Murine interferon system regulation: isolation and characterization of a mutant 3T6 cell engaged in the semiconstitutive synthesis of interferon.

We describe the isolation and characterization of a virus-resistant mutant of murine 3T6 cells. The mutant, designated 3T6-VrB2, displays a high degree of resistance to infection by members of the toga-, rhabdo- and picornavirus classes. The level of this resistance to infection is similar to the parent 3T6 pretreated with approximately 100 lU/ml of interferon. Upon co-cultivation of 3T6-VrB2 cells with interferon-sensitive mouse cells, an antiviral state is induced in the latter cells as measured by a reduction of virus yield following infection. The nature of the induction is defined by a series of experiments using anti-mouse interferon antiserum. In the presence of this antiserum, the ability of the mutant to induce an antiviral state in interferon-sensitive mouse cells upon co-cultivation is eliminated. Additionally, growth of the mutant cells in the presence of this antiserum causes a reversal of the virus-resistant phenotype. Our results indicate that 3T6-VrB2 contains a mutation affecting the regulation of the murine interferon system such that the cell is engaged in the semiconstitutive synthesis of interferon.     

A mouse cell line, which is unprotected by interferon against lytic virus infection, lacks ribonuclease F activity

A mouse cell line, NIH 3T3, does not respond to some of the activities of interferon. Even after treatment with high concentrations of interferon the replication of lytic viruses, such as encephalomyocarditis virus (EMCV) and vesicular stomatitis virus (VSV) is not inhibited in these cells. In contrast, interferon treatment of these same cells results in the inhibition of Moloney murine leukemia virus (MMuLV) production. We have analyzed enzymatic pathways which are induced by interferon in these cells. After interferon treatment, the level of the (2'-5')oligoadenylate [(2'-5)An] synthetase activity and the phosphorylation of the 67000-dalton protein (P1) are enhanced in NIH 3T3 cells to approximately the same level as interferon-sensitive mouse L-cells. Moreover, NIH 3T3 and L-cells, contain approximately the same levels of enzymes which inactivate (2'-5')An. Both exogenously added (2'-5')A3 or double-stranded RNA (dsRNA) failed to inhibit protein synthesis in NIH 3T3 extracts even though they were potent inhibitors of L-cell extract-directed protein synthesis. Direct measurements of the (2'-5')An-dependent ribonuclease F (RNase F) failed to detect such activity in NIH 3T3 cells. Our results, therefore, suggest that the presence of RNase F activity is necessary for the interferon-induced antiviral activity against EMCV and against VSV. The induction of protein kinase activity by interferon treatment of NIH 3T3 cells appears to have no direct effect on EMCV and VSV replication.     

Mechanism of interferon action. Kinetics of decay of the antiviral state and protein phosphorylation in mouse fibroblasts treated with natural and cloned interferons

The kinetics of decay of the antiviral state and protein phosphorylation induced with natural mouse interferon (IFN) and with cloned human IFN were examined in monolayer cultures of mouse Ll929 fibroblast cells. The antiviral state measured by single cycle virus yield reduction with either vesicular stomatitis virus or reovirus decayed significantly within 2 to 3 days following removal of IFN and by 5 to 8 days virus yields had returned to the level of untreated control cells. Trypsinization of IFN-treated cells did not detectably alter the rate of decay of the antiviral state; however, the decay occurred slightly more rapidly in actively growing as compared to stationary cell cultures. The decay of the IFN-induced protein kinase which catalyzes the phosphorylation of endogenous protein P1 and purified initiation factor eIF-2 alpha correlated with the decay of the antiviral state. The decay rates of the antiviral state and protein kinase observed in mouse L929 cells that had been treated with natural mouse IFN synthesized in Newcastle disease virus-induced L929 cells were comparable to the decay rates observed in L929 cells that had been treated with recombinant human IFN-alpha A/D synthesized in Escherichia coli. The induction and decay of the antiviral state and protein kinase following treatment with a single dose of IFN did not significantly affect the sensitivity of the cell population to a subsequent treatment with a single dose of IFN. However, continuous treatment of L929 cells with natural mouse IFN or recombinant human IFN prevented the decay of both the antiviral state and protein kinase but also ultimately lead to cell death. The results suggest that protein phosphorylation may play an important role in the mechanism of IFN action in mouse L929 fibroblasts.     

