H2S suppresses indoleamine 2, 3-dioxygenase 1 and exhibits immunotherapeutic efficacy in murine hepatocellular carcinoma

Gut microbial metabolites: Shaping future diagnosis and treatment against gastrointestinal cancer

Gastrointestinal cancer is a worldwide health challenge due to its dramatically increasing prevalence and as a leading cause of cancer-related mortality. Increasing evidence has illustrated the vital role of gut microbes-derived metabolites in gastrointestinal cancer progression and treatment. Microbial metabolites are produced by the gut microbiota that utilizes both extrinsic dietary components and intrinsic host-generated compounds. Meanwhile, certain categories of metabolites such as short-chain fatty acids, bile acids, tryptophan, and indole derivatives, are linked to gastrointestinal malignancy. In this review, the major classes of microbial metabolites and their impacts on various gastrointestinal cancers including colorectal cancer, gastric cancer, and hepatocellular carcinoma, have been introduced. The application of microbial metabolites as predictive biomarkers for early diagnosis and prognosis of gastrointestinal cancer has also been explored. In addition, therapeutic potential of strategies that target microbial metabolites against gastrointestinal cancer is further evaluated.

Rational design of 2-benzylsulfinyl-benzoxazoles as potent and selective indoleamine 2,3-dioxygenase 1 inhibitors to combat inflammation

Mimicking the transition state of tryptophan (Trp) and O2 in the enzymatic reaction is an effective approach to design indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors. In this study, we firstly assembled a small library of 2-substituted benzo-fused five membered heterocycles and found 2-sulfinyl-benzoxazoles with interesting IDO1 inhibitory activities. Next the inhibitory activity toward IDO1 was gradually improved. Several benzoxazoles showed potent IDO1 inhibitory activity with IC50 of 82-91 nM, and exhibited selectivity between IDO1 and tryptophan 2,3-dioxygenase (TDO2). Enzyme binding studies showed that benzoxazoles are reversible type II IDO1 inhibitors, and modeling studies suggested that the oxygen atom of the sulfoxide in benzoxazoles interacts with the iron atom of the heme group, which mimics the transition state of Fe-O-O-Trp complex. Especially, 10b can effectively inhibit the NO production in lipopolysaccharides (LPS) stimulated RAW264.7 cells, and it also shows good anti-inflammation effect on mice acute inflammation model of croton oil induced ear edema.

Cystathionine gamma-lyase deficiency exaggerates diethylnitrosamine-induced liver damage in mice

Cystathionine gamma-lyase (CSE) is a key enzyme in reverse transsulfuration pathway and contributes to the majority of H2S generation in liver tissues via cysteine metabolism. Dysfunction of the CSE/H2S system is linked to both chronic and acute liver damage. This study investigated the regulatory role of CSE deficiency on diethylnitrosamine (DEN)-induced liver damage in mice. A single injection of DEN was administered into 4-week-old male CSE knockout (CSE-KO) mice and wild-type (WT) littermates, and the mice were sacrificed at 28 weeks of age. Compared to age-matched WT mice, CSE-KO mice spontaneously developed steatosis with increased oxidative stress and higher expressions of inflammation and fibrosis-related genes at 28-weeks of age. Following DEN injection, CSE-KO mice experienced more severe liver damage in comparison with the WT group as reflected by elevated levels of lipid accumulation, increased activities of alanine aminotransferase and aspartate aminotransferase, higher oxidative stress and fibrosis development, and increased expressions of inflammation and fibrosis-related genes. No visible tumors were observed in both types of mice with DEN treatment. In addition, the expression levels of the three H2S-generating proteins (CSE, cystathionine beta-synthase, and 3-mercaptopyruvate sulfurtransferase) and the H2S production rate in liver tissues were unaffected by DEN. Taken together, our study demonstrates that CSE provides a significant hepatoprotective effect and deficiency of CSE exaggerates DEN-induced liver damage in mice. Based on these findings, it can be suggested that targeting the CSE/H2S signaling pathway could be a potential therapeutic target for the treatment of liver diseases.

