HIV-1 Tat regulates cyclin B1 by promoting both expression and degradation

Development of Responsive Nanoparticles for Cancer Therapy

Great efforts are focused on the development of safe nano-carriers for the treatment of cancer in order to overcome some of the typical limitations of conventional therapies [...].

RETRACTED: Optimized Conjugation of Fluvastatin to HIV-1 TAT Displays Enhanced Pro-Apoptotic Activity in HepG2 Cells

Accumulating evidence indicates that statins reduce the risk of different cancers and inhibit the proliferation of liver cancer cells. This study aims to explore whether the electrostatic conjugation of optimized fluvastatin (FLV) to human immunodeficiency virus type 1 (HIV-1) trans-activator transcription peptide (TAT) would enhance the anti-proliferative activity against HepG2 cells. FLV-TAT conjugation was optimized to achieve the lowest size with highest zeta potential. Nine formulae were constructed, using a factorial design with three factors-FLV concentration, TAT concentration, and pH of the medium-while the responses were zeta potential and size. The optimized formula showed a particle size of 199.24 nm and 29.14 mV zeta potential. Data indicates that conjugation of FLV to TAT (optimized formula) significantly enhances anti-proliferative activity and uptake by HepG2 cells when compared to raw FLV. Flow cytometry showed significant accumulation of cells in the pre-G phase, which highlights higher apoptotic activity. Annexin V staining indicated a significant increase in total cell death in early and late apoptosis. This was confirmed by significantly elevated caspase 3 in cells exposed to FLV-TAT preparation. In conclusion, the FLV-TAT optimized formula exhibited improved anti-proliferative action against HepG2. This is partially attributed to the enhanced apoptotic effects and cellular uptake of FLV.

Mechanisms of Human Immunodeficiency Virus-Associated Lymphocyte Regulated Cell Death

Human immunodeficiency virus-1 (HIV-1) causes CD4 T cell depletion through a number of mechanisms, including programmed cell death pathways (both apoptotic and nonapoptotic). In the setting of HIV-1 infection, the enhanced lymphocyte cell death occurs as a consequence of complex interactions between the host immune system and viral factors, which are reviewed herein. On the other hand, the main challenge to HIV-1 eradication is the development of latent infection in a subset of long lived cells, including CD4+ T cells and macrophages, which resist HIV-induced cell death. Understanding the potential mechanisms of how HIV-1 induces lymphocyte cell death is critical to the "kick and kill" cure strategy, which relies on the effective killing of reactivated, HIV-1-infected cells.

Proteasomal Degradation Machinery: Favorite Target of HIV-1 Proteins

Proteasomal degradation pathways play a central role in regulating a variety of protein functions by controlling not only their turnover but also the physiological behavior of the cell. This makes it an attractive target for the pathogens, especially viruses which rely on the host cellular machinery for their propagation and pathogenesis. Viruses have evolutionarily developed various strategies to manipulate the host proteasomal machinery thereby creating a cellular environment favorable for their own survival and replication. Human immunodeficiency virus-1 (HIV-1) is one of the most dreadful viruses which has rapidly spread throughout the world and caused high mortality due to its high evolution rate. Here, we review the various mechanisms adopted by HIV-1 to exploit the cellular proteasomal machinery in order to escape the host restriction factors and components of host immune system for supporting its own multiplication, and successfully created an infection.

Protoparvovirus Interactions with the Cellular DNA Damage Response

Protoparvoviruses are simple single-stranded DNA viruses that infect many animal species. The protoparvovirus minute virus of mice (MVM) infects murine and transformed human cells provoking a sustained DNA damage response (DDR). This DDR is dependent on signaling by the ATM kinase and leads to a prolonged pre-mitotic cell cycle block that features the inactivation of ATR-kinase mediated signaling, proteasome-targeted degradation of p21, and inhibition of cyclin B1 expression. This review explores how protoparvoviruses, and specifically MVM, co-opt the common mechanisms regulating the DDR and cell cycle progression in order to prepare the host nuclear environment for productive infection.

