Luteolin Shifts Oxaliplatin-Induced Cell Cycle Arrest at G₀/G₁ to Apoptosis in HCT116 Human Colorectal Carcinoma Cells

Certain antioxidative flavonoids are known to activate nuclear factor E2-related factor 2 (Nrf2), a transcription factor that regulates cellular antioxidants and detoxifying response and is reportedly highly activated in many types of cancers. Few studies on the potential undesired effects of flavonoid intake during chemotherapy have been conducted, yet Nrf2 activators could favor cancer cell survival by attenuating chemotherapeutic efficiency. This study aimed to examine if luteolin, an Nrf2 activator, hinders chemotherapeutic activity of oxaliplatin, a potent anticancer agent for colorectal cancer, in HCT116 cells. Luteolin treatment strongly increased the transcriptional activity of the antioxidant response element in HCT116 cells and induced the protein expression of heme oxygenase-1, which were indicative of its Nrf2-inducing potential. Intriguingly, 25 μM luteolin reduced cell viability through apoptotic induction, which was intensified in p53-expressing cells while 1 μM oxaliplatin caused cell cycle arrest at G₀/G₁-phase via the p53/p21-dependent mechanism. Moreover, luteolin treatment was found to reduce oxaliplatin-treated p53-null cell viability and colony counts further, thereby demonstrating an additional effect of luteolin in the killing of human colorectal tumor HCT116 cells not expressing functional p53 protein. The findings suggest that luteolin can induce p53-mediated apoptosis regardless of oxaliplatin treatment and may eliminate oxaliplatin-resistant p53-null colorectal cells.

Spectral studies of tert-butyl isothiocyanate-inactivated P450 2E1

Inactivation of cytochrome P450 2E1 by tert-butyl isothiocyanate (tBITC) resulted in a loss in the spectrally detectable P450-reduced CO complex. The heme prosthetic group does not appear to become modified, since little loss of the heme was observed in the absolute spectra or the pyridine hemochrome spectra, or in the amount of heme recovered from HPLC analysis of the tBITC-inactivated samples. Prolonged incubations of the inactivated P450 2E1 with dithionite and CO resulted in a recovery of both the CO complex and the enzymatic activity. Inactivated samples that were first reduced with dithionite for 1 h prior to CO exposure recovered their CO spectrum to the same extent as samples not pretreated with dithionite, suggesting that the major defect was an inability of the inactivated sample to bind CO. Spectral binding studies with 4-methylpyrazole indicated that the inactivated P450 2E1 had an impaired ability to bind the substrate. Enzymatic activity could not be restored with iodosobenzene as the alternate oxidant. EPR analysis indicated that approximately 24% of the tBITC-inactivated P450 2E1 was EPR-silent. Of the remaining tBITC-inactivated P450 2E1, approximately 45% exhibited an unusual low-spin EPR signal that was attributed to the displacement of a water molecule at the sixth position of the heme by a tBITC modification to the apoprotein. ESI-LC-MS analysis of the inactivated P450 2E1 showed an increase in the mass of the apoprotein of 115 Da. In combination, the data suggest that tBITC inactivated P450 2E1 by binding to a critical active site amino acid residue(s). This modified amino acid(s) presumably acts as the sixth ligand to the heme, thereby interfering with oxygen binding and substrate binding.

The effect of three Korean traditional medicines on the growth rate of cultured human keratinocytes

The effect of three different Korean Traditional Medicines (KTM) was studied on several functional parameters of adult human cells in culture. The cells were non-transformed strains of normal, skin epidermal cells (keratinocytes) from adult humans. Aqueous extracts of the herbal medicines were tested using two types of cell strains: one type was essential fatty acid deficient (EFAD) cells which grow rapidly in medium that was low in calcium and had no essential fatty acids; the second type was a cell strain grown in medium supplemented with essential fatty acid (EFA-supplemented). These cells had much slower, in vivo skin growth rates, and the fatty acid composition resembled that measured in epidermal biopsy tissue. The KTMs chosen for this study were tae-gang-hual-tang (for treating osteoarthritis), hual-ak-tang (for pain relief) and sip-zeon-tae-bo-tang (for fortifying immune systems). Because high proliferation rates usually correlate with skin inflammation and because many of the chemotactic agents mediating inflammatory response are modified fatty acids, this study focused on cell growth rate and membrane fatty acid composition as signals for the effects of the herbal medicines. By monitoring growth rate, these experiments measured both a stimulatory and a regulatory effect on the growth of keratinocytes. Some toxicity was seen at the highest doses of the KTMs. These effects were modeled mathematically, and the results showed varying effects on growth rate depending on dose and herbal recipe. The fitting parameters were discussed as they relate to biological function. The experimental design was also discussed and alternatives were suggested.

