Heptad repeats regulate protein phosphatase 2a recruitment to I-kappaB kinase gamma/NF-kappaB essential modulator and are targeted by human T-lymphotropic virus type 1 tax

The Protective Effects of AT2R Agonist, CGP42112A, Against Angiotensin II-Induced Oxidative Stress and Inflammatory Response in Astrocytes: Role of AT2R/PP2A/NFκB/ROS Signaling

Angiotensin II receptor type 2 (AT2R) agonists have been known to promote neuroprotection by limiting ischemic insult, neuronal proliferation, and differentiation. Further, AT2R agonists have also been associated with the suppression of neuroinflammation and neurodegeneration. Of note, brain astrocytes play a critical role in these neuroinflammatory and neurodegenerative processes. However, the role of AT2R in astrocytic activation remains elusive. Therefore, this study evaluated the role and molecular mechanism of AT2R agonist CGP42112A (CGP) against Angiotensin II (Ang II)-induced astrocytic activation in primary astrocytes, and in a rat model of hypertension. Here, we demonstrated that AT2R activation by CGP abrogated Ang II-induced astrocytic activation, by mitigating the ROS production, mitochondrial dysfunction, IκB-α degradation, NFκB nuclear translocation, and release of TNF-α in astrocytes. However, AT2R-mediated anti-inflammatory effects were reversed by AT2R antagonist, PD123319 (PD), in both in vitro and in vivo conditions. Mechanistically, AT2R via protein phosphatase-2A (PP2A) abrogated the Ang II-induced NFκB activation, ROS generation, and subsequent astrocytic activation. Importantly, PP2A antagonist, okadaic acid, reversed the anti-inflammatory effects of AT2R in Ang II-stimulated primary astrocytes and in the cortex of hypertensive rats. Thus, the present study suggests that AT2R by activating PP2A inhibits oxidative stress and NFκB activation, thereby preventing the astrocytic pro-inflammatory activation. Therefore, AT2R might be advantageous therapeutic target for neuroinflammatory/neurodegenerative diseases perpetuated by astrocytic activation.

The human T-cell leukemia virus type-1 tax oncoprotein dissociates NF-κB p65RelA-Stathmin complexes and causes catastrophic mitotic spindle damage and genomic instability

Genomic instability is a hallmark of many cancers; however, the molecular etiology of chromosomal dysregulation is not well understood. The human T-cell leukemia virus type-1 (HTLV-1) oncoprotein Tax activates NF-κB-signaling and induces DNA-damage and aberrant chromosomal segregation through diverse mechanisms which contribute to viral carcinogenesis. Intriguingly, Stathmin/oncoprotein-18 (Op-18) depolymerizes tubulin and interacts with the p65^RelA subunit and functions as a cofactor for NF-κB-dependent transactivation. We thus hypothesized that the dissociation of p65^RelA-Stathmin/Op-18 complexes by Tax could lead to the catastrophic destabilization of microtubule (MT) spindle fibers during mitosis and provide a novel mechanistic link between NF-κB-signaling and genomic instability. Here we report that the inhibition of Stathmin expression by the retroviral latency protein, p30^II, or knockdown with siRNA-stathmin, dampens Tax-mediated NF-κB transactivation and counters Tax-induced genomic instability and cytotoxicity. The Tax-G148V mutant, defective for NF-κB activation, exhibited reduced p65^RelA-Stathmin binding and diminished genomic instability and cytotoxicity. Dominant-negative inhibitors of NF-κB also prevented Tax-induced multinucleation and apoptosis. Moreover, cell clones containing the infectious HTLV-1 ACH. p30^II mutant provirus, impaired for p30^II production, exhibited increased multinucleation and the accumulation of cytoplasmic tubulin aggregates following nocodozole-treatment. These findings allude to a mechanism whereby NF-κB-signaling regulates tubulin dynamics and mitotic instability through the modulation of p65^RelA-Stathmin/Op-18 interactions, and support the notion that p30^II enhances the survival of Tax-expressing HTLV-1-transformed cells.

Protein phosphatase 2A as a therapeutic target in inflammation and neurodegeneration

Protein phosphatase 2A (PP2A) is a highly complex heterotrimeric enzyme that catalyzes the selective removal of phosphate groups from protein serine and threonine residues. Emerging evidence suggests that it functions as a tumor suppressor by constraining phosphorylation-dependent signalling pathways that regulate cellular transformation and metastasis. Therefore, PP2A-activating drugs (PADs) are being actively sought and investigated as potential novel anti-cancer treatments. Here we explore the concept that PP2A also constrains inflammatory responses through its inhibitory effects on various signalling pathways, suggesting that PADs may be effective in the treatment of inflammation-mediated pathologies.

