Cystathionine-beta-synthase inhibition for colon cancer: Enhancement of the efficacy of aminooxyacetic acid via the prodrug approach

The role of cystathionine β-synthase in cancer

Cystathionine β-synthase (CBS) occupies a key position as the initiating and rate-limiting enzyme in the sulfur transfer pathway and plays a vital role in health and disease. CBS is responsible for regulating the metabolism of cysteine, the precursor of glutathione (GSH), an important antioxidant in the body. Additionally, CBS is one of the three enzymes that produce hydrogen sulfide (H2S) in mammals through a variety of mechanisms. The dysregulation of CBS expression in cancer cells affects H2S production through direct or indirect pathways, thereby influencing cancer growth and metastasis by inducing angiogenesis, facilitating proliferation, migration, and invasion, modulating cellular energy metabolism, promoting cell cycle progression, and inhibiting apoptosis. It is noteworthy that CBS expression exhibits complex changes in different cancer models. In this paper, we focus on the CBS synthesis and metabolism, tissue distribution, potential mechanisms influencing tumor growth, and relevant signaling pathways. We also discuss the impact of pharmacological CBS inhibitors and silencing CBS in preclinical cancer models, supporting their potential as targeted cancer therapies.

Neurobehavioral dysfunction in a mouse model of Down syndrome: upregulation of cystathionine β-synthase, H2S overproduction, altered protein persulfidation, synaptic dysfunction, endoplasmic reticulum stress, and autophagy

Down syndrome (DS) is a genetic condition where the person is born with an extra chromosome 21. DS is associated with accelerated aging; people with DS are prone to age-related neurological conditions including an early-onset Alzheimer's disease. Using the Dp(17)3Yey/ + mice, which overexpresses a portion of mouse chromosome 17, which encodes for the transsulfuration enzyme cystathionine β-synthase (CBS), we investigated the functional role of the CBS/hydrogen sulfide (H2S) pathway in the pathogenesis of neurobehavioral dysfunction in DS. The data demonstrate that CBS is higher in the brain of the DS mice than in the brain of wild-type mice, with primary localization in astrocytes. DS mice exhibited impaired recognition memory and spatial learning, loss of synaptosomal function, endoplasmic reticulum stress, and autophagy. Treatment of mice with aminooxyacetate, a prototypical CBS inhibitor, improved neurobehavioral function, reduced the degree of reactive gliosis in the DS brain, increased the ability of the synaptosomes to generate ATP, and reduced endoplasmic reticulum stress. H2S levels in the brain of DS mice were higher than in wild-type mice, but, unexpectedly, protein persulfidation was decreased. Many of the above alterations were more pronounced in the female DS mice. There was a significant dysregulation of metabolism in the brain of DS mice, which affected amino acid, carbohydrate, lipid, endocannabinoid, and nucleotide metabolites; some of these alterations were reversed by treatment of the mice with the CBS inhibitor. Thus, the CBS/H2S pathway contributes to the pathogenesis of neurological dysfunction in DS in the current animal model.

Role of hydrogen sulfide in health and disease

In the past, hydrogen sulfide (H2S) was recognized as a toxic and dangerous gas; in recent years, with increased research, we have discovered that H2S can act as an endogenous regulatory transmitter. In mammals, H2S-catalyzing enzymes, such as cystathionine-β-synthase, cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase, are differentially expressed in a variety of tissues and affect a variety of biological functions, such as transcriptional and posttranslational modification of genes, activation of signaling pathways in the cell, and metabolic processes in tissues, by producing H2S. Various preclinical studies have shown that H2S affects physiological and pathological processes in the body. However, a detailed systematic summary of these roles in health and disease is lacking. Therefore, this review provides a thorough overview of the physiological roles of H2S in different systems and the diseases associated with disorders of H2S metabolism, such as ischemia-reperfusion injury, hypertension, neurodegenerative diseases, inflammatory bowel disease, and cancer. Meanwhile, this paper also introduces H2S donors and novel release modes, as well as the latest preclinical experimental results, aiming to provide researchers with new ideas to discover new diagnostic targets and therapeutic options.

Cell membrane-targeted surface enhanced Raman scattering nanoprobes for the monitoring of hydrogen sulfide secreted from living cells

Hydrogen sulfide (H2S), an important gas signal molecule, participates in intercellular signal transmission and plays a considerable role in physiology and pathology. However, in-situ monitoring of H2S level during the processes of material transport between cells remains considerably challenging. Herein, a cell membrane-targeted surface-enhanced Raman scattering (SERS) nanoprobe was designed to quantitatively detect H2S secreted from living cells. The nanoprobes were fabricated by assembling cholesterol-functionalized DNA strands and dithiobis(phenylazide) (DTBPA) molecules on core-shell gold nanostars embedded with 4-mercaptoacetonitrile (4-MBN) (AuNPs@4-MBN@Au). Thus, three functions including cell-membrane targeted via cholesterol, internal standard calibration, and responsiveness to H2S through reduction of azide group in DTBPA molecules were integrated into the nanoprobes. In addition, the nanoprobes can quickly respond to H2S within 90 s and sensitively, selectively, and reliably detect H2S with a limit of detection as low as 37 nM due to internal standard-assisted calibration and reaction specificity. Moreover, the nanoprobes can effectively target on cell membrane and realize SERS visualization of dynamic H2S released from HeLa cells. By employing the proposed approach, an intriguing phenomenon was observed: the other two major endogenous gas transmitters, carbon monoxide (CO) and nitric oxide (NO), exhibited opposite effect on H2S production in living cells stimulated by related gas release molecules. In particular, the introduction of CO inhibited the generation of H2S in HeLa cells, while NO promoted its output. Thus, the nanoprobes can provide a robust method for investigating H2S-related extracellular metabolism and intercellular signaling transmission.

Biphasic inflammatory response induced by intra-plantar injection of L-cysteine: Role of CBS-derived H2S and S1P/NO signaling

This study investigates the inflammatory response to intra-plantar injection of L-cysteine in a murine model. L-cysteine induces a two-phase response: an early phase lasting 6 h and a late phase peaking at 24 h and declining by 192 h. The early phase shows increased neutrophil accumulation at 2 h up to 24 h, followed by a reduction at 48 h. On the other hand, the late phase exhibits increased macrophage infiltration peaking at 96 h. Inhibition of cystathionine β-synthase (CBS), the first enzyme in the transsulfuration pathway, significantly reduces L-cysteine-induced edema, suggesting its dependence on CBS-derived hydrogen sulfide (H2S). Sequential formation of sphingosine-1-phosphate (S1P) preceding nitric oxide (NO) generation suggests the involvement of a CBS/S1P/NO axis in the inflammatory response. Inhibition of de novo sphingolipid biosynthesis, S1P1 receptor, and endothelial NO synthase (eNOS) attenuates L-cysteine-induced paw edema. These findings indicate a critical role of the CBS/H2S/S1P/NO signaling pathway in the development and maintenance of L-cysteine-induced inflammation. The co-presence of H2S and NO is necessary for inducing and sustaining the inflammatory response, as NaHS or L-arginine alone do not replicate the marked and prolonged inflammatory effect observed with L-cysteine. This study enhances our understanding of the complex molecular mechanisms of the interplay between NO and H2S pathways in inflammation and identifies potential therapeutic targets for inflammatory disorders.

A turn-on fluorescent probe for selective detection of H2S in environmental samples and bio-imaging in human breast cancer cells

A triphenylamine-benzothaizole-based turn-on fluorescent probe TPB-NO2 was designed and synthesized for tracking H2S in both environmental and biological samples depending upon the sensing strategy of thiolysis of 2,4-dinitrophenyl (DNP) ether. Due to PET (photoinduced electron transfer), occurring from donor triphenylamine moiety to acceptor DNP moiety, the probe TPB-NO2 itself is very weakly fluorescent and colorless in DMSO/H2O solution (1 : 1, v/v; 10 mM HEPES buffer, pH 7.4). But the addition of H2S leads to thiolysis of 2,4-dinitrophenyl ether to block the initial PET process and hence it exhibits naked eye detectable turn-on response with bright cyan fluorescence and intense brown color. Not only that, the probe exhibits excellent selectivity over other bio-thiols like Cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), fast response time (<2 min), and high sensitivity with a detection limit of 9.81 nM. Moreover, to explore the practical applicability of our probe we employed it to monitor H2S successfully in environmental water samples, solid-state TLC strip study, Quantitative determination of H2S in eggs, and in the bioimaging of human breast cancer cells (MDA-MB 231).

