Halogenated benzimidazole ribosides, Novel inhibitors of RNA synthesis

5,6-diiodo-1H-benzotriazole: new TBBt analogue that minutely affects mitochondrial activity

4,5,6,7-Tetrabromo-1H-benzotriazole is widely used as the reference ATP-competitive inhibitor of protein kinase CK2. Herein, we study its new analogs: 5,6-diiodo- and 5,6-diiodo-4,7-dibromo-1H-benzotriazole. We used biophysical (MST, ITC) and biochemical (enzymatic assay) methods to describe the interactions of halogenated benzotriazoles with the catalytic subunit of human protein kinase CK2 (hCK2α). To trace the biological activity, we measured their cytotoxicity against four reference cancer cell lines and the effect on the mitochondrial inner membrane potential. The results obtained lead to the conclusion that iodinated compounds are an attractive alternative to brominated ones. One of them retains the cytotoxicity against selected cancer cell lines of the reference TBBt with a smaller side effect on mitochondrial activity. Both iodinated compounds are candidate leaders in the further development of CK2 inhibitors.

Synthesis and Biological Evaluation of New 5,6-dichlorobenzimidazole Nucleoside Derivatives

Novel 5,6-dichlorobenzimidazole nucleoside analogues structurally related to the well-known riboside DRB have been synthesized. The 1′,2′- trans nucleosides were prepared by condensation of peracylated sugars with 5,6-dichlorobenzimidazole, whereas the 1′,2′- cis β-D-arabinofuranosyl and β-D-lyxofuranosyl nucleosides were obtained by inversion of configuration on the sugar moiety. Chiral acyclic derivatives were stereospecifically prepared by ring-opening of furano- or pyrano-nucleosides by means of periodate oxidation, followed by borohydride reduction. The in vitro activities against a range of DNA and RNA viruses, as well as the cytotoxicities in human T-lymphocyte MT-4 cells, have been determined for these novel compounds and for DRB. No truly selective activity (i.e. clearly below the cytotoxic concentration) was observed against any of the viruses used. Some of the compounds, including DRB, were cytotoxic to MT-4 cells at CC50 values of less than 10 μg ml−1.

Long-term habituation of the gill-withdrawal reflex in aplysia requires gene transcription, calcineurin and L-type voltage-gated calcium channels

Although habituation is possibly the simplest form of learning, we still do not fully understand the neurobiological basis of habituation in any organism. To advance the goal of a comprehensive understanding of habituation, we have studied long-term habituation (LTH) of the gill-withdrawal reflex (GWR) in the marine snail Aplysia californica. Previously, we showed that habituation of the GWR in a reduced preparation lasts for up to 12 h, and depends on protein synthesis, as well as activation of protein phosphatases 1 and 2A and postsynaptic glutamate receptors. Here, we have used the reduced preparation to further analyze the mechanisms of LTH in Aplysia. We found that LTH of the GWR depends on RNA synthesis because it was blocked by both the irreversible transcriptional inhibitor actinomycin-D and the reversible transcriptional inhibitor, 5,6-dichlorobenzimidazole riboside (DRB). In addition, LTH requires activation of protein phosphatase 2B (calcineurin), because it was disrupted by ascomycin. Finally, LTH was blocked by nitrendipine, which indicates that activation of l-type voltage-gated Ca²⁺ channels is required for this form of learning. Together with our previous results, the present results indicate that exclusively presynaptic mechanisms, although possibly sufficient for short-term habituation, are insufficient for LTH. Rather, LTH must involve postsynaptic, as well as presynaptic, mechanisms.

Tetrabromobenzotriazole (TBBt) and tetrabromobenzimidazole (TBBz) as selective inhibitors of protein kinase CK2: evaluation of their effects on cells and different molecular forms of human CK2

The development of selective cell-permeable inhibitors of protein kinase CK2 has represented an important advance in the field. However, it is important to not overlook the existence of discrete molecular forms of CK2 that arise from the presence of distinct isozymic forms, and the existence of the catalytic CK2 subunits as free subunits and in complexes with the regulatory CK2beta subunits and, possibly, other proteins. This review examines two recently developed, and presently widely applied, CK2 inhibitors, 4,5,6,7-tetrabromobenzotriazole (TBBt) and the related 4,5,6,7-tetrabromobenzimidazole (TBBz), the latter of which was previously shown to discriminate between different molecular forms of CK2 in yeast. We have shown, by spectrophotometric titration, that TBBt, with a pK(a) approximately 5, exists in solution at physiological pH almost exclusively (>99%) as the monoanion; whereas TBBz, with a pKa approximately 9, is predominantly (>95%) in the neutral form, both of obvious relevance to their modes of binding. In vitro, TBBt inhibits different forms of CK2 with Ki values ranging from 80 to 210 nM. TBBz better discriminates between CK2 forms, with Ki values ranging from 70 to 510 nM. Despite their general similar in vitro activities, TBBz is more effective than TBBt in inducing apoptosis and, to a lesser degree, necrosis, in transformed human cell lines. Finally, development of shRNA strategies for the selective knockdown of the CK2alpha and CK2alpha' isoforms reinforces the foregoing results, indicating that inhibition of CK2 leads to attenuation of proliferation.

Temperature-sensitive Chinese hamster fibroblast mutant with a defect in RNA metabolism

We describe a new temperature-sensitive mutant of Chinese hamster cell fibroblasts. After a shift to the nonpermissive temperature of 40.5 degrees C, the rates of DNA, RNA, and protein synthesis declined rapidly (to < or = 50% within 12 h) and the progression of unsynchronized cells through the cell cycle was affected. We believe that DNA synthesis came to a halt after a short time, because cells no longer entered the S phase. The decrease in protein synthesis at 40.5 degrees C was shown to be a consequence of a decrease in the number of polysomes, whereas free 80S ribosomes accumulated. We concluded that the components of the protein biosynthetic machinery were intact (ribosomes and soluble factors), but synthesis was limited by a shortage of mRNA. The decline in mRNA production had a significant effect on the synthesis of proteins (e.g., heat shock proteins) translated from short-lived messages. We observed that both polyadenylated and nonpolyadenylated RNA syntheses declined at 40.5 degrees C, whereas the synthesis of small RNAs (4 to 5S) was less reduced. The argument is made that the temperature-sensitive phenotype is the result of a defect affecting mRNA synthesis.