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.

Probing the Solution Structure of IκB Kinase (IKK) Subunit γ and Its Interaction with Kaposi Sarcoma-associated Herpes Virus Flice-interacting Protein and IKK Subunit β by EPR Spectroscopy

Viral flice-interacting protein (vFLIP), encoded by the oncogenic Kaposi sarcoma-associated herpes virus (KSHV), constitutively activates the canonical nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) pathway. This is achieved through subversion of the IκB kinase (IKK) complex (or signalosome), which involves a physical interaction between vFLIP and the modulatory subunit IKKγ. Although this interaction has been examined both in vivo and in vitro, the mechanism by which vFLIP activates the kinase remains to be determined. Because IKKγ functions as a scaffold, recruiting both vFLIP and the IKKα/β subunits, it has been proposed that binding of vFLIP could trigger a structural rearrangement in IKKγ conducive to activation. To investigate this hypothesis we engineered a series of mutants along the length of the IKKγ molecule that could be individually modified with nitroxide spin labels. Subsequent distance measurements using electron paramagnetic resonance spectroscopy combined with molecular modeling and molecular dynamics simulations revealed that IKKγ is a parallel coiled-coil whose response to binding of vFLIP or IKKβ is localized twisting/stiffening and not large-scale rearrangements. The coiled-coil comprises N- and C-terminal regions with distinct registers accommodated by a twist: this structural motif is exploited by vFLIP, allowing it to bind and subsequently activate the NF-κB pathway. In vivo assays confirm that NF-κB activation by vFLIP only requires the N-terminal region up to the transition between the registers, which is located directly C-terminal of the vFLIP binding site.

IKK biology

The inhibitor of nuclear factor-κB (IκB) kinase (IKK) complex is the master regulator of the NF-κB signaling pathway. The activation of the IKK complex is a tightly regulated, highly stimulus-specific, and target-specific event that is essential for the plethora of functions attributed to NF-κB. More recently, NF-κB-independent roles of IKK members have brought increased complexity to its biological function. This review highlights some of the major advances in the studies of the process of IKK activation and the biological roles of IKK family members, with a focus on NF-κB-independent functions. Understanding these complex processes is essential for targeting IKK for therapeutics.

Agnoprotein of mammalian polyomaviruses

Polyomaviruses are naked viruses with an icosahedral capsid that surrounds a circular double-stranded DNA molecule of about 5000 base-pairs. Their genome encodes at least five proteins: large and small tumor antigens and the capsid proteins VP1, VP2 and VP3. The tumor antigens are expressed during early stages of the viral life cycle and are implicated in the regulation of viral transcription and DNA replication, while the capsid proteins are produced later during infection. Members of the Polyomaviridae family have been isolated in birds (Avipolyomavirus) and mammals (Orthopolyomavirus and Wukipolyomavirus). Some mammalian polyomaviruses encode an additional protein, referred to as agnoprotein, which is a relatively small polypeptide that exerts multiple functions. This review discusses the structure, post-translational modifications, and functions of agnoprotein, and speculates why not all polyomaviruses express this protein.

The human T-lymphotropic virus type 1 tax protein inhibits nonsense-mediated mRNA decay by interacting with INT6/EIF3E and UPF1

In this report, we analyzed whether the degradation of mRNAs by the nonsense-mediated mRNA decay (NMD) pathway was affected in human T-lymphotropic virus type 1 (HTLV-1)-infected cells. This pathway was indeed strongly inhibited in C91PL, HUT102, and MT2 cells, and such an effect was also observed by the sole expression of the Tax protein in Jurkat and HeLa cells. In line with this activity, Tax binds INT6/EIF3E (here called INT6), which is a subunit of the translation initiation factor eukaryotic initiation factor 3 (eIF3) required for efficient NMD, as well as the NMD core factor upstream frameshift protein 1 (UPF1). It was also observed that Tax expression alters the morphology of processing bodies (P-bodies), the cytoplasmic structures which concentrate RNA degradation factors. The presence of UPF1 in these subcellular compartments was increased by Tax, whereas that of INT6 was decreased. In line with these effects, the level of the phosphorylated form of UPF1 was increased in the presence of Tax. Analysis of several mutants of the viral protein showed that the interaction with INT6 is necessary for NMD inhibition. The alteration of mRNA stability was observed to affect viral transcripts, such as that coding for the HTLV-1 basic leucine zipper factor (HBZ), and also several cellular mRNAs sensitive to the NMD pathway. Our data indicate that the effect of Tax on viral and cellular gene expression is not restricted to transcriptional control but can also involve posttranscriptional regulation.

