Differential transfers of reduced flavin cofactor and product by bacterial flavin reductase to luciferase

It is believed that the reduced FMN substrate required by luciferase from luminous bacteria is provided in vivo by NAD(P)H-FMN oxidoreductases (flavin reductases). Our earlier kinetic study indicates a direct flavin cofactor transfer from Vibrio harveyi NADPH-preferring flavin reductase P (FRP(H)) to the luciferase (L(H)) from the same bacterium in the in vitro coupled luminescence reaction. Kinetic studies were carried out in this work to characterize coupled luminescence reactions using FRP(H) and the Vibrio fischeri NAD(P)H-utilizing flavin reductase G (FRG(F)) in combination with L(H) or luciferase from V. fischeri (L(F)). Comparisons of K(m) values of reductases for flavin and pyridine nucleotide substrates in single-enzyme and luciferase-coupled assays indicate a direct transfer of reduced flavin, in contrast to free diffusion, from reductase to luciferase by all enzyme couples tested. Kinetic mechanisms were determined for the FRG(F)-L(F) and FRP(H)-L(F) coupled reactions. For these two and the FRG(F)-L(H) coupled reactions, patterns of FMN inhibition and effects of replacement of the FMN cofactor of FRP(H) and FRG(F) by 2-thioFMN were also characterized. Similar to the FRP(H)-L(H) couple, direct cofactor transfer was detected for FRG(F)-L(F) and FRP(H)-L(F). In contrast, despite the structural similarities between FRG(F) and FRP(H) and between L(F) and L(H), direct flavin product transfer was observed for the FRG(F)-L(H) couple. The mechanism of reduced flavin transfer appears to be delicately controlled by both flavin reductase and luciferase in the couple rather than unilaterally by either enzyme species.

Inhibition of gene expression inside cells by peptide nucleic acids: effect of mRNA target sequence, mismatched bases, and PNA length

Genome sequencing has revealed thousands of novel genes, placing renewed emphasis on chemical approaches for controlling gene expression. Antisense oligomers designed directly from the information generated by sequencing are one option for achieving this control. Here we explore the rules governing the inhibition of gene expression by peptide nucleic acids (PNAs) inside cells. PNAs are a DNA/RNA mimic in which the phosphate deoxyribose backbone has been replaced by uncharged linkages. Binding to complementary sequences is not hindered by electrostatic repulsion and is characterized by high rates of association and elevated affinities. Here we test the hypothesis that the favorable properties of PNAs offer advantages for recognition of mRNA and antisense inhibition of gene expression in vivo. We have targeted 27 PNAs to 18 different sites throughout the 5'-untranslated region (5'-UTR), start site, and coding regions of luciferase mRNA. PNAs were introduced into living cells in culture as PNA-DNA-lipid complexes, providing a convenient high throughput method for cellular delivery. We find that PNAs targeted to the terminus of the 5'-UTR are potent and sequence-specific antisense agents. PNAs fifteen to eighteen bases in length were optimal inhibitors. The introduction of one or two mismatches abolished inhibition, and complementary PNAs targeted to the sense strand were also inactive. In striking contrast to effective inhibition by PNAs directed to the terminal region, PNAs complementary to other sites within the 5'-UTR do not inhibit gene expression. We also observe no inhibition by PNAs complementary to the start site or rest of the coding region, nor do we detect inhibition by PNAs that are highly C/G rich and possess extremely high affinities for their target sequences. Our results suggest that PNAs can block binding of the translation machinery but are less able to block the progress of the ribosome along mRNA. The high specificity of antisense inhibition by PNAs emphasizes both the promise and the challenges for PNAs as antisense agents and provides general guidelines for using PNAs to probe the molecular recognition of biological targets inside cells.

Variations in mRNA content have no effect on the potency of antisense oligonucleotides

A fundamental question with regard to antisense pharmacology is the extent to which RNA content or transcription rate or both affect the potency of antisense drugs. We have addressed this by controlling RNA content and transcription rate using either an exogenous gene expressed after transfection or an endogenous gene induced with a cytokine. We have demonstrated that in both A549 and HeLa cells, varying RNA copy numbers from <1 to >100 copies per cell has no effect on the potency of RNase H-active antisense drugs transfected into cells, nor did variation in transcription rate have an effect on potency. We demonstrate that this is because the number of oligonucleotide molecules per cell is vastly in excess of the RNA copy number. These data further suggest that a significant fraction of cell-associated antisense drug molecules may be unavailable to interact with the target RNA, an observation that is not surprising, as phosphorothioate oligonucleotides interact with many cellular proteins. We suggest that these data may extrapolate to in vivo results.

