Genetic analysis of the role of cAMP in mediating effects of interferon

Fc receptor-mediated phagocytosis makes a significant contribution to clearance of influenza virus infections

Fc receptors for IgG expressed on macrophages and NK cells are important mediators of opsonophagocytosis and Ab-dependent cell-mediated cytotoxicity. Phagocyte-mediated opsonophagocytosis is pivotal for protection against bacteria, but its importance in recovery from infection with intracellular pathogens is unclear. We have now investigated the role of opsonophagocytosis in protection against lethal influenza virus infection by using FcR gamma(-/-) mice. Absence of the FcR gamma-chain did not affect the expression of IFN-gamma and IL-10 in the lungs and spleens after intranasal immunization with an influenza subunit vaccine. Titers of serum and respiratory Abs of the IgM, IgG1, IgG2a, and IgA isotypes in FcR gamma(-/-) mice were similar to levels seen in FcR gamma(+/+) mice. Nevertheless, FcR gamma(-/-) mice were highly susceptible to influenza infection, even in the presence of anti-influenza Abs from immune FcR gamma(+/+) mice. NK cells were not necessary for the observed Ab-mediated viral clearance, but macrophages were found to be capable of actively ingesting opsonized virus particles. We conclude that Fc receptor-mediated phagocytosis plays a pivotal role in clearance of respiratory virus infections.

Oligoadenylate and cyclic AMP: interrelation and mutual regulation

The data obtained are in good agreement with the hypothesis that cAMP is involved in the control of 2-5A metabolism, including the mediation of the regulation of 2-5A by IFNs; 2-5A, in turn, affects the intracellular cAMP level. The general question originating from the data is that of a biochemical mechanism connecting the activation of the cAMP/2-5A system and the effect of depression of cell division. In my opinion, this universal effect is the result of the action of the known 2-5A-dependent mechanism, namely, RNase L (see review by Pestka et al. 1987), rather than by any new 2-5A-stimulating enzyme. The RNase L activated by 2-5A decreases the total level of protein synthesis and accelerates the degradation of cellular RNA, resulting in the inhibition of cell growth. It should be mentioned that such activation of RNA turnover is generally characteristic for nondividing cells, especially for cells in the resting state (Epifanova et al. 1983). Thus, the regulatory system of cAMP/2-5A is involved evidently in the antiproliferative mechanism characteristic for the resting cells, controlling the variations in the levels of RNA turnover and protein synthesis.

Biologic agents as biochemical modulators: pharmacologic basis for the interaction of cytotoxic chemotherapeutic drugs and interferon

Biochemical modulation of cytotoxic cancer chemotherapeutic agents is one means of enhancing the activity and selectivity of antitumor drugs. Traditionally this approach has utilized detailed information regarding a particular enzymatic reaction or biochemical pathway to develop potential modulating agents. In contrast, the reported clinical therapeutic activity of IFN in combination with cytotoxic agents has prompted a reexamination of the biochemical actions of the cytokine. Interferon elicits a number of cellular actions that might contribute to its pharmacologic activity, including both direct antitumor effects and host-mediated actions. The best understood are those related to the cytotoxicity of the fluoropyrimidine antimetabolites and include enzymatic reactions involved in fluoropyrimidine metabolic activation, catabolism, and interaction with its target enzyme. However, even in this instance, a mechanistic association of a specific pharmacologic action with therapeutic activity remains to be determined. These studies demonstrate that cytokines and other biologic agents may exert specific biochemical modulations that augment (or potentially attenuate) the activity of the cytotoxic chemotherapeutic agents.