Recent advances in the role of endogenous hydrogen sulphide in cancer cells

Hydrogen sulphide (H2 S) is a gaseous neurotransmitter that can be self-synthesized by living organisms. With the deepening of research, the pathophysiological mechanisms of endogenous H2 S in cancer have been increasingly elucidated: (1) promote angiogenesis, (2) stimulate cell bioenergetics, (3) promote migration and proliferation thereby invasion, (4) inhibit apoptosis and (5) activate abnormal cell cycle. However, the increasing H2 S levels via exogenous sources show the opposite trend. This phenomenon can be explained by the bell-shaped pharmacological model of H2 S, that is, the production of endogenous (low concentration) H2 S promotes tumour growth while the exogenous (high concentration) H2 S inhibits tumour growth. Here, we review the impact of endogenous H2 S synthesis and metabolism on tumour progression, summarize the mechanism of action of H2 S in tumour growth, and discuss the possibility of H2 S as a potential target for tumour treatment.

Cystathionine β-synthase affects organization of cytoskeleton and modulates carcinogenesis in colorectal carcinoma cells

Background

Cystathionine β-synthase (CBS), one of three enzymes that endogenously produce hydrogen sulfide, is extensively studied for its relevance in the cells of various tumors. In our previous work, we observed that the immunofluorescence pattern of CBS is very similar to that of tubulin and actin. Therefore, we focused on the potential interaction of CBS with cytoskeletal proteins β-actin and β-tubulin and the functional relevance of the potential interaction of these proteins in colorectal carcinoma cell lines.

Methods

To study the potential interaction of CBS with cytoskeletal proteins and its functional consequences, a CBS-knockout DLD1 (DLDx) cell line was established by using the CRISPR/Cas9 gene editing method. The interaction of the selected cytoskeletal protein with CBS was studied by immunoprecipitation, Western blot analysis, immunofluorescence, and proximity ligation assay. The functional consequences were studied by proliferation and migration assays and by generation of xenografts in SCID/bg mice.

Results

We have found that CBS, an enzyme that endogenously produces H2S, binds to cytoskeletal β-tubulin and, to a lesser extent, also to β-actin in colorectal carcinoma-derived cells. When CBS was knocked out by the CRISPR/Cas9 technique (DLDx), we observed a de-arranged cytoskeleton compared to the unmodified DLD1 cell line. Treatment of these cells with a slow sulfide donor GYY4137 resulted in normal organization of the cytoskeleton, thus pointing to the role of CBS in microtubule dynamics. To evaluate the physiological importance of this observation, both DLD1 and DLDx cells were injected into SCID/bg mice, and the size and mass of the developed xenografts were evaluated. Significantly larger tumors developed from DLDx compared to the DLD1 cells, which correlated with the increased proliferation of these cells.

Conclusions

Taken together, in colorectal cancer DLD1 cells, CBS binds to the cytoskeleton, modulates microtubule dynamics, and thus affects the proliferation and migration in the colorectal carcinoma stable cell line.

Role of Cystathionine β-Synthase and 3-Mercaptopyruvate Sulfurtransferase in the Regulation of Proliferation, Migration, and Bioenergetics of Murine Breast Cancer Cells

Cystathionine β-synthase (CBS), CSE (cystathionine γ-lyase) and 3-mercaptopyruvate sulfurtransferase (3-MST) have emerged as three significant sources of hydrogen sulfide (H2S) in various forms of mammalian cancer. Here, we investigated the functional role of CBS’ and 3-MST’s catalytic activity in the murine breast cancer cell line EO771. The CBS/CSE inhibitor aminooxyacetic acid (AOAA) and the 3-MST inhibitor 2-[(4-hydroxy-6-methylpyrimidin-2-yl)sulfanyl]-1-(naphthalen-1-yl)ethan-1-one (HMPSNE) were used to assess the role of endogenous H2S in the modulation of breast cancer cell proliferation, migration, bioenergetics and viability in vitro. Methods included measurements of cell viability (MTT and LDH assays), cell proliferation and in vitro wound healing (IncuCyte) and cellular bioenergetics (Seahorse extracellular flux analysis). CBS and 3-MST, as well as expression were detected by Western blotting; H2S production was measured by the fluorescent dye AzMC. The results show that EO771 cells express CBS, CSE and 3-MST protein, as well as several enzymes involved in H2S degradation (SQR, TST, and ETHE1). Pharmacological inhibition of CBS or 3-MST inhibited H2S production, suppressed cellular bioenergetics and attenuated cell proliferation. Cell migration was only inhibited by the 3-MST inhibitor, but not the CBS/CSE inhibitor. Inhibition of CBS/CSE of 3-MST did not significantly affect basal cell viability; inhibition of 3-MST (but not of CBS/CSE) slightly enhanced the cytotoxic effects of oxidative stress (hydrogen peroxide challenge). From these findings, we conclude that endogenous H2S, generated by 3-MST and to a lower degree by CBS/CSE, significantly contributes to the maintenance of bioenergetics, proliferation and migration in murine breast cancer cells and may also exert a minor role as a cytoprotectant.

Implications of hydrogen sulfide in colorectal cancer: Mechanistic insights and diagnostic and therapeutic strategies

Hydrogen sulfide (H₂S) is an important signaling molecule in colorectal cancer (CRC). It is produced in the colon by the catalytic synthesis of the colonocytes' enzymatic systems and the release of intestinal microbes, and is oxidatively metabolized in the colonocytes' mitochondria. Both endogenous H₂S in colonic epithelial cells and exogenous H₂S in intestinal lumen contribute to the onset and progression of CRC. The up-regulation of endogenous synthetases is thought to be the cause of the elevated H₂S levels in CRC cells. Different diagnostic probes and combination therapies, as well as tumor treatment approaches through H₂S modulation, have been developed in recent years and have become active area of investigation for the diagnosis and treatment of CRC. In this review, we focus on the specific mechanisms of H₂S production and oxidative metabolism as well as the function of H₂S in the occurrence, progression, diagnosis, and treatment of CRC. We also discuss the present challenges and provide insights into the future research of this burgeoning field.

Regulation of CyR61 expression and release by 3-mercaptopyruvate sulfurtransferase in colon cancer cells

Cysteine-rich angiogenic inducer 61 (CYR61, also termed CCN family member 1 or CCN1), is a matricellular protein encoded by the CYR61 gene. This protein has been implicated in the regulation of various cancer-associated processes including tumor growth, angiogenesis, tumor cell adhesion, migration, and invasion as well as the regulation of anticancer drug resistance. Hydrogen sulfide (H₂S) is a gaseous endogenous biological mediator, involved in the regulation of cellular bioenergetics, angiogenesis, invasion, and chemotherapeutic resistance in several types of cancer. H₂S is produced by three enzymes: cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current studies were set up to investigate if CBS or 3-MST regulates CyR61 in colon cancer cells in the context of the regulation of proliferation, migration, and survival. The study mainly utilized HCT116 cells, in which two of the principal H₂S-producing enzymes, CBS and 3-MST, are highly expressed. The H₂S donor GYY4137 and the polysulfide donor Na₂S₃ activated the CyR61 promoter in a concentration-dependent fashion. Aminooxyacetic acid (AOAA), a pharmacological inhibitor of CBS as well as HMPSNE: 2-[(4-hydroxy-6- methylpyrimidin-2-yl)sulfanyl]-1-(naphthalen-1-yl)ethan-1-one, a pharmacological inhibitor of 3-MST inhibited CyR61 mRNA expression. This effect was more pronounced in response to HMPSNE than to AOAA and occurred through the modulation of S1PR via ATF1 and CREB. CyR61 was found to play an active, but relatively minor role in maintaining colon cell proliferation. HMPSNE markedly suppressed the secretion/release of CyR61 from the colon cancer cells. Moreover, HMPSNE promoted colon cancer cell apoptosis; endogenously produced CyR61 was found to counteract this effect, at least in part via RhoA activation. Taken together, we conclude that the upregulation of 3-MST in cancer cells exerts cytoprotective effects and confers the cancer cells a more aggressive phenotype - at least in part via the modulation of CyR61 expression and release.

Reactive sulfur species and their significance in health and disease

Reactive sulfur species (RSS) have been recognized in the last two decades as very important molecules in redox regulation. They are involved in metabolic processes and, in this way, they are responsible for maintenance of health. This review summarizes current information about the essential biological RSS, including H₂S, low molecular weight persulfides, protein persulfides as well as organic and inorganic polysulfides, their synthesis, catabolism and chemical reactivity. Moreover, the role of RSS disturbances in various pathologies including vascular diseases, chronic kidney diseases, diabetes mellitus Type 2, neurological diseases, obesity, chronic obstructive pulmonary disease and in the most current problem of COVID-19 is presented. The significance of RSS in aging is also mentioned. Finally, the possibilities of using the precursors of various forms of RSS for therapeutic purposes are discussed.