[Targeting of PP2A enzymes by viral proteins and cancer signalling]

Protein phosphatase 2A (PP2A) is a large family of holoenzymes that comprises 1% of total cellular proteins and accounts for the majority of Ser/Thr phosphatase activity in eukaryotic cells. PP2A proteins are made of a core dimer, composed of a catalytic (C) subunit and a structural (A) subunit, in association with a third variable -regulatory (B) subunit. Although initially considered as a constitutive housekeeping enzyme, PP2A is indeed highly regulated by post-translational modifications of its catalytic subunit or by the identity of a regulatory type B subunit, which determines substrate specificity, subcellular localization and enzymatic activity of a defined holoenzyme. During the two last decades, multiple studies of structural and functional regulation of PP2A holoenzymes by viral proteins led to the identification of critical pathways for both viral biology and tumorigenesis. To date a dozen of different viruses (ADN/ARN or retrovirus) have been identified that encode viral proteins associated to PP2A. In this review, we analyze a biological strategy, used by various viruses based on the targeting of PP2A enzymes by viral proteins, in order to specifically deregulate cellular pathways of their hosts. The impact of such PP2A targeting for biomedical search, and in further therapeutic developments against cancer, will also be discussed.

PP2A targeting by viral proteins: a widespread biological strategy from DNA/RNA tumor viruses to HIV-1

Protein phosphatase 2A (PP2A) is a large family of holoenzymes that comprises 1% of total cellular proteins and accounts for the majority of Ser/Thr phosphatase activity in eukaryotic cells. Although initially viewed as constitutive housekeeping enzymes, it is now well established that PP2A proteins represent a family of highly and sophistically regulated phosphatases. The past decade, multiple complementary studies have improved our knowledge about structural and functional regulation of PP2A holoenzymes. In this regard, after summarizing major cellular regulation, this review will mainly focus on discussing a particulate biological strategy, used by various viruses, which is based on the targeting of PP2A enzymes by viral proteins in order to specifically deregulate, for their own benefit, cellular pathways of their hosts. The impact of such PP2A targeting for research in human diseases, and in further therapeutic developments, is also discussed.

Proteins that bind to IKKgamma (NEMO) and down-regulate the activation of NF-kappaB

Inhibitor of kappaB kinase (IKK) gamma (IKKgamma), also referred to as nuclear factor kappaB (NF-kappaB) essential modulator (NEMO), is an important component of the IKK complex. Following the exposure of cells to NF-kappaB-inducing stimuli, the IKK complex catalyzes the phosphorylation of inhibitor of kappaB (IkappaB) proteins, which is a critical step that leads to the activation of NF-kappaB via the canonical pathway. The exact functions of IKKgamma as part of the IKK complex have not been fully elucidated. A number of proteins have been identified as directly interacting with IKKgamma and modulating the activity of the IKK complex. This mini review covers eight proteins that have been reported to bind to IKKgamma and lead to the suppression of the activities of the IKK complex and hence result in the down-regulation of the activation of NF-kappaB. The reported mechanisms by which these interactions suppress the activation of the IKK complex include the deubiquitination of IKKgamma and competition with upstream activators for binding to IKKgamma.

NEMO-binding domains of both IKKalpha and IKKbeta regulate IkappaB kinase complex assembly and classical NF-kappaB activation

