Rachel Codd

Studies on the genotoxicity of chromium: from the test tube to the cell

Abstract A critical review of the relevance of in vitro chemical studies to Cr-induced cancers is given. In particular, the nature of the most likely reductants, the species that stabilise intermediates and the mechanisms of DNA damage and damage to transcription proteins are discussed. It is concluded that the major damaging species responsible for the genotoxicity include Cr(V), Cr(IV) and/or reactive organic intermediates. These studies on the chemistry are also related to spectroscopic and other studies on mammalian cells.

The many faces of the adamantyl group in drug design.

Adamantyl-based compounds are used clinically for the treatment of neurological conditions, as anti-viral agents and as agents against type 2 diabetes. The value of the adamantyl group in drug design is multidimensional. The hydrophobic substituent constant for the adamantyl group has been estimated from the calculated partition coefficients (clogP values) of 31 adamantyl-bearing compounds in the clinic or in development as πadamantyl=3.1, which indicates that the logP value of a compound with high water solubility (logP<<0) could be moved with an adamantyl-based modification to a region that is more clinically useful. The steric bulk of the adamantyl group can: (i) restrict or modulate intramolecular reactivity; and (ii) impede the access of hydrolytic enzymes, thereby increasing drug stability and plasma half life. The value of the adamantyl group in drug design has been recognized most recently in the design of agents to treat iron overload disease (in development), malaria (in clinical trials) and type 2 diabetes (in the clinic).

Zn(II)-dependent histone deacetylase inhibitors: Suberoylanilide hydroxamic acid and trichostatin A

Suberoylanilide hydroxamic acid (SAHA, vorinostat, Zolinza) and trichostatin A (TSA) are inhibitors of the Zn(II)-dependent class I and class II histone deacetylases (HDACs), which are enzymes that operate in concert with histone acetyltransferases (HATs) to regulate the acetylation status of the epsilon-amino group of lysine residues of nucleosomal histones in chromatin. An increased level of histone acetylation resulting from the SAHA or TSA inhibition of Zn(II)-dependent HDACs relaxes the chromatin structure and upregulates transcription. The links made in the 1990s between the inhibition of HDAC activity and the suppression of tumor growth have brought the design of HDAC inhibitors (HDACi) to the forefront of oncology research. SAHA has anticancer activity against hematologic and solid tumors and has been approved by the FDA for the treatment of cutaneous T-cell lymphoma. The increased molecular-level understanding of class I and class IIa HDACs from X-ray crystallography highlights differences in the residues distal to the active site and in the cavity size, which has implications for HDACi substrate specificity and enzyme mechanism. Results from HDAC-focussed activity-based protein profiling experiments may lead to the design of molecules that are class-specific HDACi.

EPR characterisation of the CrV intermediates in the CrVI/V oxidations of organic substrates and of relevance to Cr-induced cancers

An empirical method has been developed for determining the coordination number of CrV complexes (five- or six-coordinate) and the donor groups bound to CrV from the measured giso and Aiso values. The giso values depend only on the nature of the donor groups and the number of each group, which is very useful in the characterisation of the coordination number and donors. By contrast, the Aiso values appear to be sensitive to the nature of the equatorial donor atoms and the coordination geometry. Strong correlations have been found between the giso values of isoelectronic CrV and VIV complexes and between the Aiso values of analogous complexes of these two metal ions. These, combined, have been used to understand the solution chemistry of CrV using EPR spectroscopy. Such studies have also been invaluable in unravelling the biochemistry that occurs in cell media and cells exposed to carcinogenic CrVI and, hence, possible mechanisms of Cr-induced cancers.

Sialoglycoprotein and carbohydrate complexes in chromium toxicity

Chromium(VI) compounds are amongst the most widely encountered industrial carcinogens and are of increasing concern with respect to environmental exposure. Sialoglycoproteins and carbohydrates play a crucial role in stabilizing oxoCr(V) intermediates, which are produced by extracellular and intracellular reduction of chromium(VI). Recent research has addressed the molecular characterization of oxoCr(V)-sialoglycoprotein and -carbohydrate complexes and the roles that these species may play in Cr(VI) metabolism and carcinogenesis. Particular highlights include the role of oxoCr(V) complexes of extracellular sialoglycoproteins, intracellular D-glucose, and related species and their potential roles in Cr(VI)-induced genotoxicity.