Defect in 3′-phosphoadenosine 5′-phosphosulfate synthesis in brachymorphic mice: I. Characterization of the defect

Biosynthesis of the undersulfated proteoglycan found in brachymorphic mouse (bm/ bm) cartilage has been investigated. Similar amounts of cartilage proteoglycan core protein, as measured by radioimmune inhibition assay, and comparable activity levels of four of the glycosyltransferases requisite for synthesis of chondroitin sulfate chains were found in cartilage homogenates from neonatal bm/bm and normal mice, suggesting normal production of glycosylated core protein acceptor for sulfation. When incubated with ³⁵S-labeled 3′-phosphoadenosine 5′-phosphosulfate (PAPS), bm/bm cartilage extracts showed a higher than control level of sulfotransferase activity. In contrast, when synthesis was initiated from ATP and ³⁵SO₄^2?, mutant cartilage extracts showed lower incorporation of ³⁵SO₄^2? into endogenous chondroitin sulfate proteoglycan (19% of control level) and greatly reduced formation of PAPS (10% of control level). Results from coincubations of normal and mutant cartilage extracts exhibited intermediate levels of sulfate incorporation into PAPS and endogenous acceptors, suggesting the absence of an inhibitor for sulfate-activating enzymes or sulfotransferases. Degradation rates of ³⁵S]PAPS and of ³⁵S-labeled adenosine 5′-phosphosulfate (APS) were comparable in bm/bm and normal cartilage extracts. Specific assays for both ATP sulfurylase (sulfate adenylyltransferase; ATP:sulfate adenylyltransferase, EC 2.7.7.4) and APS kinase (adenylylsulfate kinase; ATP:adenylylsulfate 3′-phosphotransferase, EC 2.7.1.25) showed decreases in the former (50% of control) and the latter (10–15% of control) enzyme activities in bm/bm cartilage extracts. Both enzyme activities were reduced to intermediate levels in extracts of cartilage from heterozygous brachymorphic mice (ATP-sulfurylase, 80% of control; APS kinase, 40–70% of control). Furthermore, the moderate reduction in ATP sulfurylase activity in bm/bm cartilage extracts was accompanied by increased lability to freezing and thawing of the residual activity of this enzyme. These results indicate that under-sulfation of chondroitin sulfate proteoglycan in bm/bm cartilage is due to a defect in synthesis of the sulfate donor (PAPS), resulting from diminished activities of both ATP sulfurylase and APS kinase, although the reduced activity of the latter enzyme seems to be primarily responsible for the defect in PAPS synthesis.     

Antiviral and antidifferentiative activities of interferon beta and gamma in relation to their induction of double-stranded RNA-dependent protein kinase activity in 3T3-L1 cells

Mouse interferons beta (IFN-beta) and gamma (IFN-gamma) inhibit the differentiation of 3T3-L1 fibroblasts into adipocytes when added to cultures at the time of induction of differentiation. Differentiation, as measured by incorporation of radiolabeled leucine into lipids, was inhibited 50% by approximately 1-3 units/ml of either IFN-beta or IFN-gamma, with maximum inhibition of differentiation achieved with 100 units/ml of either IFN. The magnitude of antiviral activity induced by IFN-beta and IFN-gamma was similar in differentiated and undifferentiated 3T3-L1 cells, although the slopes of the dose-response curves were different; IFN-gamma induced an antiviral state with greater efficiency than IFN-beta in differentiated and undifferentiated 3T3-L1 cells. By contrast, IFN-beta induced the double-stranded RNA-dependent P1 protein kinase more efficiently than did IFN-gamma in both differentiated and undifferentiated cells. However, IFN-beta and IFN-gamma both induced greater phosphorylation of protein P1 in cell-free extracts prepared from differentiated adipocytes than in extracts from undifferentiated fibroblasts. Cultures treated with either beta or gamma IFN throughout 8 days of differentiation continued to produce double-stranded RNA-dependent protein kinase in a manner dependent on IFN dose. These results suggest that the antiviral and antidifferentiative activities of IFN-beta and IFN-gamma in 3T3-L1 cells involve different molecular mechanisms.     

