Warner C. Greene

Shaping the nuclear action of NF-κB

The NF-κB/REL family of transcription factors pivotally control the inflammatory and immune responses, as well as other genetic programmes that are central to cell growth and survival. The cytoplasmic regulation of NF-κB is well characterized and, recently, significant progress has been made in understanding how its nuclear action is regulated. Post-translational modification of the NF-κB subunits as well as histones surrounding the NF-κB target genes has a key role in this regulation. Here, we review the important advances that constitute this new and exciting chapter in NF-κB biology.

Qualitative analysis of immune function in patients with the acquired immunodeficiency syndrome: evidence for a selective defect in soluble antigen recognition

We studied purified subpopulations of lymphocytes from patients with the acquired immunodeficiency syndrome (AIDS) in order to determine whether intrinsic defects in lymphocyte function, aside from those due to alterations in lymphocyte numbers, were present. Mitogen-stimulated DNA synthesis, production of gamma interferon, production of interleukin-2, and expression of interleukin-2 receptors, although variably decreased in unseparated cell populations, were normal in populations of purified T-cell subsets. In contrast, DNA synthesis in response to the soluble protein antigen tetanus toxoid was decreased in both unseparated and purified T-cell subpopulations. Cell-mixing experiments demonstrated that the hyporesponsiveness of the unfractionated lymphocytes from patients with AIDS was not due to active suppression. We conclude that the lymphocytes of patients with AIDS, although capable of undergoing a normal degree of blast transformation and lymphokine production after mitogenic stimulation, have an intrinsic defect in their ability to recognize and respond to soluble antigen.

The challenge of finding a cure for HIV infection

Although combination therapy for HIV infection represents a triumph for modern medicine, chronic suppressive therapy is required to contain persistent infection in reservoirs such as latently infected CD4+ lymphocytes and cells of the macrophage-monocyte lineage. Despite its success, chronic suppressive therapy is limited by its cost, the requirement of lifelong adherence, and the unknown effects of long-term treatment. This review discusses our current understanding of suppressive antiretroviral therapy, the latent viral reservoir, and the needs for and challenges of attacking this reservoir to achieve a cure.

HIV-1 Vif blocks the antiviral activity of APOBEC3G by impairing both its translation and intracellular stability.

The human immunodeficiency virus type 1 (HIV-1) relies on Vif (viral infectivity factor) to overcome the potent antiviral function of APOBEC3G (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G, also known as CEM15). Using an APOBEC3G-specific antiserum, we now show that Vif prevents virion incorporation of endogenous APOBEC3G by effectively depleting the intracellular levels of this enzyme in HIV-1-infected T cells. Vif achieves this depletion by both impairing the translation of APOBEC3G mRNA and accelerating the posttranslational degradation of the APOBEC3G protein by the 26S proteasome. Vif physically interacts with APOBEC3G, and expression of Vif alone in the absence of other HIV-1 proteins is sufficient to cause depletion of APOBEC3G. These findings highlight how the bimodal translational and posttranslational inhibitory effects of Vif on APOBEC3G combine to markedly suppress the expression of this potent antiviral enzyme in virally infected cells, thereby effectively curtailing the incorporation of APOBEC3G into newly formed HIV-1 virions.

Acetylation of RelA at discrete sites regulates distinct nuclear functions of NF-κB

The nuclear function of the heterodimeric NF‐κB transcription factor is regulated in part through reversible acetylation of its RelA subunit. We now demonstrate that the p300 and CBP acetyltransferases play a major role in the in vivo acetylation of RelA, principally targeting lysines 218, 221 and 310 for modification. Analysis of the functional properties of hypoacetylated RelA mutants containing lysine‐to‐arginine substitutions at these sites and of wild‐type RelA co‐expressed in the presence of a dominantly interfering mutant of p300 reveals that acetylation at lysine 221 in RelA enhances DNA binding and impairs assembly with IκBα. Conversely, acetylation of lysine 310 is required for full transcriptional activity of RelA in the absence of effects on DNA binding and IκBα assembly. Together, these findings highlight how site‐specific acetylation of RelA differentially regulates distinct biological activities of the NF‐κB transcription factor complex.

Cross-coupling of the NF-kappa B p65 and Fos/Jun transcription factors produces potentiated biological function.

NF‐kappa B and AP‐1 represent distinct mammalian transcription factors that target unique DNA enhancer elements. The heterodimeric NF‐kappa B complex is typically composed of two DNA binding subunits, NF‐kappa B p50 and NF‐kappa B p65, which share structural homology with the c‐rel proto‐oncogene product. Similarly, the AP‐1 transcription factor complex is comprised of dimers of the c‐fos and c‐jun proto‐oncogene products or of closely related proteins. We now demonstrate that the bZIP regions of c‐Fos and c‐Jun are capable of physically interacting with NF‐kappa B p65 through the Rel homology domain. This complex of NF‐kappa B p65 and Jun or Fos exhibits enhanced DNA binding and biological function via both the kappa B and AP‐1 response elements including synergistic activation of the 5′ long terminal repeat of the human immunodeficiency virus type 1. These findings support a combinatorial mechanism of gene regulation involving the unexpected cross‐coupling of two different classes of transcription factors to form novel protein complexes exhibiting potentiated biological activity.

