Adenovirus-mediated heme oxygenase-1 gene delivery inhibits injury-induced vascular neointima formation

The role of inflammation in vascular injury and repair

Inflammation plays a critical role in the vascular response to injury. In particular, mechanical injury using techniques such as balloon angioplasty and stenting results in complex inflammatory reactions which influence proliferation of vessel wall constituents such as endothelial cells, smooth muscle cells, and extracellular matrix proteins. Inflammatory cells are recruited to the injured vessel wall initially as a reparative mechanism; however, these same inflammatory processes are also pivotal in the development of restenotic lesions. Leukocytes serve as the primary inflammatory cells but we now know that platelets produce a number of important inflammatory mediators. This review describes the mechanisms that regulate endothelial cell migration, smooth muscle cell activation, and extracellular matrix protein production, all of which are key components in the inflammatory response to vascular injury.

Carbon monoxide modulates Fas/Fas ligand, caspases, and Bcl-2 family proteins via the p38alpha mitogen-activated protein kinase pathway during ischemia-reperfusion lung injury

Carbon monoxide is protective in ischemia-reperfusion organ injury, but the precise mechanisms remain elusive. We have recently shown that low levels of exogenous carbon monoxide (CO) utilize p38 MAPK and attenuate caspase 3 activity to exert an antiapoptotic effect during lung ischemia-reperfusion injury. Our current data demonstrate that CO activates the p38alpha MAPK isoform and the upstream MAPK kinase MKK3 to modulate Fas/Fas ligand expression; caspases 3, 8, and 9; mitochondrial cytochrome c release; Bcl-2 proteins; and poly(ADP-ribose) polymerase cleavage. We correlate our in vitro findings with in vivo studies using MKK3-deficient and Fas-deficient mice. Taken together, our data are the first to demonstrate that CO has an antiapoptotic effect by inhibiting Fas/Fas ligand, caspases, proapoptotic Bcl-2 proteins, and cytochrome c release via the MKK3/p38alpha MAPK pathway.

Heme oxygenase-1 in growth control and its clinical application to vascular disease

Heme oxygenase-1 (HO-1) catalyzes the degradation of heme to carbon monoxide (CO), iron, and biliverdin. Biliverdin is subsequently metabolized to bilirubin by the enzyme biliverdin reductase. Although interest in HO-1 originally centered on its heme-degrading function, recent findings indicate that HO-1 exerts other biologically important actions. Emerging evidence suggests that HO-1 plays a critical role in growth regulation. Deletion of the HO-1 gene or inhibition of HO-1 activity results in growth retardation and impaired fetal development, whereas HO-1 overexpression increases body size. Although the mechanisms responsible for the growth promoting properties of HO-1 are not well established, HO-1 can indirectly influence growth by regulating the synthesis of growth factors and by modulating the delivery of oxygen or nutrients to specific target tissues. In addition, HO-1 exerts important effects on critical determinants of tissue size, including cell proliferation, apoptosis, and hypertrophy. However, the actions of HO-1 are highly variable and may reflect a role for HO-1 in maintaining tissue homeostasis. Considerable evidence supports a crucial role for HO-1 in blocking the growth of vascular smooth muscle cells (SMCs). This antiproliferative effect of HO-1 is mediated primarily via the release of CO, which inhibits vascular SMC growth via multiple pathways. Pharmacologic or genetic approaches targeting HO-1 or CO to the blood vessel wall may represent a promising, novel therapeutic approach in treating vascular proliferative disorders.