Taking advantage of the interaction between the sulfoxide and heme cofactor to develop indoleamine 2, 3-dioxygenase 1 inhibitors

Taking advantage of key interactions between sulfoxide and heme cofactor, we used the sulfoxide as the anchor functional group to develop two series of indoleamine 2, 3-dioxygenase 1 (IDO1) inhibitors: 2-benzylsulfinylbenzoxazoles (series 1) and 2-phenylsulfinylbenzoxazoles (series 2). In vitro enzymatic screening shows that both series can inhibit the activity of IDO1 in low micromolar (series 1) or nanomolar (series 2) levels. They also show inhibitory selectivity between IDO1 and tryptophan 2, 3-dioxygenase 2. Interestingly, although series 1 is less potent IDO1 inhibitors of these two series, it exhibited stronger inhibitory activity toward kynurenine production in interferon-γ stimulated BxPC-3 cells. Enzyme kinetics and binding studies demonstrated that 2-sulfinylbenzoxazoles are non-competitive inhibitors of tryptophan, and they interact with the ferrous form of heme. These results demonstrated 2-sulfinylbenzoxazoles as type II IDO1 inhibitors. Furthermore, molecular docking studies supports the sulfoxide being of the key functional group that interacts with the heme cofactor. Compound 22 (series 1) can inhibit NO production in a concentration dependent manner in lipopolysaccharides (LPS) stimulated RAW264.7 cells, and can relieve pulmonary edema and lung injury in LPS induced mouse acute lung injury models.

Tryptophan metabolism in digestive system tumors: unraveling the pathways and implications

Tryptophan (Trp) metabolism plays a crucial role in influencing the development of digestive system tumors. Dysregulation of Trp and its metabolites has been identified in various digestive system cancers, including esophageal, gastric, liver, colorectal, and pancreatic cancers. Aberrantly expressed Trp metabolites are associated with diverse clinical features in digestive system tumors. Moreover, the levels of these metabolites can serve as prognostic indicators and predictors of recurrence risk in patients with digestive system tumors. Trp metabolites exert their influence on tumor growth and metastasis through multiple mechanisms, including immune evasion, angiogenesis promotion, and drug resistance enhancement. Suppressing the expression of key enzymes in Trp metabolism can reduce the accumulation of these metabolites, effectively impacting their role in the promotion of tumor progression and metastasis. Strategies targeting Trp metabolism through specific enzyme inhibitors or tailored drugs exhibit considerable promise in enhancing therapeutic outcomes for digestive system tumors. In addition, integrating these approaches with immunotherapy holds the potential to further enhance treatment efficacy.

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.

Indoleamine 2,3-dioxygenase 1 promotes osteosarcoma progression by regulating tumor-derived exosomal miRNA hsa-miR-23a-3p

Background: Osteosarcoma (OS) is the most common primary malignant tumor originating in bone. Immunosuppressive enzyme indoleamine 2,3-dioxygenase 1 (IDO1) participates in tumor immune tolerance and promotes tumor progression, while the study of IDO1 in OS is limited. Methods: Immunohistochemistry analysis was performed to test the expression of IDO1 and Ki67. The relationship between IDO1 or Ki67 positive count and clinical stage of the patient was analyzed. Laboratory test indexes including serum alkaline phosphatase (ALP), lactate dehydrogenase (LDH), white blood cell (WBC) count and C-reactive protein (CRP) at diagnosis of OS patients were collected. The relationship between positive count of IDO1 and Ki67 or laboratory test indexes was analyzed by Pearson's correlation analysis. IDO1 stably overexpressed cell lines of these cells (MG63 OE, 143B OE and hFOB1.19 OE) were constructed and validated by Western blot and Elisa. Exosomes were isolated from conditioned culture media of these cells and were identified by Zetaview nanoparticle tracking analyzer. Next-generation sequencing was conducted to identify miRNAs enriched in exosomes. Differentially expressed miRNAs (DE miRNAs) were verified in clinical samples and cell lines by qPCR. Biological processes and cell components analysis of DE miRNAs was conducted by GO enrichment analysis using the protein interaction network database. Results: Immunosuppressive enzyme IDO1 was highly expressed in tumor tissues. 66.7% (6/9) of the tissues showed moderately or strongly positive immunostaining signal of IDO1, and 33.3% (3/9) were weakly positive. The expression of IDO1 was positively related to Ki67 and associated with prognostic-related clinical features of OS patients. Overexpression of IDO1 significantly affected the exosome-derived miRNA subsets from MG63, 143B and hFOB1.19 cells. A total of 1244 DE miRNAs were identified, and hsa-miR-23a-3p was further screened as key DE miRNA involved in the progression of OS. GO analysis of target genes of the DE miRNA results showed that target enrichment in the functions of immune regulation and tumor progression. Discussion: Our results indicate that IDO1 has the potential to promote the progression of OS that is related to miRNAs mediated tumor immunity. Targeting IDO1-mediated hsa-miR-23a-3p may be a potential therapeutic strategy for OS treatment.

