Regulation of ROS production and vascular function by carbon monoxide

Abstract

Carbon monoxide (CO) is a gaseous molecule produced from heme by heme oxygenase (HO). CO interacts with reduced iron of heme-containing proteins, leading to its involvement in various cellular events via its production of mitochondrial reactive oxygen species (ROS). CO-mediated ROS production initiates intracellular signal events, which regulate the expression of adaptive genes implicated in oxidative stress and functions as signaling molecule for promoting vascular functions, including angiogenesis and mitochondrial biogenesis. Therefore, CO generated either by exogenous delivery or by HO activity can be fundamentally involved in regulating mitochondria-mediated redox cascades for adaptive gene expression and improving blood circulation (i.e., O(2) delivery) via neovascularization, leading to the regulation of mitochondrial energy metabolism. This paper will highlight the biological effects of CO on ROS generation and cellular redox changes involved in mitochondrial metabolism and angiogenesis. Moreover, cellular mechanisms by which CO is exploited for disease prevention and therapeutic applications will also be discussed.

Figure 1

The heme degradation pathway and roles of its byproducts in the production of angiogenic modulators. (a) Reaction intermediates in the heme oxygenase-catalyzed oxidation of heme to biliverdin. The substituents on porphyrin are vinyl (V), methyl (M), and propionate (P). The α-, β-, γ-, and δ-meso positions are labeled. The HO reaction consists of three oxidation steps and initiates with the formation of the Fe³⁺ heme-HO complex. Next, Fe³⁺ heme is reduced to the Fe²⁺ state by the electron donated from NADPH, and this step produces CO by the region-specific cleavage of the porphyrin ring of heme at the α-meso carbon atom. The final step is O₂ binding to verdoheme, which produces Fe²⁺ and biliverdin. Biliverdin is converted by biliverdin reductase to bilirubin. (b) Potential proangiogenic effects of heme-degraded products such as CO, biliverdin/bilirubin and Fe²⁺. These products possess potential proangiogenic effects by inducing proangiogenic mediators or by antagonizing antiangiogenic factors.

Figure 2

Effects of CO on targets of ROS generation in endothelial cells. CO binds to cytochrome c oxidase (COX), which is a terminal electron acceptor (complex IV) of the electron transport chain, which changes the redox state of the electron transport chain and produces ROS in mitochondria. CO-dependent mitochondrial superoxide is converted to H₂O₂ by SOD2 (Mn-SOD). Stimulation with TNF-α and LPS induces the recruitment of Nox2, p47^phox, and Rac1 into lipid rafts, thereby promoting Nox activation and ROS production. Superoxide interacts with eNOS-derived NO to produce peroxynitrite, which contributes to endothelial dysfunction. Binding of CO to the heme moiety of Nox and eNOS inhibits production of superoxide and NO, respectively.

Figure 3

CO-induced HIF-1α stabilization via ROS-dependent and -independent manners. ROS produced from mitochondria by CO inhibit PHD activity, resulting in inhibition of pVHL-mediated proteasomal degradation of HIF-1α. CO-mediated ROS production can inhibit PTEN, consequently activating Akt/mTOR-mediated HIF-1α translation. In addition, CO directly promotes the binding of HIF-1α/Hsp90, which stabilizes the HIF-1α protein in a ROS-independent manner.

Figure 4

CO prevents inflammatory responses via inhibition of NF-κB signaling pathway. (a) Inflammatory stimuli such as TNF-α and LPS lead to activation of ECs, which in turn activate inflammatory signaling cascades. The association between TLR4 and MyD88 inhibits IκB kinase activity, leading to NF-κB activation via p65/p50 nuclear translocation in TNF-α-stimulated ECs. CO significantly reduces TNF-α-induced Nox-mediated ROS generation, NF-κB activation and the expression of adhesion molecules such as ICAM-1, VCAM-1, and selectins. (b) Inflammatory stimuli induce (i) the recruitment of monocytes to the endothelium, thereby promoting their transmigration into the arterial intima, (ii) ECs apoptosis and (iii) VSMC proliferation. CO diminishes this inflammatory activation by reducing the expression of adhesion molecules, stimulating EC survival and inhibiting VSMC proliferation.