Applied gene therapy in preclinical models of vascular injury

Atherosclerosis remains the major cause of morbidity and mortality in Western countries. Atherothrombotic complications, including vascular occlusions and severe narrowing of nutrient blood vessels in the cerebral, coronary, or peripheral circulation, usually require invasive revascularization strategies. As molecular mediators contributing to these complications are being identified in more representative experimental injury models, and as gene transfer platforms and vectors acquire improved safety and efficacy profiles, there is ground for cautious optimism that gene-based interventions will likely reduce the clinical burden of these diseases. Increased generation of reactive oxygen species in diseased atherosclerotic vessels has been implicated in vasospasm, exaggerated neointima formation, and enhanced thrombosis. Ex vivo pressurized vascular gene transfer in venous bypass grafts using antisense oligonucleotides directed against cell-cycle control genes can modify the venous graft's phenotype and confer clinical benefit with improved long-term graft survival. Alternatively, percutaneous intra-arterial gene transfer is feasible, but at relatively low transgene expression levels. Although this may suffice in the case of secreted gene products with marked paracrine or bystander effects, including nitric oxide synthase and heme oxygenase-1, drug- and gene-eluting stents may provide the preferred future vehicle for well-controlled, quantifiable, and safe vascular gene transfer. Continued efforts to improve gene transfer technology in diseased human vessels and to increase our understanding of molecular targets are required before the full therapeutic potential of vascular gene therapy can be realized.

Heme oxygenase and angiogenic activity of endothelial cells: stimulation by carbon monoxide and inhibition by tin protoporphyrin-IX

The activity of heme oxygenase enzymes (HOs) is responsible for the endogenous source of carbon monoxide (CO). Their activities can be inhibited by tin protoporphyrin-IX (SnPPIX). Recent data indicate the involvement of HOs in the regulation of angiogenesis. Here, we investigated the role of the HO pathway in the production and angiogenic activity of vascular endothelial growth factor (VEGF) in endothelial cells treated with SnPPIX, or cultured in the presence of a CO-releasing molecule (CO-RM). Addition of CO-RM or induction of HO-1 by hemin resulted in a threefold elevation in CO production in culture medium (up to 20.3 microg/L) and was associated with a 30% increase in VEGF synthesis. Much higher levels of CO (up to 60 microg/L) and a further increase in VEGF production (by 277%) were measured in cells treated with prostaglandin-J(2), a potent activator of HO-1. SnPPIX prevented the induction of CO generation and inhibited VEGF synthesis. Moreover, SnPPIX reduced the VEGF-elicited angiogenic activities of endothelial cells by decreasing their proliferation (by 26%), migration (by 46%), formation of tubes on Matrigel (by 48%), and outgrowth of capillaries from endothelial spheroids (by 30%). In contrast, overexpression of HO-1 or incubation of cells with CO-RM led to an increase in capillary sprouting. Thus, HO activity up-regulates VEGF production and augments the capability of endothelial cells to respond to exogenous stimulation.

Changes of heme oxygenase-carbon monoxide system in vascular calcification in rats

The aim of the present study was to investigate the change in heme oxygenase (HO)-carbon monoxide (CO)-cyclic guanosine monophosphate (cGMP) pathway in vascular calcification. Vascular calcification model was established in rats by using vitamin D(3) and nicotine. Vascular calcium content, alkaline phosphatase (ALP) activity, HO activity, HbCO formation and content of cGMP in vessels were measured. Immunochemistry (IH) for HO 1 expression and in situ hybridization (ISH) for HO 1 mRNA were observed. Compared to those of control rats, the aortic calcium content and vascular ALP activity in rats of the calcified group (VDN group) were obviously increased, but HO 1 activity, CO concentration and cGMP content in vessels of rats in VDN group were markedly decreased. Expressions of HO-1 protein and mRNA were significantly decreased compared to control rats. Vascular calcification might induce a down regulation in vascular HO-CO-cGMP pathway.