Transforming growth factor-betas inhibit somatostatin messenger ribonucleic acid levels and somatostatin secretion in hypothalamic cells in culture

Treatment of hypothalamic cells in monolayer culture with transforming growth factor-beta1 (TGFbeta1) significantly reduced both basal and cAMP-induced somatostatin messenger RNA (mRNA) levels and somatostatin secretion. This inhibitory effect was dose- and time-dependent and not mediated by glial cells, as it was also observed in glial-free hypothalamic cell cultures treated with cytosine arabinonucleoside. TGFbeta2 and -beta3 mimicked the actions of TGFbeta1, which indicated that the three isoforms of the TGFbeta family expressed in the central nervous system displayed similar effects on the somatostatinergic neurons. The blockade of synthesis of proteins with either cycloheximide or puromycin for 24 h prevented the inhibitory effect of TGFbeta1 on somatostatin mRNA. This implied that the reduction of this mRNA by TGFbeta1 required de novo protein synthesis. We next studied whether TGFbeta1 acted at the transcriptional or posttranscriptional level by altering the stability of somatostatin mRNA. Examination of the rate of disappearance of somatostatin mRNA by Northern blot, after inhibition of mRNA transcription with either actinomycin D (AcD) or 5,6-dichloro-1beta-ribofuranosyl benzimidazole revealed that TGFbeta1 did reduce the stability of somatostatin mRNA. This effect was observed when we pretreated the cultures with TGFbeta1 4 h before the addition of AcD, but not when we administered TGFbeta1 simultaneously with AcD or 5,6-dichloro-1beta-ribofuranosyl benzimidazole. Altogether these results demonstrated that the treatment of hypothalamic cells in culture with TGFbeta1, TGFbeta2, or TGFbeta3 resulted in a decrease in somatostatin mRNA levels and somatostatin secretion. TGFbeta1 reduced the steady state levels of somatostatin mRNA by inducing the synthesis of a protein (s), that appears to accelerate the degradation of the mRNA of somatostatin. Whether TGFbeta1 has additional effects on the transcription of the somatostatin gene will require further study.

Human immunodeficiency virus type 1 Rev function requires continued synthesis of its target mRNA

Synthesis of human immunodeficiency virus structural proteins is dependent on expression of the virus-encoded Rev protein due to the constitutive nuclear sequestration of mRNAs coding for the structural proteins. The pathway by which Rev, through interaction with the Rev-responsive element (RRE) within the mRNA, achieves export of the mRNA remains unclear. To probe the mechanism by which Rev induces nuclear export of its target mRNAs, the effect of inhibiting mRNA synthesis on the function of Rev was examined. Two approaches to address this issue were pursued: (i) the use of general transcription inhibitors such as 5,6-dichlorobenzimidazole riboside (DRB) and actinomycin D, and (ii) the more selective modulation of target gene transcription permitted by the use of a tetracycline-regulated promoter. Addition of either DRB or actinomycin D inhibited Rev action despite the presence of significant quantities of the target mRNA throughout the course of drug treatment. Furthermore, prolonged DRB treatment was found to improve rather than diminish the induction observed. Subsequent analysis using the tetracycline-modulated promoter demonstrated that Rev function was dependent on the transcription rate of the target mRNA and independent of target mRNA concentration. These data strongly indicate that Rev functions through interaction with newly synthesized target mRNA, facilitating its export by preventing its interaction with the host factors that effect nuclear sequestration.

Selective control of cytosolic glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase mRNA stability by selenium supply

Selenium depletion of H4 hepatoma cells reduced cytosolic glutathione peroxidase (cGSH-Px) mRNA abundance but had no effect on phospholipid hydroperoxide glutathione peroxidase (PHGSH-Px) mRNA abundance. Actinomycin D chase experiments showed that selenium depletion had no effect on the stability of PHGSH-Px mRNA but decreased the stability of cGSH-Px mRNA. In Se-replete cells puromycin decreased the stability of both cGSH-Px and PHGSH-Px mRNAs. The results suggest that when selenium supply is limiting PHGSH-Px mRNA translation is maintained more than that of cGSH-Px mRNA, and thus more cGSH-Px mRNA is released from polysomes and degraded.

Regulation of alpha-subunit mRNA transcripts by pituitary adenylate cyclase-activating polypeptide (PACAP) in pituitary cell cultures and alpha T3-1 cells

Pituitary adenylate cyclase activating polypeptide (PACAP) increases glycoprotein hormone alpha-subunit mRNA levels suggesting a role for PACAP in maintaining the high levels of alpha-subunit protein characteristic of the pituitary. The present study used primary pituitary cell cultures and the alpha T3-1 pituitary cell line to investigate how PACAP affects alpha-subunit mRNA transcripts. Stimulation of cultured pituitary cells with 10 nM PACAP38, 10 nM GnRH, or the combination, for 24 h increased alpha-subunit mRNA levels 1.5-fold, whereas GnRH more effectively (P<0.01) stimulated alpha-subunit protein release than did PACAP38 (3.2- vs. 2.0-fold). alpha-Subunit mRNA levels in alphaT3-1 cells were also increased by PACAP38 and by GnRH to maximum values at 12 h (P<0.05), and alpha-subunit protein secretion rose proportionately and in parallel with alpha-subunit mRNA levels. PACAP38 was a 100-fold more potent stimulator of alpha-subunit mRNA than was VIP, and a VIP-antagonist failed to block the stimulatory effect of PACAP38, suggesting an effect via type PACAP 1 receptors. Type I receptor mRNA transcripts were identified by Northern analysis in alphaT3-1 cells. Depletion of PCK activity by PMA failed to block the stimulatory effect of PACAP38, but prevented GnRH from increasing alpha-subunit mRNA levels and alpha-subunit secretion. PACAP38, like 8Br-cAMP and forskolin, stimulated (P<0.05) luciferase (LUC) activity in alphaT3-1 cells transfected with a plasmid containing the first 846 of 180 base pairs of the 5'-flanking region of the human alpha-subunit gene linked upstream to a LUC reporter gene. Finally, experiments using the transcription inhibitor DRB reveal that PACAP does not appreciably change alpha-subunit mRNA half-life. These findings are consistent with the proposal that PACAP contributes to the high levels of alpha-subunit protein characteristic of the pituitary by activating Type I receptors and stimulating alpha-subunit gene transcription in part by the cAMP/PKA pathway.

