L-Cysteine promotes the proliferation and differentiation of neural stem cells via the CBS/H₂S pathway

Sulfur as a signaling nutrient through hydrogen sulfide

Hydrogen sulfide (H₂S) has emerged as an important signaling molecule with beneficial effects on various cellular processes affecting, for example, cardiovascular and neurological functions. The physiological importance of H₂S is motivating efforts to develop strategies for modulating its levels. However, advancement in the field of H₂S-based therapeutics is hampered by fundamental gaps in our knowledge of how H₂S is regulated, its mechanism of action, and its molecular targets. This review provides an overview of sulfur metabolism; describes recent progress that has shed light on the mechanism of H₂S as a signaling molecule; and examines nutritional regulation of sulfur metabolism, which pertains to health and disease.

Hydrogen sulfide and neuronal differentiation: focus on Ca2+ channels

Hydrogen sulfide (H2S) is considered the third gasotransmitter following nitric oxide (NO) and carbon monoxide (CO) in the mammalian body including the brain, heart, blood vessels, liver, kidney, pancreas, lung, gastrointestinal tract and reproductive organs. H2S is formed endogenously from L-cysteine by multiple enzymes, such as cystathionine-γ-lyase, cystathionine-β-synthase and 3-mercaptopyruvate sulfurtransferase in combination with cysteine aminotransferase, and participates in a variety of biological events through a number of target molecules. Exogenous and/or endogenous H2S enhances the activity of T-type Ca(2+) channels in NG108-15 cells and isolated dorsal root ganglion neurons that abundantly express Cav3.2, and in Cav3.2-transfected HEK293 cells. Cav3.2 mediates not only the H2S-induced enhancement of pain signals in nociceptor neurons, but also neuronal differentiation characterized by neuritogenesis and functional upregulation of high voltage-activated Ca(2+) channels in NG108-15 cells. In this review, we focus on the functional modulation by H2S of primarily Cav3.2 T-type Ca(2+) channels and the molecular mechanisms underlying the H2S-induced neuronal differentiation.

Regulation of mitochondrial bioenergetic function by hydrogen sulfide. Part II. Pathophysiological and therapeutic aspects

Emerging work demonstrates the dual regulation of mitochondrial function by hydrogen sulfide (H2 S), including, at lower concentrations, a stimulatory effect as an electron donor, and, at higher concentrations, an inhibitory effect on cytochrome C oxidase. In the current article, we overview the pathophysiological and therapeutic aspects of these processes. During cellular hypoxia/acidosis, the inhibitory effect of H2 S on complex IV is enhanced, which may shift the balance of H2 S from protective to deleterious. Several pathophysiological conditions are associated with an overproduction of H2 S (e.g. sepsis), while in other disease states H2 S levels and H2 S bioavailability are reduced and its therapeutic replacement is warranted (e.g. diabetic vascular complications). Moreover, recent studies demonstrate that colorectal cancer cells up-regulate the H2 S-producing enzyme cystathionine β-synthase (CBS), and utilize its product, H2 S, as a metabolic fuel and tumour-cell survival factor; pharmacological CBS inhibition or genetic CBS silencing suppresses cancer cell bioenergetics and suppresses cell proliferation and cell chemotaxis. In the last chapter of the current article, we overview the field of H2 S-induced therapeutic 'suspended animation', a concept in which a temporary pharmacological reduction in cell metabolism is achieved, producing a decreased oxygen demand for the experimental therapy of critical illness and/or organ transplantation.

Hydrogen sulfide in signaling pathways

For a long time hydrogen sulfide (H₂S) was considered a toxic compound, but recently H₂S (at low concentrations) has been found to play an important function in physiological processes. Hydrogen sulfide, like other well-known compounds - nitric oxide (NO) and carbon monoxide (CO) is a gaseous intracellular signal transducer. It regulates the cell cycle, apoptosis and the oxidative stress. Moreover, its functions include neuromodulation, regulation of cardiovascular system and inflammation. In this review, I focus on the metabolism of hydrogen sulfide (including enzymatic pathways of H₂S synthesis from l- and d-cysteine) and its signaling pathways in the cardiovascular system and the nervous system. I also describe how hydrogen sulfide may be used as therapeutic agent, i.e. in the cardiovascular diseases.

H2S and its role in redox signaling

Hydrogen sulfide (H2S) has emerged as an important gaseous signaling molecule that is produced endogenously by enzymes in the sulfur metabolic network. H2S exerts its effects on multiple physiological processes important under both normal and pathological conditions. These functions include neuromodulation, regulation of blood pressure and cardiac function, inflammation, cellular energetics and apoptosis. Despite the recognition of its biological importance and its beneficial effects, the mechanism of H2S action and the regulation of its tissue levels remain unclear in part owing to its chemical and physical properties that render handling and analysis challenging. Furthermore, the multitude of potential H2S effects has made it difficult to dissect its signaling mechanism and to identify specific targets. In this review, we focus on H2S metabolism and provide an overview of the recent literature that sheds some light on its mechanism of action in cellular redox signaling in health and disease. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