Epigallocatechin gallate is a potent inhibitor of cystathionine beta-synthase: Structure-activity relationship and mechanism of action

Epigallocatechin gallate (EGCG) is the main bioactive component of green tea. Through screening of a small library of natural compounds, we discovered that EGCG inhibits cystathionine β-synthase (CBS), a major H₂S-generating enzyme. Here we characterize EGCG's mechanism of action in the context of CBS-derived H₂S production. In the current project, biochemical, pharmacological and cell biology approaches were used to characterize the effect of EGCG on CBS in cellular models of cancer and Down syndrome (DS). The results show that EGCG binds to CBS and inhibits H₂S-producing CBS activity almost 30-times more efficiently than the canonical cystathionine formation (IC₅₀ 0.12 versus 3.3 μM). Through screening structural analogs and building blocks, we identified that gallate moiety of EGCG represents the pharmacophore responsible for CBS inhibition. EGCG is a mixed-mode, CBS-specific inhibitor with no effect on the other two major enzymatic sources of H₂S, CSE and 3-MST. Unlike the prototypical CBS inhibitor aminooxyacetate, EGCG does not bind the catalytic cofactor of CBS pyridoxal-5'-phosphate. Molecular modeling suggests that EGCG blocks a substrate access channel to pyridoxal-5'-phosphate. EGCG inhibits cellular H₂S production in HCT-116 colon cancer cells and in DS fibroblasts. It also exerts effects that are consistent with the functional role of CBS in these cells: in HCT-116 cells it decreases, while in DS cells it improves viability and proliferation. In conclusion, EGCG is a potent inhibitor of CBS-derived H₂S production. This effect may contribute to its pharmacological effects in various pathophysiological conditions.

H2S biogenesis by cystathionine beta-synthase: mechanism of inhibition by aminooxyacetic acid and unexpected role of serine

Cystathionine beta-synthase (CBS) is a pivotal enzyme of the transsulfuration pathway responsible for diverting homocysteine to the biosynthesis of cysteine and production of hydrogen sulfide (H₂S). Aberrant upregulation of CBS and overproduction of H₂S contribute to pathophysiology of several diseases including cancer and Down syndrome. Therefore, pharmacological CBS inhibition has emerged as a prospective therapeutic approach. Here, we characterized binding and inhibitory mechanism of aminooxyacetic acid (AOAA), the most commonly used CBS inhibitor. We found that AOAA binds CBS tighter than its respective substrates and forms a dead-end PLP-bound intermediate featuring an oxime bond. Surprisingly, serine, but not cysteine, replaced AOAA from CBS and formed an aminoacrylate reaction intermediate, which allowed for the continuation of the catalytic cycle. Indeed, serine rescued and essentially normalized the enzymatic activity of AOAA-inhibited CBS. Cellular studies confirmed that AOAA decreased H₂S production and bioenergetics, while additional serine rescued CBS activity, H₂S production and mitochondrial function. The crystal structure of AOAA-bound human CBS showed a lack of hydrogen bonding with residues G305 and Y308, found in the serine-bound model. Thus, AOAA-inhibited CBS could be reactivated by serine. This difference may be important in a cellular environment in multiple pathophysiological conditions and may modulate the CBS-inhibitory activity of AOAA. In addition, our results demonstrate additional complexities of using AOAA as a CBS-specific inhibitor of H₂S biogenesis and point to the urgent need to develop a potent, selective and specific pharmacological CBS inhibitor.

Emerging roles of cystathionine β-synthase in various forms of cancer

The expression of the reverse transsulfuration enzyme cystathionine-β-synthase (CBS) is markedly increased in many forms of cancer, including colorectal, ovarian, lung, breast and kidney, while in other cancers (liver cancer and glioma) it becomes downregulated. According to the clinical database data in high-CBS-expressor cancers (e.g. colon or ovarian cancer), high CBS expression typically predicts lower survival, while in the low-CBS-expressor cancers (e.g. liver cancer), low CBS expression is associated with lower survival. In the high-CBS expressing tumor cells, CBS, and its product hydrogen sulfide (H₂S) serves as a bioenergetic, proliferative, cytoprotective and stemness factor; it also supports angiogenesis and epithelial-to-mesenchymal transition in the cancer microenvironment. The current article reviews the various tumor-cell-supporting roles of the CBS/H₂S axis in high-CBS expressor cancers and overviews the anticancer effects of CBS silencing and pharmacological CBS inhibition in various cancer models in vitro and in vivo; it also outlines potential approaches for biomarker identification, to support future targeted cancer therapies based on pharmacological CBS inhibition.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

The CBS-H2S axis promotes liver metastasis of colon cancer by upregulating VEGF through AP-1 activation

BACKGROUND

The main therapy for colon cancer with liver metastasis is chemotherapy based on 5-fluorouracil combined with targeted drugs. However, acquired drug resistance and severe adverse reactions limit patients' benefit from standard chemotherapy. Here, we investigate the involvement of endogenous hydrogen sulfide (H₂S) in liver metastasis of colon cancer and its potential value as a novel therapeutic target.

METHODS

We used the CRISPR/Cas9 system to knockdown CBS gene expression in colon cancer cell lines. PCR arrays and proteome arrays were applied to detect the transcription and protein expression levels, respectively, of angiogenesis-related genes after knockdown. The molecular mechanism was investigated by western blot analysis, RT-qPCR, immunofluorescence staining, ChIP assays and dual-luciferase reporter assays. A liver metastasis mouse model was adopted to investigate the effect of targeting CBS on tumour metastasis in vivo.

RESULTS

Knockdown of CBS decreased the metastasis and invasion of colon cancer cells and inhibited angiogenesis both in vivo and in vitro. Tissue microarray analysis showed a positive correlation between CBS and VEGF expression in colon cancer tissues. Further analysis at the molecular level validated a positive feedback loop between the CBS-H₂S axis and VEGF.

CONCLUSIONS

Endogenous H₂S promotes angiogenesis and metastasis in colon cancer, and targeting the positive feedback loop between the CBS-H₂S axis and VEGF can effectively intervene in liver metastasis of colon cancer.

Serum metabolomic analysis of men on a low-carbohydrate diet for biochemically recurrent prostate cancer reveals the potential role of ketogenesis to slow tumor growth: a secondary analysis of the CAPS2 diet trial

BACKGROUND

Systemic treatments for prostate cancer (PC) have significant side effects. Thus, newer alternatives with fewer side effects are urgently needed. Animal and human studies suggest the therapeutic potential of low carbohydrate diet (LCD) for PC. To test this possibility, Carbohydrate and Prostate Study 2 (CAPS2) trial was conducted in PC patients with biochemical recurrence (BCR) after local treatment to determine the effect of a 6-month LCD intervention vs. usual care control on PC growth as measured by PSA doubling time (PSADT). We previously reported the LCD intervention led to significant weight loss, higher HDL, and lower triglycerides and HbA1c with a suggested longer PSADT. However, the metabolic basis of these effects are unknown.

METHODS

To identify the potential metabolic basis of effects of LCD on PSADT, serum metabolomic analysis was performed using baseline, month 3, and month 6 banked sera to identify the metabolites significantly altered by LCD and that correlated with varying PSADT.

RESULTS

LCD increased the serum levels of ketone bodies, glycine and hydroxyisocaproic acid. Reciprocally, LCD reduced the serum levels of alanine, cytidine, asymmetric dimethylarginine (ADMA) and 2-oxobutanoate. As high ADMA level is shown to inhibit nitric oxide (NO) signaling and contribute to various cardiovascular diseases, the ADMA repression under LCD may contribute to the LCD-associated health benefit. Regression analysis of the PSADT revealed a correlation between longer PSADT with higher level of 2-hydroxybutyric acids, ketone bodies, citrate and malate. Longer PSADT was also associated with LCD reduced nicotinamide, fructose-1, 6-biphosphate (FBP) and 2-oxobutanoate.

CONCLUSION

These results suggest a potential association of ketogenesis and TCA metabolites with slower PC growth and conversely glycolysis with faster PC growth. The link of high ketone bodies with longer PSADT supports future studies of ketogenic diets to slow PC growth.

