CNI-H0294, a nuclear importation inhibitor of the human immunodeficiency virus type 1 genome, abrogates virus replication in infected activated peripheral blood mononuclear cells

Nuclear translocation as a novel target for anti-HIV drugs

During recent years, remarkable progress has been achieved in the treatment of patients infected with HIV. This progress involves not only the improvement of previously known drugs but also the introduction of new classes of anti-HIV agents. Currently, drugs targeting virus entry, reverse transcription, integration and maturation are either in clinical use or in the late stages of clinical development. Nonetheless, the high mutation rate of the virus and toxicity of the drugs, which become problematic during prolonged treatment regimens characteristic of anti-HIV therapy, drive the necessity to produce new drugs that will allow physicians to keep the virus at bay in patients on lifelong anti-HIV therapy. Ideally, such drugs would target a new step in the HIV life cycle, thus avoiding crossresistance with older compounds. One such new target for anti-HIV therapy is nuclear translocation--a process critical for HIV replication. In this article, the authors will review recent literature on the mechanisms of HIV nuclear import and will describe compounds that inhibit this step of HIV replication.

Identification of critical motifs within HIV-1 integrase required for importin α3 interaction and viral cDNA nuclear import

The viral cDNA nuclear import is an important requirement for human immunodeficiency virus type 1 (HIV-1) replication in dividing and nondividing cells. Our recent study identified a specific interaction of importin α3 (Impα3) with HIV-1 integrase (IN) and its involvement in viral cDNA nuclear import. In this study, we have performed a more detailed investigation on the molecular mechanism of how HIV-1 IN interacts with Impα3. Our results revealed a reduced interaction between the two IN mutants INKK215,9AA (IN215,9) and INRK263,4AA (IN263,4) with Impα3, while an IN double mutant, IN215,9/263,4, was severely impaired for its Impα3-binding ability, even though it was still found interacting with other cofactors, IN interactor I and Transportin3. Immunostaining and fractionation analysis have shown that YFP-IN215,9/263,4 failed to localize in the nucleus of transfected cells. Also, we found that both major and minor nuclear localization signal binding grooves of Impα3 are involved in interaction with IN. All of these results suggest a cargo protein-import receptor type of interaction. Finally, the effect of IN215,9/263,4 mutations on HIV-1 replication was evaluated, and real-time quantitative PCR analysis showed that, while mutant virus (v215,9/263,4) had a slightly lowered total viral DNA, the 2-long-terminal-repeat DNA, a marker for nuclear import, was greatly reduced during v215,9/263,4 infection in both dividing and nondividing cells. Also, by cell fractionation assay, we found that a significant proportion of viral cDNA was still retained in cytoplasmic fraction of v215,9/263,4-infected cells. Overall, our study provides strong evidence that (211)KELQKQITK and (262)RRKAK regions of IN C-terminal domain are required for Impα3 interaction and HIV-1 cDNA nuclear import.

HIV-1 accessory protein Vpr: relevance in the pathogenesis of HIV and potential for therapeutic intervention

The HIV protein, Vpr, is a multifunctional accessory protein critical for efficient viral infection of target CD4⁺ T cells and macrophages. Vpr is incorporated into virions and functions to transport the preintegration complex into the nucleus where the process of viral integration into the host genome is completed. This action is particularly important in macrophages, which as a result of their terminal differentiation and non-proliferative status, would be otherwise more refractory to HIV infection. Vpr has several other critical functions including activation of HIV-1 LTR transcription, cell-cycle arrest due to DCAF-1 binding, and both direct and indirect contributions to T-cell dysfunction. The interactions of Vpr with molecular pathways in the context of macrophages, on the other hand, support accumulation of a persistent reservoir of HIV infection in cells of the myeloid lineage. The role of Vpr in the virus life cycle, as well as its effects on immune cells, appears to play an important role in the immune pathogenesis of AIDS and the development of HIV induced end-organ disease. In view of the pivotal functions of Vpr in virus infection, replication, and persistence of infection, this protein represents an attractive target for therapeutic intervention.

Strategies to inhibit viral protein nuclear import: HIV-1 as a target

Nuclear import is a critical step in the life cycle of HIV-1. During the early (preintegration) stages of infection, HIV-1 has to transport its preintegration complex into the nucleus for integration into the host cell chromatin, while at the later (postintegration) stages viral regulatory proteins Tat and Rev need to get into the nucleus to stimulate transcription and regulate splicing and nuclear export of subgenomic and genomic RNAs. Given such important role of nuclear import in HIV-1 life cycle, this step presents an attractive target for antiviral therapeutic intervention. In this review, we describe the current state of our understanding of the interactions regulating nuclear import of the HIV-1 preintegration complex and describe current approaches to inhibit it. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Interactions of human cytomegalovirus proteins with the nuclear transport machinery

