W Gao

LKB1 tumor suppressor and salt-inducible kinases negatively regulate human T-cell leukemia virus type 1 transcription

BackgroundHuman T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL). Treatment options are limited and prophylactic agents are not available. We have previously demonstrated an essential role for CREB-regulating transcriptional coactivators (CRTCs) in HTLV-1 transcription.ResultsIn this study we report on the negative regulatory role of LKB1 tumor suppressor and salt-inducible kinases (SIKs) in the activation of HTLV-1 long terminal repeats (LTR) by the oncoprotein Tax. Activation of LKB1 and SIKs effectively blunted Tax activity in a phosphorylation-dependent manner, whereas compromising these kinases, but not AMP-dependent protein kinases, augmented Tax function. Activated LKB1 and SIKs associated with Tax and suppressed Tax-induced LTR activation by counteracting CRTCs and CREB. Enforced expression of LKB1 or SIK1 in cells transfected with HTLV-1 molecular clone pX1MT repressed proviral transcription. On the contrary, depletion of LKB1 in pX1MT-transfected cells and in HTLV-1-transformed T cells boosted the expression of Tax. Treatment of HTLV-1 transformed cells with metformin led to LKB1/SIK1 activation, reduction in Tax expression, and inhibition of cell proliferation.ConclusionsOur findings revealed a new function of LKB1 and SIKs as negative regulators of HTLV-1 transcription. Pharmaceutical activation of LKB1 and SIKs might be considered as a new strategy in anti-HTLV-1 and anti-ATL therapy.

SIRT1 suppresses human T-cell leukemia virus type 1 transcription

ABSTRACT Human T-cell leukemia virus type 1 (HTLV-1)-associated diseases are poorly treatable, and HTLV-1 vaccines are not available. High proviral load is one major risk factor for disease development. HTLV-1 encodes Tax oncoprotein, which activates transcription from viral long terminal repeats (LTR) and various types of cellular promoters. Counteracting Tax function might have prophylactic and therapeutic benefits. In this work, we report on the suppression of Tax activation of HTLV-1 LTR by SIRT1 deacetylase. The transcriptional activity of Tax on the LTR was largely ablated when SIRT1 was overexpressed, but Tax activation of NF-κB was unaffected. On the contrary, the activation of the LTR by Tax was boosted when SIRT1 was depleted. Treatment of cells with resveratrol shunted Tax activity in a SIRT1-dependent manner. The activation of SIRT1 in HTLV-1-transformed T cells by resveratrol potently inhibited HTLV-1 proviral transcription and Tax expression, whereas compromising SIRT1 by specific inhibitors augmented HTLV-1 mRNA expression. The administration of resveratrol also decreased the production of cell-free HTLV-1 virions from MT2 cells and the transmission of HTLV-1 from MT2 cells to uninfected Jurkat cells in coculture. SIRT1 associated with Tax in HTLV-1-transformed T cells. Treatment with resveratrol prevented the interaction of Tax with CREB and the recruitment of CREB, CRTC1, and p300 to Tax-responsive elements in the LTR. Our work demonstrates the negative regulatory function of SIRT1 in Tax activation of HTLV-1 transcription. Small-molecule activators of SIRT1 such as resveratrol might be considered new prophylactic and therapeutic agents in HTLV-1-associated diseases. IMPORTANCE Human T-cell leukemia virus type 1 (HTLV-1) causes a highly lethal blood cancer or a chronic debilitating disease of the spinal cord. Treatments are unsatisfactory, and vaccines are not available. Disease progression is associated with robust expression of HTLV-1 genes. Suppressing HTLV-1 gene expression might have preventive and therapeutic benefits. It is therefore critical that host factors controlling HTLV-1 gene expression be identified and characterized. This work reveals a new host factor that suppresses HTLV-1 gene expression and a natural compound that activates this suppression. Our findings not only provide new knowledge of the host control of HTLV-1 gene expression but also suggest a new strategy of using natural compounds for prevention and treatment of HTLV-1-associated diseases.