Stereochemical Aspects of the anti-HCMV Activity of Cytidine Nucleoside Analogues

The remarkable selectivity of the β-L enantiomers of 2′,3′-dideoxycytidine analogues against the viral polymerases of HIV and HBV has stimulated our interest in targeting β-L enantiomers of anti-HCMV cytidine analogues. Indeed, Ara-C, FIAC and DMDC are cytidine analogues with β-D configuration that show significant potency as anti-HCMV agents but lack selectivity. β-L enantiomers have therefore been synthesized and evaluated together with four other nucleoside analogues, and the β-L. enantiomers were found not to be inhibitory to HCMV replication. However, the three α-L isomers, α-L-Ara-C, α-L-Xylo-C and α-L-FMAU, emerged with activity against HCMV and have provided new approaches for the treatment of viral diseases with nucleoside analogues possessing the unusual L-configuration.

Induction of secretogranin II mRNA by protein synthesis inhibitors in GH4C1 cells

The effect of cycloheximide (CHX) on the expression of secretogranin II (SgII), a member of the granin family of secretory proteins, was investigated in rat pituitary GH4C1 (GH) cells. The administration of CHX resulted in a dose- and time-dependent increase in SgII mRNA expression, the greatest effect (3.8-fold above control) being achieved with 1 microgram/ml CHX, which resulted in > 90% inhibition of protein synthesis. Emetine (1 microgram/ml), pactamycin (0.6 microgram/ml), anisomycin (2.5 micrograms/ml), and puromycin (100 micrograms/ml), protein synthesis inhibitors structurally and mechanistically unrelated to CHX, also increased the level of SgII mRNA expression. Treatment with forskolin (10 microM) alone had no effect on SgII mRNA levels but potentiated the effect of CHX. Neither 100 nM phorbol 12-myristate 13-acetate nor 45 mM KCl affected SgII mRNA levels in the absence or presence of 1 microgram/ml CHX. The effect of CHX was blocked by the transcription inhibitors actinomycin D (5 micrograms/ml) and 5,6-dichlorobenzimidazole riboside (20 micrograms/ml) but not by the coadministration of the polysome destabilizer pactamycin (0.6 microgram/ml), suggesting that the effect of CHX was transcriptional. These studies show that the expression of SgII mRNA is induced by protein synthesis inhibitors in GH cells, suggesting the presence of a labile repressor, which not only controls the basal expression of the SgII gene but also completely inhibits the stimulatory effect of forskolin.

Effects of interleukin-1 beta and tumor necrosis factor-alpha on the expression of glial fibrillary acidic protein and transferrin in cultured astrocytes

Recent evidence suggests that interleukin (IL)-1 and tumor necrosis factor (TNF) may play a role in astrogliosis following injury to the CNS. The short-term biochemical effects of these immune-related cytokines were determined on cultured rat polygonal and process-bearing astrocytes. Both IL-1 and TNF stimulated the rate of thymidine incorporation in polygonal astrocytes up to 137% and 215%, respectively, over the level observed in untreated controls. By contrast, thymidine incorporation was relatively unaffected by these cytokines in process-bearing astrocytes. The cytokines did not significantly affect the level of glial fibrillary acidic protein (GFAP) within polygonal astrocytes, even though they appeared to downregulate the expression of GFAP mRNA by as much as 62%. Both cytokines increased the intracellular expression of transferrin (Tf) within some polygonal astrocytes. In untreated control cultures, fewer than than 2% of polygonal astrocytes were immunoreactive for Tf. By contrast, approximately 30% of polygonal astrocytes treated with IL-1 or TNF-alpha became strongly immunoreactive for Tf. Neither IL-2 nor a number of other known growth factors appeared to alter the level of immunoreactive Tf in these cells. Process-bearing astrocytes were negative for Tf, regardless of the treatment used. Northern blot analysis demonstrated that the level of Tf mRNA in cultures of polygonal astrocytes increased 148% above the level observed in untreated controls following treatment with either IL-1 or TNF, whereas no change was observed following treatment with IL-2. These results suggest that increased levels of particular cytokines known to be present in injured CNS can produce pronounced biochemical alterations within a subtype of cultured astrocytes.

Coordinate regulation of mRNAs from multiple calmodulin genes during myoblast differentiation in vitro

Multiple genes encoding identical calmodulin molecules have been found in all mammalian species so far examined, but little is known regarding the factors involved in regulating the expression of this gene family. We have investigated the possibility of differential regulation under conditions of cell cycle withdrawal and differentiation in the nonfusing BC3H1 myoblast. Transcripts from the three genes are expressed in myoblasts and myocytes and each of the mRNA species decreases during BC3H1 differentiation. Calmodulin protein levels also decrease, although with distinct kinetics with respect to the mRNAs. Previous studies indicated that a decrease in transcription is involved (Epstein et al., Molecular Endocrinology 3:193-202, 1989). In this study, an increase in stability for each of the mRNA species is also shown to contribute to overall mRNA levels. The calmodulin mRNAs are also found to decrease under conditions of cell cycle withdrawal when differentiation is blocked. This demonstrates that the expression of mRNA from all three genes is directly coupled with the proliferation state but only indirectly with the differentiation state. Consistent with this, calmodulin expression decreases in serum deprived fibroblasts as they exit the cell cycle.

