Cellular and enzymatic activities of a synthetic heteropolymer double-stranded RNA of defined size

Inhibition of human cancer-cell proliferation by long double-stranded RNAs

Three different enzymatically synthesized long double-stranded RNAs (dsRNAs) [448 bp homologous to the third exon of c-myc messenger RNA (mRNA) (dsMyc); 473 bp homologous to enhanced green fluorescent protein (EGFP) mRNA (dsEGFP) and control interferon inducer poly(I:C)] were studied for antiproliferative and gene-silencing activities in KB-3-1, SK-N-MC, and IMR-32 human cancer cell lines. Simple incubation with these dsRNAs did not affect the expression of c-myc gene and the proliferation of KB-3-1 and IMR-32 cells, but inhibited the proliferation of SK-N-MC cells. Transfection of KB-3-1 and SK-N-MC cells using Oligofectamine-dsRNAs complexes resulted in dose-dependent inhibition of c-myc and beta-actin genes expression and proliferation. The data show that dsMyc, acting both as interferon inducer and as gene-specific interfering RNA, is more effective as c-myc inhibitor than other tested dsRNAs. The most efficient inhibition of proliferation was displayed by dsEGFP RNA, dsMyc and poly(I:C) were effective only when used in higher concentrations. Our data indicate that transfection of studied dsRNAs causes an increase in apoptotic and dead cells number in the cell population. This proapoptotic activity correlates with dsRNAs-induced antiproliferative activity. However the difference in cell growth between dsRNA-treated and Oligofectamine-only treated cells can not be attributed only to the loss of cells due to the apoptosis; it also indicates some retardation of cell cycle progression caused by dsRNA.

Viral double-stranded RNA, cytokines, and the flu

The symptoms of the flu, such as fever, drowsiness, and malaise, are the sole means by which this common clinical syndrome is defined. The syndrome is usually the first clinical manifestation of both acute bacterial and viral infections. In the case of acute bacterial infections, several proinflammatory cytokines induced by bacterial products have been implicated as the causative agents of the flu syndrome. Viruses induce similar cytokines to bacteria, plus substantial amounts of interferon-alpha (IFN-alpha), although the direct association of these cytokines with the viral flu syndrome is less clear. Furthermore, the viral inducer(s) of cytokines has not been defined. The best candidate cytokine inducer associated with a majority of viral infections is virus-associated double-stranded RNA (dsRNA). This review examines the essential physical properties of toxic dsRNA, the cytokines induced by it, its viral and cellular sources, evidence for its presence in infected cells, its quantities in normal and infected cells, its cytotoxic mechanisms, and its cell-penetration properties. Toxic effects of viruses and dsRNA are compared. Energetics and extraction artifact issues are also discussed. Whereas most research on dsRNA toxicity has employed synthetic dsRNA, studies with virus-associated dsRNA are featured when available. Finally, a model for how viral dsRNA might initiate systemic disease is presented.

The 2-5A system: modulation of viral and cellular processes through acceleration of RNA degradation

The 2-5A system is an RNA degradation pathway that can be induced by the interferons (IFNs). Treatment of cells with IFN activates genes encoding several double-stranded RNA (dsRNA)-dependent synthetases. These enzymes generate 5'-triphosphorylated, 2',5'-phosphodiester-linked oligoadenylates (2-5A) from ATP. The effects of 2-5A in cells are transient since 2-5A is unstable in cells due to the activities of phosphodiesterase and phosphatase. 2-5A activates the endoribonuclease 2-5A-dependent RNase L, causing degradation of single-stranded RNA with moderate specificity. The human 2-5A-dependent RNase is an 83.5 kDa polypeptide that has little, if any, RNase activity, unless 2-5A is present. 2-5A binding to RNase L switches the enzyme from its off-state to its on-state. At least three 2',5'-linked oligoadenylates and a single 5'-phosphoryl group are required for maximal activation of the RNase. Even though the constitutive presence of 2-5A-dependent RNase is observed in nearly all mammalian cell types, cellular amounts of 2-5A-dependent mRNA and activity can increase after IFN treatment. One well-established role of the 2-5A system is as a host defense against some types of viruses. Since virus infection of cells results in the production and secretion of IFNs, and since dsRNA is both a frequent product of virus infection and an activator of 2-5A synthesis, the replication of encephalomyocarditis virus, which produces dsRNA during its life cycle, is greatly suppressed in IFN-treated cells as a direct result of RNA decay by the activated 2-5A-dependent RNase. This review covers the organic chemistry, enzymology, and molecular biology of 2-5A and its associated enzymes. Additional possible biological roles of the 2-5A system, such as in cell growth and differentiation, human immunodeficiency virus replication, heat shock, atherosclerotic plaque, pathogenesis of Type I diabetes, and apoptosis, are presented.