Human T-Cell Lymphotropic Virus: A Model of NF-κB-Associated Tumorigenesis

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATL), whereas the highly related HTLV-2 is not associated with ATL or other cancers. In addition to ATL leukemogenesis, studies of the HTLV viruses also provide an exceptional model for understanding basic pathogenic mechanisms of virus-host interactions and human oncogenesis. Accumulating evidence suggests that the viral regulatory protein Tax and host inflammatory transcription factor NF-κB are largely responsible for the different pathogenic potentials of HTLV-1 and HTLV-2. Here, we discuss the molecular mechanisms of HTLV-1 oncogenic pathogenesis with a focus on the interplay between the Tax oncoprotein and NF-κB pro-oncogenic signaling. We also outline some of the most intriguing and outstanding questions in the fields of HTLV and NF-κB. Answers to those questions will greatly advance our understanding of ATL leukemogenesis and other NF-κB-associated tumorigenesis and will help us design personalized cancer therapies

Identification and characterization of novel NF-kB dependent genes involved in HTLV-I pathogenesis

The NF-kB transcription factor plays pivotal roles in the pathogenesis and therapy-resistance of human cancers, including adult T-cell leukemia (ATL) induced by the oncoretrovirus HTLV-I. However, the downstream target genes of NF-kB involved in cancer biology and therapy remain largely unknown. To address this important issue, we have developed a novel approach called subtraction-based complementary gene expression cloning strategy. Given the characteristic anti-apoptosis activity of cancer cells, we used this approach to identify NF-kB-dependent anti-apoptotic genes involved in HTLV-I oncogenesis. The principle of this strategy is that expression of anti-apoptotic genes induced by HTLV-I-activated NF-kB should protect normal T cells from apoptosis induced by death inducers such as FasL. Briefly, a subtractive cDNA retroviral library enriched in genes induced by HTLV-I-NF-kB was generated and used to infect FasL-sensitive T cells. The infected T cells were treated with FasL and G418 (selective marker of cDNA expression). The FasL-and G418-resistant clones were isolated by limiting dilution, and the functional genes involved in FasL-resistance were fished out by RT-PCR and DNA sequencing. Using this strategy, several known NF-kB-dependent apoptotic genes have been identified, such as IAP1, Bcl-xL, c-FLIP and DcR2, indicating the reliability of our approach. Notably, numerous novel NF-kB-dependent anti-apoptotic genes were also identified. One of these novel genes has been confirmed to be expressed highly in HTLV-I-transformed T cells and primary ATL cells, and can be induced in normal T cells by HTLV-I in an NF-kB-dependent manner. Our mechanistic studies further indicate that this novel protein binds to mitochondria and prevents FasL activation of Bid, Caspase 9 and Caspase 3 but not Caspase 8. Currently, we are actively investigating the pathophysiological role of this novel gene in the biology and therapy of ATL and other cancers associated with deregulated NF-kB

PDLIM2 restricts Th1 and Th17 differentiation and prevents autoimmune disease

Background: PDLIM2 is essential for the termination of the inflammatory transcription factors NF-κB and STAT but is dispensable for the development of immune cells and immune tissues/organs. Currently, it remains unknown whether and how PDLIM2 is involved in physiologic and pathogenic processes. Results: Here we report that naive PDLIM2 deficient CD4+ T cells were prone to differentiate into Th1 and Th17 cells. PDLIM2 deficiency, however, had no obvious effect on lineage commitment towards Th2 or Treg cells. Notably, PDLIM2 deficient mice exhibited increased susceptibility to experimental autoimmune encephalitis (EAE), a Th1 and/or Th17 cell-mediated inflammatory disease model of multiple sclerosis (MS). Mechanistic studies further indicate that PDLIM2 was required for restricting expression of Th1 and Th17 cytokines, which was in accordance with the role of PDLIM2 in the termination of NF-κB and STAT activation. Conclusion: These findings suggest that PDLIM2 is a key modulator of T-cell-mediated immune responses that may be targeted for the therapy of human autoimmune diseases

PDLIM2 is a novel E5 ubiquitin ligase enhancer that stabilizes ROC1 and recruits the ROC1-SCF ubiquitin ligase to ubiquitinate and degrade NF-κB RelA

Abstract The PDZ-LIM domain-containing protein PDLIM2 is a common tumor suppressor and a key immune modulator. One main function of PDLIM2 is to promote the ubiquitination and proteasomal degradation of nuclear activated NF-κB RelA, a physiologically indispensable transcription factor whose persistent activation has been linked to almost all cancer types and inflammation-associated diseases. However, it remains unknown how PDLIM2 exerts this physiologically and pathogenically important function. Here, we show that PDLIM2 acts as a ubiquitin ligase enhancer, termed E5. It stabilizes ROC1, an essential component of SKP1/Cullin/F-box protein (SCF) ubiquitin ligases, and chaperones the ROC1-SCFβ-TrCP ubiquitin ligase to ubiquitinate nuclear RelA for proteasomal degradation in the nucleus. Consistently, silencing of ROC1, Cullin 1 or the F-box protein β-TrCP blocks RelA ubiquitination and degradation by PDLIM2. These data provide new mechanistic insights into how PDLIM2 promotes nuclear RelA ubiquitination and degradation, thereby serving as a critical tumor suppressor and a vital immune regulator. They also improve our understanding of the complex cascade of the ubiquitination and NF-κB pathways, particularly given the well-known role of the ROC1-SCFβ-TrCP ubiquitin ligase in initiating NF-κB activation by directly binding to and ubiquitinating NF-κB inhibitors for the proteasomal degradation in the cytoplasm

SUMO1 modification of NF-κB2/p100 is essential for stimuli-induced p100 phosphorylation and processing

A primary step in activating the alternative nuclear factor-κB (NF-κB) pathway requires NF-κB2/p100 processing to generate p52. In most cases, stimuli-induced p100 processing is dependent on NF-κB-inducing kinase/IκB kinase α-mediated phosphorylation and ubiquitination. Here, we report that post-translational modification of p100 at specific sites by the small ubiquitin-like modifier (SUMO) is another determining factor for stimuli-induced p100 processing. The results show that basal SUMO modification is required for stimuli-induced p100 phosphorylation and that blocking SUMOylation of p100, either by site-directed mutation or by short interfering RNA-targeted diminution of E2 SUMO-conjugating enzyme Ubc9, inhibits various physiological stimuli-induced p100 processing and ultimate activation of the alternative NF-κB pathway. Together, these findings show the crucial role of SUMO1 modification in p100 processing and provide mechanistic insights into the participation of SUMO1 modification in the regulation of signal transduction