Benzimidazole nucleoside analogues as inhibitors of plant (maize seedling) casein kinases

Halogeno benzimidazole and benzimidazole nucleoside analogues have been screened for inhibitory activity vs. purified plant (maize seedling) casein kinases I, IIA and IIB, and the results compared with those previously reported for some of the compounds as inhibitors of the corresponding mammalian CK-1 and CK-2 (Meggio et al. (1990) Eur. J. Biochem. 187, 89-94). One new analogue, the riboside of 5,7-dibromobenzimidazole, which is sterically constrained to the anti conformation about the glycosidic bond, and is a good inhibitor, exhibited appreciable (5-7-fold) discrimination between the type I and type II enzymes. An increase in the number of halogen substituents on the benzene ring of benzimidazole from two to three led to marked enhancement of inhibitory activity, particularly against the type II enzymes, with a decrease in Ki from 24 to 4 microM. The 2-aza analogue of 5,6-dichlorobenzimidazole, i.e. 5,6-dichlorobenzotriazole, as the free base, even more effectively discriminated between the two types of plant casein kinases, with Ki approximately 100 microM for CK-I, and Ki approximately 9 microM for CK-IIA and CK-IIB. Inhibition in all instances was competitive with respect to ATP (for CK-I), and ATP and GTP (for CK-IIA and CK-IIB). The results are compared with those for halogenated isoquinolinesulfonamide inhibitors reported by Chijiwa et al. (J. Biol. Chem. (1989) 264, 4924-4927), leading to proposals for the synthesis of potentially more effective and more discriminating inhibitors. Attention is drawn to the significant role of the halogen substituents in the mechanism(s) of action of the structurally related benzimidazole, benzotriazole and naphthalene and isoquinoline, inhibitors of protein kinases.

A comparison of apparent mRNA half-life using kinetic labeling techniques vs decay following administration of transcriptional inhibitors

Several different techniques were used to determine the apparent half-lives of immunoglobulin gamma 2b heavy chain and kappa light chain mRNA's in mouse myeloma 4T001 and a mutant derived from 4T001, i.e., mutant I17. The mutant I17 Ig heavy chain mRNA lacks CH1 and has fused CH2 and CH3 domains resulting in a truncated protein. By all four techniques the Ig heavy chain mRNA from mutant I17 displays a half-life that is approximately 70% the half-life of Ig mRNA in 4T001 cells. However, the absolute values of apparent half-life varied by greater than twofold for both lines among several of the techniques employed. The half-life of Ig gamma 2b mRNA in 4T001 cells was found to be 6.4 h by measuring decay following administration of the adenosine analog DRB to block new mRNA synthesis and 5.7 hr by measuring accumulation in an approach to steady-state labeling protocol. In contrast, the observed Ig mRNA half-lives determined by measuring decay following administration of actinomycin D to block new mRNA synthesis, or in a pulse-chase analysis were 2.9 and 3.8 h, respectively. The apparent half-life for Ig kappa light chain mRNA was the same in the 4T001 and I17 lines using any one technique but the value varied depending on the technique from a high value of 5.9 h following DRB to a low value of 2.4 h with actinomycin decay. Approach to steady-state is theoretically the most accurate method to measure mRNA half-life when that value is less than the doubling time of the cells. Pulse-chase analyses are accurate for measuring mRNA half-life when that value is longer than the effective chase period. Measuring preformed message decay following administration of drugs to block new mRNA synthesis is adaptable over a range of half-lives, but the cells must be shown to retain correct RNA metabolism over the time frame of the experiment. Determining a correct half-life for a particular mRNA may not be feasible using only one method and may, in fact, require several different approaches until a consensus value emerges.

Synthesis and properties of 5,6-dichlorobenzimidazole 2'----5'- and 3'----5'-nucleotide dimers and trimers

5,6-Dichloro-1-beta-D-ribofuranosylbenzimidazole (2), synthesised by the fusion method, was used for the synthesis of 2'----5'- and 3'----5'-linked di- and tri-meric oligonucleotides. The protecting groups used were p-methoxytrityl for HO-5', tert-butyldimethylsilyl for HO-2',3', and 2,5-dichlorophenyl and 2-(4-nitrophenyl)ethyl for the phosphate group. The internucleotidic linkages were established by the phosphotriester approach to give the fully protected 2'----5' dimers (15, 17, and 18) and trimers (27 and 28), as well as the 3'----5' dimers (22 and 23) and trimers (31 and 32). Deprotection involved a sequence of steps to afford the corresponding free oligonucleotides 21, 26, 30, and 33 isolated as the triethylammonium salts in good yields. The new compounds were characterised by elemental analysis and by u.v. and 1H-n.m.r. spectroscopy.

Bioassay for trans-activation using purified human immunodeficiency virus tat-encoded protein: trans-activation requires mRNA synthesis

Expression of the human immunodeficiency virus tat-encoded protein (Tat) is required for virus replication. A genetic approach was used to facilitate the purification of biologically active Tat. A recombinant Tat protein containing a stretch of six histidine residues and a protease cleavage site was engineered and purified to greater than 95% homogeneity in a single step by immobilized metal-ion chromatography with a special affinity resin that has selectivity for proteins with neighboring histidine residues. A modified scrape loading method for introduction of protein into cell monolayers was used to demonstrate that the purified Tat retained biological activity. Tat function was completely blocked in the presence of transcription inhibitors, which demonstrates the requirement of ongoing mRNA synthesis for trans-activation. These studies indicate that the mechanism of trans-activation is unlikely to involve a direct action of Tat on mRNA stability, transport, or translation and provides the basis for a rapid assay that can be used to identify inhibitors of trans-activation. The methods described herein should be useful for the functional analysis of other proteins that do not confer activity through a receptor-mediated pathway.

Effect of RNA synthesis inhibitors on insulin-induced protein synthesis by 3T3 cells

1. The increased protein synthesis of quiescent 3T3 cells in response to insulin was separated into three distinct phases based on their response to various inhibitors of RNA synthesis. 2. The first increase in protein synthesis was insensitive to the inhibitors used, and probably resulted from activation of existing protein synthesizing mechanism. 3. The second phase was sensitive to a varying extent to alpha-amanitin and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, implying the need for new mRNA synthesis as well as the production of new ribosomes indicated by its further sensitivity to low concentration (10 ng/ml) of Actinomycin D. 4. The final phase was insensitive to inhibitors of new ribosome formation, but still depended on new mRNA. alpha-difluoromethylornithine, an inhibitor of de novo polyamine synthesis, partly inhibited the insulin induced stimulation of protein synthesis.