Mechanism of IL-4-mediated up-regulation of the polymeric Ig receptor: role of STAT6 in cell type-specific delayed transcriptional response

The polymeric IgR (pIgR) mediates transport of dimeric IgA and pentameric IgM across mucosal epithelia, thereby generating secretory Abs. Its expression is up-regulated at the transcriptional level by IL-4 in HT-29 cells. In this study, we demonstrate that IL-4 mediates up-regulation of human pIgR through a 554-bp IL-4-responsive enhancer in intron 1. Mutation of a binding site for STAT-6 within this region abolished IL-4-induced enhancement, while an adjacent putative C/EBP site was dispensable. IL-4 treatment induced binding of STAT6 to the intronic STAT6 site, but cooperation with nearby upstream and downstream DNA elements was required for IL-4 responsiveness. Furthermore, IL-4-mediated increased transcription of the pIgR-derived enhancer, like the endogenous pIgR gene, required de novo protein synthesis. Interestingly, a conditionally active form of STAT6 sufficed to activate a pIgR-derived enhancer in HT-29 cells, but not in Cos-1 cells, suggesting a requirement for cell type-specific factors. Thus, STAT6 activation mediates a delayed transcriptional enhancement of pIgR by induction of a de novo synthesized protein that cooperates with STAT6 itself bound to its cognate DNA element in intron 1. This mechanism may represent a general strategy for how pleiotropic cytokines elicit cell type-specific transcriptional responses.

Modulation of the chaperone activities of Hsc70/Hsp40 by Hsp105alpha and Hsp105beta

Hsp105alpha and Hsp105beta are stress proteins found in various mammals including human, mouse, and rat, which belong to the Hsp105/Hsp110 protein family. To elucidate their physiological functions, we examined here the chaperone activity of these stress proteins. Hsp105alpha and Hsp105beta prevented the aggregation of firefly luciferase during thermal denaturation, whereas the thermally denatured luciferase was not reactivated by itself or by rabbit reticulocyte lysate (RRL). On the other hand, Hsp105alpha and Hsp105beta suppressed the reactivation of thermally denatured luciferase by RRL and of chemically denatured luciferase by Hsc70/Hsp40 or RRL. Furthermore, although Hsp105alpha and Hsp105beta did not show ATPase activity, the addition of Hsp105alpha or Hsp105beta to Hsc70/Hsp40 enhanced the amount of hydrolysis of ATP greater than that of the Hsp40-stimulated Hsc70 ATPase activity. These findings suggest that Hsp105alpha and Hsp105beta are not only chaperones that prevent thermal aggregation of proteins, but also regulators of the Hsc70 chaperone system in mammalian cells.

Probing the mechanisms of the biological intermolecular transfer of reduced flavin

NAD(P)H-flavin oxidoreductases [flavin reductases (FR)] are a class of enzymes capable of producing reduced flavin for bacterial bioluminescence and other biological processes. Bacterial luciferase utilizes oxygen, reduced FMN (FMNH2) and a long-chain aliphatic aldehyde as substrates for light emission. The Vibrio harveyi luciferase and FRP (for which we have cloned the gene and determined the crystal structure) is a model for the elucidation of the reduced flavin transfer mechanism using both a flavin reductase single-enzyme assay monitoring the NADPH oxidation and a flavin reductase-luciferase coupled assay measuring bioluminescence intensity or quantum output. The FRP exhibits a ping-pong kinetic pattern in the single-enzyme assay but changes to a sequential pattern in the coupled assay. Furthermore, FMN at >2x10(-6) mol/L reduced both the light intensity and quantum yield of the coupled reaction by noncompetitively inhibiting NADPH and competitively inhibiting luciferase. These results support a scheme in which the luciferase forms specific complex(es) with FRP. Indeed, such complexes were shown by fluorescence anisotropy to exist between luciferase and monomeric FRP either in the holo- or apoenzyme form. Furthermore, the reduced flavin cofactor of FRP is transferred directly to luciferase for bioluminescence, whereas the reduced flavin product of FRP is inefficient in supporting the luminescence reaction. The mechanism of reduced flavin transfer is apparently flavin and flavin reductase specific.

