggests that FBXW7, normally a tumor suppressor, can act as an oncogene when mutated and may play an important role in the pathogenesis of ATL.