Control of late G0/G1 progression and protein modification by SmC/IGF I

Somatomedin C/insulin-like growth factor I (SmC/IGF I) mediates traverse of late G0/G1 in density-arrested BALB/c-3T3 cells from a distinct growth arrest point in mid-G0/G1 (the V point) to the initiation of DNA synthesis. As a prelude to future studies aimed at defining the mechanism of action of SmC/IGF I, we investigated the level (e.g., transcriptional, translational) at which SmC/IGF I modulates V to S traverse. The post-V point progression of cells arrested at the V point by amino acid starvation and released into amino acid-replenished medium containing SmC/IGF I, insulin, or platelet-poor plasma (PPP) did not require either mRNA synthesis or an increase in the overall level of protein synthesis. Although two-dimensional gel analysis of proteins prepared from SmC/IGF I-treated cells did not reveal any preferentially synthesized proteins, several SmC/IGF I-induced protein modifications, which result in an increase in isoelectric point (pI) and occur in the absence of mRNA synthesis, were evident. These findings suggest that SmC/IGF I modulates late G0/G1 progression by a posttranscriptional process that may involve protein modification.

Mitogens and protein synthesis inhibitors induce ornithine decarboxylase gene transcription through separate mechanisms in the BC3H1 muscle cell line

Ornithine decarboxylase (ODCase), the rate-limiting enzyme in polyamine biosynthesis, exhibits dramatic fluctuations in activity in response to a variety of hormones and growth factors and has been shown to be down-regulated during myogenesis. In the present study, the molecular mechanisms involved in expression of ODCase mRNA were examined in cells of the BC3H1 muscle line. Proliferating, undifferentiated cells in medium with 20% fetal calf serum displayed high levels of ODCase mRNA and enzyme activity. The transfer of proliferating cells to medium containing 0.5% serum resulted in their withdrawal from the cell cycle and a 20- to 50-fold reduction in the steady-state level of ODCase mRNA within 24 h. Down-regulation of ODCase mRNA was paralleled by a decrease in ODCase enzyme activity and ODCase gene transcription. ODCase mRNA was rapidly reinduced by exposure of quiescent, differentiated cells to medium with 20% serum or by inhibition of protein synthesis with cycloheximide. The accumulation of ODCase mRNA after mitogenic stimulation or protein synthesis inhibition was accompanied by an increase in ODCase gene transcription. The mechanisms whereby mitogens and protein synthesis inhibitors induced ODCase transcription appeared to be different since cycloheximide potentiated the effects of mitogens, resulting in superinduction of ODCase transcription to a level significantly greater than in the presence of mitogens alone. These results indicate that ODCase down-regulation during myogenesis is controlled primarily at the level of ODCase gene transcription. These data also demonstrate that ODCase expression is regulated by antagonistic signals, positive signals for transcription elicited by mitogens and negative signals from endogenous protein repressors that influence ODCase transcription.

Mitogens and protein synthesis inhibitors induce ornithine decarboxylase gene transcription through separate mechanisms in the BC3H1 muscle cell line

Ornithine decarboxylase (ODCase), the rate-limiting enzyme in polyamine biosynthesis, exhibits dramatic fluctuations in activity in response to a variety of hormones and growth factors and has been shown to be down-regulated during myogenesis. In the present study, the molecular mechanisms involved in expression of ODCase mRNA were examined in cells of the BC3H1 muscle line. Proliferating, undifferentiated cells in medium with 20% fetal calf serum displayed high levels of ODCase mRNA and enzyme activity. The transfer of proliferating cells to medium containing 0.5% serum resulted in their withdrawal from the cell cycle and a 20- to 50-fold reduction in the steady-state level of ODCase mRNA within 24 h. Down-regulation of ODCase mRNA was paralleled by a decrease in ODCase enzyme activity and ODCase gene transcription. ODCase mRNA was rapidly reinduced by exposure of quiescent, differentiated cells to medium with 20% serum or by inhibition of protein synthesis with cycloheximide. The accumulation of ODCase mRNA after mitogenic stimulation or protein synthesis inhibition was accompanied by an increase in ODCase gene transcription. The mechanisms whereby mitogens and protein synthesis inhibitors induced ODCase transcription appeared to be different since cycloheximide potentiated the effects of mitogens, resulting in superinduction of ODCase transcription to a level significantly greater than in the presence of mitogens alone. These results indicate that ODCase down-regulation during myogenesis is controlled primarily at the level of ODCase gene transcription. These data also demonstrate that ODCase expression is regulated by antagonistic signals, positive signals for transcription elicited by mitogens and negative signals from endogenous protein repressors that influence ODCase transcription.

Developmentally regulated mRNAs in 3T3-adipocytes: analysis of transcriptional control

We have investigated the regulation of mRNA synthesis during 3T3-adipocyte differentiation by measuring the transcription of specific genes in isolated preadipocyte and adipocyte nuclei. Transcription was assayed by hybridization of newly synthesized RNA to cDNA clones coding for glycerophosphate dehydrogenase (GPD), the induced protein of 13K which is shown here to be related to myelin protein P-2, the induced protein of 28K, actin, and two RNAs that are not developmentally regulated. Transcription of GPD and 13K was observed in adipocyte but not preadipocyte nuclei. Actin was transcribed in both types of nuclei but at a lower level in adipocytes. For most of the RNAs examined, there was a consistent relationship between amounts of nuclear transcription and the abundance of the corresponding cytoplasmic mRNA in adipocytes. However, 13K and 28K mRNAs are 10-100 times more abundant than would be predicted by their nuclear transcription alone. Preliminary mRNA turnover experiments in which 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole was used to inhibit mRNA synthesis suggest that these mRNAs are much more stable in the adipocyte cytoplasm than the other mRNAs examined. These results indicate that the transcription of specific genes is increased during adipocyte differentiation and suggest that other levels of control, particularly mRNA stability, may contribute to the relative abundance of certain developmentally-regulated mRNAs in adipocytes.