The role of hsp70 in protection and repair of luciferase activity in vivo; experimental data and mathematical modelling

The stably transfected rat cell line HR24 expressing high levels of the inducible human hsp70 and its parental cell line Rat-1 were used for in vivo studies to analyse the role of hsp70 during thermal protein denaturation and the subsequent renaturation. In order to monitor denaturation and renaturation of a cellular protein in vivo, both cell lines were transiently transfected with firefly luciferase (Luc). The continuous monitoring of Luc activity during and after heat stress allowed a detailed analysis of the inactivation and reactivation kinetics in cells grown in monolayers. The aim of these studies was to distinguish a protective effect of increased hsp70 levels during heat shock-induced protein inactivation from a stimulation of reactivation. In this paper we show that in cells that are stably transfected with hsp70, thermal Luc inactivation decreased, and subsequent reactivation yielded higher activity levels, compared with the parental cells. The difference in early inactivation kinetics observed in the two cell lines suggests an immediate effect of the presence of an extra amount of hsp70 on enzyme inactivation. Using different mathematical models, the heat-induced inactivation and reactivation kinetics was compared with simulations of denaturation and renaturation. It is concluded that the model in which it is assumed that hsp70 is able to interact with partially denatured proteins, which did not yet lose their enzymatic activity, most optimally explains the experimental observations.

Specific and non-specific binding of long-chain fatty acids to firefly luciferase: cutoff at octanoate

Firefly luciferase emits a burst of light when the substrates luciferin and ATP are mixed in the presence of oxygen. We (I. Ueda, A. Suzuki, Biophys. J. 75 (1998) 1052-1057) reported that long-chain fatty acids are specific inhibitors of firefly luciferase in competition with luciferin in microM ranges. They increased the thermal transition temperature. In contrast, 1-alkanols of the same carbon chain length inhibited the enzyme non-competitively in mM ranges and decreased the transition temperature. The present study showed that the action of fatty acids switched from specific to non-specific when the carbon chain length was reduced below C8 (octanoate). The fatty acids longer than C10 inhibited the enzyme in microM ranges whereas those shorter than C8 required mM ranges to inhibit it. The longer fatty acids increased whereas shorter fatty acids decreased the transition temperature. The Hill coefficients of longer chain bindings were less than one whereas those of shorter chain were more than one. The shorter fatty acids interacted with the enzyme cooperatively at multiple sites. Binding of the longer fatty acids is limited. Fatty acids longer than C10 are high-affinity specific binders and followed Koshland's induced-fit model. Those shorter than C8 are low-affinity non-specific denaturants and followed Eyring's rate process model. These results contradict the general consensus that the size of the receptor cavity discriminates specific binders.

Does pressure antagonize anesthesia? Opposite effects on specific and nonspecific inhibitors of firefly luciferase

Ueda and Suzuki (1998. Biochim. Biophys. Acta. 1380:313-319; 1998. Biophys. J. 75:1052-1057) reported that myristic acid inhibited firefly luciferase in microM range in competition with luciferin, whereas anesthetics inhibited it in millimeter ranges noncompetitively with luciferin. Myristate increased, whereas anesthetics decreased, the thermal denaturation temperature. The present study showed that high pressure increased the steady-state light intensity of the halothane-doped firefly luciferase but decreased that of the myristate-doped firefly luciferase. The steady-state light intensity showed a maximum at 19.1 degrees C. At 19.1 degrees C, high pressure did not affect the light intensity in the absence of the inhibitors. In the presence of 0.5 mM halothane, however, 25 MPa pressure (maximum effect) increased the light intensity to 106.0% of the control without the inhibitor. In the presence of 2.5 microM myristate, 40 MPa pressure decreased the light intensity to 90.9% of the control. When the temperature was 25 degrees C in the absence of inhibitors, 40 MPa pressure increased the light intensity 119.2% of the ambient value. At 0.5 mM halothane, 40 MPa pressure further increased the light intensity to 106.1% above the control 40 MPa value. At 2.5 microM myristate, 40 MPa pressure decreased the light intensity to 90.1% of the control 40 MPa value. From the pressure dependence of the light intensity, the volume change DeltaV of the enzyme was estimated at 25 degrees C: 0.5 mM halothane increased DeltaV = +3.93 cm3 mol-1, whereas 2.5 microM myristate decreased DeltaV = -7.66 cm3 mol-1. Present results show that there are distinct differences between the specific and nonspecific ligands in their response to high pressure. Myristate, which competes with luciferin, decreased the protein volume and stabilized the conformation against thermal perturbation. Halothane, which does not compete with the substrate, increased the protein volume and destabilized the conformation.

