Carbon monoxide-releasing molecule-3 (CORM-3; Ru(CO)3Cl(glycinate)) as a tool to study the concerted effects of carbon monoxide and nitric oxide on bacterial flavohemoglobin Hmp: applications and pitfalls

Metal-based carbon monoxide releasing molecules with promising cytotoxic properties

Carbon monoxide, the "silent killer" gas, is increasingly recognised as an important signalling molecule in human physiology, which has beneficial biological properties. A particular way of achieving controlled CO administration is based on the use of biocompatible molecules that only release CO when triggered by internal or external factors. These approaches include the development of pharmacologically effective prodrugs known as CO releasing molecules (CORMs), which can supply biological systems with CO in well-regulated doses. An overview of transition metal-based CORMs with cytotoxic properties is here reported. The mechanisms at the basis of the biological activities of these molecules and their potential therapeutical applications with respect to their stability and CO releasing properties have been discussed. The activation of metal-based CORMs is determined by the type of metal and by the nature and features of the auxiliary ligands, which affect the metal core electronic density and therefore the prodrug resistance towards oxidation and CO release ability. A major role in regulating the cytotoxic properties of these CORMs is played by CO and/or CO-depleted species. However, several mysteries concerning the cytotoxicity of CORMs remain as intriguing questions for scientists.

Plight of CORMs: The unreliability of four commercially available CO-releasing molecules, CORM-2, CORM-3, CORM-A1, and CORM-401, in studying CO biology

Carbon monoxide (CO) is an endogenously produced gaseous signaling molecule with demonstrated pharmacological effects. In studying CO biology, three delivery forms have been used: CO gas, CO in solution, and CO donors of various types. Among the CO donors, four carbonyl complexes with either a transition metal ion or borane (BH3) (termed CO-releasing molecules or CORMs) have played the most prominent roles appearing in over 650 publications. These are CORM-2, CORM-3, CORM-A1, and CORM-401. Intriguingly, there have been unique biology findings that were only observed with these CORMs, but not CO gas; yet these properties were often attributed to CO, raising puzzling questions as to why CO source would make such a fundamental difference in terms of CO biology. Recent years have seen a large number of reports of chemical reactivity (e.g., catalase-like activity, reaction with thiol, and reduction of NAD(P)+) and demonstrated CO-independent biological activity for these four CORMs. Further, CORM-A1 releases CO in an idiosyncratic fashion; CO release from CORM-401 is strongly influenced or even dependent on reaction with an oxidant and/or a nucleophile; CORM-2 mostly releases CO2, not CO, after a water-gas shift reaction except in the presence of a strong nucleophile; and CORM-3 does not release CO except in the presence of a strong nucleophile. All these beg the question as to what constitutes an appropriate CO donor for studying CO biology. This review critically summarizes literature findings related to these aspects, with the aim of helping result interpretation when using these CORMs and development of essential criteria for an appropriate donor for studying CO biology.

Inhibition of the carnitine acylcarnitine carrier by carbon monoxide reveals a novel mechanism of action with non-metal-containing proteins

Both toxic and physiological effects of CO are mostly caused by well described interactions with heme-groups of proteins. Interactions of CO with non-heme proteins have also been unveiled. Besides interaction of CO with mitochondrial heme containing respiratory complexes, a BK channel and the phosphate carrier which do not contain metal cofactors, have been identified as CO targets. However, the molecular mechanisms of interaction with non-metal-containing proteins are not understood. We show in this work the effect of CO on the mitochondrial carnitine carrier (SLC25A20) using CORM-3, a widely recognized CO releasing compound. CO exerts an inhibitory effect at the micromolar concentration on the transport function of the transporter extracted from treated mitochondria. The effect is due to a single Cys residue, C136 as revealed by mass spectrometry analysis. A computational approach predicted the need for vicinal Asp and Lys residues for the C136 carbonylation to occur. These data demonstrate a novel mechanism of interaction of CO with a protein not containing metal atoms and will enable the prediction of CO targets.

In Vivo Imaging and Tracking Carbon Monoxide-Releasing Molecule-3 with an NIR Fluorescent Probe

As a water-soluble carbon monoxide-releasing molecule, CORM-3 is widely used as a CO donor to study CO in the life system. CORM-3 can also replace gaseous CO as a therapeutic drug molecule to reveal the physiological and pathological effects of CO in life. Therefore, it is of great importance to visualize and track CORM-3 in the life system. We develop herein a near-infrared (NIR) fluorescent probe CORM3-NIR that can detect CORM-3 both in living cells and in vivo effectively. The probe is based on the unique fluorescent QCy7 and uses a 4-nitrobenzyl group to trap CORM-3, and importantly, it shows good water solubility and responds rapidly, selectively, and sensitively to CORM-3, releasing QCy-7 and producing distinct colorimetric and significant NIR fluorescence change signals at 743 nm. The Stokes shift is up to 81 nm. The probe is also able to detect CORM-3 ratiometrically with fluorescence at 743 and 600 nm. Besides, with low cytotoxicity, the probe also shows good NIR fluorescence bioimaging ability for CORM-3 in live cells and mice, which indicates that CORM3-NIR is an effective probe for tracking and studying CORM-3 in the life system.