Differential effect of heme oxygenase-1 in endothelial and smooth muscle cell cycle progression

Arterial remodeling in response to pathological insult is a complex process that depends in part on the balance between vascular cell apoptosis and proliferation. Studies in experimental models suggest that HO-1 mediates neointimal formation while limiting lumen stenosing, indicating a differential effect on vascular endothelial (EC) and smooth muscle cells (SMC). We investigated the effect of HO-1 expression on cell cycle progression in EC and SMC. The addition of SnMP (10 microM), an inhibitor of HO activity, to EC or SMC for 24h, resulted in significant abnormalities in DNA distribution and cell cycle progression compared to cells treated with the HO-1 inducers, heme (10 microM) or SnCl(2) (10 microM). SnMP increased G(1) phase and decreased S and G(2)/M phases in EC while heme or SnCl(2) decreased G(1) phase, but increased S and G(2)/M phases (p<0.05). Opposite effects were obtained in SMC. SnMP decreased G(1) phase and increased S and G(2)/M phases while heme or SnCl(2) increased G(1) phase but decreased S and G(2)/M phases (p<0.05). Our data demonstrate that HO-1 regulates the cell cycle in a cell-specific manner; it increases EC but decreases SMC cycle progression. The mechanisms underlying the HO-1 cell-specific effect on cell cycle progression within the vascular wall are yet to be explored. Nevertheless, these findings suggest that cell-specific targeting of HO-1 expression may provide a novel therapeutic strategy for the treatment of cardiovascular diseases.

Carbon monoxide and bile pigments: surprising mediators of vascular function

Heme oxygenase (HO) catalyzes the degradation of heme to CO, iron, and biliverdin. Biliverdin is subsequently metabolized to bilirubin by the enzyme biliverdin reductase. Although long considered irrelevant byproducts of heme catabolism, recent studies indicate that CO and the bile pigments biliverdin and bilirubin may play an important physiological role in the circulation. The release of CO by vascular cells may modulate blood flow and blood fluidity by inhibiting vasomotor tone, smooth muscle cell proliferation, and platelet aggregation. CO may also maintain the integrity of the vessel wall by directly blocking vascular cell apoptosis and by inhibiting the release of pro-apoptotic inflammatory cytokines from the vessel wall. These effects of CO are mediated via multiple pathways, including activation of soluble guanylate cyclase, potassium channels, p38 mitogen-activated protein kinase, or inhibition of cytochrome P450. In addition, the release of bile pigments may serve to sustain vascular homeostasis by protecting vascular cells from oxidative stress and by inhibiting the adhesion and infiltration of leukocytes into the vessel wall. Induction of HO-1 gene expression and the subsequent release of CO and bile pigments are observed in numerous vascular disorders and may provide an important adaptive mechanism to preserve homeostasis at sites of vascular injury. Thus, the HO-catalyzed formation of CO and bile pigments by vascular cells may function as a critical endogenous vasoprotective system. Moreover, pharmacological or genetic approaches targeting HO-1 to the vessel wall may represent a novel therapeutic approach in treating vascular disease.

Heme oxygenase-1: the "emerging molecule" has arrived

Organisms on our planet have evolved in an oxidizing environment that is intrinsically inimical to life, and cells have been forced to devise means of protecting themselves. One of the defenses used most widely in nature is the enzyme heme oxygenase-1 (HO-1). This enzyme performs the seemingly lackluster function of catabolizing heme to generate bilirubin, carbon monoxide, and free iron. Remarkably, however, the activity of this enzyme results in profound changes in cells' abilities to protect themselves against oxidative injury. HO-1 has been shown to have anti-inflammatory, antiapoptotic, and antiproliferative effects, and it is now known to have salutary effects in diseases as diverse as atherosclerosis and sepsis. The mechanism by which HO-1 confers its protective effect is as yet poorly understood, but this area of invetsigation is active and rapidly evolving. This review highlights current information on the function of HO-1 and its relevance to specific pulmonary and cardiovascular diseases.