Site-directed mutagenesis of histidine 245 in firefly luciferase: a proposed model of the active site

Firefly luciferase catalyzes the highly efficient emission of yellow-green light from substrate luciferin by a sequence of reactions that require Mg-ATP and molecular oxygen. We previously reported [Branchini, B. R., Magyar, R. A., Marcantonio, K. M., Newberry, K. J., Stroh, J. G., Hinz, L. K., and Murtiashaw, M. H. (1997) J. Biol. Chem. 272, 19359-19364] that 2-(4-benzoylphenyl)thiazole-4-carboxylic acid (BPTC), a firefly luciferin analogue, was a potent photoinactivation reagent for luciferase. We identified a luciferase peptide 244HHGF247, the degradation of which was directly correlated to the photooxidation process. We report here the construction and purification of wild-type and mutant luciferases H244F, H245F, H245A, and H245D. The results of photoinactivation and kinetic and bioluminescence studies with these proteins are consistent with His245 being the primary functional target of BPTC-catalyzed enzyme inactivation. The possibility that His245 is oxidized to aspartate during the photooxidation reaction was supported by the extremely low specific activity ( approximately 300-fold lower than WT) of the H245D mutant. Using the crystal structures of luciferase without substrates [Conti, E., Franks, N. P., and Brick, P. (1996) Structure 4, 287-298] and the functionally related phenylalanine-activating subunit of gramicidin synthetase 1 [Conti, E., Stachelhaus, T., Marahiel, M. A., and Brick, P. (1997) EMBO J. 16, 4174-4183] as a starting point, we have performed molecular-modeling studies and propose here a model for the luciferase active site with substrates luciferin and Mg-ATP bound. We have used this model to provide a structure-based interpretation of the role of 244HHGF247 in firefly bioluminescence.

Mechanism of reduced flavin transfer from Vibrio harveyi NADPH-FMN oxidoreductase to luciferase

The mechanisms of reduced flavin transfer in biological systems are poorly understood at the present. The Vibrio harveyi NADPH-FMN oxidoreductase (FRP) and the luciferase pair were chosen as a model for the delineation of the reduced flavin transfer mechanism. FRP, which uses FMN as a cofactor to mediate the reduction of the flavin substrate by NADPH, exhibited a ping-pong kinetic pattern with a Km, FMN of 8 microM and a Km,NADPH of 20 microM in a single-enzyme spectrophotometric assay monitoring the NADPH oxidation. However, the kinetic mechanism of FRP was changed to a sequential pattern with a Km,FMN of 0.3 microM and a Km,NADPH of 0.02 microM in a luciferase-coupled assay measuring light emission. In contrast, the Photobacterium fischeri NAD(P)H-FMN oxidoreductase FRG showed the same ping-pong mechanism in both the single-enzyme spectrophotometric and the luciferase-coupled assays. Moreover, for the FRP, FMN at concentrations over 2 microM significantly inhibited the coupled reaction in both light intensity and quantum yield, and showed apparent noncompetitive and competitive inhibition patterns against NADPH and luciferase, respectively. No inhibition of the NADPH oxidation was detected under identical conditions. These results are consistent with a scheme that the reduced flavin cofactor of FRP is preferentially utilized by luciferase for light emission, the reduced flavin product generated by the reductase is primarily channeled into a dark oxidation, and luciferase competes against flavin substrate in reacting with the FRP reduced flavin cofactor. An FRP derivative containing 2-thioFMN as the cofactor was also used to further examine the mechanism of flavin transfer. Results again indicate a preferential utilization of the reductase reduced flavin cofactor by luciferase for the bioluminescence reaction.

