Inhibition of mouse leukemia virus (MLV) replication by actinomycin D

A novel cell-based high-throughput screen for inhibitors of HIV-1 gene expression and budding identifies the cardiac glycosides

OBJECTIVES

Highly active antiretroviral therapy (HAART) is the mainstay of treatment for HIV-1 infection. While current HAART regimens have been extremely effective, issues of associated toxicity, cost and resistance remain and there is a need for novel antiretroviral compounds to complement the existing therapy. We sought to develop a novel high-throughput method for identifying compounds that block later steps in the life cycle not targeted by current therapy.

METHODS

We designed a high-throughput screen to identify inhibitors of post-integration steps in the HIV-1 life cycle. The screening method was applied to a library of compounds that included numerous FDA-approved small molecules.

RESULTS

Among the small molecules that inhibited late stages in HIV-1 replication were members of the cardiac glycoside family. We demonstrate that cardiac glycosides potently inhibit HIV-1 gene expression, thereby reducing the production of infectious HIV-1. We demonstrate that this inhibition is dependent upon the human Na(+)/K(+)-ATPase, but independent of cardiac glycoside-induced increases in intracellular Ca(2+).

CONCLUSIONS

We have validated a novel high-throughput screen to identify small molecule inhibitors of HIV-1 gene expression, virion assembly and budding. Using this screen, we have demonstrated that a number of FDA-approved compounds developed for other purposes potently inhibit HIV-1 replication, including the cardiac glycosides. Our work indicates that the entire cardiac glycoside family of drugs shows potential for antiretroviral drug development.

Actinomycin D inhibits human immunodeficiency virus type 1 minus-strand transfer in in vitro and endogenous reverse transcriptase assays

In this report we demonstrate that human immunodeficiency virus type 1 (HIV-1) minus-strand transfer, assayed in vitro and in endogenous reactions, is greatly inhibited by actinomycin D. Previously we showed that HIV-1 nucleocapsid (NC) protein (a nucleic acid chaperone catalyzing nucleic acid rearrangements which lead to more thermodynamically stable conformations) dramatically stimulates HIV-1 minus-strand transfer by preventing TAR-dependent self-priming from minus-strand strong-stop DNA [(-) SSDNA]. Despite this potent activity, the addition of NC to in vitro reactions with actinomycin D results in only a modest increase in the 50% inhibitory concentration (IC50) for the drug. PCR analysis of HIV-1 endogenous reactions indicates that minus-strand transfer is inhibited by the drug with an IC50 similar to that observed when NC is present in the in vitro system. Taken together, these results demonstrate that NC cannot overcome the inhibitory effect of actinomycin D on minus-strand transfer. Other experiments reveal that at actinomycin D concentrations which severely curtail minus-strand transfer, neither the synthesis of (-) SSDNA nor RNase H degradation of donor RNA is affected; however, the annealing of (-) SSDNA to acceptor RNA is significantly reduced. Thus, inhibition of the annealing reaction is responsible for actinomycin D-mediated inhibition of strand transfer. Since NC (but not reverse transcriptase) is required for efficient annealing, we conclude that actinomycin D inhibits minus-strand transfer by blocking the nucleic acid chaperone activity of NC. Our findings also suggest that actinomycin D, already approved for treatment of certain tumors, might be useful in combination therapy for AIDS.

Metabolism of viral RNA in murine leukemia virus-infected cells; evidence for differential stability of viral message and virion precursor RNA

Molecular hybridization techniques were used to examine the stability of viral message and virion precursor RNA in murine leukemia virus-infected cells treated with actinomycin D. Under the conditions used, viral RNA synthesis was inhibited, but viral protein synthesis continued, and the cells produced noninfectious particles (actinomycin D virions) lacking genomic RNA (J. G. Levin and M. J. Rosenak, Proc. Natl. Acad. Sci. U.S.A. 73:1154-1158, 1976). Analysis of total RNA in virions revealed that the amount of hybridizable viral RNA decreased steadily after the addition of actinomycin D and by 8 h was 10% of the control value. Studies on fractionated viral RNA showed that this low level of hybridization is due to residual 70S RNA in the virion population. The results indicated that viral RNA which is destined to be encapsidated into virions has a half-life of approximately 3 to 4 h. In contrast, other intracellular virus-specific RNA molecules appeared to be quite stable and persisted for a long period of time, with a half-life of at least 12 h. These observations support the idea that two independent functional pools of 35S viral RNA exist within the infected cell: one serving as message and the other as precursor to virion RNA. The existence of two viral RNA pools was further documented by the finding that 12 h after the addition of actinomycin D, when virion precursor RNA was depleted, 35S and 21S viral nRNA species could be identified in polyribosomal RNA as well as in total polyadenylated cell RNA. Surprisingly, 35S and mRNA declined more rapidly than did 21S mRNA, which appeared to be increased in amount.