Proinflammatory NF-kappaB activation requires the IkappaB (inhibitor of NF-kappaB) kinase (IKK) complex that contains two catalytic subunits named IKKalpha and IKKbeta and a regulatory subunit named NF-kappaB essential modulator (NEMO). NEMO and IKKbeta are essential for tumor necrosis factor (TNF)-induced NF-kappaB activation, and we recently demonstrated that NEMO and IKKalpha are sufficient for interleukin (IL)-1-induced signaling. IKKalpha and IKKbeta both contain a functional NEMO-binding domain (NBD); however, the role of NEMO association with each kinase in NF-kappaB signaling and IKK complex formation remains unclear. To address this question, we stably reconstituted IKKalpha(-/-) and IKKbeta(-/-) murine embryonic fibroblasts (MEFs) with wild-type (WT) or NBD-deficient (DeltaNBD) versions of IKKalpha and IKKbeta, respectively. TNF-induced classical NF-kappaB activation in IKKbeta(-/-) MEFs was rescued by IKKbeta(WT) but not IKKbeta(DeltaNBD), whereas neither IKKbeta(WT) nor IKKbeta(DeltaNBD) affected IL-1-induced NF-kappaB signaling. As previously described, classical NF-kappaB transcriptional activity was absent in IKKalpha(-/-) cells. Reconstitution with either IKKalpha(WT) or IKKalpha(DeltaNBD) rescued both IL-1 and TNF-induced transcription, demonstrating that NEMO association is not required for IKKalpha-dependent regulation of NF-kappaB-dependent transcription. Stably expressed IKKalpha(WT) or IKKbeta(WT) associated with endogenous IKKs and NEMO in IKKalpha(-/-) or IKKbeta(-/-) MEFs, respectively, resulting in formation of the heterotrimeric IKKalpha-IKKbeta-NEMO complex. In contrast, although the IKKalpha(DeltaNBD) and IKKbeta(DeltaNBD) mutants associated with endogenous IKKs containing an NBD, these dimeric endogenous IKK-IKK(DeltaNBD) complexes did not associate with NEMO. These findings therefore demonstrate that formation of the heterotrimeric IKKalpha-IKKbeta-NEMO holocomplex absolutely requires two intact NEMO-binding domains.

Mechanism of PP2A-mediated IKK beta dephosphorylation: a systems biological approach

Background

Biological effects of nuclear factor-κB (NFκB) can differ tremendously depending on the cellular context. For example, NFκB induced by interleukin-1 (IL-1) is converted from an inhibitor of death receptor induced apoptosis into a promoter of ultraviolet-B radiation (UVB)-induced apoptosis. This conversion requires prolonged NFκB activation and is facilitated by IL-1 + UVB-induced abrogation of the negative feedback loop for NFκB, involving a lack of inhibitor of κB (IκBα) protein reappearance. Permanent activation of the upstream kinase IKKβ results from UVB-induced inhibition of the catalytic subunit of Ser-Thr phosphatase PP2A (PP2Ac), leading to immediate phosphorylation and degradation of newly synthesized IκBα.

Results

To investigate the mechanism underlying the general PP2A-mediated tuning of IKKβ phosphorylation upon IL-1 stimulation, we have developed a strictly reduced mathematical model based on ordinary differential equations which includes the essential processes concerning the IL-1 receptor, IKKβ and PP2A. Combining experimental and modelling approaches we demonstrate that constitutively active, but not post-stimulation activated PP2A, tunes out IKKβ phosphorylation thus allowing for IκBα resynthesis in response to IL-1. Identifiability analysis and determination of confidence intervals reveal that the model allows reliable predictions regarding the dynamics of PP2A deactivation and IKKβ phosphorylation. Additionally, scenario analysis is used to scrutinize several hypotheses regarding the mode of UVB-induced PP2Ac inhibition. The model suggests that down regulation of PP2Ac activity, which results in prevention of IκBα reappearance, is not a direct UVB action but requires instrumentality.

Conclusion

The model developed here can be used as a reliable building block of larger NFκB models and offers comprehensive simplification potential for future modeling of NFκB signaling. It gives more insight into the newly discovered mechanisms for IKK deactivation and allows for substantiated predictions and investigation of different hypotheses. The evidence of constitutive activity of PP2Ac at the IKK complex provides new insights into the feedback regulation of NFκB, which is crucial for the development of new anti-cancer strategies.

Induction of p21(CIP1/WAF1) expression by human T-lymphotropic virus type 1 Tax requires transcriptional activation and mRNA stabilization

HTLV-1 Tax can induce senescence by up-regulating the levels of cyclin-dependent kinase inhibitors p21^CIP1/WAF1 and p27^KIP1. Tax increases p27^KIP1 protein stability by activating the anaphase promoting complex/cyclosome (APC/C) precociously, causing degradation of Skp2 and inactivation of SCF^Skp2, the E3 ligase that targets p27^KIP1. The rate of p21^CIP1/WAF1 protein turnover, however, is unaffected by Tax. Rather, the mRNA of p21^CIP1/WAF1 is greatly up-regulated. Here we show that Tax increases p21 mRNA expression by transcriptional activation and mRNA stabilization. Transcriptional activation of p21^CIP1/WAF1 by Tax occurs in a p53-independent manner and requires two tumor growth factor-β-inducible Sp1 binding sites in the -84 to -60 region of the p21^CIP1/WAF1 promoter. Tax binds Sp1 directly, and the CBP/p300-binding activity of Tax is required for p21^CIP1/WAF1 trans-activation. Tax also increases the stability of p21^CIP1/WAF1 transcript. Several Tax mutants trans-activated the p21 promoter, but were attenuated in stabilizing p21^CIP1/WAF1 mRNA, and were less proficient in increasing p21^CIP1/WAF1 expression. The possible involvement of Tax-mediated APC/C activation in p21^CIP1/WAF1 mRNA stabilization is discussed.