Minute Virus of Mice Inhibits Transcription of the Cyclin B1 Gene during Infection

Replication of minute virus of mice (MVM) induces a sustained cellular DNA damage response (DDR) which the virus then exploits to prepare the nuclear environment for effective parvovirus takeover. An essential aspect of the MVM-induced DDR is the establishment of a potent premitotic block, which we previously found to be independent of activated p21 and ATR/Chk1 signaling. This arrest, unlike others reported previously, depends upon a significant, specific depletion of cyclin B1 and its encoding RNA, which precludes cyclin B1/CDK1 complex function, thus preventing mitotic entry. We show here that while the stability of cyclin B1 RNA was not affected by MVM infection, the production of nascent cyclin B1 RNA was substantially diminished at late times postinfection. Ectopic expression of NS1 alone did not reduce cyclin B1 expression. MVM infection also reduced the levels of cyclin B1 protein, and RNA levels normally increased in response to DNA-damaging reagents. We demonstrated that at times of reduced cyclin B1 expression during infection, there was a significantly reduced occupancy of RNA polymerase II and the essential mitotic transcription factor FoxM1 on the cyclin B1 gene promoter. Additionally, while total FoxM1 levels remained constant, there was a significant decrease of the phosphorylated, likely active, forms of FoxM1. Targeting of a constitutively active FoxM1 construct or the activation domain of FoxM1 to the cyclin B1 gene promoter via clustered regularly interspaced short palindromic repeats (CRISPR)-enzymatically inactive Cas9 in MVM-infected cells increased both cyclin B1 protein and RNA levels, implicating FoxM1 as a critical target for cyclin B1 inhibition during MVM infection.IMPORTANCE Replication of the parvovirus minute virus of mice (MVM) induces a sustained cellular DNA damage response (DDR) which the virus exploits to prepare the nuclear environment for effective takeover. An essential aspect of the MVM-induced DDR is establishment of a potent premitotic block. This block depends upon a significant, specific depletion of cyclin B1 and its encoding RNA that precludes cyclin B1/CDK1 complex functions necessary for mitotic entry. We show that reduced cyclin B1 expression is controlled primarily at the level of transcription initiation. Additionally, the essential mitotic transcription factor FoxM1 and RNA polymerase II were found to occupy the cyclin B1 gene promoter at reduced levels during infection. Recruiting a constitutively active FoxM1 construct or the activation domain of FoxM1 to the cyclin B1 gene promoter via CRISPR-catalytically inactive Cas9 (dCas9) in MVM-infected cells increased expression of both cyclin B1 protein and RNA, implicating FoxM1 as a critical target mediating MVM-induced cyclin B1 inhibition.

Interaction between HIV-1 Tat and DNA-PKcs modulates HIV transcription and class switch recombination

HIV-1 tat targets a variety of host cell proteins to facilitate viral transcription and disrupts host cellular immunity by inducing lymphocyte apoptosis, but whether it influences humoral immunity remains unclear. Previously, our group demonstrated that tat depresses expression of DNA-PKcs, a critical component of the non-homologous end joining pathway (NHEJ) of DNA double-strand breaks repair, immunoglobulin class switch recombination (CSR) and V(D)J recombination, and sensitizes cells to ionizing radiation. In this study, we demonstrated that HIV-1 Tat down-regulates DNA-PKcs expression by directly binding to the core promoter sequence. In addition, Tat interacts with and activates the kinase activity of DNA-PKcs in a dose-dependent and DNA independent manner. Furthermore, Tat inhibits class switch recombination (CSR) at low concentrations (≤4 µg/ml) and stimulates CSR at high concentrations (≥8 µg/ml). On the other hand, low protein level and high kinase activity of DNA-PKcs promotes HIV-1 transcription, while high protein level and low kinase activity inhibit HIV-1 transcription. Co-immunoprecipitation results revealed that DNA-PKcs forms a large complex comprised of Cyclin T1, CDK9 and Tat via direct interacting with CDK9 and Tat but not Cyclin T1. Taken together, our results provide new clues that Tat regulates host humoral immunity via both transcriptional depression and kinase activation of DNA-PKcs. We also raise the possibility that inhibitors and interventions directed towards DNA-PKcs may inhibit HIV-1 transcription in AIDS patients.

HIV-1 Tat impairs cell cycle control by targeting the Tip60, Plk1 and cyclin B1 ternary complex

HIV-1 Tat triggers intrinsic and extrinsic apoptosis pathways in both infected and uninfected cells and plays an important role in the pathogenesis of AIDS. Knocking down Tip60, an interactive protein of Tat, leads to the impairment of cell cycle progression, indicating a key role of Tip60 in cell cycle control. We found that Tip60 interacts with Plk1 through its ZnFMYST domain, and that this interaction is enhanced in the G 2/M phase. In addition, cyclin B1 was confirmed to interact with the ZnF domain of Tip60. Immunofluorescence imaging showed that Tip60 co-localizes with both Plk1 and cyclin B1 at the centrosome during the mitotic phase and to the mid-body during cytokinesis. Further experiments revealed that Tip60 forms a ternary complex with Plk1 and cyclin B1 and acetylates Plk1 but not cyclin B1. HIV-1 Tat likely forms a quaternary complex with Tip60, cyclin B1 and Plk1. Fluorescent microscopy showed that Tat causes an unscheduled nuclear translocation of both cyclin B1 and Plk1, causing their co-localization with Tip60 in the nucleus. Tat, Tip60, cyclin B1 and Plk1 interactions provide new a mechanistic explanation for Tat-mediated cell cycle dysregulation and apoptosis.