Synthesis, structural characterization, molecular docking study, biological activity of carbon monoxide release molecules as potent antitumor agents

In this study, two series of novel carbon monoxide-releasing molecules (CO-RMs) containing Co were designed and synthesized. The synthesized complexes were characterized by IR, ESI-MS, ¹H NMR and ¹³C NMR spectroscopies. The antitumor activity of all complexes on HepG2 cells, Hela cells and MDA-MB-231 cells were assayed by MTT. IC₅₀ values of complexes 1-13 were 4.7-548.6 µM. Among these complexes, complex 1 was presented with a high selectivity to HepG2 cells (IC₅₀ = 4.7 ± 0.76 μM). Compared with iCORM (inactive CORM), CORM (complex 1) showed a remarkable activity against tumor cells owing to co-effect of CO and the ligand of COX-2 inhibitor. In addition, complex 1 increased ROS in mitochondria and caused a decrease of dose-dependent mitochondrial membrane potential against HepG2 cells. Complex 1 down-regulated the expression of COX-2 protein in western blot analysis. The molecular docking study suggested that the complex 1 formed a hydrogen bond with amino acid R120 in the active site of the Human cyclooxygenase-2 (COX-2). Therefore, the complex 1 could induce apoptosis of HepG2 cells through targeting COX-2 and mitochondria pathways, and it maybe a potential therapeutic agent for cancer.

Carbon Monoxide, a Retrograde Messenger Generated in Postsynaptic Mushroom Body Neurons, Evokes Noncanonical Dopamine Release

Dopaminergic neurons innervate extensive areas of the brain and release dopamine (DA) onto a wide range of target neurons. However, DA release is also precisely regulated. In Drosophila melanogaster brain explant preparations, DA is released specifically onto α3/α'3 compartments of mushroom body (MB) neurons that have been coincidentally activated by cholinergic and glutamatergic inputs. The mechanism for this precise release has been unclear. Here we found that coincidentally activated MB neurons generate carbon monoxide (CO), which functions as a retrograde signal evoking local DA release from presynaptic terminals. CO production depends on activity of heme oxygenase in postsynaptic MB neurons, and CO-evoked DA release requires Ca²⁺ efflux through ryanodine receptors in DA terminals. CO is only produced in MB areas receiving coincident activation, and removal of CO using scavengers blocks DA release. We propose that DA neurons use two distinct modes of transmission to produce global and local DA signaling.SIGNIFICANCE STATEMENT Dopamine (DA) is needed for various higher brain functions, including memory formation. However, DA neurons form extensive synaptic connections, while memory formation requires highly specific and localized DA release. Here we identify a mechanism through which DA release from presynaptic terminals is controlled by postsynaptic activity. Postsynaptic neurons activated by cholinergic and glutamatergic inputs generate carbon monoxide, which acts as a retrograde messenger inducing presynaptic DA release. Released DA is required for memory-associated plasticity. Our work identifies a novel mechanism that restricts DA release to the specific postsynaptic sites that require DA during memory formation.

Monitoring the dynamics of hemeoxygenase-1 activation in head and neck cancer cells in real-time using plasmonically enhanced Raman spectroscopy

Real-time monitoring of the dynamics of pharmacologically generated HO-1 in mammalian cells by using plasmonically enhanced Raman spectroscopy (PERS).