Near-Infrared Fluorescent Probe with Large Stokes Shift for Imaging of Hydrogen Sulfide in Tumor-Bearing Mice

Hydrogen sulfide (H₂S) is an important endogenous gas signal molecule in living system, which participates in a variety of physiological processes. Very recent evidence has accumulated to show that endogenous H₂S is closely associated with various cancers and can be regarded as a biomarker of cancer. Herein, we have constructed a new near-infrared fluorescent probe (DCP-H₂S) based on isophorone-xanthene dye for sensing hydrogen sulfide (H₂S). The probe shows remarkable NIR turn-on signal at 770 nm with a large Stokes shift of 200 nm, together with high sensitivity (15-fold) and rapid detection ability for H₂S (4 min). The probe also possesses excellent selectivity for H₂S over various other analytes including biothiols containing sulfhydryl (-SH). Moreover, DCP-H₂S has been successfully applied to visualize endogenous and exogenous H₂S in living cells (293T, Caco-2 and CT-26 cells). In particular, the excellent ability of DCP-H₂S to distinguish normal mice and tumor mice is shown, and it is expected to be a powerful tool for detection of H₂S in cancer diagnosis.

Methods for Suppressing Hydrogen Sulfide in Biological Systems

Significance: Hydrogen sulfide (H₂S) plays critical roles in redox biology, and its regulatory effects are tightly controlled by its cellular location and concentration. The imbalance of H₂S is believed to contribute to some pathological processes. Recent Advances: Downregulation of H₂S requires chemical tools such as inhibitors of H₂S-producing enzymes and H₂S scavengers. Recent efforts have discovered some promising inhibitors and scavengers. These advances pave the road toward better understanding of the functions of H₂S. Critical Issues: Precise H₂S downregulation is challenging. The potency and specificity of current inhibitors are still far from ideal. H₂S-producing enzymes are involved in complex sulfur metabolic pathways and ubiquitously present in biological matrices. The inhibition of these enzymes can cause unwanted side effects. H₂S scavengers allow targeted H₂S clearance, but their options are still limited. In addition, the scavenging process often results in biologically active by-products. Future Directions: Further development of potent and specific inhibitors for H₂S-producing enzymes is needed. Scavengers that can rapidly and selectively remove H₂S while generating biocompatible by-products are needed. Potential therapeutic applications of scavengers and inhibitors are worth exploring. Antioxid. Redox Signal. 36, 294-308.

Cardioprotective effects of hydrogen sulfide in attenuating myocardial ischemia‑reperfusion injury (Review)

Ischemic heart disease is one of the major causes of cardiovascular‑related mortality worldwide. Myocardial ischemia can be attenuated by reperfusion that restores the blood supply. However, injuries occur during blood flow restoration that induce cardiac dysfunction, which is known as myocardial ischemia‑reperfusion injury (MIRI). Hydrogen sulfide (H₂S), the third discovered endogenous gasotransmitter in mammals (after NO and CO), participates in various pathophysiological processes. Previous in vitro and in vivo research have revealed the protective role of H₂S in the cardiovascular system that render it useful in the protection of the myocardium against MIRI. The cardioprotective effects of H₂S in attenuating MIRI are summarized in the present review.

Hydrogen Sulfide Actions in the Vasculature

Hydrogen sulfide (H2 S) is a small, gaseous molecule with poor solubility in water that is generated by multiple pathways in many species including humans. It acts as a signaling molecule in many tissues with both beneficial and pathological effects. This article discusses its many actions in the vascular system and the growing evidence of its role to regulate vascular tone, angiogenesis, endothelial barrier function, redox, and inflammation. Alterations in some disease states are also discussed including potential roles in promoting tumor growth and contributions to the development of metabolic disease. © 2021 American Physiological Society. Compr Physiol 11:1-22, 2021.

Cystathionine β-synthase mediated PRRX2/IL-6/STAT3 inactivation suppresses Tregs infiltration and induces apoptosis to inhibit HCC carcinogenesis

Background

Hepatocellular carcinoma (HCC) is characterized by inflammation and immunopathogenesis. Accumulating evidence has shown that the cystathionine β-synthase/hydrogen sulfide (CBS/H₂S) axis is involved in the regulation of inflammation. However, roles of CBS in HCC development and immune evasion have not been systematically investigated, and their underlying mechanisms remain elusive. Here, we investigated the roles of CBS in tumor cells and tumor microenvironment of HCC.

Methods

236 HCC samples were collected to detect the expression of CBS, cleaved Caspase-3 and paired related homeobox 2 (PRRX2) and the number of immune cells. HCC cell lines were employed to examine the effects of CBS on cellular viability, apoptosis and signaling in vitro. Cbs heterozygous knockout mice, C57BL/6 mice, nude mice and non-obese diabetic severe combined immunodeficiency mice were used to investigate the in vivo functions of CBS.

Results

Downregulation of CBS was observed in HCC, and low expression of CBS predicted poor prognosis in HCC patients. CBS overexpression dramatically promoted cellular apoptosis in vitro and inhibited tumor growth in vivo. Activation of the Cbs/H₂S axis also reduced the abundance of tumor-infiltrating Tregs, while Cbs deficiency promoted Tregs-mediated immune evasion and boosted tumor growth in Cbs heterozygous knockout mice. Mechanistically, CBS facilitated the expression cleaved Caspase-3 in tumor cells, and on the other hand, suppressed Foxp3 expression in Tregs via inactivating IL-6/STAT3 pathway. As a transcription factor of IL-6, PRRX2 was reduced by CBS. Additionally, miR-24-3p was proven to be an upstream suppressor of CBS in HCC.

Conclusions

Our results indicate the antitumor function of CBS in HCC by inactivation of the PRRX2/IL-6/STAT3 pathway, which may serve as a potential target for HCC clinical immunotherapy.

A covalent organic framework-based nanoagent for H2S-activable phototherapy against colon cancer

Herin, we report a Cu(ii)-porphyrin-derived nanoscale COF, which can be triggered by endogenous H₂S via an intracellular sulfidation reaction to generate a metal-free COF-photosensitizer for PDT against H₂S-enriched colon tumors with controllable singlet oxygen release; meanwhile in situ generated CuS can be synchronously used as a photothermal agent for PTT.

Efficacy of Novel Aminooxyacetic Acid Prodrugs in Colon Cancer Models: Towards Clinical Translation of the Cystathionine β-Synthase Inhibition Concept

Upregulation of hydrogen sulfide (H₂S) biosynthesis, at least in part related to the upregulation of cystathionine β-synthetase (CBS) in cancer cells, serves as a tumor-promoting factor and has emerged as a possible molecular target for antitumor drug development. To facilitate future clinical translation, we have synthesized a variety of novel CBS-targeting, esterase-cleavable prodrugs based on the structure of the prototypical CBS inhibitor aminooxyacetic acid (AOAA). The pharmacological properties of these compounds were evaluated in cell-free assays with recombinant human CBS protein, the human colon cancer cell line HCT116, and in vivo using various tumor-bearing mice models. The prodrug YD0251 (the isopropyl ester derivative of AOAA) was selected for detailed characterization. YD0251 exhibits improved antiproliferative efficacy in cell culture models when compared to AOAA. It is up to 18 times more potent than AOAA at suppressing HCT116 tumor growth in vivo and is effective when administered to tumor-bearing mice either via subcutaneous injection or oral gavage. Patient-derived xenografts (PDTXs) with higher levels of CBS protein grew significantly larger than tumors with lower levels, and YD0251 treatment inhibited the growth of PDTXs with elevated CBS, whereas it had no significant effect on PDTXs with low CBS protein levels. The toxicity of YD0251 was assessed in mice subjected to subchronic administration of supratherapeutic doses the inhibitor; no significant alteration in circulating markers of organ injury or histopathological alterations were noted, up to 60 mg/kg/day × 5 days. In preparation to a future theranostic concept (to match CBS inhibitor therapy to high-CBS expressors), we identified a potential plasma marker of CBS-expressing tumors. Colon cancer cells produced significant levels of lanthionine, a rare metabolic intermediate of CBS-mediated H₂S biosynthesis; forced expression of CBS into non-transformed epithelial cells increased lanthionine biogenesis in vitro and in vivo (measured in the urine of tumor-bearing mice). These current results may be useful to facilitate the translation of a CBS inhibition-based antitumor concept into the clinical space.

Therapeutic potential of endogenous hydrogen sulfide inhibition in breast cancer (Review)

Hydrogen sulfide (H₂S), the third gas signal molecule, is associated with the modulation of various physiological and pathological processes. Recent studies have reevealed that endogenous H₂S may promote proliferation, induce angiogenesis and inhibit apoptosis, thereby stimulating oncogenesis. Conversely, decreased endogenous H₂S release suppresses growth of various tumors including breast cancer. This observation suggests an alternative tumor therapy strategy by inhibiting H₂S-producing enzymes to reduce the release of endogenous H₂S. Breast cancer is the most common type of cancer in women. Due to the lack of approved targeted therapy, its recurrence and metastasis still affect its clinical treatment. In recent years, significant progress has been made in the control of breast cancer by using inhibitors on H₂S-producing enzymes. This review summarized the roles of endogenous H₂S-producing enzymes in breast cancer and the effects of the enzyme inhibitors on anticancer and anti-metastasis, with the aim of providing new insights for the treatment of breast cancer.