The periplasmic nitrate reductase in Shewanella: the resolution, distribution and functional implications of two NAP isoforms, NapEDABC and NapDAGHB.

In the bacterial periplasm, the reduction of nitrate to nitrite is catalysed by a periplasmic nitrate reductase (NAP) system, which is a species-dependent assembly of protein subunits encoded by the nap operon. The reduction of nitrate catalysed by NAP takes place in the 90 kDa NapA subunit, which contains a Mo-bis-molybdopterin guanine dinucleotide cofactor and one [4Fe-4S] iron-sulfur cluster. A review of the nap operons in the genomes of 19 strains of Shewanella shows that most genomes contain two nap operons. This is an unusual feature of this genus. The two NAP isoforms each comprise three isoform-specific subunits - NapA, a di-haem cytochrome NapB, and a maturation chaperone NapD - but have different membrane-intrinsic subunits, and have been named NAP-alpha (NapEDABC) and NAP-beta (NapDAGHB). Sixteen Shewanella genomes encode both NAP-alpha and NAP-beta. The genome of the vigorous denitrifier Shewanella denitrificans OS217 encodes only NAP-alpha and the genome of the respiratory nitrate ammonifier Shewanella oneidensis MR-1 encodes only NAP-beta. This raises the possibility that NAP-alpha and NAP-beta are associated with physiologically distinct processes in the environmentally adaptable genus Shewanella.

Life on Earth. Extremophiles Continue to Move the Goal Posts

Environmental Context.Since the discovery of a diverse array of microbial life associated with hydrothermal vents on the ocean floor, where conditions are hot, reducing and acidic, scientists have been seeking insight into the mechanisms used by ‘extremophilic’ organisms (those that reside permanently under environmental extremes of temperature (hot or cold), pH (acid or alkaline), salinity, or pressure) to thrive under such seemingly inhospitable conditions. Abstract.The discovery of an abundance of microorganisms that flourish in a diverse range of environments, from the frigid waters of the Antarctic, to the superheated waters of the hydrothermal vents, at the bottom of 11-km deep ocean trenches and in salt-saturated lakes, has fuelled research aimed to understand the novel survival strategies evolved by these extreme-loving (extremophilic) organisms. Adaptations of biomolecules (proteins, nucleic acids, membranes and small molecules) evolved by extremophiles are wide ranging. Compared with a protein from a ‘regular’ organism, the extremophilic analogue might feature changes to the relative frequencies of amino acid residues that modulate the properties (e.g. conformational flexibility and stability) of the protein under conditions of the specific environmental challenge. The integrity of RNA and DNA from extremophiles may be maintained by subtle structural changes to RNA nucleobases and, in the case of (hyper)thermophiles, the expression of the enzyme reverse gyrase, which catalyses positive DNA supercoiling. The expression of small molecular weight heat-shock or related caretaker proteins also features as a common adaptive strategy for maintaining cell viability at environmental extremes. Membrane architecture in extremophiles can be modulated by the environmental temperature, with additional thermal stability in membranes from some hyperthermophiles conferred by novel (cyclised) lipid chains. In addition, a selection of osmolytes and small molecules are biosynthesised or sequestered by extremophilic organisms that have adapted to conditions of high salt and/or micronutrient deprivation.

Conjugates of Desferrioxamine B (DFOB) with Derivatives of Adamantane or with Orally Available Chelators as Potential Agents for Treating Iron Overload