β-TrCP-mediated ubiquitination and degradation of liver-enriched transcription factor CREB-H

CREB-H is an endoplasmic reticulum-resident bZIP transcription factor which critically regulates lipid homeostasis and gluconeogenesis in the liver. CREB-H is proteolytically activated by regulated intramembrane proteolysis to generate a C-terminally truncated form known as CREB-H-ΔTC, which translocates to the nucleus to activate target gene expression. CREB-H-ΔTC is a fast turnover protein but the mechanism governing its destruction was not well understood. In this study, we report on β-TrCP-dependent ubiquitination and proteasomal degradation of CREB-H-ΔTC. The degradation of CREB-H-ΔTC was mediated by lysine 48-linked polyubiquitination and could be inhibited by proteasome inhibitor. CREB-H-ΔTC physically interacted with β-TrCP, a substrate recognition subunit of the SCFβ-TrCP E3 ubiquitin ligase. Forced expression of β-TrCP increased the polyubiquitination and decreased the stability of CREB-H-ΔTC, whereas knockdown of β-TrCP had the opposite effect. An evolutionarily conserved sequence, SDSGIS, was identified in CREB-H-ΔTC, which functioned as the β-TrCP-binding motif. CREB-H-ΔTC lacking this motif was stabilized and resistant to β-TrCP-induced polyubiquitination. This motif was a phosphodegron and its phosphorylation was required for β-TrCP recognition. Furthermore, two inhibitory phosphorylation sites close to the phosphodegron were identified. Taken together, our work revealed a new intracellular signaling pathway that controls ubiquitination and degradation of the active form of CREB-H transcription factor.

Interplay between SIRT1 and hepatitis B virus X protein in the activation of viral transcription

Hepatitis B virus (HBV) genome is organized into a minichromosome known as covalently closed circular DNA (cccDNA), which serves as the template for all viral transcripts. SIRT1 is an NAD+-dependent protein deacetylase which activates HBV transcription by promoting the activity of cellular transcription factors and coactivators. How SIRT1 and viral transactivator X protein (HBx) might affect each other remains to be clarified. In this study we show synergy and mutual dependence between SIRT1 and HBx in the activation of HBV transcription. All human sirtuins SIRT1 through SIRT7 activated HBV gene expression. The steady-state levels of SIRT1 protein were elevated in HBV-infected liver tissues and HBV-replicating hepatoma cells. SIRT1 interacted with HBx and potentiated HBx transcriptional activity on precore promoter and covalently closed circular DNA (cccDNA) likely through a deacetylase-independent mechanism, leading to more robust production of cccDNA, pregenomic RNA and surface antigen. SIRT1 and HBx proteins were more abundant when both were expressed. SIRT1 promoted the recruitment of HBx as well as cellular transcriptional factors and coactivators such as PGC-1α and FXRα to cccDNA. Depletion of SIRT1 suppressed HBx recruitment. On the other hand, SIRT1 recruitment to cccDNA was compromised when HBx was deficient. Whereas pharmaceutical agonists of SIRT1 such as resveratrol activated HBV transcription, small-molecule inhibitors of SIRT1 including sirtinol and Ex527 exhibited anti-HBV activity. Taken together, our findings revealed not only the interplay between SIRT1 and HBx in the activation of HBV transcription but also new strategies and compounds for developing antivirals against HBV.

Unmasking a novel disease gene NEO1 associated with autism spectrum disorders by a hemizygous deletion on chromosome 15 and a functional polymorphism

BACKGROUND Autism spectrum disorders (ASD) are neurodevelopmental disorders with a high degree of heritability, but the genetic basis is exceedingly heterogeneous. Microdeletions of chromosome 15q24 have been demonstrated to be recurrent genomic alterations in ASD patients. Of interest, neuronal migration genes are of particular relevance to the pathogenesis of ASD. NEO1 is located in 15q24 and encodes for neogenin, a membrane receptor involved in cortical interneuron migration and axon guidance. We postulated that the ASD patient has one copy of the NEO1 gene deleted and the other copy disrupted by intragenic mutation. RESULTS We identify genetic changes in both alleles of NEO1 in two individuals from a cohort of 66 Han Chinese patients with ASD. In one patient, we detected a hemizygous 1.97-Mb deletion at 15q23q24.1 encompassing the NEO1 gene, a missense variant in NEO1, c.3388C>T (p.Arg1130Cys), and a duplication, c.2204-14_2204-2dup, in the acceptor splice site of intron 14 of NEO1. Furthermore, we identified a second patient was a compound heterozygote for NEO1. A novel missense variant in NEO1, c.302G>A (p.Arg101His), in addition to c.3388C>T and c.2204-14_2204-2dup was detected in the second patient. The c.3388C>T is a single nucleotide polymorphism with allele frequency of 0.045 in Han Chinese individuals. In silico and functional analyses indicated that p.Arg1130Cys, located at the nuclear localization signal (NLS) domain of neogenin led to defective nuclear translocation of neogenin. CONCLUSIONS The hemizygous 15q deletion unmasks the recessive functional polymorphism in NEO1 which plays a pivotal role in cortical interneuron development. Our study provides the first evidence linking NEO1 with ASD in humans.

