The transcription factor MTF-1 mediates metal regulation of the mouse ZnT1 gene

Metal regulation of the mouse zinc transporter (ZnT)-1 gene was examined in cultured cells and in the developing conceptus. Zinc or cadmium treatment of cell lines rapidly (3 h) and dramatically (about 12-fold) induced ZnT1 mRNA levels. In cells incubated in medium supplemented with Chelex-treated fetal bovine serum, to remove metal ions, levels of ZnT1 mRNA were reduced, and induction of this message in response to zinc or cadmium was accentuated (up to 31-fold induction). Changes in ZnT1 gene expression in these experiments paralleled those of metallothionein I (MT-I). Inhibition of RNA synthesis blocked metal induction of ZnT1 and MT-I mRNAs, whereas inhibition of protein synthesis did not. Metal response element-binding transcription factor (MTF)-1 mediates metal regulation of the metallothionein I gene. In vitro DNA-binding assays demonstrated that mouse MTF-1 can bind avidly to the two metal-response element sequences found in the ZnT1 promoter. Using mouse embryo fibroblasts with homozygous deletions of the MTF-1 gene, it was shown that this transcription factor is essential for basal as well as metal (zinc and cadmium) regulation of the ZnT1 gene in these cells. In vivo, ZnT1 mRNA was abundant in the midgestation visceral yolk sac and placenta. Dietary zinc deficiency during pregnancy down-regulated ZnT1 and MT-I mRNA levels (4-5-fold and >20-fold, respectively) in the visceral yolk sac, but had little effect on these mRNAs in the placenta. Homozygous knockout of the MTF-1 gene in transgenic mice also led to a 4-6-fold reduction in ZnT1 mRNA levels and a loss of MT-I mRNA in the visceral yolk sac. These results suggest that MTF-1 mediates the response to metal ions of both the ZnT1 and the MT-I genes the visceral yolk sac. Overall, these studies suggest that MTF-1 directly coordinates the regulation of genes involved in zinc homeostasis and protection against metal toxicity.

Zinc and cadmium can promote rapid nuclear translocation of metal response element-binding transcription factor-1

Metal response element-binding transcription factor-1 (MTF-1) is a six-zinc finger protein that plays an essential role in activating metallothionein expression in response to the heavy metals zinc and cadmium. Low affinity interactions between zinc and specific zinc fingers in MTF-1 reversibly regulate its binding to the metal response elements in the mouse metallothionein-I promoter. This study examined the subcellular distribution and DNA binding activity of MTF-1 in cells treated with zinc or cadmium. Immunoblot analysis of cytosolic and nuclear extracts demonstrated that in untreated cells, about 83% of MTF-1 is found in the cytosolic extracts and is not activated to bind to DNA. In sharp contrast, within 30 min of zinc treatment (100 microM), MTF-1 is detected only in nuclear extracts and is activated to bind to DNA. The activation to bind to DNA and nuclear translocation of MTF-1 occurs in the absence of increased MTF-1 content in the cell. Furthermore, immunocytochemical localization and immunoblotting assays demonstrated that zinc induces the nuclear translocation of MTF-1-FLAG, expressed from the cytomegalovirus promoter in transiently transfected dko7 (MTF-1 double knockout) cells. Immunoblot analysis of cytosolic and nuclear extracts from cadmium-treated cells demonstrated that concentrations of cadmium (10 microM) that actively induce metallothionein gene expression cause only a small increase in the amount of nuclear MTF-1. In contrast, an overtly toxic concentration of cadmium (50 microM) rapidly induced the complete nuclear translocation and activation of DNA binding activity of MTF-1. These studies are consistent with the hypothesis that MTF-1 serves as a zinc sensor that responds to changes in cytosolic free zinc concentrations. In addition, these data suggest that cadmium activation of metallothionein gene expression may be accompanied by only small changes in nuclear MTF-1.

Protein encapsulation in mesoporous silicate: the effects of confinement on protein stability, hydration, and volumetric properties

On the basis of the predictions of statistical-thermodynamic models, it is postulated that excluded volume effects may play a significant role in the stability, interaction, and function of proteins. We studied the effects of confinement on protein un/refolding and stability. Our approach was to encapsulate a model protein, RNase A, in a mesoporous silica, MCM-48, with glasslike wall structure and with well-defined pores to create a crowded microenvironment. To the best of our knowledge, this is the first report where pressure perturbation and differential scanning calorimetric techniques are employed to evaluate the stability, hydration, and volumetric properties of the confined protein. A drastic increase in protein stability ( approximately 30 degrees C increase in unfolding temperature) is observed. The increase in stability is probably not only due to a restriction in conformational space (excluded volume effect due to nonspecific interactions) but also due to an increased strength of hydration of the protein within the narrow silica pores.

