Inhaled Carbon Monoxide Prevents Graft-Induced Intimal Hyperplasia in Swine

Therapeutic potential of carbon monoxide in hypertension-induced vascular smooth muscle cell damage revisited: from physiology and pharmacology

As a chronic and progressive disorder, hypertension remains to be a serious public health problem around the world. Among the different types of hypertension, pulmonary arterial hypertension (PAH) is a devastating disease associated with pulmonary arteriole remodeling, right ventricular failure and death. The contemporary management of systemic hypertension and PAH has substantially grown since more therapeutic targets and/or agents have been developed. Evolving treatment strategies targeting the vascular remodeling lead to improving outcomes in patients with hypertension, nevertheless, significant advancement opportunities for developing better antihypertensive drugs remain. Carbon monoxide (CO), an active endogenous gasotransmitter along with hydrogen sulfide (H₂S) and nitric oxide (NO), is primarily generated by heme oxygenase (HO). Cumulative evidence suggests that CO is considered as an important signaling molecule under both physiological and pathological conditions. Studies have shown that CO confers a number of biological and pharmacological properties, especially its involvement in the pathological process and treatment of hypertension-related vascular remodeling. This review will critically outline the roles of CO in hypertension-associated vascular remodeling and discuss the underlying mechanisms for the protective effects of CO against hypertension and vascular remodeling. In addition, we will propose the challenges and perspectives of CO in hypertensive vascular remodeling. It is expected that a comprehensive understanding of CO in the vasculature might be essential to translate CO to be a novel pharmacological agent for hypertension-induced vascular remodeling.

Targeting Heme Oxygenase-1 in the Arterial Response to Injury and Disease

Heme oxygenase-1 (HO-1) catalyzes the degradation of heme into carbon monoxide (CO), iron, and biliverdin, which is rapidly metabolized to bilirubin. The activation of vascular smooth muscle cells (SMCs) plays a critical role in mediating the aberrant arterial response to injury and a number of vascular diseases. Pharmacological induction or gene transfer of HO-1 improves arterial remodeling in animal models of post-angioplasty restenosis, vascular access failure, atherosclerosis, transplant arteriosclerosis, vein grafting, and pulmonary arterial hypertension, whereas genetic loss of HO-1 exacerbates the remodeling response. The vasoprotection evoked by HO-1 is largely ascribed to the generation of CO and/or the bile pigments, biliverdin and bilirubin, which exert potent antioxidant and anti-inflammatory effects. In addition, these molecules inhibit vascular SMC proliferation, migration, apoptosis, and phenotypic switching. Several therapeutic strategies are currently being pursued that may allow for the targeting of HO-1 in arterial remodeling in various pathologies, including the use of gene delivery approaches, the development of novel inducers of the enzyme, and the administration of unique formulations of CO and bilirubin.

Emerging Delivery Strategies of Carbon Monoxide for Therapeutic Applications: from CO Gas to CO Releasing Nanomaterials

Carbon monoxide (CO) therapy has emerged as a hot topic under exploration in the field of gas therapy as it shows the promise of treating various diseases. Due to the gaseous property and the high affinity for human hemoglobin, the main challenges of administrating medicinal CO are the lack of target selectivity as well as the toxic profile at relatively high concentrations. Although abundant CO releasing molecules (CORMs) with the capacity to deliver CO in biological systems have been developed, several disadvantages related to CORMs, including random diffusion, poor solubility, potential toxicity, and lack of on-demand CO release in deep tissue, still confine their practical use. Recently, the advent of versatile nanomedicine has provided a promising chance for improving the properties of naked CORMs and simultaneously realizing the therapeutic applications of CO. This review presents a brief summarization of the emerging delivery strategies of CO based on nanomaterials for therapeutic application. First, an introduction covering the therapeutic roles of CO and several frequently used CORMs is provided. Then, recent advancements in the synthesis and application of versatile CO releasing nanomaterials are elaborated. Finally, the current challenges and future directions of these important delivery strategies are proposed.