Mechanism of interferon action. Purification and substrate specificities of the double-stranded RNA-dependent protein kinase from untreated and interferon-treated mouse fibroblasts

The double-stranded RNA (dsRNA)-dependent protein kinase which catalyzes the phosphorylation of ribosome-associated protein P1 and the alpha subunit of eukaryotic protein synthesis initiation factor 2 (eIF-2) was purified and characterized from mouse fibroblast L929 cells treated with either natural or recombinant interferon and from untreated cells. The dsRNA-dependent P1/eIF-2 alpha kinase was purified at least 1,500-fold from interferon-treated cells; the kinase activity that catalyzed the phosphorylation of eIF-2 alpha copurified with protein P1. The yield of P1/eIF-2 alpha protein kinase activity obtained following purification from cells treated with interferon was about 5-10 times greater than the yield from an equivalent number of untreated cells. The purified protein kinase remained dsRNA dependent. When P1 kinase was activated by dsRNA, a major phosphopeptide designated Xds was phosphorylated; Xds was not phosphorylated from P1 which had not been activated by dsRNA. The apparent native molecular weight of the purified mouse L929 dsRNA-dependent kinase as determined by sedimentation analysis was about 62,000, comparable to the molecular weight of 67,000 determined for denatured L929 phosphoprotein P1 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified protein kinase was highly selective for the alpha subunit of protein synthesis initiation factor eIF-2 and endogenous protein P1. Kinase activity was dependent upon Mg2+, and the Km for ATP was determined to be 5 X 10(-6) M. Histones (H1, H2A-B, H3, and H4) and protein synthesis initiation factors other than eIF-2 (eIF-3, eIF-4A, eIF-4B, and eIF-5) were not substrates or were very poor substrates for the purified dsRNA-dependent protein kinase. N-Ethylmaleimide, ethylenediaminetetraacetic acid, AMP, pyrophosphate, spermine, spermidine, and high concentrations of potassium inhibited both P1 and eIF-2 alpha phosphorylation by the purified kinase, whereas ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and phenanthroline did not significantly affect the phosphorylation of either protein P1 or eIF-2 alpha.     

Enhanced Deoxyribonucleic Acid Polymerase Activity in Human Embryonic Kidney Cultures Infected with Adenovirus 2 or 12

Deoxyribonucleic acid (DNA) polymerase activity was induced at approximately 18 to 20 hr after infection of secondary cultures of human embryonic kidney cells with adenovirus type 2 or type 12, and, at 30 to 50 hr after infection, the activity of this enzyme increased two- to threefold. The activity of thymidine kinase was also induced, but the activity of deoxycytidylic deaminase was not. The DNA content per cell at 71 hr after infection was 1.6-fold greater in adenovirus 2-infected cultures, and approximately 2.4-fold greater in adenovirus 12-infected cultures, than in the noninfected cultures. Several properties of DNA polymerase were studied. The enzymes in normal and adenovirus 2- or 12-infected cell extracts were saturated by approximately the same concentration of heat-denatured salmon sperm DNA primer (160 mug/ml); the enzyme activities had a similar broad pH optimum between 7.5 and 9. Extracts prepared from cells infected by either adenovirus did not activate DNA polymerase from noninfected cells, nor did the noninfected cell extracts inhibit enzyme activity of infected cell extracts. DNA polymerase in both normal and adenovirus 2- or 12-infected cells was located predominantly in the nucleus. In each case, the cytoplasm had only 30% of the enzyme activity of the nucleus. At 40 hr after infection with adenovirus 2 or 12, the activities of the enzyme in the nuclear and cytoplasmic fractions increased two- to threefold. Puromycin, an inhibitor of protein synthesis, prevented DNA polymerase induction when added to cultures during the 18- to 30-hr postinfection period, and it arrested the additional increase in enzyme activity when added after enzyme induction began. However, the increases in both DNA polymerase and thymidine kinase activities took place after treatment of infected cultures with 1-beta-d-arabinofuranosylcytosine, an inhibitor of DNA synthesis and adenovirus growth.     

Oligo(2'-5')adenylate synthetase in human lymphoblastoid cells

The enzyme oligo(2′–5′)adenylate synthetase, when activated by double-stranded RNA, polymerizes ATP into the novel oligonucleotide (2′–5′)ppp(Ap)_nA. We describe conditions for assay of this enzyme in crude extracts of a human lymphoblastoid cell line, Namalwa. The production of (2′–5′)ppp(Ap)_nA by Namalwa extracts was 3–5 times greater than the production by extracts of interferon pretreated mouse L cells, and 700 fold higher than the production by extracts of untreated mouse L cells. The relatively high level of oligo(2′–5′)adenylate synthetase in Namalwa cells was not attributable solely to their constitutive secretion of low levels of interferon. Analysis of the size distribution of the oligomers formed at different times suggested that the enzyme can add ATP to a free pppApA. Infection by Newcastle disease virus or treatment with interferon raised the apparent synthetase levels only marginally. Experiments that employed antibody to interferon suggested that the interferon must be externalized from the NDV-infected cell to induce maximal synthetase levels.     

