Carbon monoxide: impact on remethylation/transsulfuration metabolism and its pathophysiologic implications

Abstract

Carbon monoxide (CO) is a gaseous product generated by heme oxygenase (HO), which oxidatively degrades heme. While the stress-inducible HO-1 has well been recognized as an anti-oxidative defense mechanism under stress conditions, recent studies suggest that cancer cells utilize the reaction for their survival. HO-2, the constitutive isozyme, also plays protective roles as a tonic regulator for neurovascular function. Although protective roles of the enzyme reaction and CO have extensively been studied, little information is available on the molecular mechanisms by which the gas exerts its biological actions. Recent studies using metabolomics revealed that CO inhibits cystathionine β-synthase (CBS), which generates H(2)S, another gaseous mediator. The CO-dependent CBS inhibition may impact on the remethylation cycle and related metabolic pathways including the methionine salvage pathway and polyamine synthesis. This review focuses on the gas-responsive regulation of metabolic systems, particularly the remethylation and transsulfuration pathways, and their putative implications for cancer and ischemic diseases.

Fig. 1

Differential metabolomics reveals that CO upregulates metabolites in remethylation cycle and downregulates those in transsulfuration pathway. Metabolomic comparison of sulfur-containing amino acids and their derivatives between the heme-overloaded and vehicle-treated livers of mice. Differences in hepatic contents of the metabolites between the control and hemin-treated mice. H12 treatment with hemin at 12 h prior to sampling the liver. Note decreases in transsulfuration metabolites. In vivo pulse-chase analysis indicating conversion rates of ¹⁵N-methionine into ¹⁵N-homocysteine (Hcy) and ¹⁵N-cystathionine in livers between the groups (dotted square). The amounts of the downstream metabolites were measured at 30 min after the methionine administration. The data in the dotted square were normalized by total amounts of metabolites in remethylation cycle (¹⁵N-methionine + ¹⁵N-SAM + ¹⁵N-SAH + ¹⁵N-Hcy = ΣRM) at 30 min. ND not detected. Data indicate mean ± SE of six to eight separate experiments for each group. *P < 0.05, compared to the vehicle-treated group. Adapted by permission from Wiley: Shintani et al. Hepatology, 49: 141–150, 2009 [29]

Fig. 2

Possible metabolic pathways modulated by a CO-sensitive CBS inhibition. Not only does CBS inhibition by CO alter remethylation cycle (blue arrows) and transsulfuration pathway (pink arrows) but it may also modulate methionine salvage pathway and polyamine metabolism. Dotted arrows and a line indicate activation and inhibition of corresponding enzymes by metabolites, respectively. ALAS aminolevulinic acid synthase, FH fumarate hydratase, CSE cystathionine γ-lyase, MAT methionine adenosyl transferase, αKG α-ketoglutarate, ALA aminolevulinic acid, PBG porphobilinogen, Hcy homocysteine, SAM S-adenosylmethionine, SAH S-adenosylhomocysteine

Fig. 3

Glutathione and UDP-HexNAc as marker metabolites enriched in colon cancer metastasis. a Light-microscopic photograph of intrasplenically injected HCT116 colon cancer cell xenografts in the liver of NOG mice. Scale bar: 500 μm. b A green fluorescence image of the same specimen shown in (a). c–f Representative imaging mass spectrometry showing spatial distribution of apparent UDP-HexNAc concentration (UDP-HexNAc_app), the reduced type of glutathione (GSH_app), oxidized glutathione (GSSG_app), and (GSH_app)/(GSSG_app) ratio in the same microscopic field plotted as a heat map, respectively. Adapted by permission from Springer: Kubo et al. Anal Bioanal Chem, 400: 1895–1904, 2011 [43]

Fig. 4

Immunohistochemical localization of HO-2 and CBS in the neurovascular unit of neonatal mouse cerebellar cortex. Neurons and endothelial cells express HO-2 (a, b), the constitutive CO-producing enzyme, whereas glial cells express CBS (e–h), an H₂S producing enzyme. Note that HO-2-positive cells along vessel wall are endothelial in (b), not pericytic, since nuclei of NG2 (pericytic marker) positive cells, stained with TO-PRO-3 (a nucleic acid stain), are completely devoid of CD31 (endothelial marker) labeling in (c). The arteriolar wall is surrounded by NG2-positive pericytes in (d), key contractile cells within the neurovascular unit. i Schematic depiction of the localization of HO-2 and CBS in the neurovascular unit. GFAP glial fibrillary acidic protein, an established marker of glial cells; ml molecular layer; Pl Purkinje cell layer; gl granular layer; e endothelium; p pericyte. Adapted by permission from National Academy of Sciences, USA: Morikawa et al. PNAS, 109: 1293–1298, 2012 [68]

Fig. 5

Impaired ability of HO-2-null mice to maintain ATP levels on exposure to 10% O₂ for 1 min. a Alterations in AMP (AMP_whole), ADP (ADP_whole), ATP (ATP_whole), and energy charge (EC_whole) in the whole brain. *P < 0.05, compared to WT normoxia. †P < 0.05, compared to HO-2 null normoxia. b Representative imaging mass spectrometry showing spatial distribution of apparent ATP concentration (ATP_app) and energy charge (EC_reg). Note the basal increase in ATP in HO-2-null mice. Bottom panels—H&E staining after imaging mass spectrometry. cx cortex, hp hippocampus. c Quantitative analysis of regional ATP concentration and energy charge in WT and HO-2-null mice. *P < 0.05, compared to WT normoxia. †P < 0.05, compared to HO-2-null normoxia. Adapted by permission from National Academy of Sciences, USA: Morikawa et al. PNAS, 109: 1293–1298, 2012 [68]