Recovery of HeLa cell population growth after treatment with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB)

Dose-response curves for the inhibition of heterogeneous nuclear RNA (hnRNA) synthesis in HeLa cells by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB; 5-100 microM; 30 min) are biphasic and indicate the existence of two subpopulations of hnRNA molecules, one highly sensitive and the other completely resistant, as previously reported for molecules greater than 1,000 nucleotides long (Tamm et al., 1976; Sehgal et al., 1976a). In the short-term experiments, the drug-sensitive synthesis of hnRNA was inhibited 50% at a DRB concentration of approximately 7 microM, and 70% at 20 microM, whereas drug-resistant synthesis, which comprises approximately 20% of total, continued at DRB concentrations as high as 100 microM. After 24 hr of DRB treatment in medium containing 5% fetal calf serum, the increase in cell number in the exponentially growing population was inhibited by only 42% at 20 microM DRB, and the formation of colonies of greater than or equal to ten cells was not decreased. DRB at 40 microM concentration decreased population growth by 76% and colony formation by 63%. Treatment with 60 microM DRB was sufficient to prevent a net increase in cell number and to reduce colony formation by 78%. After termination of treatment, the time required for the surviving population to begin rapid proliferation was directly related to the concentration of DRB used to treat cells and to the duration of treatment. After 24-hr treatment with 40 microM DRB, cultures recovered within 1 day, whereas after 60 microM DRB, 3-4 days were required. After 40-hr treatment with 60 microM DRB, 5-6 days were required for recovery, and after 80 microM DRB, 9-11 days. During the "dormant" period the cell number ranged from 15 to 60% of the initial number and was fairly stable for given conditions. After the "dormant" period, recovery was rapid. The population growth rate in cultures undergoing treatment with DRB is directly related to serum concentration; however, the recovery rate during the post-treatment period is unaffected by serum concentration.

Nuclear alterations induced by cadmium chloride and L-canavanine in HeLa S3 cells. Accumulation of perichromatin granules

The effects of L-canavanine and cadmium on the ribonucleoprotein constituents of HeLa S3 cells have been analyzed. Both chemicals induce a similar pattern of alterations in different RNP structures as well as in both RNA and protein synthesis. Pulse and chase autoradiographic experiments reveal that both canavanine and cadmium induce a preferential inhibition of nucleolar RNA synthesis and a slowdown in the transport or processing of nucleolar and extranucleolar RNA. Nucleoli become round and compact. Accumulation of perichromatin granules and fibrils occurs, there is a depletion of interchromatin fibrils, and nuclear formations appear which seem to be involved in the morphogenesis of perichromatin granules accumulated during the treatments. The appearance of clusters of 29- to 35-nm granules might be related with a deficient assembling of constituents of perichromatin granules. The effects of different inhibitors of the transcriptional processes on the accumulation of perichromatin granules suggest that these granules represent a particular subpopulation of hnRNP.

Temperature-Sensitive Chinese Hamster Fibroblast Mutant with a Defect in RNA Metabolism

We describe a new temperature-sensitive mutant of Chinese hamster cell fibroblasts. After a shift to the nonpermissive temperature of 40.5°C, the rates of DNA, RNA, and protein synthesis declined rapidly (to ≤50% within 12 h) and the progression of unsynchronized cells through the cell cycle was affected. We believe that DNA synthesis came to a halt after a short time, because cells no longer entered the S phase. The decrease in protein synthesis at 40.5°C was shown to be a consequence of a decrease in the number of polysomes, whereas free 80S ribosomes accumulated. We concluded that the components of the protein biosynthetic machinery were intact (ribosomes and soluble factors), but synthesis was limited by a shortage of mRNA. The decline in mRNA production had a significant effect on the synthesis of proteins (e.g., heat shock proteins) translated from short-lived messages. We observed that both polyadenylated and nonpolyadenylated RNA syntheses declined at 40.5°C, whereas the synthesis of small RNAs (4 to 5S) was less reduced. The argument is made that the temperature-sensitive phenotype is the result of a defect affecting mRNA synthesis.

Specific inhibition of hnRNA synthesis by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Requirement of a free 3'-hydroxyl group, but not 2'- or 5'-hydroxyls

Five structural analogues of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), all with modified sugar moieties, have been examined for their inhibitory activities on RNA transcription in salivary glands of Chironomus tentans. The well-known ability of the parent DRB at 65 microM concentration to selectively inhibit hnRNA/mRNA synthesis by approx. 90% was essentially abolished on methylation of the 3'-OH; but, at an overdose the analogue suppressed labeling of all RNA classes examined (hnRNA/mRNA, rRNA, 4-5 S RNA) by 70-80%. By contrast, the 2'-O-methyl derivative of DRB was almost as effective as DRB itself in blocking transcription of hnRNA/mRNA genes. Blocking of both the 2' and 3' hydroxyls (2',3'-O-isopropylidene-DRB) completely abolished inhibitory activity, irrespective of the concentration employed. The 5'-deoxy-5'-chloro derivative of DRB was only slightly less effective than the parent DRB. An unusual aspect of the activities of 2'-O-methyl-DRB and 5'-deoxy-5'-chloro-DRB was their ability to stimulate synthesis of 4-5 S RNA by 25-45%. Also investigated was the influence of the various analogues on the rate of formation of [3H]UTP from [3H]uridine used as an RNA precursor. The rate of such formation of [3H]UTP was suppressed 2-6-fold by treatment with 2'-O-methyl or 3'-O-methyl-DRB, but was unaffected by 5'-deoxy-5'-chloro-DRB or 5,6-dichloro-1-alpha-D-arabinofuranosylbenzimidazole. The overall data point to the importance of a free 3'-OH in the ribose moiety of DRB for selective inhibitory activity. The alpha-D-arabinofuranosyl analogue, although less selective in inhibition of RNA transcription, still exhibits about 50% of the activity of DRB.