We report for the first time the usage of plasmonically enhanced Raman spectroscopy (PERS) to directly monitor the dynamics of pharmacologically generated hemeoxygenase-1 (HO-1) by evaluating the kinetics of formation of carbon monoxide (CO), one of the metabolites of HO-1 activation, in live cells during cisplatin treatment. Being an endogenous signaling molecule, CO plays an important role in cancer regression. Many aspects of HO-1's and CO's functions in biology are still unclear largely due to the lack of technological tools for the real-time monitoring of their dynamics in live cells and tissues. In this study, we found that, together with nuclear region-targeted gold nanocubes (AuNCs), cisplatin treatment can dramatically trigger the activation of HO-1 and thereby the rate and production of CO in mammalian cells in a dose-dependent manner. Though quantitative molecular data revealed that a lower concentration of cisplatin up-regulates HO-1 expression in cancer cells, PERS data suggest that it poorly facilitates the activation of HO-1 and thereby the production of CO. However, at a higher dose, cisplatin along with AuNCs could significantly enhance the activation of HO-1 in cancer cells, which could be probed in real-time by monitoring the CO generation by using PERS. Under the same conditions, the rate of formation of CO in healthy cells was relatively higher in comparison to the cancer cells. Additionally, molecular data revealed that AuNCs have the potential to suppress the up-regulation of HO-1 in cancer cells during cisplatin treatment at a lower concentration. As up-regulation of HO-1 has a significant role in cell adaptation to oxidative stress in cancer cells, the ability of AuNCs in suppressing the HO-1 overexpression will have a remarkable impact in the development of nanoformulations for combination cancer therapy. This exploratory study demonstrates the unique possibilities of PERS in the real-time monitoring of endogenously generated CO and thereby the dynamics of HO-1 in live cells, which could expedite our understanding of the signaling action of CO and HO-1 in cancer progression.

Critical Roles of Carbon Monoxide and Nitric Oxide in Ca2+ Signaling for Insulin Secretion in Pancreatic Islets

AIMS

Glucagon-like peptide-1 (GLP-1) increases intracellular Ca²⁺ concentrations, resulting in insulin secretion from pancreatic β-cells through the sequential production of Ca²⁺ mobilizing messengers nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (cADPR). We previously found that NAADP activates the neuronal type of nitric oxide (NO) synthase (nNOS), the product of which, NO, activates guanylyl cyclase to produce cyclic guanosine monophosphate (cGMP), which, in turn, induces cADPR formation. Our aim was to explore the relationship between Ca²⁺ signals and gasotransmitters formation in insulin secretion in β-cells upon GLP-1 stimulation.

RESULTS

We show that NAADP-induced cGMP production by nNOS activation is dependent on carbon monoxide (CO) formation by heme oxygenase-2 (HO-2). Treatment with exogenous NO and CO amplifies cGMP formation, Ca²⁺ signal strength, and insulin secretion, whereas this signal is impeded when exposed to combined treatment with NO and CO. Furthermore, CO potentiates cGMP formation in a dose-dependent manner, but higher doses of CO inhibited cGMP formation. Our data with regard to zinc protoporphyrin, a HO inhibitor, and HO-2 knockdown, revealed that NO-induced cADPR formation and insulin secretion are dependent on HO-2. Consistent with this observation, the administration of NO or CO donors to type 2 diabetic mice improved glucose tolerance, but the same did not hold true when both were administered concurrently.

INNOVATION

Our research reveals the role of two gas transmitters, CO and NO, for Ca²⁺ second messengers formation in pancreatic β-cells.

CONCLUSION

These results demonstrate that CO, the downstream regulator of NO, plays a role in bridging the gap between the Ca²⁺ signaling messengers during insulin secretion in pancreatic β-cells.

Carbon monoxide and its donors - their implications for medicine

Inhalation of high concentrations of carbon monoxide (CO) is known to lead to serious systemic complications and neuronal disturbances. However, it has been found that not only is CO produced endogenously, but also that low concentrations can bestow beneficial effects which may be of interest in biology and medicine. As translocation of CO through the human organism is difficult, small molecules known as CO-releasing molecules (CORMs) deliver controlled amounts of CO to biological systems, and these are of great interest from a medical point of view. These actions may prevent vascular dysfunction, regulate blood pressure, inhibit blood platelet aggregation or have anti-inflammatory effects. This review summarizes the functions of various CO-releasing molecules in biology and medicine.

Nitric Oxide Stress as a Metabolic Flux

Nitric oxide (NO) is an antimicrobial metabolite produced by immune cells to prohibit infection. Due to its reactivity, NO has numerous reaction routes available to it in biological systems with some leading to cellular damage and others producing innocuous compounds. Pathogens have evolved resistance mechanisms toward NO, and many of these take the form of enzymes that chemically passivate the molecule. In essence, bacteria have channeled NO flux toward useful or harmless compounds, and away from pathways that damage cellular components. Pathogens devoid of detoxification enzymes have been found to have compromised survival in different infection models, which suggests that diverting flux away from NO defenses could be a viable antiinfective strategy. From this perspective, potentiation of NO stress mirrors challenges in metabolic engineering where researchers endeavor to divert flux away from endogenous pathways and toward those that produce desirable biomolecules. In this review, we cast NO stress as a metabolic flux and discuss how the tools and methodologies of metabolic engineering are well suited for analysis of this bacterial stress response. We provide examples of such interdisciplinary applications, discuss the benefits of considering NO stress from a flux perspective, as well as the pitfalls, and offer a vision for how metabolic engineering analyses can assist in deciphering the economics underlying bacterial responses to multistress conditions that are characteristic of the phagosomes of immune cells.