Cysteine metabolic circuitries: druggable targets in cancer

To enable survival in adverse conditions, cancer cells undergo global metabolic adaptations. The amino acid cysteine actively contributes to cancer metabolic remodelling on three different levels: first, in its free form, in redox control, as a component of the antioxidant glutathione or its involvement in protein s-cysteinylation, a reversible post-translational modification; second, as a substrate for the production of hydrogen sulphide (H2S), which feeds the mitochondrial electron transfer chain and mediates per-sulphidation of ATPase and glycolytic enzymes, thereby stimulating cellular bioenergetics; and, finally, as a carbon source for epigenetic regulation, biomass production and energy production. This review will provide a systematic portrayal of the role of cysteine in cancer biology as a source of carbon and sulphur atoms, the pivotal role of cysteine in different metabolic pathways and the importance of H2S as an energetic substrate and signalling molecule. The different pools of cysteine in the cell and within the body, and their putative use as prognostic cancer markers will be also addressed. Finally, we will discuss the pharmacological means and potential of targeting cysteine metabolism for the treatment of cancer.

Inhibition of cystathionine β-synthase promotes apoptosis and reduces cell proliferation in chronic myeloid leukemia

Increased endogenous hydrogen sulfide (H₂S) level by cystathionine β-synthase (CBS) has been shown to closely relate tumorigenesis. H₂S promotes angiogenesis, stimulates bioenergy metabolism and inhibits selective phosphatases. However, the role of CBS and H₂S in chronic myeloid leukemia (CML) remains elusive. In this study, we found that CBS and H₂S levels were increased in the bone marrow mononuclear cells of pediatric CML patients, as well as in the CML-derived K562 cells and CBS expression levels were correlated with different disease phases. Inhibition of CBS reduced the proliferation of the CML primary bone marrow mononuclear cells and induced growth inhibition, apoptosis, cell cycle arrest, and migration suppression in K562 cells and tumor xenografts. The knockdown of CBS expression by shRNA and inhibiting CBS activity by AOAA decreased the endogenous H₂S levels, promoted mitochondrial-related apoptosis and inhibited the NF-κB-mediated gene expression. Our study suggests that inhibition of CBS induces cell apoptosis, as well as limits cell proliferation and migration, a potential target for the treatment of chronic myeloid leukemia.

Hydrogen Sulfide, an Endogenous Stimulator of Mitochondrial Function in Cancer Cells

Hydrogen sulfide (H₂S) has a long history as toxic gas and environmental hazard; inhibition of cytochrome c oxidase (mitochondrial Complex IV) is viewed as a primary mode of its cytotoxic action. However, studies conducted over the last two decades unveiled multiple biological regulatory roles of H₂S as an endogenously produced mammalian gaseous transmitter. Cystathionine γ-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST) are currently viewed as the principal mammalian H₂S-generating enzymes. In contrast to its inhibitory (toxicological) mitochondrial effects, at lower (physiological) concentrations, H₂S serves as a stimulator of electron transport in mammalian mitochondria, by acting as an electron donor—with sulfide:quinone oxidoreductase (SQR) being the immediate electron acceptor. The mitochondrial roles of H₂S are significant in various cancer cells, many of which exhibit high expression and partial mitochondrial localization of various H₂S producing enzymes. In addition to the stimulation of mitochondrial ATP production, the roles of endogenous H₂S in cancer cells include the maintenance of mitochondrial organization (protection against mitochondrial fission) and the maintenance of mitochondrial DNA repair (via the stimulation of the assembly of mitochondrial DNA repair complexes). The current article overviews the state-of-the-art knowledge regarding the mitochondrial functions of endogenously produced H₂S in cancer cells.

Pharmacological induction of mesenchymal-epithelial transition via inhibition of H2S biosynthesis and consequent suppression of ACLY activity in colon cancer cells

Hydrogen sulfide (H₂S) is an important endogenous gaseous transmitter mediator, which regulates a variety of cellular functions in autocrine and paracrine manner. The enzymes responsible for the biological generation of H₂S include cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MST). Increased expression of these enzymes and overproduction of H₂S has been implicated in essential processes of various cancer cells, including the stimulation of metabolism, maintenance of cell proliferation and cytoprotection. Cancer cell identity is characterized by so-called "transition states". The progression from normal (epithelial) to transformed (mesenchymal) state is termed epithelial-to-mesenchymal transition (EMT) whereby epithelial cells lose their cell-to-cell adhesion capacity and gain mesenchymal characteristics. The transition process can also proceed in the opposite direction, and this process is termed mesenchymal-to-epithelial transition (MET). The current project was designed to determine whether inhibition of endogenous H₂S production in colon cancer cells affects the EMT/MET balance in vitro. Inhibition of H₂S biosynthesis in HCT116 human colon cancer cells was achieved either with aminooxyacetic acid (AOAA) or 2-[(4-hydroxy-6-methylpyrimidin-2-yl)sulfanyl]-1-(naphthalen-1-yl)ethan-1-one (HMPSNE). These inhibitors induced an upregulation of E-cadherin and Zonula occludens-1 (ZO-1) expression and downregulation of fibronectin expression, demonstrating that H₂S biosynthesis inhibitors can produce a pharmacological induction of MET in colon cancer cells. These actions were functionally reflected in an inhibition of cell migration, as demonstrated in an in vitro "scratch wound" assay. The mechanisms involved in the action of endogenously produced H₂S in cancer cells in promoting (or maintaining) EMT (or tonically inhibiting MET) relate, at least in part, in the induction of ATP citrate lyase (ACLY) protein expression, which occurs via upregulation of ACLY mRNA (via activation of the ACLY promoter). ACLY in turn, regulates the Wnt-β-catenin pathway, an essential regulator of the EMT/MET balance. Taken together, pharmacological inhibition of endogenous H₂S biosynthesis in cancer cells induces MET. We hypothesize that this may contribute to anti-cancer / anti-metastatic effects of H₂S biosynthesis inhibitors.

Production of hydrogen sulfide by the intestinal microbiota and epithelial cells and consequences for the colonic and rectal mucosa

Among bacterial metabolites, hydrogen sulfide (H₂S) has received increasing attention. The epithelial cells of the large intestine are exposed to two sources of H₂S. The main one is the luminal source that results from specific bacteria metabolic activity toward sulfur-containing substrates. The other source in colonocytes is from the intracellular production mainly through cystathionine β-synthase (CBS) activity. H₂S is oxidized by the mitochondrial sulfide oxidation unit, resulting in ATP synthesis, and, thus, establishing this compound as the first mineral energy substrate in colonocytes. However, when the intracellular H₂S concentration exceeds the colonocyte capacity for its oxidation, it inhibits the mitochondrial respiratory chain, thus affecting energy metabolism. Higher luminal H₂S concentration affects the integrity of the mucus layer and displays proinflammatory effects. However, a low/minimal amount of endogenous H₂S exerts an anti-inflammatory effect on the colon mucosa, pointing out the ambivalent effect of H₂S depending on its intracellular concentration. Regarding colorectal carcinogenesis, forced CBS expression in late adenoma-like colonocytes increased their proliferative activity, bioenergetics capacity, and tumorigenicity; whereas, genetic ablation of CBS in mice resulted in a reduced number of mutagen-induced aberrant crypt foci. Activation of endogenous H₂S production and low H₂S extracellular concentration enhance cancerous colorectal cell proliferation. Higher exogenous H₂S concentrations markedly reduce mitochondrial ATP synthesis and proliferative capacity in cancerous cells and enhance glycolysis but do not affect their ATP cell content or viability. Thus, it appears that, notably through an effect on colonocyte energy metabolism, endogenous and microbiota-derived H₂S are involved in the host intestinal physiology and physiopathology.

Role of Hydrogen Sulfide and 3-Mercaptopyruvate Sulfurtransferase in the Regulation of the Endoplasmic Reticulum Stress Response in Hepatocytes

It is estimated that over 1.5 billion people suffer from various forms of chronic liver disease worldwide. The emerging prevalence of metabolic syndromes and alcohol misuse, along with the lack of disease-modifying agents for the therapy of many severe liver conditions predicts that chronic liver disease will continue to be a major problem in the future. Better understanding of the underlying pathogenetic mechanisms and identification of potential therapeutic targets remains a priority. Herein, we explored the potential role of the 3-mercaptopyruvate sulfurtransferase/hydrogen sulfide (H₂S) system in the regulation of the endoplasmic reticulum (ER) stress and of its downstream processes in the immortalized hepatic cell line HepG2 in vitro. ER stress suppressed endogenous H₂S levels and pharmacological supplementation of H₂S with sodium hydrogen sulfide (NaHS) mitigated many aspects of ER stress, culminating in improved cellular bioenergetics and prevention of autophagic arrest, thereby switching cells’ fate towards survival. Genetic silencing of 3-MST or pharmacological inhibition of the key enzymes involved in hepatocyte H₂S biosynthesis exacerbated many readouts related to ER-stress or its downstream functional responses. Our findings implicate the 3-MST/H₂S system in the intracellular network that governs proteostasis and ER-stress adaptability in hepatocytes and reinforce the therapeutic potential of pharmacological H₂S supplementation.