Frequency of circulating regulatory T cells increases during chronic HIV infection and is largely controlled by highly active antiretroviral therapy

Regulatory T cells (Tregs) act by suppressing the activation and effector functions of innate and adaptive immune responses. HIV infection impacts Treg proportion and phenotype, although discrepant results have been reported depending on the patient population and the way Tregs were characterized. The effects of highly active antiretroviral therapy (HAART) on Treg frequency have not been thoroughly documented. We performed a detailed longitudinal analysis of Treg frequency and phenotype in 11 HIV-infected individuals enrolled in a single, prospective clinical trial, in which all patients underwent the same treatment protocol and were sampled at the same time points. Tregs were characterized for their expression of molecules associated with activation, cell cycle, apoptosis, or function, and compared to circulating Tregs from a group of age-matched healthy individuals.

Our results revealed increased proportions, but reduced absolute numbers of circulating CD3⁺CD4⁺FOXP3⁺ Tregs in chronically infected HIV-infected patients. Treg frequency was largely normalized by HAART. Importantly, we show that similar conclusions were drawn regardless of the combination of markers used to define Tregs. Our results also showed increased expression of cell cycle markers (Ki67 and cyclin B) in Tregs from untreated infected individuals, which were decreased by HAART. However, the Treg phenotype in untreated patients was not consistent with a higher level of generalized activation, as they expressed very low levels of CD69, slightly elevated levels of HLA-DR and similar levels of GARP compared to Tregs from uninfected donors. Moreover, none of these markers was significantly changed by HAART. Treg expression of CTLA-4 and cytotoxic molecules was identical between patients and controls. The most striking difference in terms of functional molecules was the high expression of CD39 by Tregs in untreated patients, which HAART only partially controlled.

CD4+ T cell depletion in human immunodeficiency virus (HIV) infection: role of apoptosis

Human immunodeficiency virus (HIV) infection is principally a mucosal disease and the gastrointestinal (GI) tract is the major site of HIV replication. Loss of CD4+ T cells and systemic immune hyperactivation are the hallmarks of HIV infection. The end of acute infection is associated with the emergence of specific CD4+ and CD8+ T cell responses and the establishment of a chronic phase of infection. Abnormal levels of immune activation and inflammation persist despite a low steady state level of viremia. Although the causes of persistent immune hyperactivation remain incompletely characterized, physiological alterations of gastrointestinal tract probably play a major role. Failure to restore Th17 cells in gut-associated lymphoid tissues (GALT) might impair the recovery of the gut mucosal barrier. This review discusses recent advances on understanding the contribution of CD4+ T cell depletion to HIV pathogenesis.

The anaphase-promoting complex is a dual integrator that regulates both MicroRNA-mediated transcriptional regulation of cyclin B1 and degradation of Cyclin B1 during Arabidopsis male gametophyte development

The anaphase-promoting complex/cyclosome (APC/C), an essential ubiquitin protein ligase, regulates mitotic progression and exit by enhancing degradation of cell cycle regulatory proteins, such as CYCB1;1, whose transcripts are upregulated by DUO POLLEN1 (DUO1). DUO1 is required for cell division in male gametophytes and is a target of microRNA 159 (miR159) in Arabidopsis thaliana. Whether APC/C is required for DUO1-dependent CYCB1;1 regulation is unknown. Mutants in both APC8 and APC13 had pleiotrophic phenotypes resembling those of mutants affecting microRNA biogenesis. We show that these apc/c mutants had reduced miR159 levels and increased DUO1 and CYCB1;1 transcript levels and that APC/C is required to recruit RNA polymerase II to MIR159 promoters. Thus, in addition to its role in degrading CYCB1;1, APC/C stimulates production of miR159, which downregulates DUO1 expression, leading to reduced CYCB1;1 transcription. Both MIR159 and APC8-yellow fluorescent protein accumulated in unicellular microspores and bicellular pollen but decreased in tricellular pollen, suggesting that spatial and temporal regulation of miR159 by APC/C ensures mitotic progression. Consistent with this, the percentage of mature pollen with no or single sperm-like cells increased in apc/c mutants and plants overexpressing APC8 partially mimicked the duo1 phenotype. Thus, APC/C is an integrator that regulates both microRNA-mediated transcriptional regulation of CYCB1;1 and degradation of CYCB1;1.