Hydrogen sulfide and its donors: Novel antitumor and antimetastatic agents for liver cancer

Hepatocellular carcinoma (HCC) is the sixth most frequent human cancer and the world's third most significant cause of cancer mortality. HCC treatment has recently improved, but its mortality continues to increase worldwide due to its extremely complicated and heterogeneous genetic abnormalities. After nitric oxide (NO) and carbon monoxide (CO), the third gas signaling molecule discovered is hydrogen sulfide (H₂S), which has long been thought to be a toxic gas. However, numerous studies have proven that H₂S plays many pathophysiological roles in mammals. Endogenous or exogenous H₂S can decrease cell proliferation, promote apoptosis, block cell cycle, invasion and migration through various cellular signaling pathways. This review analyzes and discusses the recent literature on the function and molecular mechanism of H₂S and H₂S donors in HCC, so as to provide convenience for the scientific research and clinical application of H₂S in the treatment of liver cancer.

The Role of Hydrogen Sulfide in the Development and Progression of Lung Cancer

Lung cancer is one of the 10 most common cancers in the world, which seriously affects the normal life and health of patients. According to the investigation report, the 3-year survival rate of patients with lung cancer is less than 20%. Heredity, the environment, and long-term smoking or secondhand smoke greatly promote the development and progress of the disease. The mechanisms of action of the occurrence and development of lung cancer have not been fully clarified. As a new type of gas signal molecule, hydrogen sulfide (H₂S) has received great attention for its physiological and pathological roles in mammalian cells. It has been found that H₂S is widely involved in the regulation of the respiratory system and digestive system, and plays an important role in the occurrence and development of lung cancer. H₂S has the characteristics of dissolving in water and passing through the cell membrane, and is widely expressed in body tissues, which determines the possibility of its participation in the occurrence of lung cancer. Both endogenous and exogenous H₂S may be involved in the inhibition of lung cancer cells by regulating mitochondrial energy metabolism, mitochondrial DNA integrity, and phosphoinositide 3-kinase/protein kinase B co-pathway hypoxia-inducible factor-1α (HIF-1α). This article reviews and discusses the molecular mechanism of H₂S in the development of lung cancer, and provides novel insights for the prevention and targeted therapy of lung cancer.

The double-edged role of hydrogen sulfide in the pathomechanism of multiple liver diseases

In mammalian systems, hydrogen sulfide (H₂S)-one of the three known gaseous signaling molecules in mammals-has been found to have a variety of physiological functions. Existing studies have demonstrated that endogenous H₂S is produced through enzymatic and non-enzymatic pathways. The liver is the body's largest solid organ and is essential for H₂S synthesis and elimination. Mounting evidence suggests H₂S has essential roles in various aspects of liver physiological processes and pathological conditions, such as hepatic lipid metabolism, liver fibrosis, liver ischemia‒reperfusion injury, hepatocellular carcinoma, hepatotoxicity, and acute liver failure. In this review, we discuss the functions and underlying molecular mechanisms of H₂S in multiple liver pathophysiological conditions.