The interaction between ferroptosis and lipid metabolism in cancer

Ferroptosis is a new form of programmed cell death characterized by the accumulation of iron-dependent lethal lipid peroxides. Recent discoveries have focused on alterations that occur in lipid metabolism during ferroptosis and have provided intriguing insights into the interplay between ferroptosis and lipid metabolism in cancer. Their interaction regulates the initiation, development, metastasis, therapy resistance of cancer, as well as the tumor immunity, which offers several potential strategies for cancer treatment. This review is a brief overview of the features characterizing the interaction between ferroptosis and lipid metabolism, and highlights the significance of this interaction in cancer.

Effect of 3-mercaptopyruvate Sulfurtransferase Deficiency on the Development of Multiorgan Failure, Inflammation, and Wound Healing in Mice Subjected to Burn Injury

The gaseous transmitter hydrogen sulfide (H2S) has been implicated in various forms of critical illness. Here, we have compared the outcome of scald burn injury in wild-type mice and in mice deficient in 3-mercaptopyruvate sulfurtransferase (3-MST), a mammalian H2S-generating enzyme. Outcome variables included indices of organ injury, clinical chemistry parameters, and plasma levels of inflammatory mediators. Plasma levels of H2S significantly increased in response to burn in wild-type mice, but remained unchanged in 3-MST-/- mice. The capacity of tissue homogenates to produce H2S from 3-mercaptopyruvate was unaffected by burn injury. In 3-MST-/- mice, compared to wild-type controls, there was a significant enhancement in the accumulation of polymorphonuclear cells (as assessed by the quantification of myeloperoxidase) in the liver (but not heart, lung, or skin) at 7 days postburn. Oxidative tissue damage (as assessed by malon dialdehyde content) was comparable between wild-type and 3-MST-deficient mice in all tissues studied. 3-MST-/- and wild-type mice exhibited comparable burn-induced elevations in circulating plasma levels of hepatic injury; however, 3-MST-/- mice exhibited a higher degree of renal injury (as reflected by elevated blood urea nitrogen levels) at 7 days postburn. Inflammatory mediators (eg, TNF-α, IL-1β, IL-2, IL-6, IL-10, and IL-12) increased in burn injury, but without significant differences between the 3-MST-/- and wild-type groups. The healing of the burn wound was also unaffected by 3-MST deficiency. In conclusion, the absence of the H2S-producing enzyme 3-MST slightly exacerbates the development of multiorgan dysfunction but does not affect inflammatory mediator production or wound healing in a murine model of burn injury.

Cystathionine-β-Synthase: Molecular Regulation and Pharmacological Inhibition

Cystathionine-β-synthase (CBS), the first (and rate-limiting) enzyme in the transsulfuration pathway, is an important mammalian enzyme in health and disease. Its biochemical functions under physiological conditions include the metabolism of homocysteine (a cytotoxic molecule and cardiovascular risk factor) and the generation of hydrogen sulfide (H2S), a gaseous biological mediator with multiple regulatory roles in the vascular, nervous, and immune system. CBS is up-regulated in several diseases, including Down syndrome and many forms of cancer; in these conditions, the preclinical data indicate that inhibition or inactivation of CBS exerts beneficial effects. This article overviews the current information on the expression, tissue distribution, physiological roles, and biochemistry of CBS, followed by a comprehensive overview of direct and indirect approaches to inhibit the enzyme. Among the small-molecule CBS inhibitors, the review highlights the specificity and selectivity problems related to many of the commonly used "CBS inhibitors" (e.g., aminooxyacetic acid) and provides a comprehensive review of their pharmacological actions under physiological conditions and in various disease models.

The re-emerging pathophysiological role of the cystathionine-β-synthase - hydrogen sulfide system in Down syndrome

Down syndrome (DS) is associated with significant perturbances in many morphological and biochemical features. Cystathionine-β-synthase (CBS) is one of the key mammalian enzymes that is responsible for the biological production of the gaseous transmitter hydrogen sulfide (H₂ S). When H₂ S is overproduced, it can exert detrimental cellular effects, in part due to inhibition of mitochondrial Complex IV activity. An increased expression of CBS and the consequent overproduction of H₂ S are well documented in individuals with DS. Two decades ago, it has been proposed that a toxic overproduction of H₂ S importantly contributes to the metabolic and neurological deficits associated with DS. However, until recently, this hypothesis has not yet been tested experimentally. Recent data generated in human dermal fibroblasts show that DS cells overproduce H₂ S, which, in turn, suppresses mitochondrial Complex IV activity and impairs mitochondrial oxygen consumption and ATP generation. Therapeutic CBS inhibition lifts the tonic (and reversible) suppression of Complex IV: This results in the normalization of mitochondrial function in DS cells. H₂ S may also contribute to the cellular dysfunction via several other molecular mechanisms through interactions with various mitochondrial and extramitochondrial molecular targets. The current article provides a historical background of the field, summarizes the recently published data and their potential implications, and outlines potential translational approaches (such as CBS inhibition and H₂ S neutralization) and future experimental studies in this re-emerging field of pathobiochemistry.

Activation of the reverse transsulfuration pathway through NRF2/CBS confers erastin-induced ferroptosis resistance

Background

Ferroptosis is an iron-dependent, lipid peroxide-mediated cell death that may be exploited to selective elimination of damaged and malignant cells. Recent studies have identified that small-molecule erastin specifically inhibits transmembrane cystine–glutamate antiporter system x_c⁻, prevents extracellular cystine import and ultimately causes ferroptosis in certain cancer cells. In this study, we aimed to investigate the molecular mechanism underlying erastin-induced ferroptosis resistance in ovarian cancer cells.

Methods

We treated ovarian cancer cells with erastin and examined cell viability, cellular ROS and metabolites of the transsulfuration pathway. We also depleted cystathionine β-synthase (CBS) and NRF2 to investigate the CBS and NRF2 dependency in erastin-resistant cells.

Results

We found that prolonged erastin treatment induced ferroptosis resistance. Upon exposure to erastin, cells gradually adapted to cystine deprivation via sustained activation of the reverse transsulfuration pathway, allowing the cells to bypass erastin insult. CBS, the biosynthetic enzyme for cysteine, was constantly upregulated and was critical for the resistance. Knockdown of CBS by RNAi in erastin-resistant cells caused ferroptotic cell death, while CBS overexpression conferred ferroptosis resistance. We determined that the antioxidant transcriptional factor, NRF2 was constitutively activated in erastin-resistant cells and NRF2 transcriptionally upregulated CBS. Genetically repression of NRF2 enhanced ferroptosis susceptibility.

Conclusions

Based on these results, we concluded that constitutive activation of NRF2/CBS signalling confers erastin-induced ferroptosis resistance. This study demonstrates a new mechanism underlying ferroptosis resistance, and has implications for the therapeutic response to erastin-induced ferroptosis.

Molecular isomerization triggered by H2S to an NIR accessible first direct visualization of Ca2+-dependent production in living HeLa cells

Few studies determined the role of intracellular labile Ca²⁺ in H₂S homeostasis. Undoubtedly, fluorescent probes are powerful tools for exploring the question because of their unique advantages: non-destruction, visualization, and multi-levels imaging. Herein, a near-infrared (λ_em = 687 nm) and methylene blue chromophore-based fluorescent probe (MB1) for H₂S was rationally developed. Based on its high sensitivity and selectivity, MB1 was employed to image the concentration change of H₂S, upon stimulating it with ionomycin (a specific calcium ionophore). We found that the intracellular labile Ca²⁺ acted as a promotor for H₂S production in living cells. Furthermore, cystathionine γ-lyase (CSE) might have functioned as a positive mediator of Ca²⁺-dependent H₂S production. These direct and visible links for H₂S/Ca²⁺ will help us to understand the complex signaling in a better way.

A Review of Hydrogen Sulfide Synthesis, Metabolism, and Measurement: Is Modulation of Hydrogen Sulfide a Novel Therapeutic for Cancer?