Desferrioxamine B (DFOB) conjugates with adamantane-1-carboxylic acid, 3-hydroxyadamantane-1-carboxylic acid, 3,5-dimethyladamantane-1-carboxylic acid, adamantane-1-acetic acid, 4-methylphenoxyacetic acid, 3-hydroxy-2-methyl-4-oxo-1-pyridineacetic acid (N-acetic acid derivative of deferiprone), or 4-[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]benzoic acid (deferasirox) were prepared and the integrity of Fe(III) binding of the compounds was established from electrospray ionization mass spectrometry and RP-HPLC measurements. The extent of intracellular (59)Fe mobilized by the DFOB-3,5-dimethyladamantane-1-carboxylic acid adduct was 3-fold greater than DFOB alone, and the IC(50) value of this adduct was 6- or 15-fold greater than DFOB in two different cell types. The relationship between logP and (59)Fe mobilization for the DFOB conjugates showed that maximal mobilization of intracellular (59)Fe occurred at a logP value approximately 2.3. This parameter, rather than the affinity for Fe(III), appears to influence the extent of intracellular (59)Fe mobilization. The low toxicity-high Fe mobilization efficacy of selected adamantane-based DFOB conjugates underscores the potential of these compounds to treat iron overload disease in patients with transfusional-dependent disorders such as beta-thalassemia.

Proteomics of Pseudomonas aeruginosa Australian epidemic strain 1 (AES-1) cultured under conditions mimicking the cystic fibrosis lung reveals increased iron acquisition via the siderophore pyochelin.

Pseudomonas aeruginosa is an opportunistic pathogen that is the major cause of morbidity and mortality in patients with cystic fibrosis (CF). While most CF patients are thought to acquire P. aeruginosa from the environment, person-to-person transmissible strains have been identified in CF clinics worldwide, and the molecular basis for transmissibility remains poorly understood. We undertook a complementary proteomics approach to characterize protein profiles from a transmissible, acute isolate of the Australian epidemic strain 1 (AES-1R), the virulent burns/wound isolate PA14, and the poorly virulent, laboratory-associated strain PAO1 when grown in an artificial medium that mimics the CF lung environment compared to growth in standard laboratory medium. Proteins elevated in abundance in AES-1R included those involved in methionine and S-adenosylmethionine biosynthesis and in the synthesis of phenazines. Proteomic data were validated by measuring culture supernatant levels of the virulence factor pyocyanin, which is the final product of the phenazine pathway. AES-1R and PAO1 released higher extracellular levels of pyocyanin compared to PA14 when grown in conditions that mimic the CF lung. Proteins associated with biosynthesis of the iron-scavenging siderophore pyochelin (PchDEFGH and FptA) were also present at elevated abundance in AES-1R and at much higher levels than in PAO1, whereas they were reduced in PA14. These protein changes resulted phenotypically in increased extracellular iron acquisition potential and, specifically, elevated pyochelin levels in AES-1R culture supernatants as detected by chrome azurol-S assay and fluorometry, respectively. Transcript analysis of pyochelin genes (pchDFG and fptA) showed they were highly expressed during the early stage of growth in artificial sputum medium (18 h) but returned to basal levels following the establishment of microcolony growth (72 h) consistent with that observed in the CF lung. This provides further evidence that iron acquisition by pyochelin may play a role in the early stages of transmissible CF infection associated with AES-1R.

Lipophilic adamantyl- or deferasirox-based conjugates of desferrioxamine B have enhanced neuroprotective capacity: implications for Parkinson disease.

Parkinson disease (PD) is a neurodegenerative disease characterized by death of dopaminergic neurons in the substantia nigra region of the brain. Iron content is also elevated in this region in PD and is implicated in the pathobiology of the disease. Desferrioxamine B (DFOB) is a high-affinity iron chelator and has shown efficacy in animal models of Parkinson disease. The high water solubility of DFOB, however, attenuates its ability to enter the brain. In this study, we have conjugated DFOB to derivatives of adamantane or the clinical iron chelator deferasirox to produce lipophilic compounds designed to increase the bioavailability of DFOB to brain cells. We found that the novel compounds are highly effective in preventing iron-mediated paraquat and hydrogen peroxide toxicity in neuronal-like BE2-M17 dopaminergic cells, primary neurons, and iron-loaded or glutathione-depleted primary astrocytes. The compounds also alleviated paraquat toxicity in BE2-M17 cells that express the PD-causing A30P mutation of α-synuclein. This protection was ∼66-fold more potent than DFOB alone and also more effective than other cell-permeative metal chelators, clioquinol and phenanthroline. These results demonstrate that increasing the bioavailability of DFOB through the conjugation of lipophilic fragments greatly enhances its protective capacity. These novel compounds have potential as therapeutics for the treatment of PD and other conditions of Fe dyshomeostasis.