Interferons induce xanthine dehydrogenase gene expression in L929 cells

Human interferon-alpha A/D (Bg/II) (IFN-alpha A/D) and mouse interferon-gamma (IFN-gamma) are shown to induce xanthine dehydrogenase (XD) mRNA in L929 fibroblastic cells. XD mRNA accumulation after IFN-alpha A/D treatment is relatively fast, being already evident after 4 h and reaching its maximum after 24 h. IFN-alpha A/D is active in inducing XD mRNA at 0.1 unit/ml and it is maximally active at 10(3) units/ml. The half-life of the XD message is unaffected by IFN-alpha A/D treatment, whereas the transcriptional activity of the XD gene and the concentrations of XD heterogeneous nuclear RNA are increased by 2- and 6-fold respectively. The effect of IFN-alpha A/D on XD mRNA is insensitive to cycloheximide, suggesting that protein synthesis de novo is not required. Experiments conducted with specific inhibitors suggest that protein kinase C, cyclic AMP and arachidonic acid metabolites derived from lipoxygenase or cyclooxygenase do not act as second-messenger molecules in the induction of XD mRNA by IFN-alpha A/D. XD mRNA is also induced in NIH3T3 fibroblastic cells, but not in F9 teratocarcinoma or B16 melanoma cells after treatment with IFN-alpha A/D. NIH3T3 are the only cells so far tested that have detectable XD and xanthine oxidase activities under basal conditions and after IFN-alpha A/D treatment, although their responsiveness to the cytokine is much less than that observed in L929 cells.

A collection of mRNA species that are inducible in the RAW 264.7 mouse macrophage cell line by gamma interferon and other agents

To identify genes induced during macrophage activation, a cDNA library was prepared from cultures of the RAW 264.7 mouse macrophage cell line that had been treated with conditioned medium from mitogen-stimulated spleen cells, and the cDNA library was screened by differential plaque hybridization. Eleven cDNA clones, designated CRG-1 through CRG-11, corresponding to mRNA species inducible in RAW 264.7 cells by the spleen cell conditioned medium, were isolated. Inductions were not blocked by cycloheximide. All of the mRNAs were inducible by gamma interferon, and some were also inducible by alpha and beta interferons, by lipopolysaccharide, by phorbol 12-myristate 13-acetate, and by the calcium ionophore A23187. Sequencing of the cDNAs revealed that CRG-1, CRG-3, and CRG-5 are cDNAs of recently identified transcription factors IRF-1, zif/268, and LRF-1 respectively. As previously reported, CRG-2 and CRG-10 (MIG) encode new members of the platelet factor 4 family of cytokines. CRG-6 corresponds to a new member of a family of interferon-inducible genes clustered on mouse chromosome 1, CRG-9 corresponds to a prostaglandin synthase homolog, CRG-8 corresponds to beta 2-microglobulin, and CRG-4 corresponds to metallothionein II. CRG-11 contains sequences of a truncated L1Md repetitive element as well as nonrepetitive sequences. The nonrepetitive sequence of CRG-11 as well as the sequences of CRG-7 are not closely related to published sequences. The CRG genes and proteins are of interest because of their involvement in macrophage activation, because of their roles as mediators of the effects of gamma interferon and other pleiotropic agents, and because of their usefulness as tools for studying the signal pathways through which gamma interferon and other inducers exert their effects on gene and protein expression.

A collection of mRNA species that are inducible in the RAW 264.7 mouse macrophage cell line by gamma interferon and other agents