Significance: Hydrogen sulfide (H2S) has been recognized as the third gaseous transmitter alongside nitric oxide and carbon monoxide. In the past decade, numerous studies have demonstrated an active role of H2S in the context of cancer biology. Recent Advances: The three H2S-producing enzymes, namely cystathionine γ-lyase (CSE), cystathionine β-synthase (CBS), and 3-mercaptopyruvate sulfurtransferase (3MST), have been found to be highly expressed in numerous types of cancer. Moreover, inhibition of CBS has shown anti-tumor activity, particularly in colon cancer, ovarian cancer, and breast cancer, whereas the consequence of CSE or 3MST inhibition remains largely unexplored in cancer cells. Intriguingly, H2S donation at high amounts or a long time duration has also been observed to induce cancer cell apoptosis in vitro and in vivo while sparing noncancerous fibroblast cells. Therefore, a bell-shaped model has been proposed to explain the role of H2S in cancer development. Specifically, endogenous H2S or a relatively low level of exogenous H2S may exhibit a pro-cancer effect, whereas exposure to H2S at a higher amount or for a long period may lead to cancer cell death. This indicates that inhibition of H2S biosynthesis and H2S supplementation serve as two distinct ways for cancer treatment. This paradoxical role of H2S has stimulated the enthusiasm for the development of novel CBS inhibitors, H2S donors, and H2S-releasing hybrids. Critical Issues: A clear relationship between H2S level and cancer progression remains lacking. The possibility that the altered levels of these byproducts have influenced the cell viability of cancer cells has not been excluded in previous studies when modulating H2S producing enzymes. Future Directions: The consequence of CSE or 3MST inhibition in cancer cells need to be examined in the future. Better portrayal of the crosstalk among these gaseous transmitters may not only lead to an in-depth understanding of cancer progression but also shed light on novel strategies for cancer therapy.

Screening Pyridine Derivatives against Human Hydrogen Sulfide-synthesizing Enzymes by Orthogonal Methods

Biosynthesis of hydrogen sulfide (H₂S), a key signalling molecule in human (patho)physiology, is mostly accomplished by the human enzymes cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (MST). Several lines of evidence have shown a close correlation between increased H₂S production and human diseases, such as several cancer types and amyotrophic lateral sclerosis. Identifying compounds selectively and potently inhibiting the human H₂S-synthesizing enzymes may therefore prove beneficial for pharmacological applications. Here, the human enzymes CBS, CSE and MST were expressed and purified from Escherichia coli, and thirty-one pyridine derivatives were synthesized and screened for their ability to bind and inhibit these enzymes. Using differential scanning fluorimetry (DSF), surface plasmon resonance (SPR), circular dichroism spectropolarimetry (CD), and activity assays based on fluorimetric and colorimetric H₂S detection, two compounds (C30 and C31) sharing structural similarities were found to weakly inhibit both CBS and CSE: 1 mM C30 inhibited these enzymes by approx. 50% and 40%, respectively, while 0.5 mM C31 accounted for CBS and CSE inhibition by approx. 40% and 60%, respectively. This work, while presenting a robust methodological platform for screening putative inhibitors of the human H₂S-synthesizing enzymes, highlights the importance of employing complementary methodologies in compound screenings.

Effects of PKM2 on global metabolic changes and prognosis in hepatocellular carcinoma: from gene expression to drug discovery

BACKGROUND

Hepatocellular carcinoma (HCC) is a malignant tumor that threatens global human health. High PKM2 expression is widely reported in multiple cancers, especially in HCC. This study aimed to explore the effects of PKM2 on global gene expression, metabolic damages, patient prognosis, and multiple transcriptional regulation relationships, as well as to identify several key metabolic genes and screen some small-molecule drugs.

METHODS

Transcriptome and clinical HCC data were downloaded from the NIH-GDC repository. Information regarding the metabolic genes and subsystems was collected from the Recon 2 human metabolic model. Drug-protein interaction data were obtained from the DrugBank and UniProt databases. We defined patients with PKM2 expression levels ≥11.25 as the high-PKM2 group, and those with low PKM2 expression (< 11.25) were defined as the low-PKM2 group.

RESULTS

The results showed that the global metabolic gene expression levels were obviously divided into the high- or low-PKM2 groups. In addition, a greater number of affected metabolic subsystems were observed in the high-PKM2 group. Furthermore, we identified 98 PKM2-correlated deregulated metabolic genes that were associated with poor overall patient survival. Together, these findings suggest more comprehensive influences of PKM2 on HCC. In addition, we screened several small-molecule drugs that target these metabolic enzymes, some of which have been used in antitumor clinical studies.

CONCLUSIONS

HCC patients with high PKM2 expression showed more severe metabolic damage, transcriptional regulation imbalance and poor prognosis than low-PKM2 individuals. We believe that our study provides valuable information for pathology research and drug development for HCC.

A pharmacological probe identifies cystathionine β-synthase as a new negative regulator for ferroptosis

Cystathionine β-synthase (CBS) is responsible for the first enzymatic reaction in the transsulfuration pathway of sulfur amino acids. The molecular function and mechanism of CBS as well as that of transsulfuration pathway remain ill-defined in cell proliferation and death. In the present study, we designed, synthesized and obtained a bioactive inhibitor CH004 for human CBS, which functions in vitro and in vivo. CH004 inhibits CBS activity, elevated the cellular homocysteine and suppressed the production of hydrogen sulfide in a dose-dependent manner in cells or in vivo. Chemical or genetic inhibition of CBS demonstrates that endogenous CBS is closely coupled with cell proliferation and cell cycle. Moreover, CH004 substantially retarded in vivo tumor growth in a xenograft mice model of liver cancer. Importantly, inhibition of CBS triggers ferroptosis in hepatocellular carcinoma. Overall, the study provides several clues for studying the interplays amongst transsulfuration pathway, ferroptosis and liver cancer.

Hydrogen Sulfide (H2S)-Releasing Compounds: Therapeutic Potential in Cardiovascular Diseases

Cardiovascular disease is the main cause of death worldwide, but its pathogenesis is not yet clear. Hydrogen sulfide (H₂S) is considered to be the third most important endogenous gasotransmitter in the organism after carbon monoxide and nitric oxide. It can be synthesized in mammalian tissues and can freely cross the cell membrane and exert many biological effects in various systems including cardiovascular system. More and more recent studies have supported the protective effects of endogenous H₂S and exogenous H₂S-releasing compounds (such as NaHS, Na₂S, and GYY4137) in cardiovascular diseases, such as cardiac hypertrophy, heart failure, ischemia/reperfusion injury, and atherosclerosis. Here, we provided an up-to-date overview of the mechanistic actions of H₂S as well as the therapeutic potential of various classes of H₂S donors in treating cardiovascular diseases.

Hydrogen Sulfide and Glucose Homeostasis: A Tale of Sweet and the Stink

SIGNIFICANCE

Among many endogenous mediators, the gasotransmitter hydrogen sulfide (H₂S) plays an important role in the regulation of glucose homeostasis. In this article we discuss different functional roles of H₂S in several metabolic organs/tissues required in the maintenance of glucose homeostasis. Recent Advances: New evidence has emerged revealing the insulin sensitizing role of H₂S in adipose tissue and skeletal muscle biology. In addition, H₂S was demonstrated to be a potent stimulator of gluconeogenesis via the induction and stimulation of various glucose-producing pathways in the liver.

CRITICAL ISSUES

Similar to its other physiological effects, H₂S exhibits paradoxical characteristics in the regulation of glucose homeostasis: (1) H₂S stimulates glucose production via activation of gluconeogenesis and glycogenolysis in hepatocytes, yet inhibits lipolysis in adipocytes; (2) H₂S stimulates glucose uptake into adipocytes and skeletal muscle but inhibits glucose uptake into hepatocytes; (3) H₂S inhibits insulin secretion from pancreatic β cells, yet sensitizes insulin signaling and insulin-triggered response in adipose tissues and skeletal muscle. It is also unclear the impact H₂S may have on glucose metabolism and utilization by other vital organs, such as the brain.

FUTURE DIRECTIONS

Recent reports and ongoing studies lay the foundation for a general, although highly incomplete, understanding of the effect of H₂S on regulating glucose homeostasis. In this review, we describe the molecular mechanisms and physiological outcomes of the gasotransmitter H₂S on organs and tissues required for homeostatic maintenance of blood glucose. Future directions highlighting the H₂S-mediated homeostatic control of glucose metabolism under physiological and insulin-resistant conditions are also discussed. Antioxid. Redox Signal. 28, 1463-1482.