Dichlorobenzimidazole-riboside inhibition of nuclear RNA accumulation initiated with gamma-thio analogues of ATP and GTP

The gamma-thio analogues of ATP and GTP, ATP[S] and GTP[S], have been used as affinity probes to measure RNA synthesis initiated in vitro in L cell nuclei. 5,6-Dichlororibofuranosylbenzimidazole was found to inhibit total RNA synthesis in vitro and the amount bound by mercury-affinity chromatography. Initiation in vitro was also shown by [35S]ATP[S] and [35S]GTP[S] incorporation in RNA molecules with a larger size distribution. Although 66% of RNA molecules labeled with [35S]GTP[S] and 74% of those labeled with [35S]ATP[S] were 3-12 S n size, the remainder of the label was recovered in RNA molecules larger than 12 S and a significant portion in RNA larger tha 18 S. The specificity of the initiation process seems to be indicated by the finding of molecules of the size of pre-tRNA on gel electrophoresis which were labeled with [35S]GTP[S] but not [35S]ATP[S] under our experimental conditions. 5,6-Dichlororibofuranosylbenzimidazole severely inhibited the incorporation of [35S]GTP[S] and [35S]AT[[S] by all size classes, indicating that it can decrease accumulation of RNA initiated in vitro in the L cell nuclear system.

Serum enhances the cycling and survival of HeLa cells treated with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole

We have defined the growth kinetics of HeLa cell populations by determining the frequencies of mitoses and deaths and the lengths of intermitotic intervals. This was done by time-lapse cinemicrography. Untreated control cells proliferated at closely similar rates in medium enriched with 5% or 15% fetal calf serum, with an average of 4% dividing and less than 0.1% dying per hr. The mean intermitotic interval was 16 hr during exponential growth of the control populations. In contrast, in cultures treated with 40 or 60 microM 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), a selective inhibitor of heterogeneous nuclear RNA synthesis, the frequency of mitoses was markedly and directly dependent on serum concentration, whereas the frequency of deaths was inversely dependent. DRB prolonged the intermitotic interval in cells cycling in the presence of the drug, but the effect was less in 15% than in 5% serum. After prolonged treatment of HeLa cells with DRB, the inhibition of heterogeneous nuclear RNA synthesis by DRB appeared to be reduced, which was not due to inactivation of DRB in the culture medium.

Modulation of 75S RNA synthesis in the Balbiani rings of Chironomus tentans with galactose treatment

Galactose has been used as a tool to modify gene activity in the giant puffs Balbiani ring 2 (BR2) and Balbiani ring 1 (BR1) on chromosome IV in the salivary glands of Chironomus tentans. BR2 decreased gradually and was absent or almost absent after a four day galactose treatment. Concomitant with this morphological change, the labelling of the population of growing 75S RNA molecules in BR2 decreased, and was essentially abolished after four days in galactose. Since the elongation rate at the 75S RNA genes proved to be the same in the galactose treated glands as in the control glands, the decreased labelling in BR2 was likely to correspond to a decreased production of 75S RNA. No changes in the size distribution of the growing 75S RNA molecules were noted during the galactose treatment, suggesting that the modulation of the activity was most likely accomplished at the initiation level, but regulation of a very early premature termination could not be excluded. When galactose was removed from the medium, BR2 attained its normal size and its ordinary RNA labelling. BR1 was studied in parallel with BR2 and it behaved strikingly different: BR1 expanded during the galactose treatment and the amount of growing 75S RNA increased, indicating an enhanced production of this 75S RNA species. Also the modulation of BR1 RNA synthesis was reversible. During the galactose treatment no changes in the labelling of chromosome I-III and of nucleolar RNA were observed suggesting that during the four day treatment, galactose exerted its effect mainly on the synthesis of BR2 and BR1 transcription products. The significance of these observations are considered in relation to the information available on the synthesis of the corresponding secretory polypeptides and the formation of the tube-like burrows. We also discuss the implications of the results for models of the regulation of gene activity and of the puffing process.

Induction of premature termination of transcription of the mouse beta-globin gene by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB)

Hybridization of pulse-labeled RNA from DRB-treated Friend cells to mouse beta-globin cDNA revealed that the appearance of beta-globin mRNA in the cytoplasm was inhibited by greater than 87%. To examine the effect of DRB (125 microM) on HnRNA synthesis, nuclear RNA was electrophoresed in methyl mercuric hydroxide gels, transferred to nitrocellulose, and hybridized with beta-globin specific probes. Full-length nuclear transcripts, while present in untreated cells, were not detected in DRB-treated cells. Using restriction enzymes, the cloned beta-globin gene was divided into fragments proceeding from the 5' gene region to the 3' gene region. RNA labeled in vitro by transcription in nuclei isolated from DRB-treated cells hybridized only to the promoter proximal DNA fragment. Transcripts hybridizing to fragments from both the 5' and 3' regions of the gene were produced in nuclei from untreated cells. Together these results indicate that DRB causes premature termination of transcription within the beta-globin gene.

Effects of inhibitors of de novo protein synthesis on UV-mutagenesis in Chinese hamster cells. Evidence against mutagenesis via inducible, error-prone DNA repair

Experiments were designed to test whether UV-mutagenesis in animal cells requires an inducible error-phone DNA-repair system similar to the "SOS system" in E. coli. Chinese hamster (V79) cells were exposed to either 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB, a transcription inhibitor), cycloheximide, or puromycin for various times (3-6 h) following UV-irradiation (2-8 J/m2). Post-irradiation treatment with DRB resulted in a reproducible enhancement of UV-induced mutagenesis, whereas post-irradiation treatment with either cycloheximide or puromycin resulted in decreased UV-mutagenesis. Thus, the frequency of UV-mutagenesis does not appear to be dependent on an inducible error-prone DNA-repair pathway, since all 3 agents share the ability to inhibit de novo protein synthesis. In order to understand the effects of these inhibitors on mutation frequency, DNA synthesis in the presence of these agents was examined. DRB stimulated DNA synthesis in both irradiated and unirradiated cells. On the other hand, cycloheximide and puromycin caused an immediate inhibition of DNA synthesis in both irradiated and unirradiated cells. Therefore, it appears that UV-mutagenesis reflects changes in post-irradiation DNA synthesis rather than post-irradiation de novo protein synthesis.