Aggregation of bovine insulin probed by DSC/PPC calorimetry and FTIR spectroscopy

Pressure perturbation calorimetry (PPC), differential scanning calorimetry (DSC), and time-resolved Fourier transform infrared spectroscopy (FTIR) have been employed to investigate aggregation of bovine insulin at pH 1.9. The aggregation process exhibits two distinguished phases. In the first phase, an intermediate molten globule-like conformational state is transiently formed, reflected by loose tertiary contacts and a robust H/D-exchange. This is followed by unfolding of the native secondary structure. The unfolding of insulin is fast, endothermic, partly reversible, and accompanied by a volume expansion of approximately 0.2%. The second phase consists of actual aggregation: an exothermic irreversible process revealing typical features of nucleation-controlled kinetics. The volumetric changes associated with the second phase are small. The concentration-dependence of DSC scans does not support a monomer intermediate model. While insulin aggregation under ambient pressure is fast and quantitative, pressure as low as 300 bar is sufficient to prevent the aggregation completely, as high-pressure FTIR spectroscopy revealed. This is explained in terms of the high pressure having an adverse effect on the thermal unfolding of insulin, and therefore preventing occurrence of the aggregation-prone intermediate. A comparison of the aggregation in H(2)O and D(2)O shows that the isotopic substitution has diverse effects on both the phases of aggregation. In heavy water, a more pronounced volume expansion accompanies the unfolding stage, while only the second phase shifts to higher temperature.

On the temperature--pressure free-energy landscape of proteins

We studied the thermodynamic stability of a small monomeric protein, staphylococcal nuclease (Snase), as a function of both temperature and pressure, and expressed it as a 3D free-energy surface on the p,T-plane using a second-order Taylor expansion of the Gibbs free-energy change delta G upon unfolding. We took advantage of a series of different techniques (small-angle X-ray scattering, Fourier-transform infrared spectroscopy, differential thermal analysis, pressure perturbation calorimetry and densitometry) in the evaluation of the conformation of the protein and in evaluating the changes in the thermodynamic parameters upon unfolding, such as the heat capacity, enthalpy, entropy, volume, isothermal compressibility and expansivity. The calculated results of the free-energy landscape of the protein are in good agreement with experimental data of the p,T-stability diagram of the protein over a temperature range from 200 to 400 K and at pressures from ambient pressure to 4000 bar. The results demonstrate that combined temperature--pressure-dependent studies can help delineate the free-energy landscape of proteins and hence help elucidate which features and thermodynamic parameters are essential in determining the stability of the native conformational state of proteins. The approach presented may also be used for studying other systems with so-called re-entrant or Tamman loop-shaped phase diagrams.

Pressure perturbation calorimetric studies of the solvation properties and the thermal unfolding of proteins in solution—experiments and theoretical interpretation

We used pressure perturbation calorimetry (PPC), a relatively new and efficient technique, to study the solvation and volumetric properties of amino acids and peptides as well as of proteins in their native and unfolded state. In PPC, the coefficient of thermal expansion of the partial volume of the protein is deduced from the heat consumed or produced after small isothermal pressure jumps, which strongly depends on the interaction of the protein with the solvent or cosolvent at the protein-solvent interface. Furthermore, the effects of various chaotropic and kosmotropic cosolvents on the volume and expansivity changes of proteins were measured over a wide concentration range with high precision. Depending on the type of cosolvent and its concentration, specific differences were found for the solvation properties and unfolding behaviour of the proteins, and the volume change upon unfolding may even change sign. To yield a molecular interpretation of the different terms contributing to the partial protein volume and its temperature dependence, and hence a better understanding of the PPC data, molecular dynamics computer simulations on SNase were also carried out and compared with the experimental data. The PPC studies introduced aim to obtain more insight into the basic thermodynamic properties of protein solvation and volume effects accompanying structural transformations of proteins in various cosolvents on one hand, as these form the basis for understanding their physiological functions and their use in drug designing and formulations, but also to initiate further valuable applications in studies of other biomolecular and chemical systems.