Carbon Monoxide and Its Controlled Release: Therapeutic Application, Detection, and Development of Carbon Monoxide Releasing Molecules (CORMs)

Carbon monoxide (CO) is attracting increasing attention because of its role as a gasotransmitter with cytoprotective and homeostatic properties. Carbon monoxide releasing molecules (CORMs) are spatially and temporally controlled CO releasers that exhibit superior and more effective pharmaceutical traits than gaseous CO because of their chemistry and structure. Experimental and preclinical research in animal models has shown the therapeutic potential of inhaled CO and CORMs, and the biological effects of CO and CORMs have also been observed in preclinical trials via the genetic modulation of heme oxygenase-1 (HO-1). In this review, we describe the pharmaceutical use of CO and CORMs, methods of detecting CO release, and developments in CORM design and synthesis. Many valuable clinical CORMs formulated using macromolecules and nanomaterials are also described.

T-Type Ca2+ Channel Regulation by CO: A Mechanism for Control of Cell Proliferation

T-type Ca(2+) channels regulate proliferation in a number of tissue types, including vascular smooth muscle and various cancers. In such tissues, up-regulation of the inducible enzyme heme oxygenase-1 (HO-1) is often observed, and hypoxia is a key factor in its induction. HO-1 degrades heme to generate carbon monoxide (CO) along with Fe(2+) and biliverdin. Since CO is increasingly recognized as a regulator of ion channels (Peers et al. 2015), we have explored the possibility that it may regulate proliferation via modulation of T-type Ca(2+) channels.Whole-cell patch-clamp recordings revealed that CO (applied as the dissolved gas or via CORM donors) inhibited all 3 isoforms of T-type Ca(2+) channels (Cav3.1-3.3) when expressed in HEK293 cells with similar IC(50) values, and induction of HO-1 expression also suppressed T-type currents (Boycott et al. 2013). CO/HO-1 induction also suppressed the elevated basal [Ca(2+) ](i) in cells expressing these channels and reduced their proliferative rate to levels seen in non-transfected control cells (Duckles et al. 2015).Proliferation of vascular smooth muscle cells (both A7r5 and human saphenous vein cells) was also suppressed either by T-type Ca(2+) channel inhibitors (mibefradil and NNC 55-0396), HO-1 induction or application of CO. Effects of these blockers and CO were non additive. Although L-type Ca(2+) channels were also sensitive to CO (Scragg et al. 2008), they did not influence proliferation. Our data suggest that HO-1 acts to control proliferation via CO modulation of T-type Ca(2+) channels.

Ovine femoral artery bypass grafting using saphenous vein: a new model

BACKGROUND

Saphenous vein grafts (SVGs) are frequently used for multi-vessel coronary artery bypass grafting and peripheral arterial bypasses; however, the estimated 40% failure rate within the first 5 y due to intimal hyperplasia (IH) and the subsequent failure rate of 2%-4% per year pose a significant clinical problem. Here, we report a surgical model in sheep intended to study IH development in SVGs, which can also be used for the evaluation of potential alternative treatments.

MATERIALS AND METHODS

Autologous bilateral SVGs were implanted as femoral artery interposition grafts using end-to-side anastomoses in adult sheep (n = 23), which were survived for 30 (n = 6), 90 (n = 7), 180 (n = 7), or 365 (n = 3) days. Post-implant, mid-term, and pretermination angiograms were quantified, and harvested SVGs were evaluated using quantitative histomorphometry.

RESULTS

We describe a peripheral arterial surgical technique that models the progression of SVG pathology. Angiographic analysis showed a progressive dilation of SVGs leading to worsening diametrical matching to the target artery and reduced blood flow; and histomorphometry data showed an increase in IH over time. Multivariable regression analysis suggested that statistically significant (P < 0.05) time-dependent relationships exist between SVG dilation and both reduction in blood flow and IH development.

CONCLUSIONS

Bilateral SVGs implanted onto the femoral arteries of sheep produced, controlled and consistent angiographic and histomorphometric results for which direct correlations could be made. This preclinical investigation model can be used as a robust tool to evaluate therapies intended for cardiovascular pathologies such as occlusive IH in SVGs.