Induction by nitriles of sex chromosome aneuploidy: tests of mechanism

Genetic and biochemical assays were conducted to determine if nitrile induced adult paralysis and germline aneuploidy in female Drosophila melanogaster requires a biochemical activation mechanism which results in the release of free cyanide. Two nitriles predicted to differ substantially in their susceptibility to enzymatic cyanide release were found to be equally effective inducers of aneuploidy. Regardless of differences in chemical structure, nitriles seem to be affecting a common cellular target as judged by the lack of synergistic effects when two nitriles are presented simultaneously. Mitochondrial respiration was not inhibited by acetonitrile under conditions in which sodium cyanide completely blocked respiration. A sensitive luciferase enzyme inhibition assay suggests that some, but not all, nitriles may affect hydrophobic protein interactions. These results suggest that there is no single biochemical mechanism by which all nitriles induce aneuploidy, although the cellular target disrupted is probably the same for each chemical. The implications of these findings for structural alert based pre-screening of mutagens are discussed.

Characterization of mouse retinoid X receptor (RXR)-beta gene promoter: negative regulation by tumor necrosis factor (TNF)-alpha

A genomic clone of mouse retinoid X receptor (RXR)-beta (Rxrb) has recently been isolated and mapped within the H2-K region of the mouse major histocompatibility complex. A putative 250-bp promoter, which is located between Rxrb and H2-Ke4, and may possibly be their common promoter, has also been identified. In order to study the gene regulation of Rxrb, we analyzed the transcriptional function of the Rxrb promoter with chimeric constructs containing the Rxrb promoter fragments fused upstream of a firefly luciferase cDNA, which were transiently transfected into rat GH3 cells. We found that 1) a part of the H2-Ke4 genomic region (1.9-kb), as well as the 250-bp promoter, was transcriptionally active as an Rxrb promoter; 2) tumor necrosis factor (TNF)-alpha significantly repressed the activity of the 250-bp promoter although thyroid hormone, 9-cis retinoic acid, interleukin (IL)-1beta, and IL-6 did not affect the activity; 3) either the change in orientation or point mutations of a consensus NF-kappaB site located in the 250-bp promoter did not affect the repression; 4) SB 203580, a highly specific inhibitor of p38 mitogen-activated protein (MAP) kinase, completely abolished the repression by TNF-alpha. These data suggest that TNF-alpha represses the promoter activity of the 250-bp region, and the repression is mediated by p38 MAP kinase independent of NF-kappaB. We thus have first shown a relation between the retinoic acid receptor and a cytokine TNF-alpha.

Targeted inhibition of hepatitis C virus-directed gene expression in human hepatoma cell lines

BACKGROUND & AIMS

The 5'-nontranslated region (NTR) of hepatitis C virus (HCV) contains important elements that control HCV translation. The aim of this study was to determine whether antisense oligonucleotides against the NTR of the HCV genome can be targeted to inhibit HCV gene expression.

METHODS

Antisense oligonucleotides directed against a sequence in the internal ribosomal binding site of the NTR (anti-III) and a portion of the NTR overlapping the core protein translational start site of HCV (anti-IV) were prepared. In transient transfections of a plasmid containing a luciferase gene immediately downstream from an HCV NTR insert, oligonucleotides anti-III and anti-IV in the form of asialoglycoprotein-polylysine complexes were administered to Huh7 cells, and luciferase activity generated by cytomegalovirus (CMV) HCVluc was measured.

RESULTS

Anti-III inhibited luciferase activity by 75% and 99% at 0.01 mumol/L and 0.1 mumol/L, respectively. Similarly, anti-IV inhibited luciferase activity 88% and 99% at 0.01 mumol/L and 0.1 mumol/L, respectively. In cell lines stably transfected with CMV HCVluc plasmid, complexed anti-III inhibited luciferase activity in Huh7 cells by 20% at 10 mumol/L and 85% at 60 mumol/L, and was competable by an excess of asialoglycoprotein.

CONCLUSIONS

Antisense oligonucleotides that bind to the NTR of HCV can be targeted by receptor-mediated endocytosis, and they specifically inhibit HCV-directed protein synthesis under intracellular conditions.