Ethanol-Perturbed Amyloidogenic Self-Assembly of Insulin: Looking for Origins of Amyloid Strains

A model cosolvent, ethanol, has profound and diversified effects on the amyloidogenic self-assembly of insulin, yielding spectroscopically and morphologically distinguishable forms of beta-aggregates. The alcohol reduces hydrodynamic radii of insulin molecules, decreases enthalpic costs associated with aggregation-prone intermediate states, and accelerates the aggregation itself. Increasing the concentration of the cosolvent promotes curved, amorphous, and finally donut-shaped forms. According to FT-IR data, inter-beta-strand hydrogen bonding is stronger in fibrils formed in the presence of ethanol. Mechanisms underlying the polymorphism of insulin aggregates were investigated by spectroscopic (CD, FT-IR, and fluorescence anisotropy) and calorimetric (DSC and PPC) methods. The nonmonotonic character of the influence of ethanol on insulin aggregation suggests that both preferential exclusion (predominant at the low concentrations) and direct alcohol-protein interactions are involved. The perturbed hydration of aggregation nuclei appears to be a decisive factor in selection of a dominant mode of beta-strand alignment. It may override unfavorable structural consequences of an alternative strand-to-strand stacking, such as strained hydrogen bonding. A hypothetical mechanism of inducing different amyloid "strains" has been put forward. The cooperative character of fibril assembly creates enormous energy barriers for any interstrain transition, which renders the energy landscape comblike-shaped.

Hydration and packing effects on prion folding and beta-sheet conversion. High pressure spectroscopy and pressure perturbation calorimetry studies

The main hypothesis for prion diseases proposes that the cellular protein (PrP(C)) can be altered into a misfolded, beta-sheet-rich isoform (PrP(Sc)), which undergoes aggregation and triggers the onset of transmissible spongiform encephalopathies. Here, we compare the stability against pressure and the thermomechanical properties of the alpha-helical and beta-sheet conformations of recombinant murine prion protein, designated as alpha-rPrP and beta-rPrP, respectively. High temperature induces aggregates and a large gain in intermolecular antiparallel beta-sheet (beta-rPrP), a conformation that shares structural similarity with PrP(Sc). alpha-rPrP is highly stable, and only pressures above 5 kilobars (1 kilobar = 100 MegaPascals) cause reversible denaturation, a process that leads to a random and turnrich conformation with concomitant loss of alpha-helix, as measured by Fourier transform infrared spectroscopy. In contrast, aggregates of beta-rPrP are very sensitive to pressure, undergoing transition into a dissociated species that differs from the denatured form derived from alpha-rPrP. The higher susceptibility to pressure of beta-rPrP can be explained by its less hydrated structure. Pressure perturbation calorimetry supports the view that the accessible surface area of alpha-rPrP is much higher than that of beta-rPrP, which explains the lower degree of hydration of beta-rPrP. Our findings shed new light on the mechanism of prion conversion and show how water plays a prominent role. Our results allow us to propose a volume and free energy diagram of the different species involved in the conversion and aggregation. The existence of different folded conformations as well as different denatured states of PrP may explain the elusive character of its conversion into a pathogenic form.

The diastereomeric assembly of polylysine is the low-volume pathway for preferential formation of beta-sheet aggregates

The interaction of left- and right-handed polylysine chains (poly(D-lysine) and poly(L-lysine)) results in a dramatic increase in the propensity to form aggregated beta-sheet structure (and amyloid-like fibrils), which is reflected by an approximately 15 degrees C decrease of temperature of the alpha-helix-to-beta-sheet transition. While a relative volume expansion of 13-19 mL x mol(-1) accompanies the alpha-to-beta-transition in a single enantiomer, this does not hold true for the mixture, which, along with substantially more negative heat capacity changes, points to a lower solvent-entropy cost of the transition as the possible thermodynamic driving force of the diastereomeric aggregation. The underlying solvational mechanism may be one of the decisive factors responsible for the spontaneous protein aggregation in vivo and, as such, may shed new light on the molecular basis of amyloid-associated diseases.