The IkappaB kinase complex: master regulator of NF-kappaB signaling

The Nuclear Factor-kappa B (NF-kappaB) family of transcription factors regulates the expression of a wide range of genes critical for immune and inflammatory responses, cell survival, immune development, and cell proliferation. Dysregulated NF-kappaB activity occurs in a number of chronic inflammatory diseases and certain types of cancers making NF-kappaB signaling an attractive target for the development of anti-inflammatory and anti-cancer drugs. A pivotal regulator of all inducible NF-kappaB signaling pathways is the IkappaB kinase (IKK) complex that consists of two kinases (IKKalpha and IKKbeta) and a regulatory subunit named NF-kappaB essential modulator (NEMO). Genetic analysis of the IKK complex has identified two separate pathways named the classical and non-canonical mechanisms that are dependent on either NEMO and IKKbeta (classical) or IKKalpha alone (non-canonical). To better understand the mechanisms that regulate IKK complex activity and to address the differential functions of IKKalpha and IKKbeta we have molecularly dissected the IKKs. We describe here how these studies have identified a unique inhibitor of pro-inflammatory NF-kappaB signaling, an unforeseen role for IKKalpha in the classical NF-kappaB pathway, and a novel functional domain in IKKbeta that is not present in IKKalpha.

The HTLV-1 Tax interactome

The Tax1 oncoprotein encoded by Human T-lymphotropic virus type I is a major determinant of viral persistence and pathogenesis. Tax1 affects a wide variety of cellular signalling pathways leading to transcriptional activation, proliferation and ultimately transformation. To carry out these functions, Tax1 interacts with and modulates activity of a number of cellular proteins. In this review, we summarize the present knowledge of the Tax1 interactome and propose a rationale for the broad range of cellular proteins identified so far.

PP2A-dependent disruption of centrosome replication and cytoskeleton organization in Drosophila by SV40 small tumor antigen

Viruses of the DNA tumor virus family share the ability to transform vertebrate cells through the action of virus-encoded tumor antigens that interfere with normal cell physiology. They accomplish this very efficiently by inhibiting endogenous tumor suppressor proteins that control cell proliferation and apoptosis. Simian virus 40 (SV40) encodes two oncoproteins, large tumor antigen, which directly inhibits the tumor suppressors p53 and Rb, and small tumor antigen (ST), which interferes with serine/threonine protein phosphatase 2A (PP2A). We have constructed a Drosophila model for SV40 ST expression and show that ST induces supernumerary centrosomes, an activity we also demonstrate in human cells. In early Drosophila embryos, ST also caused increased microtubule stability, chromosome segregation errors, defective assembly of actin into cleavage furrows, cleavage failure, a rise in cyclin E levels and embryonic lethality. Using ST mutants and genetic interaction experiments between ST and PP2A subunit mutations, we show that all of these phenotypes are dependent on ST's interaction with PP2A. These analyses demonstrate the validity and utility of Drosophila as a model for viral oncoprotein function in vivo.

Identification of PP2A as a crucial regulator of the NF-kappaB feedback loop: its inhibition by UVB turns NF-kappaB into a pro-apoptotic factor

Although nuclear factor-kappaB (NF-kappaB) usually exerts anti-apoptotic activity, upon activation by interleukin-1 (IL-1) it enhances ultraviolet-B radiation (UVB)-induced apoptosis. This paradoxical effect is associated with NF-kappaB-dependent pronounced secretion of tumour necrosis factor-alpha (TNF) which activates TNF-R1 in an autocrine fashion to enhance UVB-induced apoptosis. We demonstrate that sustained TNF transcription in UVB+IL-1-treated cells involves complete abrogation of the negative feedback loop of NF-kappaB preventing IkappaBalpha resynthesis, hence allowing uncontrolled NF-kappaB activity. We show that IkappaBalpha is not transcriptionally inhibited but resynthesized protein is immediately marked for degradation due to persistent inhibitor of kappaB kinasebeta (IKKbeta) activity. Continuous IKKbeta phosphorylation and activation is caused by UVB-mediated inhibition of the phosphatase PP2A. This study demonstrates that the cellular response to different NF-kappaB activators may be converted to the opposite reaction when both stimuli act in concert. Our data shed new light on the significance of negative feedback regulation of NF-kappaB and identifies PP2A as the key regulator of this process.