Heme oxygenase-1 regulates cell proliferation via carbon monoxide-mediated inhibition of T-type Ca2+ channels

Induction of the antioxidant enzyme heme oxygenase-1 (HO-1) affords cellular protection and suppresses proliferation of vascular smooth muscle cells (VSMCs) associated with a variety of pathological cardiovascular conditions including myocardial infarction and vascular injury. However, the underlying mechanisms are not fully understood. Over-expression of Ca_v3.2 T-type Ca²⁺ channels in HEK293 cells raised basal [Ca²⁺]_i and increased proliferation as compared with non-transfected cells. Proliferation and [Ca²⁺]_i levels were reduced to levels seen in non-transfected cells either by induction of HO-1 or exposure of cells to the HO-1 product, carbon monoxide (CO) (applied as the CO releasing molecule, CORM-3). In the aortic VSMC line A7r5, proliferation was also inhibited by induction of HO-1 or by exposure of cells to CO, and patch-clamp recordings indicated that CO inhibited T-type (as well as L-type) Ca²⁺ currents in these cells. Finally, in human saphenous vein smooth muscle cells, proliferation was reduced by T-type channel inhibition or by HO-1 induction or CO exposure. The effects of T-type channel blockade and HO-1 induction were non-additive. Collectively, these data indicate that HO-1 regulates proliferation via CO-mediated inhibition of T-type Ca²⁺ channels. This signalling pathway provides a novel means by which proliferation of VSMCs (and other cells) may be regulated therapeutically.

Carbon monoxide alleviates ethanol-induced oxidative damage and inflammatory stress through activating p38 MAPK pathway

Stress-inducible protein heme oxygenase-1(HO-1) is well-appreciative to counteract oxidative damage and inflammatory stress involving the pathogenesis of alcoholic liver diseases (ALD). The potential role and signaling pathways of HO-1 metabolite carbon monoxide (CO), however, still remained unclear. To explore the precise mechanisms, ethanol-dosed adult male Balb/c mice (5.0g/kg.bw.) or ethanol-incubated primary rat hepatocytes (100mmol/L) were pretreated by tricarbonyldichlororuthenium (II) dimmer (CORM-2, 8mg/kg for mice or 20μmol/L for hepatocytes), as well as other pharmacological reagents. Our data showed that CO released from HO-1 induction by quercetin prevented ethanol-derived oxidative injury, which was abolished by CO scavenger hemoglobin. The protection was mimicked by CORM-2 with the attenuation of GSH depletion, SOD inactivation, MDA overproduction, and the leakage of AST, ALT or LDH in serum and culture medium induced by ethanol. Moreover, CORM-2 injection or incubation stimulated p38 phosphorylation and suppressed abnormal Tnfa and IL-6, accompanying the alleviation of redox imbalance induced by ethanol and aggravated by inflammatory factors. The protective role of CORM-2 was abolished by SB203580 (p38 inhibitor) but not by PD98059 (ERK inhibitor) or SP600125 (JNK inhibitor). Thus, HO-1 released CO prevented ethanol-elicited hepatic oxidative damage and inflammatory stress through activating p38 MAPK pathway, suggesting a potential therapeutic role of gaseous signal molecule on ALD induced by naturally occurring phytochemicals.

Carbon monoxide: an unusual drug

The highly toxic gas carbon monoxide (CO) displays many physiological roles in several organs and tissues. Although many diseases, including cancer, hematological diseases, hypertension, heart failure, inflammation, sepsis, neurodegeneration, and sleep disorders, have been linked to abnormal endogenous CO metabolism and functions, CO administration has therapeutic potential in inflammation, sepsis, lung injury, cardiovascular diseases, transplantation, and cancer. Here, insights into the CO-based therapy, characterized by the induction or gene transfer of heme oxygenase-1 and either gas or CO-releasing molecule administration, are reviewed.

Protective role of heme oxygenase-1 against inflammation in atherosclerosis

Heme oxygenase-1 (HO-1) catalyzes the first and rate-limiting step in the metabolism of free heme into equimolar amounts of ferrous iron, carbon monoxide (CO), and biliverdin. Biliverdin is subsequently converted to bilirubin by biliverdin reductase. HO-1 has recently been identified as a promising therapeutic target in the treatment of vascular inflammatory disease, including atherosclerosis. HO-1 represses inflammation by removing the pro-inflammatory molecule heme and by generating CO and the bile pigments, biliverdin and bilirubin. These HO-1 reaction products are capable of blocking innate and adaptive immune responses by modifying the activation, differentiation, maturation, and/or polarization of numerous immune cells, including endothelial cells, monocytes/macrophages, dendritic cells, T lymphocytes, mast cells, and platelets. These cellular actions by CO and bile pigments result in diminished leukocyte recruitment and infiltration, and pro-inflammatory mediator production within atherosclerotic lesions. This review highlights the mechanisms by which HO-1 suppresses vascular inflammation in atherosclerosis, and explores possible therapeutic modalities by which HO-1 and its reaction products can be employed to ameliorate vascular inflammatory disease.