Characterization of the temperature- and pressure-induced inverse and reentrant transition of the minimum elastin-like polypeptide GVG(VPGVG) by DSC, PPC, CD, and FT-IR spectroscopy

We investigated the temperature- and pressure-dependent structure and phase behavior of a solvated oligopeptide, GVG(VPGVG), which serves as a minimalistic elastin-like model system, over a large region of the thermodynamic phase field, ranging from 2 to 120 degrees C and from ambient pressure up to approximately 10 kbar, applying various spectroscopic (CD, FT-IR) and thermodynamic (DSC, PPC) measurements. We find that this octapeptide behaves as a two-state system which undergoes the well-known inverse-temperature folding transition occurring at T approximately 36 degrees C, and, in addition, a slow trend reversal at higher temperatures, finally leading to a reentrant unfolding close to the boiling point of water. Furthermore, the pressure-dependence of the folding/unfolding transition was studied to yield a more complete picture of the p, T-stability diagram of the system. A molecular-level picture of these processes, in particular on the role of water for the folding and unfolding events of the peptide, presented with the help of molecular-dynamics simulations, is presented in a companion article in this issue.

Folding and unfolding of an elastinlike oligopeptide: "inverse temperature transition," reentrance, and hydrogen-bond dynamics

The temperature-dependent behavior of a solvated oligopeptide, GVG(VPGVG), is investigated. Spectroscopic measurements, thermodynamic measurements, and molecular dynamics simulations find that this elastinlike octapeptide behaves as a two-state system that undergoes an "inverse temperature" folding transition and reentrant unfolding close to the boiling point of water. A molecular picture of these processes is presented, emphasizing changes in the dynamics of hydrogen bonding at the protein/water interface and peptide backbone librational entropy.

Pressure perturbation and differential scanning calorimetric studies of bipolar tetraether liposomes derived from the thermoacidophilic archaeon Sulfolobus acidocaldarius

Differential scanning calorimetry (DSC) and pressure perturbation calorimetry (PPC) were used to characterize thermal phase transitions, membrane packing, and volumetric properties in multilamellar vesicles (MLVs) composed of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius grown at different temperatures. For PLFE MLVs derived from cells grown at 78 degrees C, the first DSC heating scan exhibits an endothermic transition at 46.7 degrees C, a small hump near 60 degrees C, and a broad exothermic transition at 78.5 degrees C, whereas the PPC scan reveals two transitions at approximately 45 degrees C and 60 degrees C. The endothermic peak at 46.7 degrees C is attributed to a lamellar-to-lamellar phase transition and has an unusually low DeltaH (3.5 kJ/mol) and DeltaV/V (0.1%) value, as compared to those for the main phase transitions of saturated diacyl monopolar diester lipids. This result may arise from the restricted trans-gauche conformational changes in the dibiphytanyl chain due to the presence of cyclopentane rings and branched methyl groups and due to the spanning of the lipid molecules over the whole membrane. The exothermic peak at 78.5 degrees C probably corresponds to a lamellar-to-cubic phase transition and exhibits a large and negative DeltaH value (-23.2 kJ/mol), which is uncommon for normal lamellar-to-cubic phospholipid phase transformations. This exothermic transition disappears in the subsequent heating scans and thus may involve a metastable phase, which is irreversible at the scan rate used. Further, there is no distinct peak in the plot of the thermal expansion coefficient alpha versus temperature near 78.5 degrees C, indicating that this lamellar-to-cubic phase transition is not accompanied by any significant volume change. For PLFE MLVs derived from cells grown at 65 degrees C, similar DSC and PPC profiles and thermal history responses were obtained. However, the lower growth temperature yields a higher DeltaV/V ( approximately 0.25%) and DeltaH (14 kJ/mol) value for the lamellar-to-lamellar phase transition measured at the same pH (2.1). A lower growth temperature also generates a less negative temperature dependence of alpha. The changes in DeltaV/V, DeltaH, and the temperature dependence of alpha can be attributed to the decrease in the number of cyclopentane rings in PLFE at the lower growth temperature. The relatively low DeltaV/V and small DeltaH involved in the phase transitions help to explain why PLFE liposomes are remarkably thermally stable and also echo the proposal that PLFE liposomes are generally rigid and tightly packed. These results help us to understand why, despite the occurrence of thermal-induced phase transitions, PLFE liposomes exhibit a remarkably low temperature sensitivity of proton permeation and dye leakage.

