One of the mediators of interferon action is a latent endoribonuclease (ribonuclease L) that is activated by (2'-5')oligoadenylates. Among the homopolymers of the four common ribonucleotides, activated ribonuclease L degrades at an appreciable rate only polyuridylic acid. In two natural RNA's tested the most frequent ribonuclease L cleavages occur after UA, UG, and UU (A, adenine; U, uracil; and G, guanine) and much less frequent cleavages after CA and AC (C, cytosine).

The mammalian 2'-5' oligoadenylate synthetases (2'-5'OASs) are enzymes that are crucial in the interferon-induced antiviral response. They catalyze the polymerization of ATP into 2'-5'-linked oligoadenylates which activate a constitutively expressed latent endonuclease, RNaseL, to block viral replication at the level of mRNA degradation. A molecular evolutionary analysis of available OAS sequences suggests that the vertebrate genes are members of a multigene family with its roots in the early history of tetrapods. The modern mammalian 2'-5'OAS genes underwent successive gene duplication events resulting in three size classes of enzymes, containing one, two, or three homologous domains. Expansion of the OAS gene family occurred by whole-gene duplications to increase gene content and by domain couplings to produce the multidomain genes. Evolutionary analyses show that the 2'-5'OAS genes in rodents underwent gene duplications as recently as 11 MYA and predict the existence of additional undiscovered OAS genes in mammals.

The growth inhibitory effect of IFN-beta was evaluated in 5 human glioma cell lines (AO2V4, GJC, GJR, NN and NNR) and in normal astrocyte cultures (SC and TM). All 5 glioma cell lines showed an anti-proliferative response to IFN-beta whereas normal glial cells were non-responsive. IFN-beta at 10, 100 and 500 U/ml lead to a 30%, 70% and 80% relative decrease in cell number after 12 days, respectively in AO2V4 cells. GJC and GJR cell lines also responded significantly to the lowest concentration of IFN-beta tested and at 500 U/ml the relative cell number decreased 55%. The NN and NNR cells were the least responsive to IFN-beta with maximum growth inhibition of 30% at 500 U IFN-beta/ml. Following treatment with IFN-beta, AO2V4, GJC, GJR and normal astrocytes all expressed mRNA encoding the anti-viral protein, 2-5A synthetase demonstrating that IFN-beta bound to receptors on all four cell lines and activated signal transduction pathways required for induction of an anti-viral protein. A determination of the relative number of viable cells showed that none of these cells exhibited a significant decrease in cell viability. Since the antiproliferative response to IFN-beta was not primarily due to cell death, the effect of IFN-beta on cell cycle progression was evaluated by flow cytometry. All treated glioma cell lines showed a relative increase in proportion of cells in S phase. AO2V4 cells had a 50%-80% increase in the percentage of cells in S phase, whereas GJC, GJR and NNR had percentage increases of 20%-40%. IFN-beta treatment of normal astrocytes did not significantly alter their cell cycle profile. These data suggest that IFN-beta exerts its antiproliferative effect on glioma cells by arresting the ordered progression through S phase or decreasing entry into G2/M phase of the cell cycle.

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