Cell death by pyroptosis drives CD4 T-cell depletion in HIV-1 infection

The pathway causing CD4 T-cell death in HIV-infected hosts remains poorly understood although apoptosis has been proposed as a key mechanism. We now show that caspase-3-mediated apoptosis accounts for the death of only a small fraction of CD4 T cells corresponding to those that are both activated and productively infected. The remaining over 95% of quiescent lymphoid CD4 T cells die by caspase-1-mediated pyroptosis triggered by abortive viral infection. Pyroptosis corresponds to an intensely inflammatory form of programmed cell death in which cytoplasmic contents and pro-inflammatory cytokines, including IL-1β, are released. This death pathway thus links the two signature events in HIV infection—CD4 T-cell depletion and chronic inflammation—and creates a pathogenic vicious cycle in which dying CD4 T cells release inflammatory signals that attract more cells to die. This cycle can be broken by caspase 1 inhibitors shown to be safe in humans, raising the possibility of a new class of ‘anti-AIDS’ therapeutics targeting the host rather than the virus.

Cellular APOBEC3G restricts HIV-1 infection in resting CD4+ T cells

In contrast to activated CD4+ T cells, resting human CD4+ T cells circulating in blood are highly resistant to infection with human immunodeficiency virus (HIV). Whether the inability of HIV to infect these resting CD4+ T cells is due to the lack of a key factor, or alternatively reflects the presence of an efficient mechanism for defence against HIV, is not clear. Here we show that the anti-retroviral deoxycytidine deaminase APOBEC3G strongly protects unstimulated peripheral blood CD4+ T cells against HIV-1 infection. In activated CD4+ T cells, cytoplasmic APOBEC3G resides in an enzymatically inactive, high-molecular-mass (HMM) ribonucleoprotein complex that converts to an enzymatically active low-molecular-mass (LMM) form after treatment with RNase. In contrast, LMM APOBEC3G predominates in unstimulated CD4+ T cells, where HIV-1 replication is blocked and reverse transcription is impaired. Mitogen activation induces the recruitment of LMM APOBEC3G into the HMM complex, and this correlates with a sharp increase in permissivity for HIV infection in these stimulated cells. Notably, when APOBEC3G-specific small interfering RNAs are introduced into unstimulated CD4+ T cells, the early replication block encountered by HIV-1 is greatly relieved. Thus, LMM APOBEC3G functions as a potent post-entry restriction factor for HIV-1 in unstimulated CD4+ T cells. Surprisingly, sequencing of the reverse transcripts slowly formed in unstimulated CD4+ T cells reveals only low levels of dG → dA hypermutation, raising the possibility that the APOBEC3G-restricting activity may not be strictly dependent on deoxycytidine deamination.

A sensitive and specific enzyme-based assay detecting HIV-1 virion fusion in primary T lymphocytes

As an early event in the viral life cycle, the entry of enveloped viruses into target cells has received considerable attention. Viral fusion to cellular targets has been studied principally with fusion assays in which cells engineered to express the viral envelope are cultured with the target cells. These assays yield valuable information but do not fully recapitulate all of the variables governing the fusion of actual virions to their cellular targets. The virion membrane and the plasma membrane, for example, differ strikingly in their lipid and protein compositions. Two virion-based fusion assays have been described. One is based on the redistribution of a self-quenching fluorophore, whereas the second depends on photosensitized activation of a hydrophobic probe by a fluorescent lipid loaded into the target membrane. These assays are complex and have not been adapted to study fusion in complex cell populations. We have developed a simple, rapid assay allowing the detection of HIV-1 virion fusion to biologically relevant target cells, including primary CD4+ T lymphocytes. It is based on the incorporation of β-lactamase–Vpr chimeric proteins (BlaM-Vpr) into HIV-1 virions and their subsequent delivery into the cytoplasm of target cells as a result of virion fusion. This transfer is then detected by enzymatic cleavage of the CCF2 dye, a fluorescent substrate of β-lactamase (BlaM), loaded in the target cells. BlaM cleaves the β-lactam ring in CCF2, changing its fluorescence emission spectrum from green (520 nm) to blue (447 nm) and thereby allowing fusion to be detected by fluorescence microscopy, flow cytometry, or UV photometry.

NF‐κB p50 promotes HIV latency through HDAC recruitment and repression of transcriptional initiation

Cells latently infected with HIV represent a currently insurmountable barrier to viral eradication in infected patients. Using the J‐Lat human T‐cell model of HIV latency, we have investigated the role of host factor binding to the κB enhancer elements of the HIV long terminal repeat (LTR) in the maintenance of viral latency. We show that NF‐κB p50–HDAC1 complexes constitutively bind the latent HIV LTR and induce histone deacetylation and repressive changes in chromatin structure of the HIV LTR, changes that impair recruitment of RNA polymerase II and transcriptional initiation. Knockdown of p50 expression with specific small hairpin RNAs reduces HDAC1 binding to the latent HIV LTR and induces RNA polymerase II recruitment. Similarly, inhibition of histone deacetylase (HDAC) activity with trichostatin A promotes binding of RNA polymerase II to the latent HIV LTR. This bound polymerase complex, however, remains non‐processive, generating only short viral transcripts. Synthesis of full‐length viral transcripts can be rescued under these conditions by expression of Tat. The combination of HDAC inhibitors and Tat merits consideration as a new strategy for purging latent HIV proviruses from their cellular reservoirs.