Pressure Perturbation Calorimetic Studies of the Solvation Properties and the Thermal Unfolding of Proteins in Solution

In this study, pressure perturbation calorimetry (PPC), a relatively new and efficient technique, was used to study the solvation and volumetric properties of the enzyme ribonuclease A (RNase A) in its native and unfolded state. In PPC, the coefficient of thermal expansion of the partial volume of the protein is deduced from the heat consumed or produced after small isothermal pressure jumps (±5bar), which strongly depends on the interaction of the protein with the solvent or co-solvent at the protein-solvent interface. The effects of various chaotropic and kosmotropic co-solvents (glycerol, sorbitol, urea, guanidinium hydrochloride, K2SO4) on the solvation and unfolding behaviour of RNase A was investigated as well, and the observed volume and expansivity changes are correlated with further thermodynamic properties obtained from differential scanning calorimetry (DSC). Depending on the type of co-solvent and its concentration, specific differences are found for the solvation properties of the protein, and the volume change upon unfolding may even change sign. Taken together, the data obtained lead to a deeper understanding of the solvation process of proteins in different co-solvents in their native and unfolded states.

Role of microtubules in glucose uptake by C6 glioma cells

Drugs that influence tubulin function were used to investigate the role of microtubules in hexose uptake by C6 glioma cells. In C6 cells, colchicine and vinblastine (which inhibit tubulin polymerization) inhibited radioactive [3H]2-deoxy-D-glucose uptake by about 30%. Paclitaxel (which promotes tubulin polymerization) stimulated hexose uptake by about 25%. To further demonstrate that microtubules play a role in hexose uptake, C6 cells were transfected with GLUT1 cDNA and then challenged with 100 nM paclitaxel. In GLUT1-transfected cells paclitaxel stimulated 2-deoxy-D-glucose uptake by about 35%. To study the role of tubulin in agonist-stimulated hexose uptake, the effect of colchicine on carbachol-induced uptake was next examined. Hexose uptake was increased with carbachol in concentration-dependent manner which was abolished by pretreatment with colchicine. To examine the specificity of the inhibitory effect of colchicine on G protein-mediated signal transduction pathway, the influence of colchicine on insulin (which acts via tyrosine kinase pathway) stimulation of 2-deoxy-D-glucose uptake was investigated. Hexose uptake was increased by insulin in a concentration-dependent manner which was unaffected by pretreatment with colchicine. These results suggest that microtubules are involved in basal and carbachol-stimulated glucose uptake by C6 cells.

Progesterone, testosterone and estradiol-17 beta inhibit gonadotropin-releasing hormone stimulation of G protein GTPase activity in plasma membranes from rat anterior pituitary lobe

In order to understand the biochemical mechanisms underlying the rapid, non-genomic effects of gonadal steroids on gonadotropin secretion, we examined the effects of progesterone, testosterone and estradiol-17 beta on the low Km GTPase activity associated with transducer G proteins coupled to gonadotropin-releasing hormone (GnRH) receptors. Homogenates of anterior pituitary lobes from adult male rats were processed by discontinuous sucrose gradient centrifugation to isolate plasma membranes. The low Km GTPase activity (EC 3.6.1.-) was assayed in 5 micrograms membrane protein using [gamma-32P]GTP at 37 degrees C in an ATP-regenerating buffer containing 1 mumol/l unlabeled GTP. One hundred nmol/l each of progesterone, testosterone and estradiol-17 beta maximally stimulated low Km GTPase activity by 61%, 59% and 45%, respectively (p less than 0.05). Time course studies revealed that 100 nmol/l progesterone stimulated the enzyme activity by 93% and 62% at 5 and 30 min, respectively; 100 nmol/l testosterone stimulated GTPase activity by 100% and 72% at 5 and 30 min, respectively; 100 nmol/l estradiol-17 beta stimulated GTPase activity by 80% and 70% at 5 and 30 min, respectively. GnRH stimulated the low Km GTPase activity by about 60% in a concentration-dependent manner. In the presence of the gonadal steroids, the ability of GnRH to stimulate the GTPase activity was inhibited. For example, stimulation ranged from 36% to 60% with 0.1-100 nmol/l GnRH alone, but only from 7% to 20% in the presence of GnRH and 100 nmol/l progesterone (p less than 0.05). Similarly, in the presence of 100 nmol/l estradiol-17 beta, GnRH stimulation of the enzymatic activity ranged from 12% to 19%.(ABSTRACT TRUNCATED AT 250 WORDS)