Impairment of human immunodeficiency virus type 1 (HIV-1) entry into Jurkat T cells by constitutive expression of the HIV-1 vpr protein: role of CD4 down-modulation

Signal peptide-binding drug as a selective inhibitor of co-translational protein translocation

In eukaryotic cells, surface expression of most type I transmembrane proteins requires translation and simultaneous insertion of the precursor protein into the endoplasmic reticulum (ER) membrane for subsequent routing to the cell surface. This co-translational translocation pathway is initiated when a hydrophobic N-terminal signal peptide (SP) on the nascent protein emerges from the ribosome, binds the cytosolic signal recognition particle (SRP), and targets the ribosome-nascent chain complex to the Sec61 translocon, a universally conserved protein-conducting channel in the ER-membrane. Despite their common function in Sec61 targeting and ER translocation, SPs have diverse but unique primary sequences. Thus, drugs that recognise SPs could be exploited to inhibit translocation of specific proteins into the ER. Here, through flow cytometric analysis the small-molecule macrocycle cyclotriazadisulfonamide (CADA) is identified as a highly selective human CD4 (hCD4) down-modulator. We show that CADA inhibits CD4 biogenesis and that this is due to its ability to inhibit co-translational translocation of CD4 into the lumen of the ER, both in cells as in a cell-free in vitro translation/translocation system. The activity of CADA maps to the cleavable N-terminal SP of hCD4. Moreover, through surface plasmon resonance analysis we were able to show direct binding of CADA to the SP of hCD4 and identify this SP as the target of our drug. Furthermore, CADA locks the SP in the translocon during a post-targeting step, possibly in a folded state, and prevents the translocation of the associated protein into the ER lumen. Instead, the precursor protein is routed to the cytosol for degradation. These findings demonstrate that a synthetic, cell-permeable small-molecule can be developed as a SP-binding drug to selectively inhibit protein translocation and to reversibly regulate the expression of specific target proteins.

Improving potencies and properties of CD4 down-modulating CADA analogs

INTRODUCTION

CADA is a synthetic small molecule that inhibits HIV replication in cell cultures through down-modulating cell surface CD4 by inhibiting cotranslational translocation of nascent CD4 across the ER membrane in a signal sequence-specific manner. Analogs have been prepared mainly to increase potency and investigate the mechanism of action.

AREAS COVERED

This article reviews progress on discovery of more potent CADA analogs, including symmetrical and unsymmetrical compounds, as well as fluorescent derivatives. The article also discusses some properties of CADA and a more potent analog (KKD023) that are relevant to drug development, including aqueous solubility, permeability, metabolism and oral bioavailability.

EXPERT OPINION

Further studies on CADA analogs should focus on improving both potency and drug-like properties, and on elucidating the detailed mechanism of action. Solubility and permeability may be improved by reducing molecular weight, decreasing molecular flexibility and symmetry, or by a prodrug approach inducing active transport. Identifying the molecular mechanism of CD4 down-modulation may aid in assessing potential side effects of such immunomodulatory/anti-HIV drugs, and it could potentially lead to a general approach to designing drugs for specifically down-modulating other cell-surface proteins.

Unsymmetrical cyclotriazadisulfonamide (CADA) compounds as human CD4 receptor down-modulating agents

Cyclotriazadisulfonamide (CADA) inhibits HIV at submicromolar levels by specifically down-modulating cell-surface and intracellular CD4. The specific biomolecular target of CADA compounds is unknown, but previous studies led to an unsymmetrical binding model. To test this model, methods were developed for effective synthesis of diverse, unsymmetrical CADA compounds. A total of 13 new, unsymmetrical target compounds were synthesized, as well as one symmetrical analogue. The new compounds display a wide range of potency for CD4 down-modulation in CHO·CD4-YFP cells. VGD020 (IC(50) = 46 nM) is the most potent CADA compound discovered to date, and VGD029 (IC(50) = 730 nM) is the most potent fluorescent analogue. Structure-activity relationships are analyzed from the standpoint of additive or nonadditive energy effects of different substituents. They appear to be consistent with the zipper-type mechanism in which entropy costs are reduced for additional stabilizing interactions between the small molecule and its protein target.