To identify genes induced during macrophage activation, a cDNA library was prepared from cultures of the RAW 264.7 mouse macrophage cell line that had been treated with conditioned medium from mitogen-stimulated spleen cells, and the cDNA library was screened by differential plaque hybridization. Eleven cDNA clones, designated CRG-1 through CRG-11, corresponding to mRNA species inducible in RAW 264.7 cells by the spleen cell conditioned medium, were isolated. Inductions were not blocked by cycloheximide. All of the mRNAs were inducible by gamma interferon, and some were also inducible by alpha and beta interferons, by lipopolysaccharide, by phorbol 12-myristate 13-acetate, and by the calcium ionophore A23187. Sequencing of the cDNAs revealed that CRG-1, CRG-3, and CRG-5 are cDNAs of recently identified transcription factors IRF-1, zif/268, and LRF-1 respectively. As previously reported, CRG-2 and CRG-10 (MIG) encode new members of the platelet factor 4 family of cytokines. CRG-6 corresponds to a new member of a family of interferon-inducible genes clustered on mouse chromosome 1, CRG-9 corresponds to a prostaglandin synthase homolog, CRG-8 corresponds to beta 2-microglobulin, and CRG-4 corresponds to metallothionein II. CRG-11 contains sequences of a truncated L1Md repetitive element as well as nonrepetitive sequences. The nonrepetitive sequence of CRG-11 as well as the sequences of CRG-7 are not closely related to published sequences. The CRG genes and proteins are of interest because of their involvement in macrophage activation, because of their roles as mediators of the effects of gamma interferon and other pleiotropic agents, and because of their usefulness as tools for studying the signal pathways through which gamma interferon and other inducers exert their effects on gene and protein expression.

The control of interferon-inducible gene expression

The alpha beta interferons (IFNs) transiently induce genes through an IFN-stimulable DNA response element (ISRE). IFN-cell surface receptor interaction triggers the cytoplasmic activation of the complex primary transcription factor E, which on translocation and interaction with the ISRE initiates transcription. Whether E is activated directly through the receptor(s) or through a more classical second message pathway(s) and the roles of additional factors in the alpha beta and gamma IFN responses remain to be established. Meanwhile analysis of mutants has revealed complexity and overlap in the alpha, beta and gamma IFN response pathways and the products of at least two viruses have been shown to inhibit IFN-inducible gene expression.

Cyclic AMP mediates the direct antiproliferative action of mismatched double-stranded RNA

Previous experiments have demonstrated that double-stranded RNAs (dsRNAs) can exert an antiproliferative effect on human tumor cells, independent of interferon (IFN) induction. However, the mechanism by which dsRNAs inhibit tumor growth has not been elucidated. As a first step in determining the molecular events responsible for growth arrest, we have explored the role of signal transduction through the cAMP system in the antiproliferative effect of the mismatched dsRNA, r(I)n.r(C12,U)n (Ampligen). These studies utilized the human glioma cell line A1235, which does not produce detectable levels of IFN-alpha, -beta, or -gamma in response to mismatched dsRNA treatment. Treatment of A1235 cells with mismatched dsRNA in combination with either 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), which inhibits cAMP-dependent protein kinase and protein kinase C, or N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA1004), which preferentially inhibits the cAMP-dependent protein kinase, yielded an antagonism of the mismatched dsRNA-induced antiproliferative effect. Measurement of adenylate cyclase activation showed a dose-dependent increase in activity at antiproliferative mismatched dsRNA concentrations, but not at lower, nonantiproliferative doses. This increase in activity was rapid, seen as early as 30 sec after initiation of treatment, and it was sustained at peak levels for 1-2 hr. Analysis of the intracellular cAMP concentration gave similar kinetics of induction. Exposure of cells to the stable cAMP analogue dibutyryl cAMP yielded dose-dependent inhibition of cell growth. The cAMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine also inhibited proliferation. In contrast, neither H-7 nor HA1004 had an effect on growth inhibition induced by human natural IFN-alpha treatment. In addition, antiproliferative doses of IFN-alpha did not increase cAMP concentrations. These results indicate that the cAMP system is utilized by mismatched dsRNA as an early signal transduction mechanism for growth control. Furthermore, the antiproliferative effects induced by mismatched dsRNA and IFN can occur by different mechanisms of action.