Accurate cellular localization is crucial for the effective function of most viral macromolecules and nuclear translocation is central to the function of herpesviral proteins that are involved in processes such as transcription and DNA replication. The passage of large molecules between the cytoplasm and nucleus, however, is restricted, and this restriction affords specific mechanisms that control nucleocytoplasmic exchange. In this review, we focus on two cytomegalovirus-encoded proteins, pUL69 and pUL84, that are able to shuttle between the nucleus and the cytoplasm. Both viral proteins use unconventional interactions with components of the cellular transport machinery: pUL69 binds to the mRNA export factor UAP56, and this interaction is crucial for pUL69-mediated nuclear export of unspliced RNA; pUL84 docks to importin-alpha proteins via an unusually large protein domain that contains functional leucine-rich nuclear export signals, thus serving as a complex bidirectional transport domain. Selective interference with these unconventional interactions, which disturbs the intracellular trafficking of important viral regulatory proteins, may constitute a novel and attractive principle for antiviral therapy.

Oxadiazols: a new class of rationally designed anti-human immunodeficiency virus compounds targeting the nuclear localization signal of the viral matrix protein

Despite recent progress in anti-human immunodeficiency virus (HIV) therapy, drug toxicity and emergence of drug-resistant isolates during long-term treatment of HIV-infected patients necessitate the search for new targets that can be used to develop novel antiviral agents. One such target is the process of nuclear translocation of the HIV preintegration complex. Previously we described a class of arylene bis(methylketone) compounds that inhibit HIV-1 nuclear import by targeting the nuclear localization signal (NLS) in the matrix protein (MA). Here we report a different class of MA NLS-targeting compounds that was selected using computer-assisted drug design. The leading compound from this group, ITI-367, showed potent anti-HIV activity in cultures of T lymphocytes and macrophages and also inhibited HIV-1 replication in ex vivo cultured lymphoid tissue. The virus carrying inactivating mutations in MA NLS was resistant to ITI-367. Analysis by real-time PCR demonstrated that the compound specifically inhibited nuclear import of viral DNA, measured by two-long terminal repeat circle formation. Evidence of the existence of this mechanism was provided by immunofluorescent microscopy, using fluorescently labeled HIV-1, which demonstrated retention of the viral DNA in the cytoplasm of drug-treated macrophages. Compounds inhibiting HIV-1 nuclear import may be attractive candidates for further development.

Heat-shock protein 70 exerts opposing effects on Vpr-dependent and Vpr-independent HIV-1 replication in macrophages

HIV-1 viral protein R (Vpr) shuttles between the nucleus and the cytoplasm and is believed to contribute to the process of nuclear translocation of the viral preintegration complex, thus facilitating HIV-1 replication in macrophages. In this report, we demonstrate that Hsp70, a heat-shock protein contributing to cellular stress responses, inhibits nuclear translocation of HIV-1 Vpr. In macrophages, Hsp70 is induced shortly after HIV-1 infection. Recombinant Hsp70 or a mild heat shock diminished replication of the wild-type HIV-1, suggesting that Hsp70 might function as an innate antiviral factor. Surprisingly, Hsp70 stimulated nuclear import and replication in macrophages of the Vpr-deficient HIV-1 construct. This finding suggests that Hsp70 and Vpr may function in a similar manner when expressed separately, but they neutralize each other's activity when present together. Consistent with this interpretation, Hsp70 coprecipitated with Vpr from HIV-1–infected cells.

Inhibition of HIV-1 nuclear import via schiff base formation with arylene bis(methylketone) compounds

Arylene bis(methylketone) compounds specifically block nuclear translocation of the HIV-1 pre-integration complex by forming Schiff-base adducts with contiguous lysines within nuclear localization signal.

Selective regulation of human immunodeficiency virus-infected CD4(+) lymphocytes by a synthetic immunomodulator leads to potent virus suppression in vitro and in hu-PBL-SCID mice

We have previously observed that the synthetic immunomodulator Murabutide inhibits human immunodeficiency virus type 1 (HIV-1) replication at multiple levels in macrophages and dendritic cells. The present study was designed to profile the activity of Murabutide on CD8-depleted phytohemagglutinin-activated lymphocytes from HIV-1-infected subjects and on the outcome of HIV-1 infection in severe combined immunodeficiency mice reconstituted with human peripheral blood leukocytes (hu-PBL-SCID mice). Maintaining cultures of CD8-depleted blasts from 36 patients in the presence of Murabutide produced dramatically reduced levels of viral p24 protein in the supernatants. This activity correlated with reduced viral transcripts and proviral DNA, was evident in cultures harboring R5, X4-R5, or X4 HIV-1 isolates, was not linked to inhibition of cellular DNA synthesis, and did not correlate with beta-chemokine release. Moreover, c-myc mRNA expression was down-regulated in Murabutide-treated cells, suggesting potential interference of the immunomodulator with the nuclear transport of viral preintegration complexes. On the other hand, daily treatment of HIV-1-infected hu-PBL-SCID mice with Murabutide significantly reduced the viral loads in plasma and the proviral DNA content in human peritoneal cells. These results are the first to demonstrate that a clinically acceptable synthetic immunomodulator with an ability to enhance the host's nonspecific immune defense mechanisms against infections can directly regulate cellular factors in infected lymphocytes, leading to controlled HIV-1 replication.