Crystal structure of a vFlip-IKKgamma complex: insights into viral activation of the IKK signalosome

Key to the pathogenicity of several viruses is activation of the canonical nuclear factor-kappaB (NF-kappaB) transcriptional pathway. Subversion of this tightly regulated mechanism is achieved through the production of host mimetic viral proteins that deregulate the transcription process. One such protein is ks-vFLIP (produced by the Kaposi's sarcoma herpes virus [KSHV]), which associates with IKKgamma, an essential component of the IKK complex or signalosome. This interaction renders the canonical NF-kappaB pathway constitutively active and has been linked to Kaposi's sarcoma and other malignancies. In order to elucidate the molecular basis underpinning ks-vFLIP-induced activation of the IKK signalosome, we have determined the crystal structure of a complex involving a fragment of IKKgamma bound to ks-vFLIP at 3.2 A. In addition to identifying and subsequently probing the ks-vFLIP-IKKgamma interface, we have also investigated the effects of a mutation implicated in the genetic disorder anhydrotic ectodermal dysplasia with immunodeficiency (EDA-ID).

Induced TRPC1 expression increases protein phosphatase 2A sensitizing intestinal epithelial cells to apoptosis through inhibition of NF-kappaB activation

Transient receptor potential canonical-1 (TRPC1) functions as a store-operated Ca2+ channel in intestinal epithelial cells (IECs), and induced TRPC1 expression sensitizes IECs to apoptosis by inhibiting NF-kappaB activation. However, the exact mechanism by which increased TRPC1 results in NF-kappaB inactivation remains elusive. Protein phosphatase 2A (PP2A) is a widely conserved protein serine/threonine phosphatase that is implicated in the regulation of a wide array of cellular functions including apoptosis. The present study tests the hypothesis that induced TRPC1 expression inhibits NF-kappaB activation by increasing PP2A activity through Ca2+ influx in IECs. The expression of TRPC1 induced by stable transfection with the wild-type TRPC1 gene increased PP2A activity as indicated by increases in levels of PP2A proteins and their phosphatase activity. Increased levels of PP2A activity in stable TRPC1-transfected IEC-6 cells (IEC-TRPC1) were associated with decreased nuclear levels of NF-kappaB proteins and a reduction in NF-kappaB-dependent transcriptional activity, although there were no changes in total NF-kappaB protein levels. Inhibition of PP2A activity by treatment with okadaic acid or PP2A silencing with small interfering RNA not only enhanced NF-kappaB transactivation but also prevented the increased susceptibility of IEC-TRPC1 cells to apoptosis induced by treatment with tumor necrosis factor-alpha (TNF-alpha)/cycloheximide (CHX). Decreasing Ca2+ influx by exposure to the Ca2+-free medium reduced PP2A mRNA levels, destabilized PP2A proteins, and induced NF-kappaB activation, thus blocking the increased sensitivity of IEC-TRPC1 cells to TNF-alpha/CHX-induced apoptosis. These results indicate that induced TRPC1 expression increases PP2A activity through Ca2+ influx and that increased PP2A sensitizes IECs to apoptosis as a result of NF-kappaB inactivation.

Constitutive activation of TAK1 by HTLV-1 tax-dependent overexpression of TAB2 induces activation of JNK-ATF2 but not IKK-NF-kappaB

HTLV-1 Tax oncoprotein induces persistent activation of the transcription factor NF-kappaB and CREB (cAMP-response element-binding protein)/ATF. Transforming growth factor-beta-activated kinase 1 (TAK1) has been shown to play a critical role in these transcription factors. Here, we found that TAK1 was constitutively activated in Tax-positive HTLV-1-transformed T cells. Tax induced persistent overexpression of TAK1-binding protein 2 (TAB2), but not TAB3, which is essential for TAK1 activation. Surprisingly, TAK1 was not involved in the activation of NF-kappaB. On the other hand, JNK and p38 mitogen-activated protein kinases were activated by TAK1. In addition, ATF2, but not CREB, was a target for the TAK1-JNK pathway, and p38 negatively regulated TAK1 activity through TAB1 phosphorylation. These results indicate that Tax-mediated TAK1 activation is important for the activation of ATF2 rather than NF-kappaB.