Role of cyclic AMP and polypeptide growth regulators in growth inhibition by interferon in PC-3 cells

Participation of growth factors and intracellular cAMP in direct antiproliferative action of interferon alpha (IFN-alpha) was investigated in PC-3 human prostate carcinoma cell line. IFN-alpha inhibited proliferation of PC-3 cells in a dose-dependent manner in vitro, and the effect was reversible. Fibroblast growth factor, epidermal growth factor and platelet-derived growth factor, when added to the culture medium, showed no effect on growth of PC-3 cells in presence or absence of IFN-alpha. Transforming growth factor beta (TGF-beta) significantly inhibited PC-3 cell growth, and the effect was additived to that of IFN-alpha. TGF-beta content in conditioned medium of PC-3 cells was not affected by treatment with IFN-alpha. On the other hand, IFN-alpha increased intracellular cAMP concentration about 20-fold. Dibutyryl cAMP and reagents which elevated intracellular cAMP level also inhibited PC-3 cell growth. These indicated that direct antiproliferative effect of IFN-alpha on PC-3 cells was at least partly mediated by cAMP, and that neither growth factors nor a growth inhibitor participated in the action of IFN-alpha.

Alpha interferon and gamma interferon stimulate transcription of a single gene through different signal transduction pathways

Interferons (IFNs) play a key role in the defense against virus infection and the regulation of cell growth and differentiation, in part through changes in specific gene transcription in target cells. We describe several differences between the signal transduction events that result in transcriptional activation of the human gene coding for a guanylate-binding protein (GBP) by alpha interferon (IFN-alpha) and gamma interferon (IFN-gamma). Activation by IFN-alpha was rapid, transient, and cycloheximide resistant. Activation by IFN-gamma was slower, sustained, and delayed by cycloheximide. IFN-gamma led to the formation of a stable intracellular signal which led to continued GBP transcription even if the ligand was withdrawn, whereas IFN-alpha-induced GBP transcription decayed rapidly if IFN-alpha was withdrawn. Perturbations of signaling pathways involving classical second messengers (cyclic AMP, Ca2+, protein kinase C) did not induce GBP transcription. However, various kinase inhibitors blocked the transcriptional response to IFN-gamma but not IFN-alpha, suggesting that a specific and possibly novel kinase is involved in gene activation by IFN-gamma.

Changes in indole metabolism in organ cultured rat pineal glands induced by interferon-gamma

The purpose of this study was to test the effects of recombinant rat interferon-gamma (IFN-gamma) on rat pineal function in organ culture. The glands were pre-incubated with IFN-gamma (50, 100, and 1,000 antiviral units/ml) for two hours, followed by a stimulation with isoproterenol (ISO, 10(-6) M) (in the presence of IFN-gamma) for an additional three hours. The effect of IFN-gamma alone on the pineal function was also tested by adding another group incubated with IFN-gamma (1,000 antiviral units/ml) without ISO stimulation. The following parameters were estimated at the end of the incubation: the activities of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), the levels of melatonin, 5-hydroxytryptophan (5HTP), 5-hydroxytryptamine (5HT), and 5-hydroxyindole acetic acid (5HIAA), and the 5HT/5HIAA ratio. IFN-gamma had no effect on either NAT or HIOMT activities in the pineal glands. At 50 units/ml, IFN-gamma increased the melatonin content over the ISO-stimulated level in the pineal but not in the media. IFN-gamma alone, at 1,000 units/ml, also increased the melatonin content significantly over the control level. The levels of 5HTP and 5HIAA were significantly suppressed by IFN-gamma, either alone or in combination with ISO. The level of 5HT was not altered by any of the treatments. The 5HT/5HIAA ratio was significantly increased by IFN-gamma, either when given alone or when given in combination with ISO. The results suggest that IFN-gamma enhances melatonin production in the pineal gland by suppressing the oxidative deamination of 5HT to 5HIAA and shunting the biosynthetic pathway toward melatonin production.