Indoleamine 2,3-dioxygenase 1 signaling orchestrates immune tolerance in Echinococcus multilocularis-infected mice

The cestode Echinococcus multilocularis larva infection causes lethal zoonotic alveolar echinococcosis (AE), a disease posing a great threat to the public health worldwide. This persistent hepatic tumor-like disease in AE patients has been largely attributed to aberrant T cell responses, of which Th1 responses are impeded, whilst Th2 and regulatory T cell responses are elevated, creating an immune tolerogenic microenvironment in the liver. However, the immune tolerance mechanisms are not fully understood. Dendritic cells (DCs) are key cellular components in facilitating immune tolerance in chronic diseases, including AE. Here, we demonstrate that indoleamine 2,3-dioxygenase 1-deficient (IDO1-/-) mice display less severe AE as compared to wild-type (WT) mice during the infection. Mechanistically, IDO1 prevents optimal T cells responses by programming DCs into a tolerogenic state. Specifically, IDO1 prevents the maturation and migration potential of DCs, as shown by the significantly enhanced expression of the antigen-presenting molecule (MHC II), costimulatory molecules (CD80 and CD86), and chemokine receptors (CXCR4 and CCR7) in infected IDO1-/- mice as compared to infected wild-type mice. More importantly, the tolerogenic phenotype of DCs is partly reverted in IDO1-/- mice, as indicated by enhanced activation, proliferation, and differentiation of both CD4+ and CD8+ - T cells upon infection with Echinococcus multilocularis, in comparison with WT mice. Interestingly, in absence of IDO1, CD4+ T cells are prone to differentiate to effector memory cells (CD44+CD62L-); in contrast, CD8+ T cells are highly biased to the central memory phenotype (CD44+CD62L+). Overall, these data are the first to demonstrate the essential role of IDO1 signaling in inducing immunosuppression in mice infected with Echinococcus multilocularis.

Hepatic stellate cell mediates transcription of TNFSF14 in hepatocellular carcinoma cells via H2S/CSE-JNK/JunB signaling pathway

Hepatic stellate cells (HSC) and hydrogen sulfide (H₂S) both play important roles in the development of hepatocellar carcinoma (HCC). Whereas, in the microenvironment of HCC, whether HSC participate in regulating the biological process of HCC cells by releasing H₂S remains elusive. In vitro, Flow cytometry (FCM), CCK-8, RNA-sequencing, Western blotting, RT-qPCR, immunofluorescence and ChIP assays were carried out in the HCC cells to investigate the effect of H₂S on biological functions and JNK/JunB-TNFSF14 signaling pathway. Specimens from HCC patients were analyzed by RT-qPCR and Western blotting assays for evaluating the expression of TNFSF14 and CSE. Statistical analysis was used to analyze the correlation between TNFSF14 expression and clinical data of HCC patients. Based on the FCM and CCK-8 results, we found the LX-2 cells were able to induce HCC cells apoptosis through releasing H₂S. RNA-sequencing, RT-qPCR, and Western blotting results showed that TNFSF14 gene was upregulated in both LX-2 and NaHS group. NaHS treated in HCC cells led to JNK/JunB signaling pathway activating and greater binding of p-JunB to its responsive elements on TNFSF14 promoter. Impairment of TNFSF14 induction alleviated LX-2 and NaHS induced apoptosis of HepG2 and PLC/PRF/5 cells. Furthermore, TNFSF14 expression in HCC tissues was lower than the adjacent tissue. HCC patients with low expression of TNFSF14 had higher malignant degree and poor prognosis. In summary, demonstration of the involvement of HSC-derived H₂S in JNK/JunB mediated expression of TNFSF14 gene strongly indicates H₂S palys an important role in the regulation of HCC apoptosis.

Targeting Immune Cells in the Tumor Microenvironment of HCC: New Opportunities and Challenges

Immune associated cells in the microenvironment have a significant impact on the development and progression of hepatocellular carcinoma (HCC) and have received more and more attention. Different types of immune-associated cells play different roles, including promoting/inhibiting HCC and several different types that are controversial. It is well known that immune escape of HCC has become a difficult problem in tumor therapy. Therefore, in recent years, a large number of studies have focused on the immune microenvironment of HCC, explored many mechanisms worth identifying tumor immunosuppression, and developed a variety of immunotherapy methods as targets, laying the foundation for the final victory in the fight against HCC. This paper reviews recent studies on the immune microenvironment of HCC that are more reliable and important, and provides a more comprehensive view of the investigation of the immune microenvironment of HCC and the development of more immunotherapeutic approaches based on the relevant summaries of different immune cells.