Kinetic Analysis and Structural Interpretation of Competitive Ligand Binding for NO Dioxygenation in Truncated Hemoglobin N

The conversion of nitric oxide (NO) into nitrate (NO₃^- ) by dioxygenation protects cells from lethal NO. Starting from NO-bound heme, the first step in converting NO into benign NO₃^- is the ligand exchange reaction FeNO+O₂ →FeO₂ +NO, which is still poorly understood at a molecular level. For wild-type (WT) truncated hemoglobin N (trHbN) and its Y33A mutant, the calculated barriers for the exchange reaction differ by 1.5 kcal mol^-1 , compared with 1.7 kcal mol^-1 from experiment. It is directly confirmed that the ligand exchange reaction is rate-limiting in trHbN and that entropic contributions account for 75 % of the difference between the WT and the mutant. Residues Tyr 33, Phe 46, Val 80, His 81, and Gln 82 surrounding the active site are expected to control the reaction path. By comparison with electronic structure calculations, the transition state separating the two ligand-bound states was assigned to a ² A state.

Carbon monoxide (CO) inhibits hydrogen peroxide (H2O2)-induced oxidative stress and the activation of NF-κB signaling in lens epithelial cells

Lens epithelial cells (LECs) play a critical role in the maintenance of clear crystalline lens. Previously, we reported that heme oxygenase-1 can protect LECs from hydrogen peroxide (H₂O₂)-induced apoptosis and oxidative stress; however, to the best of our knowledge, these protection mechanisms have not yet been explained. As carbon monoxide (CO) is an active by-product of heme degradation, we investigated its cytoprotective mechanism in both H₂O₂-treated human LECs (SRA 01/04) and primary rabbit LECs. CO-releasing molecule-3 was used as a CO releasing vehicle. The nuclear translocation of nuclear factor kappa B (NF-κB) p65 was monitored by Western blot and immunofluorescence staining. In addition, the levels of intracellular reactive oxygen species (ROS), antioxidants, and apoptotic molecules (Bax, Bcl-2, and caspase-3) were measured. Furthermore, cell apoptosis rate was quantified by flow cytometry. Our results disclosed that low concentrations of CO released from CO-releasing molecule-3 can attenuate NF-κB p65 nuclear translocation, reduce ROS generation, and enhance intracellular glutathione and superoxide dismutase levels. Moreover, low concentrations of CO inhibited H₂O₂-induced apoptotic molecules, thereby decreasing the apoptosis of LECs. These findings suggest that low concentrations of CO protect LECs from H₂O₂-induced oxidative damage by attenuating NF-κB p65 nuclear translocation, reducing the generation of ROS and apoptotic molecules, and restoring antioxidant enzyme levels, thereby inhibiting LECs apoptosis.

The Microbiology of Ruthenium Complexes

Ruthenium is seldom mentioned in microbiology texts, due to the fact that this metal has no known, essential roles in biological systems, nor is it generally considered toxic. Since the fortuitous discovery of cisplatin, first as an antimicrobial agent and then later employed widely as an anticancer agent, complexes of other platinum group metals, such as ruthenium, have attracted interest for their medicinal properties. Here, we review at length how ruthenium complexes have been investigated as potential antimicrobial, antiparasitic and chemotherapeutic agents, in addition to their long and well-established roles as biological stains and inhibitors of calcium channels. Ruthenium complexes are also employed in a surprising number of biotechnological roles. It is in the employment of ruthenium complexes as antimicrobial agents and alternatives or adjuvants to more traditional antibiotics, that we expect to see the most striking developments in the future. Such novel contributions from organometallic chemistry are undoubtedly sorely needed to address the antimicrobial resistance crisis and the slow appearance on the market of new antibiotics.