Gene regulation in reverse transformation: cyclic AMP-induced actin homolog in CHO cells

Reverse transformation (RT) presents a challenge in understanding of the role of protein-genome interaction in regulating gene expression in normal and transformed cells. Early during RT of CHO-K1 cells by cyclic AMP a new protein, mol wt = 43,000 and pI = 5.3 +/- 0.2, was rapidly and specifically induced. This cAMP-induced protein (CIP) is a phosphorylated actin homolog. Induction required new protein synthesis. Actinomycin D treatment failed to inhibit CIP induction, suggesting the existence of an untranslated or sequestered mRNA in untreated cells. Expression of CIP was not dependent upon cell shape or cytoskeletal integrity as are other steps in RT. CIP was detectable only in cAMP-treated cells, whether transformed or nontransformed, and cAMP treatment inhibited growth of both cell types. CIP was associated with soluble cell fractions and not with F-actin. We propose that CIP plays an early role in RT, that is necessary but not sufficient for the complete RT process, and that it participates in the cAMP signaling pathway of cells through changes in the cytoskeleton. This pathway inhibits cell growth as required in the differentiated phenotype. A molecular model is presented for the RT reaction in CHO-K1, which also explains cAMP effects on transformed cells such as the S49 lymphoma and other malignancies.

Alpha interferon and gamma interferon stimulate transcription of a single gene through different signal transduction pathways

Interferons (IFNs) play a key role in the defense against virus infection and the regulation of cell growth and differentiation, in part through changes in specific gene transcription in target cells. We describe several differences between the signal transduction events that result in transcriptional activation of the human gene coding for a guanylate-binding protein (GBP) by alpha interferon (IFN-alpha) and gamma interferon (IFN-gamma). Activation by IFN-alpha was rapid, transient, and cycloheximide resistant. Activation by IFN-gamma was slower, sustained, and delayed by cycloheximide. IFN-gamma led to the formation of a stable intracellular signal which led to continued GBP transcription even if the ligand was withdrawn, whereas IFN-alpha-induced GBP transcription decayed rapidly if IFN-alpha was withdrawn. Perturbations of signaling pathways involving classical second messengers (cyclic AMP, Ca2+, protein kinase C) did not induce GBP transcription. However, various kinase inhibitors blocked the transcriptional response to IFN-gamma but not IFN-alpha, suggesting that a specific and possibly novel kinase is involved in gene activation by IFN-gamma.

Regulation of muscle acetylcholine receptor-channel function by interferon

The effect of the antiviral agent interferon (IFN) on the function of the nicotinic acetylcholine receptor (AChR) channel, has been investigated in both mammalian cultured myotubes and adult fibres, using the single channel recording patch-clamp technique. Shortened AChR-channel lifetime, and occasionally reduced channel conductance and slowed opening frequency were seen with fibroblast IFN (IFN-beta) in the mouse myotubes, and with IFN-beta and leucocytes IFN (IFN-alpha), in the rat muscle fibres. These effects paralleled an increase in the cytosolic level of cAMP. This suggests that IFN exerts a regulatory action on AChR function. A similar regulatory action on other receptor may be responsible for some of the neurological side effects observed in patients treated with IFN.

Regulation of macrophage growth and antiviral activity by interferon-gamma

Interferons, in addition to their antiviral activity, induce a multiplicity of effects on different cell types. Interferon (IFN)-gamma exerts a unique regulatory effect on cells of the mononuclear phagocyte lineage. To investigate whether the antiviral and antiproliferative effects of IFN-gamma in macrophages can be genetically dissociated, and whether IFN-alpha and IFN-gamma use the same cellular signals and/or effector mechanisms to achieve their biologic effects, we have derived a series of somatic cell genetic variants resistant to the antiproliferative and/or antiviral activities of IFN-gamma. Two different classes of variants were found: those resistant to the antiproliferative and antiviral effects of IFN-gamma against vesicular stomatitis virus (VSV) and those resistant to the antiproliferative effect, but protected against VSV and encephalomyocarditis virus (EMCV) lysis by IFN-gamma. In addition, a third class of mutants was obtained that was susceptible to the growth inhibitory activity, but resistant to the antiviral activity of IFN-gamma. Analysis of these mutants has provided several insights regarding the regulatory mechanisms of IFN-gamma and IFN-alpha on the murine macrophage cell lines. The antiproliferative activity of IFN-gamma on these cells, in contrast to that of IFN-alpha, is mediated by a cAMP-independent pathway. The antiproliferative and antiviral activities of IFN-gamma were genetically dissociated. Variants were obtained that are growth resistant but antivirally protected, or are growth inhibited but not antivirally protected against VSV or EMCV. The genetic analysis indicated that IFN-alpha and IFN-gamma regulate the induction of the dsRNA-dependent P1/eIF-2 alpha protein kinase and 2',5'-oligoadenylate synthetase enzymatic activities via different pathways. Finally, a unique macrophage mutant was obtained that was protected by IFN-gamma against infection by VSV, but not EMCV, suggesting that antiviral mechanisms involved in protection against these different types of RNA viruses must be distinct at some level.