Downregulation of CD4 is required for maintenance of viral infectivity of HIV-1

Downregulation of virus receptors on the cell surface is considered to be important in preventing superinfection. HIV-1 encodes multiple gene products, Env, Vpu, and Nef, involved in downregulation of CD4, a major HIV-1 receptor. We found that simultaneous mutations in both vpu and nef severely impaired virus replication. We examined the involvement of CD4 downregulation mediated by Vpu and Nef in the modification of virus infectivity. The mutation in vpu increased CD4 incorporation into virions without affecting the Env content in it, inhibiting the attachment step of virions to the CD4-positive cell surface. Although a single mutation in nef suppresses virus infectivity via a CD4-independent mechanism, it could augment CD4 incorporation in virions in combination with a vpu mutation. These results indicated that CD4 downregulation was necessary for maintenance of Env function in the virion.

Surface CD4 expression modulated by a cellular factor induced by HIV type 1 infection

Human immunodeficiency virus type 1 (HIV-1) alters gene expression in infected cells, leading to cellular dysfunction. We uncovered a number of host cell genes that are modulated in both CD4(+) T cell lines and primary CD4(+) T lymphocytes infected with HIV-1, using high-density oligonucleotide probe microarray technology. We focused on one gene in particular, nuclear factor I-B2 (NFI-B2), because of its high level of expression. NFI-B2 is a member of the nuclear factor I family of nuclear proteins, which are known to be involved in viral and cellular transcription. To better understand the role of NFI-B2 during HIV-1 infection, we generated a Jurkat T cell line that constitutively expressed NFI-B2. After infection with HIV-1, these cells produced fewer viruses because of a downregulation of surface CD4 expression. The surface expression of the coreceptor, CXCR4, remained unchanged. Furthermore, levels of CD4 mRNA were reduced in NFI-B2-producing cells, suggesting that expression of NFI-B2 impairs CD4 transcription. Modulation of NFI-B2 by HIV-1 may represent yet another mechanism by which HIV infection reduces cell surface expression of CD4.

Human immunodeficiency virus type 1 Vpr protein does not modulate surface expression of the CD4 receptor

The CD4 receptor is required for the entry of human immunodeficiency virus (HIV) into target cells. It has long been known that Nef, Env, and Vpu participate in the removal of the viral receptor from the cell surface. Recently, it has been proposed that the HIV type 1 (HIV-1) Vpr protein may also play a role in the downmodulation of CD4 from the surfaces of infected cells (L. Conti, B. Varano, M. C. Gauzzi, P. Matarrese, M. Federico, W. Malorani, F. Belardelli, and S. Gessani, J. Virol. 74:10207-10211, 2000). To investigate the possible role of Vpr in the downregulation of the viral receptor Vpr alleles from HIV-1 and simian immunodeficiency virus were transiently expressed in transformed T cells and in 293T fibroblasts, and their ability to modulate surface CD4 was evaluated. All Vpr alleles efficiently arrested cells in the G(2) stage of the cell cycle. However, none of the tested Vpr proteins altered the expression of CD4 on the cell surface. In comparison, HIV-1 Nef efficiently downmodulated surface CD4 in all the experimental settings. Transformed T cells and primary lymphocytes were challenged with wild-type, Nef-defective, and Vpr-defective viruses. A significant reduction in the HIV-induced downmodulation of surface CD4 was observed in viruses lacking Nef. However, Vpr-deletion-containing viruses showed no defect in their ability to remove CD4 from the surfaces of infected cells. Our results indicate that Vpr does not play a role in the HIV-induced downmodulation of the CD4 receptor.

Reciprocal regulatory interaction between human herpesvirus 8 and human immunodeficiency virus type 1

Human herpesvirus 8 (HHV8) is the primary viral etiologic agent in Kaposi's sarcoma (KS). However, individuals dually infected with both HHV8 and human immunodeficiency virus type 1 (HIV-1) show an enhanced prevalence of KS when compared with those singularly infected with HHV8. Host immune suppression conferred by HIV infection cannot wholly explain this increased presentation of KS. To better understand how HHV8 and HIV-1 might interact directly in the pathogenesis of KS, we queried for potential regulatory interactions between the two viruses. Here, we report that HHV8 and HIV-1 reciprocally up-regulate the gene expression of each other. We found that the KIE2 immediate-early gene product of HHV8 interacted synergistically with Tat in activating expression from the HIV-1 long terminal repeat. On the other hand, HIV-1 encoded Tat and Vpr proteins increased intracellular HHV8-specific expression. These results provide molecular insights correlating coinfection with HHV8 and HIV-1 with an unusually high incidence of KS.