EPR spectrometry of cytochrome P450 2B4: effects of mutations and substrate binding

The EPR spectra of NH(2)-terminal-truncated P450 cytochrome 2B4 and of several active site mutants that were previously shown to be profoundly altered in catalytic properties were determined. From these spectra it was seen that the truncated P450 2B4, like the full length cytochrome, exists as the low spin ferric form, but upon mutation of threonine 302 to alanine approximately 40% of the cytochrome is present as the high spin ferric form (g approximately 8, 4, 2). A similar situation was observed in the double mutant E310L T302A, but not in the single mutant E301L. A rhombic high spin signal (g approximately 8, 4, 2) was observed when a substrate such as styrene, benzphetamine, or cyclohexane was added to the truncated cytochrome. Accompanying this change was the appearance of a signal at g = 1.98. Conversely, an axial high spin signal was observed (g approximately 6, 6, 2) when cyclohexanecarboxaldehyde or 3-phenylpropionaldehyde was added to the truncated P450 2B4.

Rearrangement of L-2-hydroxyglutarate to L-threo-3-methylmalate catalyzed by adenosylcobalamin-dependent glutamate mutase

Adenosylcobalamin-dependent enzymes catalyze a variety of chemically difficult isomerizations in which a nonacidic hydrogen on one carbon is interchanged with an electron-withdrawing group on an adjacent carbon. We describe a new isomerization, that of L-2-hydroxyglutarate to L-threo-3-methylmalate, involving the migration of the carbinol carbon. This reaction is catalyzed by glutamate mutase, but k(cat) = 0.05 s(-)(1) is much lower than that for the natural substrate, L-glutamate (k(cat) = 5.6 s(-)(1)). EPR spectroscopy confirms that the major organic radical that accumulates on the enzyme is the C-4 radical of L-2-hydroxyglutarate. Pre-steady-state kinetic measurements revealed that L-2-hydroxyglutarate-induced homolysis of AdoCbl occurs very rapidly, with a rate constant approaching those measured previously with glutamate and methylaspartate as substrates. These observations are consistent with the rearrangement of the 2-hydroxyglutaryl radical being the rate-determining step in the reaction. The slow rearrangement of the 2-hydroxyglutaryl radical can be attributed to the poor stabilization by the hydroxyl group of the migrating glycolyl moiety of the radical transiently formed on the migrating carbon. In contrast, with the normal substrate the migrating carbon atom bears a nitrogen substituent that better stabilizes the analogous glycyl moiety. These studies point to the importance of the functional groups attached to the migrating carbon in facilitating the carbon skeleton rearrangement.

Synthesis, spectroscopic, and structural characterization of the first aqueous cobalt(II)-citrate complex: toward a potentially bioavailable form of cobalt in biologically relevant fluids