Ascorbate increases the synthesis of procollagen hydroxyproline by cultured fibroblasts from chick embryo tendons without activation of prolyl hydroxyla.

An improved procedure was developed to extract prolyl hydroxylase from tendon cells of chick embryos with detergent, and improved assays were developed for both the activity of the enzyme and the amount of enzyme protein. Freshly isolated tendon cells were found to contain approx. 100 mug of enzyme protein per 10(8) cells and 40-50% of the enzyme protein was active. When the cells were cultured, they were found to contain the same amount of enzyme protein but only 15-20% of the enzyme protein was active. Gel filtration of cell extracts indicated that the active form of prolyl hydroxylase in freshly isolated tendon cells and incultured tendon cells had the same apparent size and the same activity per mug of immunoreactive protein as enzyme which was shown to be a tetramer. The inactive form was found to have about the same apparent size as subunits of the enzyme. When freshly isolated cells were incubated for 2 h in the presence of 40 mug per ml of ascorbate, there was a slight increase in the rate of hydroxyproline synthesis. In cultured cells, ascorbate at a concentration of 40 mug per ml caused a 2-fold increase in the rate of hydroxyproline synthesis within 30 min. However, ascorbate did not icrease the activity of prolyl hydroxylase in extracts from either cell system. Therefore it appears that the influence of ascorbate on synthesis of procollagen hydroxyproline by the cells studied here must be ascribed to a cofactor effect on the hydroxylation reaction similar to that observed with purified enzyme, and it does not involve "activation" of inactive enzyme protein to active enzyme as has been observed in cultures of L-929 and 3T6 mouse fibroblasts.     

Virogenic Properties of Bromodeoxyuridine-sensitive and Bromodeoxyuridine-resistant Simian Virus 40-transformed Mouse Kidney Cells

When simian virus 40 (SV40)-transformed mouse kidney cells (mKS) were grown in the presence of susceptible indicator cells, SV40 was readily recovered from: (i) 15 transformed cell lines, (ii) transformed cells subcultured 45 times over a 7-month period in medium containing antiviral serum and bromodeoxyuridine (dBU), (iii) 45 of 46 clonal lines isolated in the presence of antiviral serum, (iv) 19 of 19 secondary clones isolated from two clonal lines, and (v) dBU-resistant transformed cell lines. dBU-resistant SV40-transformed mouse kidney cell lines were selected and shown to contain the T antigen and to have normal levels of thymidylate kinase and deoxyribonucleic acid (DNA) polymerase, but to be deficient in thymidine (dT) kinase. Radioautographic and biochemical experiments demonstrated that very little (3)H-dT was incorporated into DNA of dBU-resistant cells during a 6-hr labeling period. After infection of dT kinase-deficient mKS cells with vaccinia virus, high levels of dT kinase were induced. The properties of SV40 recovered from dBU-sensitive and dBU-resistant cells were studied. SV40 recovered from transformed cells was shown to express in CV-1 cells at least six functions characteristic of parental virus: synthesis of capsid antigen, synthesis of T antigen, synthesis of viral DNA, induction of dT kinase, induction of DNA polymerase, and induction of host cell DNA synthesis. In addition, SV40 recovered from the transformed cells induced T antigen, dT kinase, deoxycytidylate deaminase, thymidylate kinase, and DNA polymerase in abortively infected mouse kidney cultures, and the virus was also capable of transforming primary cultures of mouse kidney cells.     

Ascorbate increases the synthesis of procollagen hydroxyproline by cultured fibroblasts from chick embryo tendons without activation or prolyl hydroxylase

An improved procedure was developed to extract prolyl hydroxylase from tendon cells of chick embryos with detergent, and improved assays were developed for both the activity of the enzyme and the amount of enzyme protein. Freshly isolated tendon cells were found to contain approx. 100 μg of enzyme protein per 10⁸ cells and 40–50% of the enzyme protein was active. When the cells were cultured, they were found to contain the same amount of enzyme protein by only 15–20% of the enzyme protein was active. Gel filtration of cell extracts indicated that the active form of prolyl hydroxylase in freshly isolated tendon cells and in cultured tendon cells had the same apparent size and the same activity per μg of immunoreactive protein as enzyme which was shown to be a tetramer. The inactive form was found to have about the same apparent size as subunits of the enzyme.When freshly isolated cells were incubated for 2 h in the presence of 40 μg per ml of ascorbate, there was a slight increase in the rate of hydroxyproline synthesis. In cultured cells, ascorbate at a concentration of 40 μg per ml caused a 2-fold increase in the rate of hydroxyproline synthesis within 30 min. However, ascorbate did not increase the activity of prolyl hydroxylase in extracts from either cell system. Therefore it appears that the influence of ascorbate on synthesis of procollagen hydroxyproline by the cells studied here must be ascribed to a cofactor effect on the hydroxylation reaction similar to that observed with purified enzyme, and it does not involve “activation” of inactive enzyme protein to active enzyme as has been observed in cultures of L-929 and 3T6 mouse fibroblasts.     