Viral entry into the nucleus

Because many viruses replicate in the nucleus of their host cells, they must have ways of transporting their genome and other components into and out of this compartment. For the incoming virus particle, nuclear entry is often one of the final steps in a complex transport and uncoating program. Typically, it involves recognition by importins (karyopherins), transport to the nucleus, and binding to nuclear pore complexes. Although all viruses take advantage of cellular signals and factors, viruses and viral capsids vary considerably in size, structure, and in how they interact with the nuclear import machinery. Influenza and adenoviruses undergo extensive disassembly prior to genome import; herpesviruses release their genome into the nucleus without immediate capsid disassembly. Polyoma viruses, parvoviruses, and lentivirus preintegration complexes are thought to enter in intact form, whereas the corresponding complexes of onco-retroviruses have to wait for mitosis because they cannot infect interphase nuclei.

Heat-shock protein 70 can replace viral protein R of HIV-1 during nuclear import of the viral preintegration complex

Heat-shock proteins (Hsp's) are a family of molecular chaperones that contribute to protection from environmental stress. In this report, we demonstrate that a member of this family, Hsp70, facilitates nuclear import of HIV-1 preintegration complexes (PICs). The mechanism of this activity appears to be similar to the one used by Vpr, an HIV-1 protein regulating viral nuclear import and replication in macrophages. Indeed Hsp70 stimulated binding of HIV-1 matrix antigen to GST-karyopherin alpha fusion protein and rescued nuclear import of a Vpr-defective HIV-1 strain in vitro. Binding studies with truncated forms of GST-karyopherin alpha demonstrated that both Vpr and Hsp70 bind to a region in the amino-terminal part of the karyopherin alpha molecule. This region appears to be distinct from the binding sites for two other karyopherin alpha cargoes, basic-type NLS-containing proteins and transcription factor STAT-1. Vpr competed with Hsp70 for binding to karyopherin alpha. These results suggest the presence of a novel regulatory site on karyopherin alpha which is used by Hsp70 and Vpr to stimulate interaction between the HIV-1 PIC and karyopherin alpha and thus promote viral nuclear import.

Small molecule inhibitor of HIV-1 nuclear import suppresses HIV-1 replication in human lymphoid tissue ex vivo: a potential addition to current anti-HIV drug repertoire

Despite recent progress in anti-HIV therapy, which has to do mainly with introduction of protease inhibitors into clinical practice, drug toxicity and emergence of drug-resistant isolates during the long-term treatment of the patients necessitates search for new drugs that can be added to currently used components of a multi-drug cocktail in highly active anti-retroviral therapy (HAART). Recently, we described a class of arylene bis(methylketone) compounds that inhibit nuclear import of HIV-1 pre-integration complexes and suppress viral replication in macrophages and PBMC in vitro. In this report, we demonstrate that one of these compounds, CNI-H1194, inhibited HIV-1 replication in primary lymphoid tissue ex vivo. The compound did not antagonize the activity of currently used anti-HIV drugs that inhibit viral reverse transcriptase or protease. These results suggest that arylene bis(methylketone) compounds might be a valuable addition to HAART.

Two nuclear localization signals in the HIV-1 matrix protein regulate nuclear import of the HIV-1 pre-integration complex

Replication of HIV-1 in non-dividing and slowly proliferating cell populations depends on active import of the viral pre-integration complex (PIC) into the cell nucleus. While it is commonly accepted that this process is mediated by an interaction between the HIV-1 PIC and the cellular nuclear import machinery, controversial results have been reported concerning the mechanisms of this interaction. Here, we demonstrate that a recently identified nuclear localization signal within the HIV-1 matrix protein (MA), MA NLS-2, together with previously described MA NLS-1, mediates nuclear import of the HIV-1 PIC. Inactivation of both MA NLSs precluded nuclear translocation of MA and rendered the virus defective in nuclear import and replication in non-dividing macrophage cultures, even when functional Vpr and integrase (IN), two more viral proteins implicated in HIV-1 nuclear import, were present. Taken together, these results indicate that Vpr does not function as an independent nuclear import factor and demonstrate that HIV-1 MA, by virtue of its two nuclear localization signals, regulates HIV-1 nuclear import.