Heme oxygenase-1-derived carbon monoxide is an autocrine inhibitor of vascular smooth muscle cell growth

Vascular smooth muscle cells (SMCs) generate carbon monoxide (CO) via the catabolism of heme by the enzyme heme oxygenase (HO). In the present study, we found that serum stimulated a time- and concentration-dependent increase in the levels of HO-1 messenger RNA (mRNA) and protein in vascular SMCs. The induction of HO-1 expression by serum was inhibited by actinomycin D or cycloheximide. In addition, serum stimulated HO activity, as reflected by an increase in the concentration of bilirubin in the culture media. Treatment of vascular SMCs with serum stimulated DNA synthesis and this was potentiated by the HO inhibitors, zinc and tin protoporphyrin-IX as well as by the CO scavenger, hemoglobin. The iron chelator desferrioxamine had no effect on DNA synthesis. However, exposure of vascular SMCs to exogenous CO inhibited serum-stimulated SMC proliferation and the phosphorylation of retinoblastoma protein. In addition, CO arrested SMCs at the G(1)/S transition phase of the cell cycle and selectively blocked the serum-stimulated expression of cyclin A mRNA and protein without affecting the expression of cyclin D1 and E. CO also inhibited the serum-stimulated activation of cyclin A-associated kinase activity and cyclin-dependent kinase 2 activity. These results demonstrate that serum stimulates HO-1 gene expression and CO synthesis. Furthermore, they show that CO acts in a negative feedback fashion to inhibit vascular SMC growth by regulating specific components of the cell cycle machinery. The capacity of vascular mitogens to induce CO synthesis may provide a novel mechanism by which these agents modulate cell growth.

Overexpression of heme oxygenase-1 protects smooth muscle cells against oxidative injury and inhibits cell proliferation

To investigate whether the expression of exogenous heme oxygenase-1 (HO-1) gene within vascular smooth muscle cells (VSMC) could protect the cells from free radical attack and inhibit cell proliferation, we established an in vitro transfection of human HO-1 gene into rat VSMC mediated by a retroviral vector. The results showed that the profound expression of HO-1 protein as well as HO activity was 1.8- and 2.0-fold increased respectively in the transfected cells compared to the non-transfected ones. The treatment of VSMC with different concentrations of H2O2 led to the remarkable cell damage as indicated by survival rate and LDH leakage. However, the resistance of the HO-1 transfected VSMC against H2O2 was significantly raised. This protective effect was dramatically diminished when the transfected VSMC were pretreated with ZnPP-IX, a specific inhibitor of HO, for 24 h. In addition, we found that the growth potential of the transfected cells was significantly inhibited directly by increased activity of HO-1, and this effect might be related to decreased phosphorylation of MAPK. These results suggest that the overexpression of introduced hHO-1 is potentially able to reduce the risk factors of atherosclerosis, partially due to its cellular protection against oxidative injury and to its inhibitory effect on cellular proliferation.

YC-1-mediated vascular protection through inhibition of smooth muscle cell proliferation and platelet function

YC-1, a synthetic benzyl indazole derivative, is capable of stimulating endogenous vessel wall cyclic guanosine monophosphate (cGMP) production and attenuating the remodeling response to experimental arterial angioplasty. In an effort to investigate the mechanisms of this YC-1-mediated vasoprotection, we examined the influence of soluble YC-1 or YC-1 incorporated in a polyethylene glycol (PEG) hydrogel on cultured rat vascular smooth muscle cell (SMC) cGMP synthesis, SMC proliferation, and platelet function. Results demonstrate that soluble YC-1 stimulated SMC cGMP production in a dose-dependent fashion, while both soluble and hydrogel-released YC-1 inhibited vascular SMC proliferation in a dose-dependent fashion without effects on cell viability. Platelet aggregation and adherence to collagen were both significantly inhibited in a dose-dependent fashion by soluble and hydrogel-released YC-1. Arterial neointima formation following experimental balloon injury was significantly attenuated by perivascular hydrogel-released YC-1. These results suggest that YC-1 is a potent, physiologically active agent with major anti-proliferative and anti-platelet properties that may provide protection against vascular injury through cGMP-dependent mechanisms.