Regulation of the stability of poly(I)xpoly(C)-induced human fibroblast interferon mRNA: selective inactivation of interferon mRNA and lack of involvement of 2',5'-oligo(A) synthetase activation during the shutoff of interferon production

The inactivation of interferon mRNA during the shutoff phase of interferon production in poly(I)xpoly(C)-induced human fibroblast cultures is selective. We have determined that the shutoff of interferon production, which takes place from 3 to 8 hr after the beginning of induction, is not associated with an appreciable declined in the rate of bulk cellular protein synthesis or of cellular protein secretion. While the amount of translatable interferon mRNA declined markedly during the shutoff phase, the level of translatable bulk cellular mRNA and the stability of [3H]uridine-labeled mRNA were unaffected. Superinduction with actinomycin D selectively stabilized interferon mRNA with no apparent effect on the stability of bulk cellular mRNA. Furthermore, an activation of the 2',5'-oligo(A) synthetase/endonuclease system does not appear to be involved in the shutoff phenomenon. Uninduced FS-4 cells contained a low basal level of 2'5'-oligo(A) synthetase activity, which was unchanged in poly(I)xpoly(C)-induced cells during the shutoff phase. Treatment of FS-4 cells with interferon for 16-18 hr prior to induction increased the enzyme activity by approximately 200-fold. However, this did not inhibit interferon production after induction with poly(I)xpoly(C) alone or after superinduction with cycloheximide or actinomycin D or both. Furthermore, the rates of decay of interferon production were comparable in cells with either a basal or an increased level of 2',5'-oligo(A) synthetase. Thus a 200-fold increase in 2',5'-oligo(A) synthetase level did not affect either the stability of interferon mRNA or the efficacy of interferon superinduction by metabolic inhibitors.

Inhibition of interferon production and antiviral action in mouse cells by 4,5',8-trimethylpsoralen activated with near ultraviolet light. Brief report

The DNA of mammalian cells treated with 4,5',8-trimethylpsoralen (Trioxsalen) is crosslinked (preventing transcription) only after photoactivation with near ultraviolet light. Suppression of gene action in cocultivated or fused cultures, including both induction and antiviral action of fibroblast interferon, can consequently be limited to pretreated cells, an advantage over actinomycin treatment.

The addition of 5' cap structures occurs early in hnRNA synthesis and prematurely terminated molecules are capped

After cells were labeled by brief exposure to 3H-methyl-L-methionine, the majority of labeled 5' terminal cap I (m7GpppN1mpN2p) oligonucleotide structures were in nuclear RNA (hnRNA) molecules approximately 750 nucleotides or less in length. After longer label times, the proportion of cap I structures in nuclear molecules longer than mRNA rose to approximately 60% of the total, but approximately 40% of the cap I structures were still in molecules shorter than approximately 750 nucleotides. The cap I structures in both long and short hnRNA chains contained all four 2' methylated nucleotides in the N1 position in about the same proportion as in mRNA. None of the large hnRNA molecules could be demonstrated to contain 5' pppX p termini; the only such terminus in high molecular weight RNA was pppAp which was decreased markedly by low doses of actinomycin and is presumably the terminus of pre-rRNA. These results raise the possibilities that hnRNA chains can initiate with any of the four nucleotides, that capping occurs very close to or at the start of hnRNA chain synthesis and that approximately 40% of the hnRNA chains may be prematurely terminated.

Kinetics of decay in the expression of interferon-dependent mRNAs responsible for resistance to virus

We used 5,6-dichloro-beta D-ribofuranosyl-benzimidazole (DRB), a selective and reversible inhibitor of mRNA production, to investigate the regulation of the pathway leading to resistance to viruses in cells treated with interferon (IF). DRB allows initiation of transcription but promotes premature termination of the nucleotide chains, so that it abolishes interferon-dependent protection against viruses. When the DRB is removed, synthesis of complete mRNAs can resume. Mouse L-929 cells were exposed to 100 microM DRB before and during a 1-hr pulse of IF followed by treatment with antibody to IF to prevent cell-to-cell spread of IF after that time. At different intervals thereafter the cells were washed and the DRB was replaced by medium; after further incubation, the cells were infected with vesicular stomatitis virus. Resistance to virus was inversely proportional to the duration of the block imposed by DRB. When the DRB was removed soon after the IF pulse, substantial protection from virus ensued, but none developed when removal of the DRB was deferred for 5-6 hr. Cells exposed to DRB for 5 hr, then pulsed with IF for 1 hr, still mounted a strong antiviral response. The data show that the ability of cells to resist viral infection decays within 5-6 hr after treatment with IF. Whether the decay is due to shutoff of transcription of mRNAs, or to their destruction or degradation, or whether regulation takes place at one or more subsequent steps in the antiviral pathway, remains to be determined.

Interferon induction with Newcastle disease virus in FS-4 cells: effect of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB)

DRB is an inhibitor of heterogeneous nuclear RNA (hnRNA) and messenger RNA (MRNA) synthesis. The effect of DRB on interferon production stimulated by Newcastle disease virus (NDV) in the human FS-4 cells was studied. Interferon production in cells primed by treatment with interferon was markedly enhanced (superinduced) in the presence of DRB. This superinduction was essentially due to an inhibition of the rapid decline (shutoff) of interferon production observed in primed cells not treated with DRB. Continuous presence of DRB was required for maximal superinduction. In this and other respects the interferon response induced by NDV in primed cells resembled poly(I). poly(C)-induced interferon production. In contrast interferon production in cells not primed with interferon was virtually abolished by DRB treatment. Since neither virus specific RNA synthesis nor virus replication were significantly affected by DRB, the inhibition of interferon production is likely to result from the inhibitory action of DRB on a cellular, rather than viral, function. Apparently some differences exist in the synthesis or processing of the mRNAs for interferons in primed and unprimed cells and these determine the different sensitivities of these two responses to DRB.