Assay of (2'-5')-oligo(A) synthesized in vitro and the analysis of naturally occurring (2'-5')-oligo(A) from intact cells

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.

Fractionation and characterization of 2',5'-oligoadenylates by polyacrylamide gel electrophoresis: an alternative method for assaying 2',5'-oligoadenylate synthetase

2',5'-Linked oligoadenylates of varying chain lengths (2,5As) are formed from ATP by an interferon (IFN)-induced enzyme, 2',5'-oligoadenylate synthetase (2-5A synthetase). To identify these multiple forms, a method was devised utilizing electrophoretic separation of 32P-labeled 2,5As in a thin 20% polyacrylamide gel containing 7 M urea. A mixture of 2,5As synthesized from rat liver nuclear suspension was fractionated by this method. Each species was eluted from the gel for characterization by specific nucleotidylic enzymes. All major species in the gel were identified, including dimeric, trimeric, and tetrameric forms with either 5' tri- or diphosphates, as well as dephosphorylated species. Thus, in a single step, this method produced a more complete assessment of newly synthesized 2,5As and their degradative products than conventional, multistep DEAE-cellulose chromatography. It also allowed more rapid screening of multiple samples than high-pressure liquid chromatography (HPLC). This method, used to assay 2-5A synthetase induction by IFN-alpha in human T-cell H9 and CEM-CM3 lines, should be applicable for routine analysis of clinical specimens.

Constitutive expression of a 2',5'-oligoadenylate synthetase cDNA results in increased antiviral activity and growth suppression

The interferon (IFN)-induced enzyme 2',5'-oligoadenylate (2-5A) synthetase has been implicated in the development of antiviral activity in human and animal cells. However, its role in IFN-mediated growth inhibition remains unclear. To elucidate the function of 2-5A synthetase, we have stably introduced a human 2-5A synthetase cDNA into a human glioblastoma cell line (T98G). Constitutive expression of the cDNA in these cells is associated with increased levels of resistance to infection by encephalomyocarditis virus. One transfected subclone, which expresses elevated levels of 2-5A synthetase enzyme activity, also shows a reduced rate of cellular proliferation.

Elevation of 2',5'-oligoadenylate synthetase activity and HLA-I associated beta 2-microglobulin in response to recombinant interferon-gamma administration in chronic HBeAg-positive hepatitis

We have analyzed the immunomodulatory effect of 5 and 2 MU of recombinant interferon-gamma (rIFN-gamma) administered to 8 carriers of HBsAg with histologically proven chronic active liver disease. After the rIFN-gamma administration, T8 lymphocyte subsets showed a significant decrease (basal vs. 4 weeks, p less than 0.05) and T4/T8 ratios were higher than the basal values in 6/8 patients. Serum levels of the HLA class I-associated beta 2-microglobulin increased significantly in all patients within the first week of treatment, both with the high (p less than 0.01) and the low (p less than 0.05) rIFN-gamma dose. Then, differences between the two doses reached statistical significance (p less than 0.03). Similar results (p less than 0.05) were obtained by measuring the 2',5'-oligoadenylate (2-5A) synthetase activity, co-occurring with the decreases in HBV-DNA polymerase and HBV-DNA, although no differences were found between the two doses. In addition, levels of 2-5A synthetase correlated significantly with those of beta 2-microglobulin (r = 0.743, p less than 0.01). On the other hand, after the rIFN-gamma administration, all the patients had liver membrane antibodies (LMA) in their serum (p less than 0.05); only two patients (who were anti-HD positive) showed LMA at the end of the follow-up. rIFN-gamma has both antiviral and immunomodulatory effects in HBeAg carriers with chronic liver disease.

2',5'-Oligoadenylate synthetase activity in peripheral blood lymphocytes as a clinical marker in interferon therapy for chronic hepatitis B

Treatment of chronic hepatitis B (CHB) with human leukocyte interferon (IFN-alpha) was studied in terms of increase of 2',5'-oligoadenylate (2-5A) synthetase activity in peripheral blood lymphocytes (PBL) after IFN-alpha administration. The 2-5A synthetase activity in PBL increased to a maximum 16-24 h after IFN-alpha injection and then gradually decreased, while serum HB virus DNA polymerase (HBV-DNAP) activity, which corresponds to the amount of HBV virions in the serum, decreased to a minimum after about 48 h. Increase in 2-5A synthetase activity was followed about 1 day later by decrease in DNAP activity. Furthermore, of 9 patients treated with IFN for at least 22 weeks and observed for over 1 year after IFN treatment, the effective 5 cases in which HBeAg disappeared during IFN therapy and did not reappear showed rather high increases in 2-5A synthetase activity, whereas the ineffective 4 cases in which HBeAg remained positive at the end of IFN therapy showed little increases in enzyme activity. The present study suggests that prescreening by 2-5A synthetase assay before IFN therapy should be useful for obtaining better results in IFN treatment of CHB patients.