Endogenous H2S producing enzymes are involved in apoptosis induction in clear cell renal cell carcinoma

BACKGROUND

Knowledge about the expression and thus a role of enzymes that produce endogenous H₂S - cystathionine-β-synthase, cystathionine γ-lyase and mercaptopyruvate sulfurtransferase - in renal tumors is still controversial. In this study we aimed to determine the expression of these enzymes relatively to the expression in unaffected part of kidney from the same patient and to found relation of these changes to apoptosis. To evaluate patient's samples, microarray and immunohistochemistry was used.

METHODS

To determine the physiological importance, we used RCC4 stable cell line derived from clear cell renal cell carcinoma, where apoptosis induction by a mixture of five chemotherapeutics with/without silencing of H₂S-producing enzymes was detected. Immunofluorescence was used to determine each enzyme in the cells.

RESULTS

In clear cell renal cell carcinomas, expression of H₂S-producing enzymes was mostly decreased compared to a part of kidney that was distal from the tumor. To evaluate a potential role of H₂S-producing enzymes in the apoptosis induction, we used RCC4 stable cell line. We have found that silencing of cystathionine-β-synthase and cystathionine γ-lyase prevented induction of apoptosis. Immunofluorescence staining clearly showed that these enzymes were upregulated during apoptosis in RCC4 cells.

CONCLUSION

Based on these results we concluded that in clear cell renal cell carcinoma, reduced expression of the H₂S-producing enzymes, mainly cystathionine γ-lyase, might contribute to a resistance to the induction of apoptosis. Increased production of the endogenous H₂S, or donation from the external sources might be of a therapeutic importance in these tumors.

Role of endogenous and exogenous nitric oxide, carbon monoxide and hydrogen sulfide in HCT116 colon cancer cell proliferation

The role of the three gasotransmitter systems - nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) - in cancer cells has not yet been studied simultaneously in the same experimental system. We measured the expression of NO and CO and H2S generating enzymes in primary colon cancer tissues and HCT116 colon cancer cells, and evaluated the effect of their pharmacological inhibition or pharmacological donation on cell proliferation. Increased expression of iNOS, nNOS, HO-1, CBS and 3-MST was detected in colon cancer. Inhibitors of NOS, HO-1/2, CBS/CSE and 3-MST, at lower concentrations, slightly stimulated HCT116 cell proliferation, but inhibited proliferation at higher concentrations. Donors of NO, CO or H2S inhibited HCT116 proliferation in a concentration-dependent manner. Inhibition of the cGMP/VASP pathway, Akt and p44/42 MAPK (Erk1/2) inhibited HCT116 cell proliferation. Endogenous NO and H2S biosynthesis were found to play a role in the maintenance of the activity of the cGMP/VASP pathway in HCT116 cells. We conclude that each of the three gasotransmitters play similar, bell-shaped roles in the control of HCT116 cell proliferation: endogenously produced NO, CO and H2S, at an optimal concentration, support HCT116 proliferation; inhibition of their production (which decreases gasotransmitter levels below optimal concentrations) as well as exogenous delivery of these gasotransmitters (which increases gasotransmitter levels above optimal concentrations) suppresses colon cancer cell proliferation. The current data give a mechanistic explanation for the paradoxical finding that both inhibitors and donors of NO, CO and H2S exert anticancer actions in cancer cells.

Antitumor effect of sikokianin C, a selective cystathionine β-synthase inhibitor, against human colon cancer in vitro and in vivo

Cystathionine β-synthase (CBS) overexpression is related to the proliferation and migration of human colon cancers. Targeted therapy that inhibits CBS has achieved promising effects in colon cancer treatments, but no selective inhibitor of CBS is available. In our previous study, a natural biflavonoid compound, sikokianin C, was identified as a potent and selective inhibitor of CBS. However, the mode of action of this compound and its antitumor efficacy in vivo remain unknown. In the present study, we have demonstrated that sikokianin C selectively inhibits CBS activity in a competitive manner, and the five key residues involved in the binding of sikokianin C to the substrate channel of CBS protein were identified via a combination of molecular docking and site-directed mutagenesis. Additionally, we analyzed the antitumor efficacy of sikokianin C against human colon cancer cells in vitro and in vivo. Sikokianin C greatly suppressed the proliferation of HT29 colon cancer cells with an IC50 value of 1.6 μM, and CBS is the target of sikokianin C in mammalian cells, as evidenced by CBS knockdown analyses. Moreover, sikokianin C induced the apoptosis of HT29 cancer cells in a dose dependent manner. Treating mice with sikokianin C dramatically reduced the tumor volume and the weight of the colon cancer xenograft in vivo. These results indicate that the selective CBS inhibitor sikokianin C can potentially be used for the treatment of colon cancer.

Drug resistance induces the upregulation of H2S-producing enzymes in HCT116 colon cancer cells

Hydrogen sulfide (H₂S) production in colon cancer cells supports cellular bioenergetics and proliferation. The aim of the present study was to investigate the alterations in H₂S homeostasis during the development of resistance to 5-fluorouracil (5-FU), a commonly used chemotherapeutic agent. A 5-FU-resistant HCT116 human colon cancer cell line was established by serial passage in the presence of increasing 5-FU concentrations. The 5-FU-resistant cells also demonstrated a partial resistance to an unrelated chemotherapeutic agent, oxaliplatin. Compared to parental cells, the 5-FU-resistant cells rely more on oxidative phosphorylation than glycolysis for bioenergetic function. There was a significant increase in the expression of the drug-metabolizing cytochrome P450 enzymes CYP1A2 and CYP2A6 in 5-FU-resistant cells. The CYP450 inhibitor phenylpyrrole enhanced 5-FU-induced cytotoxicity in 5-FU-resistant cells. Two major H₂S-generating enzymes, cystathionine-β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST) were upregulated in the 5-FU-resistant cells. 5-FU-resistant cells exhibited decreased sensitivity to the CBS inhibitor aminooxyacetate (AOAA) in terms of suppression of cell viability, inhibition of cell proliferation and inhibition of oxidative phosphorylation. However, 5FU-resistant cells remained sensitive to the antiproliferative effect of benserazide (a recently identified, potentially repurposable CBS inhibitor). Taken together, the current data suggest that 5-FU resistance in HCT116 cells is associated with the upregulation of drug-metabolizing enzymes and an enhancement of endogenous H₂S production. The anticancer effect of prototypical H₂S biosynthesis inhibitor AOAA is impaired in 5-FU-resistant cells, but benserazide remains efficacious. Pharmacological approaches aimed at restoring the sensitivity of 5-FU-resistant cells to chemotherapeutic agents may be useful in the formulation of novel therapeutic strategies against colorectal cancer.

International Union of Basic and Clinical Pharmacology. CII: Pharmacological Modulation of H2S Levels: H2S Donors and H2S Biosynthesis Inhibitors

Over the last decade, hydrogen sulfide (H2S) has emerged as an important endogenous gasotransmitter in mammalian cells and tissues. Similar to the previously characterized gasotransmitters nitric oxide and carbon monoxide, H2S is produced by various enzymatic reactions and regulates a host of physiologic and pathophysiological processes in various cells and tissues. H2S levels are decreased in a number of conditions (e.g., diabetes mellitus, ischemia, and aging) and are increased in other states (e.g., inflammation, critical illness, and cancer). Over the last decades, multiple approaches have been identified for the therapeutic exploitation of H2S, either based on H2S donation or inhibition of H2S biosynthesis. H2S donation can be achieved through the inhalation of H2S gas and/or the parenteral or enteral administration of so-called fast-releasing H2S donors (salts of H2S such as NaHS and Na2S) or slow-releasing H2S donors (GYY4137 being the prototypical compound used in hundreds of studies in vitro and in vivo). Recent work also identifies various donors with regulated H2S release profiles, including oxidant-triggered donors, pH-dependent donors, esterase-activated donors, and organelle-targeted (e.g., mitochondrial) compounds. There are also approaches where existing, clinically approved drugs of various classes (e.g., nonsteroidal anti-inflammatories) are coupled with H2S-donating groups (the most advanced compound in clinical trials is ATB-346, an H2S-donating derivative of the non-steroidal anti-inflammatory compound naproxen). For pharmacological inhibition of H2S synthesis, there are now several small molecule compounds targeting each of the three H2S-producing enzymes cystathionine-β-synthase (CBS), cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase. Although many of these compounds have their limitations (potency, selectivity), these molecules, especially in combination with genetic approaches, can be instrumental for the delineation of the biologic processes involving endogenous H2S production. Moreover, some of these compounds (e.g., cell-permeable prodrugs of the CBS inhibitor aminooxyacetate, or benserazide, a potentially repurposable CBS inhibitor) may serve as starting points for future clinical translation. The present article overviews the currently known H2S donors and H2S biosynthesis inhibitors, delineates their mode of action, and offers examples for their biologic effects and potential therapeutic utility.