Examining the antimicrobial activity and toxicity to animal cells of different types of CO-releasing molecules

Transition metal carbonyl complexes used as CO-releasing molecules (CORMs) for biological and therapeutic applications may exhibit interesting antimicrobial activity. However, understanding the chemical traits and mechanisms of action that rule this activity is required to establish a rationale for the development of CORMs into useful antibiotics. In this work the bactericidal activity, the toxicity to eukaryotic cells, and the ability of CORMs to deliver CO to bacterial and eukaryotic cells were analysed for a set of seven CORMs that differ in the transition metal, ancillary ligands and the CO release profile. Most of these CORMs exhibited bactericidal properties that decrease in the following order: CORM-2 > CORM-3 > ALF062 > ALF850 > ALF186 > ALF153 > [Fe(SBPy3)(CO)](BF4)2. A similar yet not entirely coincident decreasing order was found for their induction of intracellular reactive oxygen species (ROS) in E. coli. In contrast, studies in model animal cells showed that for any given CORM, the level of intracellular ROS generated was negligible when compared with that measured inside bacteria. Importantly, these CORMs were in general not toxic to eukaryotic cells, namely murine macrophages, kidney LLC-PK1 epithelial cells, and liver cell line HepG2. CORM-2 and CORM-3 delivered CO to the intracellular space of both E. coli and the two types of tested eukaryotic cells, yet toxicity was only elicited in the case of E. coli. CO delivered by ALF186 into the intercellular space did not enter E. coli cells and the compound was not toxic to either bacteria or to eukaryotic cells. The Fe(ii) carbonyl complex [Fe(SBPy3)(CO)](2+) had the reverse, undesirable toxicity profile, being unexpectedly toxic to eukaryotic cells and non-toxic to E. coli. ALF153, the most stable complex in the whole set, was essentially devoid of toxicity or ROS induction ability in all cells. These results suggest that CORMs have a relevant therapeutic potential as antimicrobial drugs since (i) they can show opposite toxicity profiles towards bacteria and eukaryotic cells; (ii) their activity can be modulated through manipulation of the ancillary ligands, as shown with the three {Ru(CO)3}(2+) and two zerovalent Mo based CORMs; and (iii) their toxicity to eukaryotic cells can be made acceptably low. With this new approach, this work contributes to the understanding of the roots of the bactericidal action of CORMs and helps in establishing strategies for their development into a new class of antibiotics.

Antimicrobial Activity of the Manganese Photoactivated Carbon Monoxide-Releasing Molecule [Mn(CO)3(tpa-κ(3)N)](+) Against a Pathogenic Escherichia coli that Causes Urinary Infections

AIMS

We set out to investigate the antibacterial activity of a new Mn-based photoactivated carbon monoxide-releasing molecule (PhotoCORM, [Mn(CO)3(tpa-κ(3)N)](+)) against an antibiotic-resistant uropathogenic strain (EC958) of Escherichia coli.

RESULTS

Activated PhotoCORM inhibits growth and decreases viability of E. coli EC958, but non-illuminated carbon monoxide-releasing molecule (CORM) is without effect. NADH-supported respiration rates are significantly decreased by activated PhotoCORM, mimicking the effect of dissolved CO gas. CO from the PhotoCORM binds to intracellular targets, namely respiratory oxidases in strain EC958 and a bacterial globin heterologously expressed in strain K-12. However, unlike previously characterized CORMs, the PhotoCORM is not significantly accumulated in cells, as deduced from the cellular manganese content. Activated PhotoCORM reacts avidly with hydrogen peroxide producing hydroxyl radicals; the observed peroxide-enhanced toxicity of the PhotoCORM is ameliorated by thiourea. The PhotoCORM also potentiates the effect of the antibiotic, doxycycline.

INNOVATION

The present work investigates for the first time the antimicrobial activity of a light-activated PhotoCORM against an antibiotic-resistant pathogen. A comprehensive study of the effects of the PhotoCORM and its derivative molecules upon illumination is performed and mechanisms of toxicity of the activated PhotoCORM are investigated.

CONCLUSION

The PhotoCORM allows a site-specific and time-controlled release of CO in bacterial cultures and has the potential to provide much needed information on the generality of CORM activities in biology. Understanding the mechanism(s) of activated PhotoCORM toxicity will be key in exploring the potential of this and similar compounds as antimicrobial agents, perhaps in combinatorial therapies with other agents. Antioxid. Redox Signal. 24, 765-780.

CO-releasing Metal Carbonyl Compounds as Antimicrobial Agents in the Post-antibiotic Era

The possibility of a “post-antibiotic era” in the 21st century, in which common infections may kill, has prompted research into radically new antimicrobials. CO-releasing molecules (CORMs), mostly metal carbonyl compounds, originally developed for therapeutic CO delivery in animals, are potent antimicrobial agents. Certain CORMs inhibit growth and respiration, reduce viability, and release CO to intracellular hemes, as predicted, but their actions are more complex, as revealed by transcriptomic datasets and modeling. Progress is hindered by difficulties in detecting CO release intracellularly, limited understanding of the biological chemistry of CO reactions with non-heme targets, and the cytotoxicity of some CORMs to mammalian cells.