Hydrogen Sulfide and the Immune System

Hydrogen sulfide (H₂S) is the "third gasotransmitter" recognized alongside nitric oxide (NO) and carbon monoxide (CO). H₂S exhibits an array of biological effects in mammalian cells as revealed by studies showing important roles in the cardiovascular system, in cell signalling processes, post-translational modifications and in the immune system. Regarding the latter, using pharmacological and genetic approaches scientists have shown this molecule to have both pro- and anti-inflammatory effects in mammalian systems. The anti-inflammatory effects of H₂S appeared to be due to its inhibitory action on the nuclear factor kappa beta signalling pathway; NF-kB representing a transcription factor involved in the regulation pro-inflammatory mediators like nitric oxide, prostaglandins, and cytokines. In contrast, results from several animal model describe a more complicated picture and report on pro-inflammatory effects linked to exposure to this molecule; linked to dosage used and point of administration of this molecule. Overall, roles for H₂S in several inflammatory diseases spanning arthritis, atherosclerosis, sepsis, and asthma have been described by researchers. In light this work fascinating research, this chapter will cover H₂S biology and its many roles in the immune system.

Brassinin Inhibits Proliferation in Human Liver Cancer Cells via Mitochondrial Dysfunction

Brassinin is a phytochemical derived from Chinese cabbage, a cruciferous vegetable. Brassinin has shown anticancer effects on prostate and colon cancer cells, among others. However, its mechanisms and effects on hepatocellular carcinoma (HCC) have not been elucidated yet. Our results confirmed that brassinin exerted antiproliferative effects by reducing proliferating cell nuclear antigen (PCNA) activity, a proliferation indicator and inducing cell cycle arrest in human HCC (Huh7 and Hep3B) cells. Brassinin also increased mitochondrial Ca²⁺ levels and depolarized the mitochondrial membrane in both Huh7 and Hep3B cells. Moreover, brassinin generated high amounts of reactive oxygen species (ROS) in both cell lines. The ROS scavenger N-acetyl-L-cysteine (NAC) inhibited this brassinin-induced ROS production. Brassinin also regulated the AKT and mitogen-activated protein kinases (MAPK) signaling pathways in Huh7 and Hep3B cells. Furthermore, co-administering brassinin and pharmacological inhibitors for JNK, ERK1/2 and P38 decreased cell proliferation in both HCC cell lines more than the pharmacological inhibitors alone. Collectively, our results demonstrated that brassinin exerts antiproliferative effects via mitochondrial dysfunction and MAPK pathway regulation on HCC cells.

The footprint of kynurenine pathway in every cancer: a new target for chemotherapy

Treatment of cancers has always been a challenge for physicians. Typically, several groups of anti-cancer medications are needed for effective management of an invasive and metastatic cancer. Recently, therapeutic potentiation of immune system markedly improved treatment of cancers. Kynurenine pathway has an interwoven correlation with immune system. Kynurenine promotes T Reg (regulatory) differentiation, which leads to increased production of anti-inflammatory cytokines and suppression of cytotoxic activity of T cells. Overactivation of kynurenine pathway in cancers provides an immunologically susceptible microenvironment for mutant cells to survive and invade surrounding tissues. Interestingly, kynurenine pathway vigorously interacts with other molecular pathways involved in tumorigenesis. For instance, kynurenine pathway interacts with phospoinosisitide-3 kinase (PI3K), extracellular signal-regulated kinase (ERK), Wnt/β-catenin, P53, bridging integrator 1 (BIN-1), cyclooxygenase 2 (COX-2), cyclin-dependent kinase (CDK) and collagen type XII α1 chain (COL12A1). Overactivation of kynurenine pathway, particularly overactivation of indoleamine 2,3-dioxygenase (IDO) predicts poor prognosis of several cancers such as gastrointestinal cancers, gynecological cancers, hematologic malignancies, breast cancer, lung cancer, glioma, melanoma, prostate cancer and pancreatic cancer. Furthermore, kynurenine increases the invasion, metastasis and chemoresistance of cancer cells. Recently, IDO inhibitors entered clinical trials and successfully passed their safety tests and showed promising therapeutic efficacy for cancers such as melanoma, brain cancer, renal cell carcinoma, prostate cancer and pancreatic cancer. However, a phase III trial of epacadostat, an IDO inhibitor, could not increase the efficacy of treatment with pembrolizumab for melanoma. In this review the expanding knowledge towards kynurenine pathway and its application in each cancer is discussed separately.