Irreversible phase transition of firefly luciferase: contrasting effects of volatile anesthetics and myristic acid

Firefly luciferase (FFL) has been used as a lipid-free protein model to study direct interaction of anesthetics with proteins. FFL emits a burst of light when luciferin and ATP are added in the presence of oxygen. Volatile anesthetics inhibited FFL at mM ranges, while myristic acid inhibited it at microM range. Despite the large difference, octanol/water partition coefficients of both myristic acid and halothane are 199. Differential scanning calorimetry (DSC) showed that thermal transition occurred at 38.5 degreesC with excess enthalpy of denaturation of 91.9 kcal mol-1. The transition, however, was irreversible. According to the irreversible transition kinetics, the anesthetic effects were evaluated by the temperature where the irreversible transition is half completed (T1/2). Volatile anesthetics decreased T1/2 at mM ranges, while myristic acid and oxyluciferyladenylate (luciferin competitor) increased it at microM ranges. Luciferin is a heterocyclic carboxylate and acylates AMP. Carboxyl group of myristic acid appears to make a high affinity contact to the luciferin-recognition sites. The induced-fit theory states that binding of substrates induces the enzyme into high-energy transition state. Myristic acid stabilized FFL at the transition state, which resisted thermal denaturation. Anesthetics destabilized FFL by reversibly unfolding the protein into less active intermediate states and promoted irreversible transition when the temperature is elevated.

Bioluminescence as a possible auxiliary oxygen detoxifying mechanism in elaterid larvae

This work examines the hypothesis that beetle bioluminescent reactions may primarily have evolved to provide an auxiliary O2 detoxifying mechanism. The activities of antioxidant enzymes and of luciferase in the prothorax (bright) and abdomen (dim) of luminous larval Pyrearinus termitilluminans (Coleoptera: Elateridae) were measured after previous challenge with either hyperoxia, hypoxia, or the firefly luciferase inhibitor luciferin 6'-methyl ether (LME). Upon exposure to pure O2 for 72 h, the prothorax activities of total superoxide dismutase (SOD) and catalase were found to increase by 85% and 50%, respectively. Concomitantly, levels of luciferase and luciferin increased 80% and 50%. Assays of thiobarbituric acid reactive substances (TBARS) showed significantly augmented lipid peroxidation only in the abdomen (30%) where levels of antioxidant enzymes and especially luciferase are low. In contrast, exposure to hypoxia (2% O2) led to significant increases in prothorax citrate synthase (85%), succinate dehydrogenase (25%), and lactate dehydrogenase (30%) activities, but not in luciferase or antioxidant enzyme levels. LME administration alone decreased luciferase activities 20% but did not alter prothorax SOD activity. Prothorax SOD activity was increased by concomitant LME and hyperoxia treatments (30%), along with higher levels of TBARS (25%) and protein reactive carbonyl groups (50%). Altogether these data suggest that in elaterids, bioluminescence and reactions catalyzed by antioxidant enzymes may cooperate to minimize oxidative stress.

Identification of a firefly luciferase active site peptide using a benzophenone-based photooxidation reagent

Firefly luciferase catalyzes the highly efficient emission of yellow-green light from substrate luciferin by a series of reactions that require MgATP and molecular oxygen. We prepared 2-(4-benzoylphenyl)thiazole-4-carboxylic acid (BPTC), a novel benzophenone-based substrate analog, intending to use it in photoaffinity labeling studies to probe the luciferase active site. Instead, we found that while BPTC was a potent photoinactivating reagent for firefly luciferase, it was not a photoaffinity labeling agent. Using proteolysis, reverse phase high-performance liquid chromatography, tandem high performance liquid chromatography-electrospray ionization mass spectrometry, and Edman sequencing, we identified a single luciferase peptide, 244HHGF247, the degradation of which was directly correlated to luciferase photoinactivation. Results of enzyme kinetics and related studies were consistent with this peptide being at or near the luciferin binding site. Further, peptide model studies and additional investigations on the nature of the photoinactivation process strongly suggested that BPTC catalyzed the formation of singlet oxygen at the active site of the enzyme. We describe here an uncommon example of active site-directed photooxidation of an enzyme by singlet oxygen.