Production and characterization of a monoclonal antibody to a human interferon-induced double-stranded RNA-binding Mr 68,000 protein kinase

One of the interferon-induced proteins thought to be involved in the antiviral effects of interferon is a double-stranded RNA-dependent protein kinase. This paper reports the development of a monoclonal antibody, 10A5, that recognizes a protein that co-migrates with the double-stranded RNA-dependent protein kinase at an approximate molecular weight of 68,000. Levels of this protein and of the protein kinase activity increase 3-fold on interferon treatment of T98G cells. The specificity of the monoclonal antibody was determined by ELISA, immunoblotting, and immunoprecipitation procedures. Furthermore, immunoaffinity chromatography of an interferon-induced T98G cell extract previously phosphorylated in the presence of double-stranded RNA and radiolabeled ATP resulted in the specific elution of a phosphorylated Mr 68,000 protein from the monoclonal antibody 10A5-Sepharose column. Monoclonal antibody 10A5 recognizes both native and denatured protein kinase, independent of double-stranded RNA binding or phosphorylation, and should therefore serve as a useful tool in analyzing the role of the double-stranded RNA-dependent protein kinase in the mechanism of interferon action.

Isolation and characterization of an interferon-resistant cell line deficient in the induction of (2'-5')oligoadenylate synthetase activity

To screen for cells with different sensitivities to interferon (IFN), NIH 3T3 mouse fibroblasts were subcloned and examined for their response to IFN treatment. Of 30 clones tested, 2 appeared to be relatively resistant to IFN, since the replication of both vesicular stomatitis virus and mengovirus was not inhibited, even in the presence of 1,000 U of IFN per ml. One resistant (A10) and one sensitive (A5) clone were further analyzed. In both clones, murine leukemia virus replication was equally inhibited by IFN, indicating the presence of functional receptors for IFN in the resistant clone. Using the (2'-5')oligoadenylate (2-5A) radiobinding assay, we could demonstrate that both clones contained the RNase L protein. Furthermore, this enzyme appears to be active, since a similar reduction in the rate of protein synthesis was evident after the introduction of exogenous 2-5A to the cells. We also analyzed the activity of another enzyme in the 2-5A pathway, namely, 2-5A synthetase. In the sensitive cells (A5), the induction of enzyme activity was proportional to the IFN concentration used, reaching a maximum of more than a 10-fold increase over the background of untreated cells. However, little if any induction over the basal activity was observed in the resistant cells (A10) when similar doses of IFN were used. It is thus probable that the lack of induction of 2-5A synthetase activity by IFN in A10 cells is at least partly responsible for their relative resistance to IFN treatment.

Isolation and characterization of an interferon-resistant cell line deficient in the induction of (2'-5')oligoadenylate synthetase activity

To screen for cells with different sensitivities to interferon (IFN), NIH 3T3 mouse fibroblasts were subcloned and examined for their response to IFN treatment. Of 30 clones tested, 2 appeared to be relatively resistant to IFN, since the replication of both vesicular stomatitis virus and mengovirus was not inhibited, even in the presence of 1,000 U of IFN per ml. One resistant (A10) and one sensitive (A5) clone were further analyzed. In both clones, murine leukemia virus replication was equally inhibited by IFN, indicating the presence of functional receptors for IFN in the resistant clone. Using the (2'-5')oligoadenylate (2-5A) radiobinding assay, we could demonstrate that both clones contained the RNase L protein. Furthermore, this enzyme appears to be active, since a similar reduction in the rate of protein synthesis was evident after the introduction of exogenous 2-5A to the cells. We also analyzed the activity of another enzyme in the 2-5A pathway, namely, 2-5A synthetase. In the sensitive cells (A5), the induction of enzyme activity was proportional to the IFN concentration used, reaching a maximum of more than a 10-fold increase over the background of untreated cells. However, little if any induction over the basal activity was observed in the resistant cells (A10) when similar doses of IFN were used. It is thus probable that the lack of induction of 2-5A synthetase activity by IFN in A10 cells is at least partly responsible for their relative resistance to IFN treatment.