Decreased expression of cystathionine β-synthase promotes glioma tumorigenesis

Cystathionine β-synthase (CBS) catalyzes metabolic reactions that convert homocysteine to cystathionine. To assess the role of CBS in human glioma, cells were stably transfected with lentiviral vectors encoding shRNA targeting CBS or a nontargeting control shRNA, and subclones were injected into immunodeficient mice. Interestingly, decreased CBS expression did not affect proliferation in vitro but decreased the latency period before rapid tumor xenograft growth after subcutaneous injection and increased tumor incidence and volume following orthotopic implantation into the caudate-putamen. In soft-agar colony formation assays, CBS knockdown subclones displayed increased anchorage-independent growth. Molecular analysis revealed that CBS knockdown subclones expressed higher basal levels of the transcriptional activator hypoxia-inducible factor 2α (HIF2α/EPAS1). HIF2α knockdown counteracted the effect of CBS knockdown on anchorage-independent growth. Bioinformatic analysis of mRNA expression data from human glioma specimens revealed a significant association between low expression of CBS mRNA and high expression of angiopoietin-like 4 (ANGPTL4) and VEGF transcripts, which are HIF2 target gene products that were also increased in CBS knockdown subclones. These results suggest that decreased CBS expression in glioma increases HIF2α protein levels and HIF2 target gene expression, which promotes glioma tumor formation.

IMPLICATIONS

CBS loss-of-function promotes glioma growth.

Homocysteine induces cytotoxicity and proliferation inhibition in neural stem cells via DNA methylation in vitro

Mild to moderate hyperhomocysteinemia has been implicated in neurodevelopmental disorders and neurodegenerative diseases in human studies. Although the molecular mechanisms underlying the effects of homocysteine (Hcy) neurotoxicity on the nervous system are not yet fully understood, inhibition of neural stem cell (NSC) proliferation and alterations in DNA methylation may be involved. The aim of the present study was to characterize the effects of Hcy on DNA methylation in NSCs, and to explore how Hcy-induced changes in DNA methylation patterns affect NSC proliferation. We found that D,L-Hcy (30-1000 μm) but not L-cysteine inhibited cell proliferation and reduced levels of global DNA methylation in NSCs from neonatal rat hippocampus and increased cell injury. High levels of Hcy also induced an increase in S-adenosylhomocysteine (SAH), a decrease in the ratio of S-adenosylmethionine (SAM) to SAH, and a reduction in protein expression of the DNA methyltransferases DNMT1, DNMT3a and DNMT3b and their enzymatic activity. Moreover, the DNMT inhibitor zebularine reduced the global DNA methylation level and inhibited NSC proliferation. Our results suggest that alterations in DNA methylation may be an important mechanism by which high levels of Hcy inhibit NSC viability in vitro. Hcy-induced DNA hypomethylation may be caused by a reduction in the DNMT activity which is regulated by the cellular concentrations of SAM and SAH, or their protein expression levels. Our results also suggest that Hcy may play a role in the pathogenesis of certain nervous system diseases via a molecular mechanism that involves negative regulation of NSC proliferation and alterations in DNA methylation.

Palmitic acid affects proliferation and differentiation of neural stem cells in vitro

High-lipid diet composed of saturated fatty acids (SFAs) has significant detrimental effects on brain homeostasis, and deleterious effects of SFAs on various cells have been well documented. However, the effects of SFAs on neural stem Cells (NSCs) function have not been fully explored. The aim of this study was to determine whether palmitic acid (PA) affected the proliferation and differentiation of murine-derived NSCs. The results showed that PA dose dependently suppressed viability of NSCs and was cytotoxic at high concentrations. The toxic levels of PA inhibited the proliferation of NSCs as shown by reduced bromodeoxyuridine labeling of NSCs, which is correlated with reactive oxygen species generation. Pretreatment of the cells with the antioxidant N-acetyl-L-cysteine inhibitor significantly attenuated the effects of PA on the proliferation of NSCs. Furthermore, nontoxic levels of PA promoted astrocytogenesis in the differentiated NSCs, associated with Stat3 activation and altered expression of serial of basic helix-loop-helix transcription factor genes. Altogether, our data have demonstrated that PA has a significant impact on proliferation and differentiation of NSCs in vitro and may be useful for elucidating the role of SFAs in regulating NSCs fate in physiological and pathological settings.

A metabolic signature of colon cancer initiating cells

Colon cancer initiating cells (CCICs) are more tumorigenic and metastatic than the majority of colorectal cancer (CRC) cells. CCICs have also been associated with stem cell-like properties. However, there is a lack of system-level understanding of what mechanisms distinguish CCICs from common CRC cells. We compared the transcriptomes of CD133+ CCICs and CD133- CRC cells from multiple sources, which identified a distinct metabolic signature for CD133(high) CCICs. High-resolution unbiased metabolomics was then performed to validate this CCIC metabolic signature. Specifically, levels of enzymes and metabolites involved in glycolysis, the citric acid (TCA) cycle, and cysteine and methionine metabolism are altered in CCICs. Analyses of the alterations further suggest an epigenetic link. This metabolic signature provides mechanistic insights into CCIC phenotypes and may serve as potential biomarkers and therapeutic targets for future CRC treatment.