Indoleamine 2,3-dioxygenase 1 limits hepatic inflammatory cells recruitment and promotes bile duct ligation-induced liver fibrosis

Liver fibrosis is a course of chronic liver dysfunction, can develop into cirrhosis and hepatocellular carcinoma. Inflammatory insult owing to pathogenic factors plays a crucial role in the pathogenesis of liver fibrosis. Indoleamine 2,3-dioxygenase 1 (IDO1) can affect the infiltration of immune cells in many pathology processes of diseases, but its role in liver fibrosis has not been elucidated completely. Here, the markedly elevated protein IDO1 in livers was identified, and dendritic cells (DCs) immune-phenotypes were significantly altered after BDL challenge. A distinct hepatic population of CD11c+DCs was decreased and presented an immature immune-phenotype, reflected by lower expression levels of co-stimulatory molecules (CD40, MHCII). Frequencies of CD11c+CD80+, CD11c+CD86+, CD11c+MHCII+, and CD11c+CD40+ cells in splenic leukocytes were reduced significantly. Notably, IDO1 overexpression inhibited hepatic, splenic CD11c+DCs maturation, mature DCs-mediated T-cell proliferation and worsened liver fibrosis, whereas above pathological phenomena were reversed in IDO1-/- mice. Our data demonstrate that IDO1 affects the process of immune cells recruitment via inhibiting DCs maturation and subsequent T cells proliferation, resulting in the promotion of hepatic fibrosis. Thus, amelioration of immune responses in hepatic and splenic microenvironment by targeting IDO1 might be essential for the therapeutic effects on liver fibrosis.

Mutual antagonism between indoleamine 2,3-dioxygenase 1 and nuclear factor E2-related factor 2 regulates the maturation status of DCs in liver fibrosis

Liver fibrosis can develop into liver cirrhosis and hepatocellular carcinoma substantially without effective available treatment currently due to rarely characterized molecular pathogenesis. Indoleamine 2,3-dioxygenase 1(IDO1) can be detected on antigen-presenting cells (APCs) and modulates various immune responses. However, the role of IDO1 in the regulation of dendritic cells (DCs) during liver fibrosis is rarely reported. Here, we found that hepatic IDO1 was up-regulated during CCL4-induced liver fibrosis, which accompanied by a significant decrease in the frequencies of CD11c⁺CD80⁺, CD11c⁺CD86⁺, CD11c⁺CD40⁺ and CD11c⁺MHCII⁺ cells and a reduction in the subsequent T cell proliferation rate, whereas these changes were reversed significantly in IDO1^-/- mice. Overexpressing IDO1 by adeno-associated viral vector serotype 9 (AAV9) significantly inhibited the maturation status of DCs, worsened fibrosis. In vitro studies showed that significantly elevated CD80, CD86, CD40 and MHCII expression were observed in BMDCs derived from IDO1^-/- mice. Moreover, the maturation of BMDCs derived from WT mice were significantly increased after stimulated with IDO1 inhibitor (1-methyl- D -tryptophan). Nuclear factor E2-related factor 2 (Nrf2), a key regulator of the cellular adaptive response to oxidative insults and inflammation, exhibited a markedly decrease in the liver of WT fibrotic mice, nevertheless, knockout of IDO1 enhanced the protein level of Nrf2. Moreover, the expression of IDO1 and Nrf2 exhibited inverse colocalization pattern suggesting that ectopically expressed IDO1 down-regulated Nrf2. Additionally, up-regulation of IDO1 was also observed in the livers of Nrf2^-/- fibrotic mice. Taken together, these data uncovered mutual antagonism between IDO1 and Nrf2 on the maturation status of DCs during hepatic fibrosis.