Targeting heme oxygenase-1 in vascular disease

Heme oxygenase-1 (HO-1) metabolizes heme to generate carbon monoxide (CO), biliverdin, and iron. Biliverdin is subsequently metabolized to bilirubin by biliverdin reductase. HO-1 has recently emerged as a promising therapeutic target in the treatment of vascular disease. Pharmacological induction or gene transfer of HO-1 ameliorates vascular dysfunction in animal models of atherosclerosis, post-angioplasty restenosis, vein graft stenosis, thrombosis, myocardial infarction, and hypertension, while inhibition of HO-1 activity or gene deletion exacerbates these disorders. The vasoprotection afforded by HO-1 is largely attributable to its end products: CO and the bile pigments, biliverdin and bilirubin. These end products exert potent anti-inflammatory, antioxidant, anti-apoptotic, and anti-thrombotic actions. In addition, CO and bile pigments act to preserve vascular homeostasis at sites of arterial injury by influencing the proliferation, migration, and adhesion of vascular smooth muscle cells, endothelial cells, endothelial progenitor cells, or leukocytes. Several strategies are currently being developed to target HO-1 in vascular disease. Pharmacological induction of HO-1 by heme derivatives, dietary antioxidants, or currently available drugs, is a promising near-term approach, while HO-1 gene delivery is a long-term therapeutic goal. Direct administration of CO via inhalation or through the use of CO-releasing molecules and/or CO-sensitizing agents provides an attractive alternative approach in targeting HO-1. Furthermore, delivery of bile pigments, either alone or in combination with CO, presents another avenue for protecting against vascular disease. Since HO-1 and its products are potentially toxic, a major challenge will be to devise clinically effective therapeutic modalities that target HO-1 without causing any adverse effects.

Ex vivo carbon monoxide delivery inhibits intimal hyperplasia in arterialized vein grafts

AIMS

Veins are still the best conduits available for arterial bypass surgery. When these arterialized vein grafts fail, it is often due to the development of intimal hyperplasia (IH). We investigated the feasibility and efficacy of the ex vivo pre-treatment of vein grafts with soluble carbon monoxide (CO) in the inhibition of IH.

METHODS AND RESULTS

The inferior vena cava was excised from donor rats and placed as an interposition graft into the abdominal aorta of syngeneic rats. Prior to implantation, vein grafts were stored in cold Lactated Ringer (LR) solution with or without CO saturation (bubbling of 100% CO) for 2 h. Three and 6 weeks following grafting, vein grafts treated with cold LR for 2 h developed IH, whereas grafts implanted immediately after harvest demonstrated significantly less IH. Treatment in CO-saturated LR significantly inhibited IH and reduced vascular endothelial cell (VEC) apoptosis. Electron microscopy revealed improved VEC integrity with less platelet/white blood cell aggregation in CO-treated grafts. The effects of CO in preventing IH were associated with activation of hypoxia inducible factor-1α (HIF-1α) and an increase in vascular endothelial growth factor (VEGF) expression at 3-6 h after grafting. Treatment with a HIF-1α inhibitor completely abrogated the induction of VEGF by CO and reversed the protective effects of CO on prevention of IH.

CONCLUSION

Ex vivo treatment of vein grafts in CO-saturated LR preserved VEC integrity perioperatively and significantly reduced neointima formation. These effects appear to be mediated through the activation of the HIF1α/VEGF pathway.

Carbon Monoxide: Vascular Therapeutic for the Future

In the past decade, carbon monoxide has lost its reputation as a toxic gas and has gained a following of scientists who now appreciate its potential therapeutic utility as an anti-inflammatory and cytoprotective agent. Its vasoprotective properties have also gained a tremendous amount of attention, especially its ability to inhibit intimal hyperplasia. This review will provide a historical perspective of carbon monoxide biology and summarize its function in disease modulation with a special focus on its potential role as a vascular therapeutic.