Effect of transforming growth factor-beta1 on expression of aryl hydrocarbon receptor and genes of Ah gene battery: clues for independent down-regulation in A549 cells

An inhibitory effect on both constitutive and inducible expression of cytochrome P450 isoenzymes has been shown for different cytokines and growth factors. We previously described an inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced CYP1A1 mRNA and enzyme activity by transforming growth factor-beta1 (TGF-beta1) in human lung cancer A549 cells. In the present study, we report that not only TCDD-induced expression of CYP1A1 but also basal mRNA expression of CYP1A1, CYP1B1, and aryl hydrocarbon receptor (AHR) was down-regulated by TGF-beta1 in cells not treated with TCDD. In contrast, mRNA expression of the AHR partner protein Arnt (aryl hydrocarbon receptor nuclear translocator) was not influenced. Furthermore, TCDD-induced expression of CYP1B1 and NMO-1 was inhibited, and the IC50 values of 5-10 pM TGF-beta1 were in the same range as observed for inhibition of CYP1A1 and AHR mRNA expression. Transfection studies with a plasmid containing a luciferase reporter gene under control of two dioxin-responsive elements indicate an effect on AHR protein expression. Results of time-course studies revealed a parallel inhibition of AHR and CYP1 mRNA expression, indicating that TGF-beta1 is a direct negative regulator of transcription of these genes. The treatment of cells with cycloheximide led to a superinduction of TCDD-induced CYP1A1 and CYP1B1 mRNA expression and abolished the inhibitory effect of TGF-beta1 on basal as well as TCDD-induced CYP1 and AHR mRNA expression. TGF-beta1 seems not to influence the stability of AHR mRNA. The results suggest that TGF-beta1 induces rapid transcription and translation of an as-yet-unknown negative regulatory factor or factors that may directly regulate expression of AHR and genes of Ah gene battery.

o,p'-DDT and its metabolites inhibit progesterone-dependent responses in yeast and human cells

Using a combination of in vitro assays we have evaluated whether DDT metabolites can interact with the progesterone receptor pathway in yeast expressing human progesterone receptor (hPR) and in T47D human breast cancer cells which express endogenous hPR. In transactivation assays using both yeast and T47D cells, o,p'-DDT and the metabolites p,p'-DDT, o,p'-DDD, p,p'-DDD, o,p'-DDE, p,p'-DDE, p,p'-DDA, and DDOH inhibited progesterone-induced reporter gene activity in a dose-dependent manner. None of the DDT metabolites functioned as hPR agonists. Whole cell competition binding assays using T47D cells indicated that the inhibitory effects of DDT metabolites on progesterone-dependent activites may occur through both hPR-dependent and hPR-independent pathways. Our results and previous reports of DDT metabolites interacting with estrogen and androgen receptors suggests that this class of environmental chemicals may interact with numerous hormone receptor signaling pathways.

Inactivation of firefly luciferase with N-(iodoacetyl)-N'- (5-sulfo-1-naphthyl)ethylenediamine (I-AEDANS)

N-Iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine (I-AEDANS), a fluorescent reagent that selectively modifies cysteine residues, was demonstrated to irreversibly inhibit native Photinus pyralis luciferase purified from firefly lanterns. Complete inactivation of luciferase activity was accompanied by the blockage of all four cysteine thiols and the concomitant incorporation of 4 mol of N-acetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine (AEDANS) per mole of enzyme. Employing proteolytic digestions of AEDANS-labeled luciferase and reverse-phase-high-performance liquid chromatography (RP-HPLC), seven tagged peptides were isolated. The AEDANS label provided a convenient spectroscopic marker for the identification of the modified peptides. The sequences of the labeled peptides were deduced from electrospray ionization mass spectrometry (ESMS) and N-terminal sequencing. The fluorescent peptides included cysteine residues and spanned sequences composed of amino acids Leu78-Lys85, Thr214-Arg218, Asp224-Arg275, and Gly388-Met396. The luciferin substrate provided substantial protection against luciferase inactivation resulting in a 60-67% decrease in the labeling of all four cysteine thiols. Thus, it does not appear that a specific cysteine mediates the loss of luciferase activity. Additional LC/ESMS studies permitted the identification of 78% of the native luciferase molecule, which, unlike the recombinant protein, was found to contain an acetylated N-terminus. The AEDANS labeling results and the identification of well-defined proteolytic fragments should facilitate future structure-function investigations of the firefly luciferases.