STAT3: A key signaling molecule for converting cold to hot tumors

Tumors can be classified as cold or hot according to the degree of immune cell infiltration into tumor tissues; cold tumors are insensitive to either chemotherapy or immunotherapy and are associated with poor prognosis. Recent studies have shown that STAT3 signaling molecules hinder the conversion of cold to hot tumors by regulating immunosuppressive molecule secretion and immunosuppressive cell functions. This review aims to present the most recent studies on how STAT3 regulates cold tumor formation and discuss its research status in cancer therapy. We also present insight for designing new therapeutic strategies to "heat" tumors and provide a reference for tumor immunotherapy.

Nitric Oxide-Mediated Enhancement and Reversal of Resistance of Anticancer Therapies

In the last decade, immune therapies against human cancers have emerged as a very effective therapeutic strategy in the treatment of various cancers, some of which are resistant to current therapies. Although the clinical responses achieved with many therapeutic strategies were significant in a subset of patients, another subset remained unresponsive initially, or became resistant to further therapies. Hence, there is a need to develop novel approaches to treat those unresponsive patients. Several investigations have been reported to explain the underlying mechanisms of immune resistance, including the anti-proliferative and anti-apoptotic pathways and, in addition, the increased expression of the transcription factor Yin-Yang 1 (YY1) and the programmed death ligand 1 (PD-L1). We have reported that YY1 leads to immune resistance through increasing HIF-1α accumulation and PD-L1 expression. These mechanisms inhibit the ability of the cytotoxic T-lymphocytes to mediate their cytotoxic functions via the inhibitory signal delivered by the PD-L1 on tumor cells to the PD-1 receptor on cytotoxic T-cells. Thus, means to override these resistance mechanisms are needed to sensitize the tumor cells to both cell killing and inhibition of tumor progression. Treatment with nitric oxide (NO) donors has been shown to sensitize many types of tumors to chemotherapy, immunotherapy, and radiotherapy. Treatment of cancer cell lines with NO donors has resulted in the inhibition of cancer cell activities via, in part, the inhibition of YY1 and PD-L1. The NO-mediated inhibition of YY1 was the result of both the inhibition of the upstream NF-κB pathway as well as the S-nitrosylation of YY1, leading to both the downregulation of YY1 expression as well as the inhibition of YY1-DNA binding activity, respectively. Also, treatment with NO donors induced the inhibition of YY1 and resulted in the inhibition of PD-L1 expression. Based on the above findings, we propose that treatment of tumor cells with the combination of NO donors, at optimal noncytotoxic doses, and anti-tumor cytotoxic effector cells or other conventional therapies will result in a synergistic anticancer activity and tumor regression.

Design and Synthesis of Indoleamine 2,3-Dioxygenase 1 Inhibitors and Evaluation of Their Use as Anti-Tumor Agents

Indoleamine 2,3-dioxygenase (IDO) 1 is the key enzyme for regulating tryptophan metabolism and is an important target for interrupting tumor immune escape. In this study, we designed four series of compounds as potential IDO1 inhibitors by attaching various fragments or ligands to indole or phenylimidazole scaffolds to improve binding to IDO1. The compounds were synthesized and their inhibitory activities against IDO1 and tryptophan 2,3-dioxygenase were evaluated. The cytotoxicities of the compounds against two tumor cell lines were also determined. Two compounds with a phenylimidazole scaffold (DX-03-12 and DX-03-13) showed potent IDO1 inhibition with IC₅₀ values of 0.3–0.5 μM. These two IDO1 inhibitors showed low cell cytotoxicity, which indicated that they may exert their anti-tumor effect via immune modulation. Compound DX-03-12 was investigated further by determining the in vivo pharmacokinetic profile and anti-tumor efficacy. The pharmacokinetic study revealed that DX-03-12 had satisfactory properties in mice, with rapid absorption, moderate plasma clearance (∼36% of hepatic blood flow), acceptable half-life (∼4.6 h), and high oral bioavailability (∼96%). Daily oral administration of 60 mg/kg of compound DX-03-12 decreased tumor growth by 72.2% after 19 days in a mouse melanoma cell B16-F10 xenograft model compared with the untreated control. Moreover, there was no obvious weight loss in DX-03-12-treated mice. In conclusion, compound DX-03-12 is a potent lead compound for developing IDO1 inhibitors and anti-tumor agents.