2,5-A synthetase

The molecular basis by which interferons promote their characteristic effects, in particular antiviral activity, is understood at least in part. Interferon stimulation of the JAK-STAT pathway induces synthesis of at least 30 different gene products, many of which cooperate to inhibit viral replication. These antiviral gene products are generally enzymes, the most important of which are 2 -5 oligoadenylate synthetase (2,5-A synthetase) and the eIF-2a protein kinase. 

These intracellular enzymes remain in an inactive state after their initial induction. They are activated only when the cell comes under viral attack, and their activation can inhibit viral replication in that cell. The 2,5-A synthetase acts in concert with two additional enzymes, i.e. an endori-bonuclease and a phosphodiesterase, to promote and regulate the antiviral state (Figure 8.4). 

The sole biochemical function of 2 -5A (and hence 2 -5 A synthetase) appears to be as an activator of a dormant endo-RNase, which is expressed constitutively in the cell. This RNase, known as RNase L or RNase F, cleaves all types of single-stranded RNA (ssRNA). This inhibits production of both viral and cellular proteins, thus paralyzing viral replication. Presumably, cellular destruction of the invading ssRNA will be accompanied by destruction of any additional viral components. Removal of dsRNA would facilitate deactivation of the endo-RNase, allowing translation of cellular mRNA to resume. A 2-5 phosphodiesterase represents a third enzymatic... 

Figure 8.5 (a) Structural detail of the 2 -5 oligonucleotides (2 -5 A ) generated by 2 -5 A synthetase. Compare the 2 -5 phosphodiester linkages with the 3 -5 linkages characteristic of normal cellular oligonucleotides such as mRNA (b)... 

The ability of interferons (especially type I interferons) to induce an antiviral state is unlikely to be solely dependent upon the enzymatic mechanisms discussed above. Furthermore the 2 -5 A synthetase and eIF-2a kinase systems may play important roles in mediating additional interferon actions. The ability of such systems to stall protein synthesis in cells may play a role in interferon-induced alterations of cellular differentiation or cell cycle progression. They may also be involved in mediating interferon-induced anti-proliferative effects on various transformed cells. 

Figure 11. Effect of poly(ICLC) (100 ug/kg) injection to humans on 2 5 A. synthetase. 

The antiviral state induced by different types of IFNs is mediated by various IFN-induced proteins. The best-known antiviral effectors produced as a result of IFN cascade induction are shown in Table 2. They include 2 -5 oligoadenylate synthetase (2 -5 OAS), double-stranded RNA activated protein kinase (PKR), and myxovirus (Mx) proteins. Additional effectors include RNA-specific adenosine deaminase 1 (ADARl), the 20-kDa ISG product (ISG20), ISG54 and ISG56, and IFN-stimulated micro RNAs (Pedersen et al. 2007). 

OAS 2 -5 oligoadenylate synthetase Activate a latent endoribonuclease, RNAse L, which degrades viral and cellular mRNAs and rRNAs... 

Kajaste-Rudnitski, A., Mashimo, T., ErenMel, M. P., Guenet, J. L., Lucas, M., and Despres, P. (2006) The 2, 5"-oligoadenylate synthetase lb is a potent inhibitor of West Nile virus replication inside infected cells. J. Biol.Chem. 281, 4624-4637. 

Mashimo, T., Lucas, M., Simon-Chazottes, D., et al. (2002) A nonsense mutation in the gene encoding 2, 5 -oligoadenylate synthetase/Ll isoform is associated with West Nile virus susceptibility in laboratory mice. Proc. Natl. Acad. Sci. U. S. A. 99, 11311-11316. 

Merritt, J.A., L.A. Ball, K.M. Sielaff, D.M. Meltzer, and E.C. Borden, Modulation of 2, 5 -oligoadenylate synthetase in patients treated with alpha-interferon effects of dose, schedule, and route of administration. J Interferon Res, 1986. 6(3) 189-98. 

Pharmacokinetics The pharmacokinetic properties of Infergen have not been evaluated in patients with chronic hepatitis C. Pharmacokinetic profiles were evaluated in normal, healthy volunteer subjects after subcutaneous injection of interferon alfacon-1 at doses up to 9 j,g. Plasma levels of interferon alfacon-1 at any dose were too low to be detected. However, analysis of Infergen-induced cellular products— induction of 2 5 oligoadenylate synthetase and (beta)-2 microglobulin—after treatment in these subjects revealed a statistically significant, dose-related increase in the area under the curve (AUC). 

The crystal structure of succinyl-CoA synthetase from Es-cherichia coli at 2.5-A resolution. J. Biol. Chem. 269, 10,883-10,890. 

Binding of human interferon-lb to specific cell receptors induces the expression of a number of interferon-induced gene products (e.g., 2, 5 -oligoandenylate synthetase, protein kinase, and indoleamine 2,3-dioxygenase that are believed to be mediators of the biological actions... 

R. A second class of synthetase structure revealed by X-ray analysis of Escherichia coli seryl-tRNA synthetase at 2.5 A. Nature 1990 347 249-255. 

Although it is not possible to delineate the mechanisms by which interferon beta-lb exerts its activity in MS. it is known that the interferon binds to specific receptors on cell surfaces and induces the expression of a number of interferon-induced gene products, such as 2. 5 -oligoadenylate synthetase and protein kinase. Additionally, interferon beta-lb blocks the synthesis of INF-y. which is believed to be involved in MS attacks. 

A second polynucleotide that has been studied extensively is ampligen, a mismatched poly rLpoly rC 12U resulting from mispairing of the duplex (16). This compound has been shown to induce interferon and to activate 2 -5 -adenosine synthetase (17). Ampligen has properties similar to those of poly rLpoly rC, although it has been reported to have broader activities and lower toxicities and is still in development. 

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