A novel transcript isoform of STING that sequesters cGAMP and dominantly inhibits innate nucleic acid sensing

Abstract STING is a core adaptor in innate nucleic acid sensing in mammalian cells, on which different sensing pathways converge to induce type I interferon (IFN) production. Particularly, STING is activated by 2′3′-cGAMP, a cyclic dinucleotide containing mixed phosphodiester linkages and produced by cytoplasmic DNA sensor cGAS. Here, we reported on a novel transcript isoform of STING designated STING-β that dominantly inhibits innate nucleic acid sensing. STING-β without transmembrane domains was widely expressed at low levels in various human tissues and viral induction of STING-β correlated inversely with IFN-β production. The expression of STING-β declined in patients with lupus, in which type I IFNs are commonly overproduced. STING-β suppressed the induction of IFNs, IFN-stimulated genes and other cytokines by various immunostimulatory agents including cyclic dinucleotides, DNA, RNA and viruses, whereas depletion of STING-β showed the opposite effect. STING-β interacted with STING-α and antagonized its antiviral function. STING-β also interacted with TBK1 and prevented it from binding with STING-α, TRIF or other transducers. In addition, STING-β bound to 2′3′-cGAMP and impeded its binding with and activation of STING-α, leading to suppression of IFN-β production. Taken together, STING-β sequesters 2′3′-cGAMP second messenger and other transducer molecules to inhibit innate nucleic acid sensing dominantly.

Inhibition of AIM2 inflammasome activation by a novel transcript isoform of IFI16

Mouse p202 is a disease locus for lupus and a dominant‐negative inhibitor of AIM2 inflammasome activation. A human homolog of p202 has not been identified so far. Here, we report a novel transcript isoform of human IFI16‐designated IFI16‐β, which has a domain architecture similar to that of mouse p202. Like p202, IFI16‐β contains two HIN domains, but lacks the pyrin domain. IFI16‐β is ubiquitously expressed in various human tissues and cells. Its mRNA levels are also elevated in leukocytes of patients with lupus, virus‐infected cells, and cells treated with interferon‐β or phorbol ester. IFI16‐β co‐localizes with AIM2 in the cytoplasm, whereas IFI16‐α is predominantly found in the nucleus. IFI16‐β interacts with AIM2 to impede the formation of a functional AIM2‐ASC complex. In addition, IFI16‐β sequesters cytoplasmic dsDNA and renders it unavailable for AIM2 sensing. Enforced expression of IFI16‐β inhibits the activation of AIM2 inflammasome, whereas knockdown of IFI16‐β augments interleukin‐1β secretion triggered by dsDNA but not dsRNA. Thus, cytoplasm‐localized IFI16‐β is functionally equivalent to mouse p202 that exerts an inhibitory effect on AIM2 inflammasome.

Suppression of gluconeogenic gene transcription by SIK1-induced ubiquitination and degradation of CRTC1

CRTCs are a group of three transcriptional coactivators required for CREB-dependent transcription. CREB and CRTCs are critically involved in the regulation of various biological processes such as cell proliferation, metabolism, learning and memory. However, whether CRTC1 efficiently induces gluconeogenic gene expression and how CRTC1 is regulated by upstream kinase SIK1 remain to be understood. In this work, we demonstrated SIK1-induced phosphorylation, ubiquitination and degradation of CRTC1 in the context of the regulation of gluconeogenesis. CRTC1 protein was destabilized by SIK1 but not SIK2 or SIK3. This effect was likely mediated by phosphorylation at S155, S167, S188 and S346 residues of CRTC1 followed by K48-linked polyubiquitination and proteasomal degradation. Expression of gluconeogenic genes such as that coding for phosphoenolpyruvate carboxykinase was stimulated by CRTC1, but suppressed by SIK1. Depletion of CRTC1 protein also blocked forskolin-induced gluconeogenic gene expression, knockdown or pharmaceutical inhibition of SIK1 had the opposite effect. Finally, SIK1-induced ubiquitination of CRTC1 was mediated by RFWD2 ubiquitin ligase at a site not equivalent to K628 in CRTC2. Taken together, our work reveals a regulatory circuit in which SIK1 suppresses gluconeogenic gene transcription by inducing ubiquitination and degradation of CRTC1. Our findings have implications in the development of new antihyperglycemic agents.