Identification of multiple epidermal growth factor-stimulated protein serine/threonine kinases from Swiss 3T3 cells

Growth factor activation of serine/threonine protein kinases was studied by treating quiescent Swiss 3T3 cells with epidermal growth factor (EGF) and examining cytosolic extracts for protein kinase activity under conditions inhibitory to calcium- and cyclic nucleotide-dependent kinases. Cytosolic extracts of cells stimulated for 5 min were fractionated by Mono Q fast protein liquid chromatography. Eight peaks of kinase activity were resolved, of which five were stimulated by EGF treatment of cells. These peaks were revealed using the synthetic peptide Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala (S6 peptide), 40 S ribosomal S6 protein, glycogen synthase, microtubule-associated protein 2, and myelin basic protein as substrates. The peaks varied in the kinetics of their activation by EGF and in their response to insulin. Selected peaks were resolved further by sizing gel chromatography. The results together indicate that at least seven distinct fractions of cytosolic kinase activities are stimulated in Swiss 3T3 cells by EGF. One of these, which phosphorylates both S6 protein and S6 peptide, is similar to the S6 kinase characterized previously in this cell line by others. Four additional activities that also phosphorylate the S6 protein and S6 peptide appear unrelated to this enzyme. Finally, two kinase activities that phosphorylate both myelin basic protein and microtubule associated protein 2 are EGF stimulated. One is similar to an insulin-stimulated microtubule-associated protein 2 kinase described in other cell lines whereas the other seems to represent a novel activity. Several of these EGF-stimulated activities were inactivated by protein phosphatases, suggesting that they might be regulated by phosphorylation.     

Properties of mitochondrial thymidine kinases of parental and enzyme-deficient HeLa cells

HeLa(BU25), a mutant subline of HeLa S3 cells, contains mitochondrial thymidine (dT) kinase, despite a marked deficiency in the dT kinase activity of the “cytosol” (high-speed supernatant) cell fraction. The HeLa(BU25) mitochondrial dT kinase differs from the “cytosol” enzyme of parental HeLa S3 cells in sedimentation coefficient, ability to utilize ribonucleoside 5′-triphosphates other than ATP as phosphate donors, sensitivity to inhibition by dCTP, and in disc polyacrylamide gel electrophoretic (disc PAGE) patterns. Two dT kinase activities [relative mobilities (Rm) of 0.4 and 0.6–0.7] were detected after disc PAGE of HeLa(BU25) mitochondrial extracts and both activities migrated more rapidly than the typical cytosol enzyme (Rm = 0.2) of dT kinase-positive human cells. The 0.6 to 0.7-Rm dT kinase of HeLa(BU25) mitochondria, but not the 0.4-Rm activity, utilized GTP and UTP, as well as ATP, as phosphate donors. HeLa S3 mitochondrial fractions contained the 0.6–0.7 Rm and the 0.4-Rm activities, and in addition, a “cytosol-like” 0.2-Rm activity. The 0.6 to 0.7-Rm dT kinase of HeLa S3 mitochondria utilized either UTP or ATP as phosphate donors, but the 0.4- and 0.2-Rm dT kinases utilized only ATP. Similarly, the HeLa S3 cytosol dT kinase efficiently utilized ATP, but not UTP, as a phosphate donor.     

2',5'-Oligoadenylate synthetase activity in lymphocytes from normal mouse.

The activity of 2',5'-oligoadenylate synthetase, an enzyme recently discovered in interferon-treated cells, was found in lymphocytes from normal mouse spleen that had received neither exogenous interferon nor its inducers. The oligoadenylate synthesized by lymphocyte cell extracts inhibited protein synthesis in rabbit reticulocyte lysates. The oligomers were composed mainly of trimer and were resistant to digestion by T2 ribonuclease. The level of the enzyme in lymphocytes was about 20 to 30% of that in L929 cells treated with interferon. The activity of the enzyme was further enhanced in lymphocytes in vitro by addition of interferon. The 2',5'-oligoadenylate synthetase was distributed among several lymphoid tissues, but was not detected in cell extracts from brain or liver. The enzyme may play an important role in the regulation of the immune system.     