Inhibition of ornithine decarboxylase induction of interferon (alpha + beta) and its reversal by dibutyryl adenosine 3',5'-monophosphate

Interferon (alpha + beta) given to C3H/HeN mice intraperitoneally inhibited increases in the activities of adenylate cyclase and ornithine decarboxylase after partial hepatectomy. The inhibition of ornithine decarboxylase was prevented by administration of dibutyryl cAMP. Core (2'-5')oligo(adenylate), i.e. A2'p5'A2'p5'A or (A2'p)2A, as well as interferon inhibited the increases in these two enzymes caused by partial hepatectomy. The inhibition by (A2'p)2A of ornithine decarboxylase activity was reversed by dibutyryl cAMP. These results suggested that the activity of interferon was similar to that of (A2'p)2A and that the inhibition of ornithine decarboxylase induction caused by these agents resulted from the inhibition of adenylate cyclase activity.

Protein kinase C potentiates isoproterenol-mediated cyclic AMP production without modifying the homologous desensitization process in J774 cells

The J774 murine macrophage cells possess a beta 2-adrenergic receptor coupled to adenylate cyclase, which can be regulated by homologous desensitization. Stimulation of protein kinase C by phorbol esters or oleoyl acetyl glycerol potentiates two-to-threefold the isoproterenol-induced cyclic AMP accumulation. These promoters act at a post-receptor level, since the number and affinity of the beta-adrenergic receptors, measured by use of the hydrophilic ligand [3H]CGP-12177, are not modified. In addition, the effect of cholera toxin is similarly increased and pretreatment of the cells with pertussis toxin prevents the action of phorbol esters. On the other hand, these promoters are ineffective on isoproterenol-induced desensitization and the rates of receptor segregation and recovery remain unchanged. Therefore, protein kinase C modulates the isoproterenol-stimulated adenylate cyclase, whereas it is inactive on the homologous desensitization process.

Regulation of cell growth by interferon

Interferons can regulate growth and differentiation in a wide range of cell types. These mechanisms are currently being examined. Interferons inhibit the growth of tumour cells and are thus potential anti-cancer agents. They can also inhibit normal cell growth in vitro, and stimulate tumour cell growth in vitro. They may also be involved in some autoimmune diseases. This review examines the effect of interferons on cell proliferation, function, and growth, focusing primarily on in vitro cell systems.

Transcriptional induction of two genes in human cells by beta interferon

The binding of interferons to distinct cell surface receptors leads to the induction of synthesis of several unique polypeptides and their corresponding mRNAs (1-6). We have isolated two cDNAs that are complementary to nuclear RNA whose synthesis is induced from undetectable levels to maximal rates of transcription within 30-60 min after the addition of beta interferon to human fibroblasts or to HeLa cells. These results prove that a single polypeptide can, by binding to a specific plasma membrane receptor, promptly activate the transcription of a defined set of genes.