Low temperature enhancement of reporter genes expression directed by human immunodeficiency virus type 1 long terminal repeat

Bacteria and eukaryotic cells respond to cold stress by inducing and enhancing the synthesis of specific arrays of proteins. We describe here cold-induced enhancement of expression for two reporter genes; luciferase and beta-galactosidase, both under the control of HIV-1 LTR sequences, observed in mouse fibroblasts and human HeLa cells respectively. Increased expression of luciferase in fibroblasts when shifted to 25 degrees C was detectable at 30 degrees C but was not observed following cold shock at 4 degrees C. To sustain the cold-induced effect, cells had to be kept at subphysiological temperature. The observed enhancement of luciferase activity did not result from a particular site of integration of the reporter gene and was evident whether cold-stressed cells were stationary or growing. Cold-induced expression of luciferase was evidenced at the protein level, enzymatic activity and RNA level, furthermore, active transcription and translation were required for overexpression. The cold effect which has been generalized with the reporter gene beta-galactosidase appears to be a process involving, at least in part, the HIV-1 LTR sequences and might correspond to an increase in the half-life of mRNA. The cold-dependent enhanced expression of luciferase and beta-galactosidase reported here, together with data describing the activation of HIV-1 LTR by hyperthermia, point out the particular temperature sensitivity of these regulatory sequences. This potential thermal modulation may be useful in the comprehension of regulatory processes in latency and reactivation of viral expression during HIV-1 infection.

Regulation of the heat-shock protein 70 reaction cycle by the mammalian DnaJ homolog, Hsp40

The effects of the human DnaJ homolog, Hsp40, on the ATPase and chaperone functions of the constitutively expressed Hsp70 homolog, Hsc70, were analyzed. Hsp40 stimulates the hydrolysis of ATP by Hsc70, causing a approximately 7-fold increase in its steady-state ATPase activity. In contrast to the prokaryotic Hsp70 system, ATP-hydrolysis and not the release of bound ADP is the rate-limiting step in the overall ATPase cycle of mammalian Hsc70. The ability to activate the Hsc70 ATPase is partially preserved in a deletion mutant containing the J-domain and the G/F region of Hsp40 but not in a deletion mutant that contains the J-domain alone. As a result of its ATPase stimulating activity, addition of Hsp40 allows Hsc70 to bind peptide in the presence of ATP, whereas in the absence of Hsp40, peptide is efficiently released upon ATP binding to Hsc70. The functional cooperation of Hsp40 with Hsc70 is essential to ensure the ATP hydrolysis-dependent binding of aggregation-sensitive denatured polypeptides, such as thermally denatured firefly luciferase and chemically denatured rhodanese. Binding of these proteins results in the formation of ternary complexes of Hsc70, Hsp40, and substrates. Hsc70 and Hsp40 cooperate with further factors in protein renaturation, as demonstrated by the finding that luciferase, thermally denatured in the presence of Hsc70, Hsp40, and ATP, refolds upon addition of rabbit reticulocyte cytosol. Our results indicate that Hsp40 has a critical regulatory function in the Hsc70 ATPase cycle that is required for the efficient loading of peptide substrate onto Hsc70.

Firefly luciferase can use L-luciferin to produce light

L-Luciferin is a competitive inhibitor of firefly luciferase with a K1 between 3 and 4 microM. Furthermore L-luciferin can serve as an alternative substrate for light production. Catalysis of L-luciferin can be observed in the absence of, or at low concentrations of, D-luciferin. The light production from L-luciferin increases slowly (maximal half-time 8 min) to a stable plateau. At low concentrations of enzyme and L-luciferin, maximal light production is about half of that observed at corresponding D-luciferin concentrations. Increasing the concentration of enzyme or L-luciferin reduces the light production relative to that obtained by D-luciferin catalysis. In contrast to the catalysis of D-luciferin the light production from L-luciferin can be effectively stimulated by the addition of PP1 provided that luciferase is premixed with inorganic pyrophosphatase (PP1-ase). A flash is emitted if PP1 is injected into a mixture of luciferase, L-luciferin, ATP and PP1-ase. The system maintains its responsiveness and emits further flashes of about equal duration and intensity upon repeated additions of PP1. It is proposed that PP1 induces a racemization of enzyme-bound L-luciferyl adenylate. The potential usefulness of PP1-dependent intracellular ATP monitoring is discussed. The proposed activation of firefly luciferase by PP1 may be part of the regulation of in vivo flashing.