Citric acid represents a class of carboxylic acids present in biological fluids and playing key roles in biochemical processes in bacteria and humans. Its ability to promote diverse coordination chemistries in aqueous media, in the presence of metal ions known to act as trace elements in human metabolism, earmarks its involvement in a number of physiological functions. Cobalt is known to be a central element of metabolically important biomolecules, such as B12, and therefore its biospeciation in biological fluids constitutes a theme worthy of chemical and biological perusal. In an effort to unravel the aqueous chemistry of cobalt in the presence of a physiologically relevant ligand, citrate, the first aqueous, soluble, mononuclear complex has been synthesized and isolated from reaction mixtures containing Co(II) and citrate in a 1:2 molar ratio at pH approximately 8. The crystalline compound (NH4)4[Co(C6H5O7)2] (1) has been characterized spectroscopically (UV/vis, EPR) and crystallographically. Its X-ray structure consists of a distorted octahedral anion with two citrate ligands fulfilling the coordination requirements of the Co(II) ion. The magnetic susceptibility measurements of 1 in the range from 6 to 295 K are consistent with a high-spin complex containing Co(II) with a ground state S=3/2. Corroborating this result is the EPR spectrum of 1, which shows a signal consistent with the presence of a Co(II) system. The spectroscopic and structural properties of the complex signify its potential biological relevance and participation in speciation patterns arising under conditions consistent with those employed for its synthesis and isolation.

Electron paramagnetic resonance measurements of the ferrous mononuclear site of phthalate dioxygenase substituted with alternate divalent metal ions: direct evidence for ligation of two histidines in the copper(II)-reconstituted protein

The metalloenzyme phthalate dioxygenase (PDO) contains two iron-based sites. A Rieske-type [2Fe-2S] cluster serves as an electron-transferring cofactor, and a mononuclear iron site is the putative site of substrate oxygenation. A reductase, which contains FMN and a plant-type [2Fe-2S] ferredoxin domain, transfers electrons from NADH to the Rieske center. Any of the metal ions, Fe(II), Cu(II), Co(II), Mn(II), and Zn(II), can be used to populate the mononuclear site, but only Fe(II) is competent for effecting hydroxylation. Nevertheless, studies of how these metal ions affect both the EPR spectra of the reduced Rieske site and the kinetics of electron transfer in the PDO system indicated that each of these metal ions binds tightly and affects the protein similarly. In this study, EPR spectra were obtained from samples in which iron of the mononuclear site was replaced with Cu(II). The use of (63)Cu(II), in combination with PDO obtained from cultures grown on media enriched in (15)N [using ((15)NH(4))(2)SO(4) as a sole nitrogen source], [delta,epsilon-(15)N]histidine, as well as natural abundance sources of nitrogen, enabled detailed spectral analysis of the superhyperfine structure of the Cu(II) EPR lines. These studies clearly show that two histidines are coordinated to the mononuclear site. Coupled with previous studies [Bertini, I., Luchinat, C., Mincione, G., Parigi, G., Gassner G. T., and Ballou, D. P. (1996) J. Bioinorg. Chem. 1, 468-475] that show the presence of one or two water molecules coordinated to the iron, it is suggested that the mononuclear site is similar to several other mononuclear nonheme iron proteins, including naphthalene dioxygenase, for which crystal structures are available. The lack of observable EPR interaction signals between Cu(II) in the mononuclear site and the reduced Rieske center of PDO suggest that the two sites are at least 12 A apart, which is similar to that found in the naphthalene dioxygenase crystal structure.

The high-potential flavin and heme of nitric oxide synthase are not magnetically linked: implications for electron transfer

BACKGROUND

The homodimeric nitric oxide synthase (NOS) catalyzes conversion of L-arginine to L-citrulline and nitric oxide. Each subunit contains two flavins and one protoporphyrin IX heme. A key component of the reaction is the transfer of electrons from the flavins to the heme. The NOS gene encodes two domains linked by a short helix containing a calmodulin-recognition sequence. The reductase domain binds the flavin cofactors, while the oxygenase domain binds heme and L-arginine and additionally mediates the dimerization of the NOS subunits. We investigated the origin of the unusual magnetic properties (rapid-spin relaxation) of an air-stable free radical localized to a reductase domain flavin cofactor.

RESULTS

We characterized the air-stable flavin in wild-type NOS, both in the presence and absence of calcium and calmodulin, the imidazole-bound heme complex of wild-type NOS, the NOS Cys415-->Ala mutant, and the isolated reductase domain. All preparations of NOS had the same flavin electron-spin relaxation behavior. No half-field transitions or temperature-dependent changes in the linewidth of the radical spin signal were detected.