Multiple components in an epidermal growth factor-stimulated protein kinase cascade. In vitro activation of a myelin basic protein/microtubule-associated protein 2 kinase.

Epidermal growth factor stimulates the activity of several cytosolic serine/threonine protein kinases in quiescent Swiss 3T3 cells. Two of these, which use myelin basic protein (MBP) as substrate, act as kinase kinases in that they are able to activate a separate peptide kinase activity in vitro by a mechanism involving protein phosphorylation. In this study, we have identified two activities from extracts of epidermal growth factor-treated cells that stimulate an ATP-dependent activation of both of the MBP kinases, derived in their inactive precursor forms from extracts of untreated cells. The resulting MBP kinase activities are stable to further purification and can be inactivated with either tyrosine or serine/threonine protein phosphatases and then reactivated to their original levels of activity. Thus, we propose that the in vitro activation involves protein phosphorylation, stimulated by the action of novel MBP kinase activating factors that represent intermediate components in a growth factor-stimulated kinase cascade.     

Antiviral and cell multiplication inhibitory activities of mouse interferon poreparations tested on an interferon sensitive murine sarcoma cell line

The cell multiplication inhibitory effect of SDS-treated mouse interferon separated into antiviral (AV) and cell multiplication inhibitory (CMI) fractions was compared to that of untreated similar interferon on a line of murine osteosarcoma cells. The untreated interferon poreparatin and the CMI fractions dose-dependently inhibited the multiplication of the cells as measured by cell count and incorporation of 3H-thymidine into the cultures. The AV fractions, containing comparable antiviral activites as the untreated interferon preparations, had only a minor effect on cell multiplication. The biochemical properties of the fractions studied remain unknown.     

Altered regulation of cyclic AMP-dependent protein kinase in a mouse lymphoma cell line.

The ability of cyclic AMP to inhibit growth, cause cytolysis and induce synthesis of cyclic AMP-phosphodiesterase in S49.1 mouse lymphoma cells is deficient in cells selected on the basis of their resistance to killing by 2 mM dibutyryl cyclic AMP. The properties of the cyclic AMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) in the cyclic AMP-sensitive (S) and cyclic AMP-resistant (R) lymphoma cells were comparatively studied. The cyclic AMP-dependent protein kinase activity or R cells cytosol exhibits an apparent Ka for activation by cyclic AMP 100-fold greater than that of the enzyme from the parental S cells. The free regulatory and catalytic subunits from both S and R kinase are thermolabile, when associated in the holoenzyme the two subunits are more stable to heat inactivation in R kinase than in S kinase. The increased heat stability of R kinase is observed however only for the enzyme in which the catalytic and cyclic AMP-binding activities are expressed at high cyclic AMP concentrations (10(-5)--10(-4) M), the activities expressed at low cyclic AMP concentrations (10(-9)--10(-6) M) being thermolabile. The regulatory subunit of S kinase can be stabilized against heat inactivation by cyclic AMP binding both at 2-10(-7) and 10(-5) M cyclic AMP concentrations. In contrast, the regulatory subunit-cyclic AMP complex from R kinase is stable to heat inactivation only when formed in the presence of high cyclic AMP concentrations (10(-5)M). The findings indicate that the transition from a cyclic AMP-sensitive to a cyclic AMP-resistant lymphoma cell phenotype is related to a structural alteration in the regulatory subunit of the cyclic AMP-dependent protein kinase which has affected the protein's affinity for cyclic AMP and its interaction with the catalytic subunit.     

Dissociation of protein kinase activity and the induction of the antiviral state in a cell line responsive to the antiviral effects of interferon

Interferons or oxidized glutathione were found to induce double-stranded RNA-dependent protein kinase activity in mouse L cells that phosphorylates the α subunit of eukaryotic peptide initiation factor 2. A mixture of leukocyte/fibroblast interferons as well as immune interferon induced the protein kinase and also suppressed virus replication in the L cells. Oxidized glutathione was equally effective in inducing protein kinase activity, but it did not induce an antiviral state in these cells. The data suggest that a simple cause and effect relationship does not exist between protein kinase induction and the establishment of the antiviral state in a cell that is responsive to the antiviral effects of interferon.