Cyclic AMP does not mediate inhibition of DNA synthesis by interferon in mouse Swiss 3T3 cells

The purpose of this study was to determine whether cyclic AMP (cAMP) plays any direct or indirect role in the antiproliferative effect of mouse L-cell interferon in Swiss 3T3 cells. Firstly, we found that interferon did not affect intracellular levels of cAMP in these cells in the absence or the presence of cAMP-elevating agents. Secondly, we examined the effect of interferon on the stimulation of DNA synthesis of quiescent 3T3 cells by a range of cyclic AMP-elevating agents, including cholera toxin, cAMP derivatives, and prostaglandin E, added in the presence of insulin or vasopressin. Interferon inhibited cyclic AMP-stimulated DNA synthesis as measured by incorporation of radioactive thymidine into acid-insoluble material and autoradiographic analysis of the fraction of labelled cells. Dose-response curves and kinetics of inhibition were identical to those obtained in cultures stimulated by combinations of growth factors that do not increase the intracellular level of cAMP. The inhibition by interferon of cAMP-stimulated DNA synthesis was also observed in secondary cultures of mouse embryo fibroblasts, where cAMP-elevating agents provide a mitogenic signal in the absence of other added growth factors. These results show that the inhibitory effect of interferon on DNA synthesis in Swiss 3T3 cells is not mediated by cyclic AMP.

Biochemical analysis of mutants of a macrophage cell line resistant to the growth-inhibitory activity of interferon

While a multiplicity of cellular and biochemical effects are mediated by interferons on cultured cells, the mechanisms involved in the direct growth-inhibitory activity of interferons remain problematic. We have previously found that variants in cAMP metabolism in a macrophage cell line, J774.2, were at least 50-fold less sensitive to the growth inhibitory activity of interferons (IFN) than the parental clone. To test the hypothesis that cAMP mediates the growth inhibition produced by IFN in these cells, interferon-resistant variants were selected and characterized with respect to cAMP synthesis and function. Approximately one-third of the IFN-resistant clones were found to be resistant to growth inhibition produced by cholera toxin, but not 8Br-cAMP. IFN was fully able to protect all of the interferon-resistant/choleratoxin-resistant (IFNr/CTr) clones against infection by vesicular stomatitis virus and markedly stimulated 2', 5'-oligodenylate synthetase activity. These IFNr/CTr variants were shown to have a defect in adenylate cyclase. The remaining IFN-resistant clones were fully susceptible to the growth-inhibitory effects of cholera toxin because their basal and stimulated adenylate cyclase activity is similar to that of the parental clone. IFN failed to protect these IFNr/choleratoxin sensitive clones against infection by vesicular stomatitis virus and failed to stimulate 2', 5-oligodenylate synthetase, suggesting that they have defective or deficient IFN receptors. In addition, IFN failed to increase intracellular cAMP levels in both IFNr/CTr and IFNr/choleratoxin sensitive clones. These results provide firm genetic and biochemical evidence that the growth inhibitory effects of IFN on this cell line are mediated by cAMP.

Cyclic AMP-dependent regulation of activities of synthetase and phosphodiesterase of 2',5'-oligoadenylate in NIH 3T3 cells

Treatment of NIH 3T3 cells with adenylate cyclase activator adrenaline (10(-6) M) or cAMP phosphodiesterase inhibitor theophylline (10(-3) M) was shown to lead to intracellular cAMP elevation followed by a 2.0-to 2.5-fold increase in the 2',5'-oligoadenylate synthetase activity. This process was blocked by actinomycin D. The rise in the intracellular cAMP level was also followed by a 3-4-fold decrease in the activity of 2'-phosphodiesterase. Propranolol prevented this inhibition but actinomycin D produced only a negligible effect on the process. Incubation of the cell homogenate with purified catalytic subunit of cAMP-dependent protein kinase and ATP also resulted in a decrease of 2'-phosphodiesterase activity. These results indicate that cAMP is involved in the regulation of enzymes of the 2',5'-oligoadenylate system. The possibility that certain biological functions of cAMP are implemented via 2',5'-oligoadenylate-dependent processes is discussed.