CONCLUSIONS

These data suggest that the observed relaxation enhancement of the NOS flavin radical is caused by the environment provided by the reductase domain. No magnetic interaction between the heme and flavin cofactors was detected, suggesting that the flavin and heme centers are probably separated by more than 15 A.

Identification and functional requirement of Cu(I) and its ligands within coagulation factor VIII

Coagulation factor VIII (FVIII) is a heterodimer consisting of a light chain of 80 kDa (domains A3-C1-C2) in a metal ion-dependent association with a 220-kDa heavy chain (domains A1-A2-B). The nature of the metal ion-dependent association between the heavy and light chains was investigated using atomic absorption spectroscopy, electron paramagnetic resonance spectroscopy (EPR), and site-directed mutagenesis and expression of the FVIII cDNA. Whereas copper ion was not detected in intact recombinant FVIII, EDTA dissociation of the chains yielded an EPR signal consistent with 1 mol of Cu(I)/mol of active protein, supporting the hypothesis that a single molecule of reduced copper ion is buried within intact FVIII and is released and oxidized upon treatment with EDTA. Cu(I), and not Cu(II), was able to reconstitute FVIII activity from dissociated chains, demonstrating a requirement for Cu(I) in FVIII function. Three potential copper ion binding sites exist within FVIII: one type-2 site and two type-1 sites. The importance of these potential copper ion ligands was tested by studying the effect of site-directed mutants. Of the two histidines that compose the type-2 binding site, the His-1957 --> Ala mutant displayed secretion, light and heavy chain assembly, and activity similar to wild-type FVIII, while mutant His-99 --> Ala was partially defective for secretion and had low levels of heavy and light chain association and activity. In contrast, FVIII having the mutation Cys-310 --> Ser within the type-1 copper binding site in the A1 domain was inactive and partially defective for secretion from the cell, and the heavy and light chains of the secreted protein were not associated. Mutant Cys-2000 --> Ser within the A3 domain displayed secretion, assembly, and activity similar to that for wild-type FVIII. These results support the hypothesis that Cu(I) is buried within the type-1 copper binding site within the A1 domain and is required for FVIII chain association and activity.

Regulation of the soxRS oxidative stress regulon. Reversible oxidation of the Fe-S centers of SoxR in vivo

SoxR protein, a transcriptional activator of the soxRS (superoxide response) regulon of Escherichia coli, contains autooxidizable [2Fe-2S] centers that are presumed to serve as redox sensors. In vitro transcription experiments previously demonstrated that only the oxidized form is active. Reduced SoxR was detected in overproducing strains by EPR spectroscopy of suspensions of intact cells. Oxidized Fe-S centers were determined by lysing the cells and treating them with the reducing agent sodium dithionite prior to EPR measurements. In uninduced cells, 90% of the SoxR was in the reduced form. Treatment with the redox cycling agents phenazine methosulfate or plumbagin was accompanied by reversible oxidation of the Fe-S centers. Mutant SoxR derivatives that were constitutively activated existed constitutively in an oxidized state. The results indicate the presence of a cellular pathway for countering the autooxidation of SoxR and confirm the hypothesis that induction of the regulon is mediated by a shift in the redox equilibrium of SoxR rather than by assembly of its Fe-S clusters.

The iron-sulfur clusters 2 and ubisemiquinone radicals of NADH:ubiquinone oxidoreductase are involved in energy coupling in submitochondrial particles