Selective decrease in the rate of cleavage of an intracellular precursor to Rauscher leukemia virus p30 by treatment of infected cells with actinomycin D

The cleavage of an intracellular 67,000- to 70,000-dalton precursor, termed Pr4 to Rauscher leukemia virus (RLV) p30 protein proceeded at a slower rate when virus-producing cells were treated with actinomycin D (AMD). Treatment with AMD also caused a slight accumulation of Pr4 in purified early virus particles produced by a cell line which usually produces virions that contain little Pr4. The cleavage of other intracellular viral precursor polypeptides was not affected by treatment with AMD. Treatment of infected cells with cycloheximide, on the other hand, allowed the cleavage of Pr4 to proceed at the usual rate for a short period of time before further cleavage was drastically slowed or prevented. The cleavage of several other viral precursor polypeptides was also inhibited by treatment with cycloheximide. Different lines of evidence suggest that the mechanism of action of AMD is not due to a possible indirect effect on protein synthesis. Thus, the rate of cleavage of Pr4 was not affected by the length of pretreatment with AMD between 1 to 8 h. In addition, the combined effect of AMD and cycloheximide, at their maximal inhibitory concentrations, was greater than the effect of either drug alone, indicating the involvement of two at least partially different mechanisms in the action of AMD and cycloheximide. Furthermore, AMD did not affect the pulse labeling of viral precursor polypeptides. These results suggest that the interaction with viral RNA, whose production is inhibited by AMD, accelerates the cleavage of Pr4 to p30 during virus assembly. A hypothetical model is presented to illustrate th possible advantages of having a step in virus assembly in which genomic RNA interacts with a precursor to capsid proteins before the cleavage of that precursor.

Deficiency of 60 to 70S RNA in murine leukemia virus particles assembled in cells treated with actinomycin D

Production of particles with the ultrastructural appearance of C-type virions persisted for at least 6 h in actinomycin D-treated cells infected with murine leukemia virus. This phenomenon occurred despite severe inhibition of viral RNA synthesis. Virus particles present in a 6-h harvest sedimented in sucrose gradients with the buoyant density characteristic of RNA tumor viruses (1.16 g/cm(3)) and exhibited high levels of reverse transcriptase activity in response to the exogenous template polyriboadenylic acid.oligo deoxythymidylic acid in the range of untreated controls. However, RNase-sensitive endogenous activity was only (1/5) the level found in controls. This observation correlated with a marked reduction in infectivity. Kinetic studies on the appearance of labeled RNA in banded virions revealed that within the first hour after addition of actinomycin D, particles contained 60 to 70S RNA and two low-molecular-weight RNA species corresponding to 8 and 4S RNA. After approximately 1 h of incubation with actinomycin D, 60 to 70S RNA could not be detected and 4S RNA was the predominant species. These findings suggest that murine leukemia virus particles assembled in the presence of actinomycin D are deficient in 60 to 70S viral RNA.

Free and membrane-bound polyribosomes in normal and Rauscher-virus-infected mouse spleen cells

1. Free and membrane-bound polyribosomes and ribosomal monomers were isolated from normal and Rauscher-virus-infected mouse spleens by means of discontinuous sucrose density gradients. 2. The addition of ribonuclease inhibitor from rat liver was essential to protect these polyribosomes from degradation. To separate the smooth and rough membranes from ribosomal monomers an additional centrifugation step through a continuous sucrose density gradient was necessary. 3. After infection a marked increase in rRNA from both membrane-bound and free polyribosomes was observed. Treatment of the membrane-bound polyribosomes with sodium deoxycholate yielded only 80S particles even when ribonuclease inhibitor was added. 4. A striking feature of the infected spleen was the occurrence of large polyribosomes. Up to 40 monomers per polyribosome could be counted on electron micrographs.

On the role of DNA synthesis in avian tumor virus infection

Preinfection of chick embryo fibroblasts with an avian tumor virus does not eliminate the requirement of early DNA synthesis for other avian tumor viruses superinfecting the same cells several days after the primary inoculum. The function served by the early DNA in avian tumor virus infection either fails to persist into late infection or is genetically specific for each viral type.

Characterization of intracellular ribonucleic acid specific for the murine sarcoma-leukemia virus complex

Virus particles were continuously produced by a cell line (78A1) of rat embryo fibroblasts that had been transformed by the murine sarcoma-leukemia virus complex. Since most of the mature virions were found in the extracellular fluid and were not cell-associated, a measurable quantity of viral ribonucleic acid (RNA) could not be extracted from these cells. Cycloheximide, a protein inhibitor, was successfully used to accumulate viral RNA within the cells. This ribonuclease-sensitive RNA, with a sedimentation coefficient of 71S, had the same base composition as the high molecular weight RNA (S(20,w) = 71) isolated from purified virions released by the transformed cells.