Suppression of HTLV-1 transcription by SIRT1 deacetylase

Infection with HTLV-1 causes adult T-cell leukemia and tropical spastic paraparesis in different subsets of infected people. Treatments for HTLV-1-associated diseases are unspecific and unsatisfactory. Prophylactic measures have not been developed. Although HTLV-1 pathogenesis involves multiple stages and factors, high proviral load has been singled out as a major risk factor which predicts disease. HTLV-1 encodes Tax transactivator that potently activates transcription from viral long terminal repeats (LTR) and cellular promoters harbouring cAMP-responsive or KB element. Blocking Tax activity in infected cells should provide prophylactic and therapeutic benefits. In this study, we characterize the negative regulatory role of SIRT1 deacetylase in Tax-induced activation of HTLV-1 LTR. SIRT1 is a sirtuin with anti-cancer and anti-aging activity. Whereas over expression of SIRT1 abolished the activity of Tax to activate the LTR, compromising SIRT1 by RNA interference augmented Tax activation of the LTR. Resveratrol is a natural inhibitor of SIRT1 widely sold as a nutritional supplement and extensively tested for beneficial effects in various diseases. A SIRT1-dependent inhibition of the transcriptional activity of Tax was observed in cells treated with resveratrol. Consistently, treatment of HTLV-1-transformed T cells with resveratrol led to the activation of SIRT1 and the suppression of HTLV-1 proviral transcription. On the contrary, specific inhibition of SIRT1 by Sirtinol or Ex527 promoted HTLV-1 mRNA expression. The amount of HTLV-1 virion collected from culture supernatant was decreased in MT2 cells treated with resveratrol. Tax was found to interact with SIRT1 in HTLV-1-transformed T cells. Administration of resveratrol blocked the interaction of Tax with CREB and suppressed the recruitment of CREB and CRTC1 to the LTR. Taken together, our findings document the suppression of Tax activation of HTLV-1 transcription by SIRT1 and reveal potential benefits of small-molecule activators of SIRT1 such as resveratrol in the prevention and intervention of HTLV-1-associated diseases. Supported by SKY-MRF (2011), HK-RGC (HKU7674/12M, HKU7686/13M and HKU1/CRF/11G) and HK-HMRF (13121052) grants.

Metformin inhibits human T-cell leukemia virus type 1 transcription through activation of LKB1 and salt-inducible kinases

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL). Specific therapeutic and prophylactic agents are not available. Epidemiological studies have established a correlation between long-term use of the commonly prescribed anti-diabetic drug metformin and a decrease in the incidence of breast and other cancers. Whether metformin might also have anti-HTLV-1 and anti-ATL activity is unclear. In this study we demonstrate an inhibitory effect of metformin on HTLV-1 transcription mediated through the activation of LKB1 tumor suppressor and downstream salt-inducible kinases (SIKs). Treatment of HTLV-1- transformed ATL cells or cells transfected with HTLV-1 molecular clone pX1MT with metformin led to reduction in cell-free virion production and inhibition of cell proliferation. This effect was attributed to the activation of LKB1/SIK1 which compromised Tax expression and HTLV-1 transcription. LKB1 and SIKs function as host restriction factors that counteract Tax activation of HTLV-1 long terminal repeats. Expression of LKB1 and activated SIKs effectively blunted Tax activity in a phosphorylation-dependent manner in LKB1-null HeLa cells, whereas compromising these kinases, but not AMP-dependent protein kinases, augmented Tax function in LKB1-proficient HEK293T cells. Activated LKB1 and SIKs associated with Tax. Enforced expression of LKB1 or SIK1 in pX1MT-transfected cells and HTLV-1-transformed ATL cells repressed proviral transcription, whereas depletion of LKB1 boosted Tax expression. Taken together, our findings revealed a potential therapeutic and prophylactic agent for ATL as well as a new negative regulatory function of LKB1 and SIKs in HTLV-1 transcription. (Supported by HKU7661/08M, HKU7674/12M, HKU1/CRF/11G and SKY-MRF-2011).