The behavior of ubisemiquinone radicals and the iron-sulfur clusters 2 of NADH:ubiquinone oxidoreductase (Complex I) in coupled and uncoupled submitochondrial particles (SMP), oxidizing either NADH or succinate under steady-state conditions, was studied. Multifrequency EPR spectra revealed that the two new g2 lines of the clusters 2, only observed during coupled electron transfer under conditions where energy dissipation is rate-limiting [De Jong, A. M. Ph., Kotlyar, A. B., & Albracht, S. P. J. (1994) Biochim. Biophys. Acta 1186, 163-171], are the result of a spin-spin interaction of 2.8 mT. Investigation of the radical signals present in coupled SMP indicated that more than 90% of the radicals can be ascribed to two types of semiquinones which are bound to Complex I (QI-radicals) or ubiquinol:cytochrome c oxidoreductase (Complex III; QIII-radicals). The presence of QIII-radicals, but not that of QI-radicals, was completely abolished by uncoupler. Part of the QI-radicals weakly interact with the clusters 2 of Complex I. This uncoupler-sensitive interaction can amount to a splitting of the radical EPR signal of at most 1 mT, considerably weaker than the 2.8 mT splitting of the g2 lines of the clusters 2. We propose that the 2.8 mT splitting of these g2 lines results from an energy-induced spin-spin interaction between the two clusters 2 within the TYKY subunit of Complex I. The two clusters 2 show no interaction during electron transfer is uncoupled SMP or in fully-reduced anaerobic-coupled SMP. The results point to a direct role of the Fe-S clusters 2 and the QI-radicals in the mechanism of coupled electron transfer catalyzed by Complex I.

Antibiotic-induced lethal enterocolitis in hamsters: studies with eleven agents and evidence to support the pathogenic role of toxin-producing Clostridia

Clindamycin-induced enterocolitis in hamsters was studied, using a tissue culture assay to detect clostridial toxin. It was found that animals with lethal enterocolitis had a cytopathogenic substance in cecal contents and blood that was neutralized by clostridial antitoxins. Cultures of the cecal flora yielded numerous species of clostridia, but only 1 organism was detected which produced a toxin which was cytopathic in tissue culture. This organism, Clostridium difficile, was consistently present in high concentrations, and the cell-free supernate of these strains caused enterocolitis if injected intracecally into hamsters. Ten additional antimicrobials were tested ih hamsters. Ampicillin, vancomycin, erythromycin, cephalosporins, and oral gentamicin caused lethal enterocolitis in most recipients, and all animals which died had evidence of clostridia toxin in cecal contents at necropsy. Tetracycline and metronidazole were well tolerated, and the animals given these antimicrobials had no evidence of the toxin. We conclude that toxin-producing clostridia are responsible for lethal enterocolitis due to a variety of antimicrobials in hamsters.

Occurrence of human pancreatic polypeptide in pancreatic endocrine tumors. Possible implication in the watery diarrhea syndrome

Eighteen endocrine pancreatic tumors were examined for the occurrence of cells producing insulin, glucagon, gastrin, human pancreatic polypeptide (HPP), and vasoactive intestinal polypeptide (VIP) and for A1 cells. More than half of the tumors were mixed, i.e., they contained more than one type of hormone-producing cell. The clinical symptoms were attributable only to one of the hormones produced by the mixed tumors. Three of four tumors causing the watery diarrhea syndrome contained both VIP and HPP cells. In one such tumor there was a strong predominance of HPP cells; the serum HPP levels of this patient were a thousandfold elevated, whereas her VIP levels were within the normal range. Several lines of evidence point to HPP as a possible agent causing the watery diarrhea syndrome. In many of our patients, HPP cells hyperplasia was present in the extratumoral pancreas. Such hyperplasia may give rise to the raised serum HPP levels seen in many patients having endocrine pancreatic tumors.

Pancreatic-polypeptide response to food in duodenal-ulcer patients before and after vagotomy

In 21 normal fasting subjects basal serum-pancreatic-polypeptide (H.P.P.), as measured by a specific radioimmunoassay, was 43 +/- 4 pmol/l (mean +/- S.E.M). This basal concentration was of the same magnitude as that of other gastrointestinal peptide hormones. In normal subjects the H.P.P. response to food was bisphasic. A rapid eightfold increase, reaching a maximum 20--30 min after beginning of the meal, was followed by a secondary, prolonged increase. H.P.P. did not return to basal concentration within five hours. 14 duodenal-ulcer (D.U.) patients were studied before and after truncal vagotomy. Before operation the D.U. patients had higher basal H.P.P. concentrations than the normal group (110 +/- 40 pmol/l) whereas their H.P.P. response pattern to food was similar to that of the normal subjects. After vagotomy the primary rapid increase in H.P.P. concentration was eliminated and the secondary, prolonged response was significantly reduced (P less than 0-005). The results indicate that H.P.P. is a digestive hormone under vagal control.