Hinokitiol Inhibits Breast Cancer Cells In Vitro Stemness-Progression and Self-Renewal with Apoptosis and Autophagy Modulation via the CD44/Nanog/SOX2/Oct4 Pathway.

Breast cancer (BC) represents one of the most prevalent malignant threats to women globally. Tumor relapse or metastasis is facilitated by BC stemness progression, contributing to tumorigenicity. Therefore, comprehending the characteristics of stemness progression and the underlying molecular mechanisms is pivotal for BC advancement. Hinokitiol (ß-thujaplicin), a tropolone-related compound abundant in the heartwood of cupressaceous plants, exhibits antimicrobial activity. In our study, we employed three BC cell lines (MDA-MB-231, MCF-7, and T47D) to assess the expression of stemness-, apoptosis-, and autophagy-related proteins. Hinokitiol significantly reduced the viability of cancer cells in a dose-dependent manner. Furthermore, we observed that hinokitiol enhances apoptosis by increasing the levels of cleaved poly-ADP-ribose polymerase (PARP) and phospho-p53. It also induces dysfunction in autophagy through the upregulation of LC3B and p62 protein expression. Additionally, hinokitiol significantly suppressed the number and diameter of cancer cell line spheres by reducing the expression of cluster of differentiation44 (CD44) and key transcription factors. These findings underscore hinokitiol's potential as a therapeutic agent for breast cancer, particularly as a stemness-progression inhibitor. Further research and clinical studies are warranted to explore the full therapeutic potential of hinokitiol in the treatment of breast cancer.

Hinokitiol protects gastric injury from ethanol exposure via its iron sequestration capacity.

Hinokitiol is a natural bioactive tropolone derivative isolated from Chamaecyparis obtusa and Thuja plicata, which exhibits promising potential in terms of antioxidant and anti-inflammatory properties and possesses potent iron-binding capacity. In this study, we aimed to investigate the potential role of hinokitiol in protecting against ethanol-induced gastric injury and elucidate the underlying mechanism. Our results demonstrated that hinokitiol effectively attenuated hemorrhagic gastric lesions, epithelial cell loss, and inflammatory response in mice with ethanol-induced gastric injury. Intriguingly, we found that ethanol exposure affects iron levels both in vivo and in vitro. Moreover, the disturbed iron homeostasis was involved in the development of ethanol-induced injury. Iron depletion was found to enhance defense against ethanol-induced damage, while iron repletion showed the opposite effect. To further explore the role of iron sequestration in the protective effects of hinokitiol, we synthesized methylhinokitiol, a compound that shields the iron binding capacity of hinokitiol with a methyl group. Interestingly, this compound significantly diminishes the protective effect against ethanol-induced injury. These findings collectively demonstrated that hinokitiol could potentially be used to prevent or improve gastric injury induced by ethanol through regulating cellular iron homeostasis.

Investigation of the activity of a novel tropolone in osteosarcoma.

Osteosarcoma (OS) is a primary malignant bone tumor characterized by frequent metastasis, rapid disease progression, and a high rate of mortality. Treatment options for OS have remained largely unchanged for decades, consisting primarily of cytotoxic chemotherapy and surgery, thus necessitating the urgent need for novel therapies. Tropolones are naturally occurring seven-membered non-benzenoid aromatic compounds that possess antiproliferative effects in a wide array of cancer cell types. MO-OH-Nap is an α-substituted tropolone that has activity as an iron chelator. Here, we demonstrate that MO-OH-Nap activates all three arms of the unfolded protein response (UPR) pathway and induces apoptosis in a panel of human OS cell lines. Co-incubation with ferric chloride or ammonium ferrous sulfate completely prevents the induction of apoptotic and UPR markers in MO-OH-Nap-treated OS cells. MO-OH-Nap upregulates transferrin receptor 1 (TFR1) protein levels, as well as TFR1, divalent metal transporter 1 (DMT1), iron-regulatory proteins (IRP1, IRP2), ferroportin (FPN), and zinc transporter 14 (ZIP14) transcript levels, demonstrating the impact of MO-OH-Nap on iron-homeostasis pathways in OS cells. Furthermore, MO-OH-Nap treatment restricts the migration and invasion of OS cells in vitro. Lastly, metabolomic profiling of MO-OH-Nap-treated OS cells revealed distinct changes in purine and pyrimidine metabolism. Collectively, we demonstrate that MO-OH-Nap-induced cytotoxic effects in OS cells are dependent on the tropolone's ability to alter cellular iron availability and that this agent exploits key metabolic pathways. These studies support further evaluation of MO-OH-Nap as a novel treatment for OS.

In vitro evaluation of tropolone absorption, metabolism, and clearance.

Tropolone compounds can inhibit hepatitis B virus (HBV) replication at sub-micromolar levels and are synergistic upon co-treatment with nucleos(t)ide analog drugs. However, only a few compounds within this chemotype have been screened for their pharmacological properties. Here, we chose 36 structurally diverse tropolones from six subclasses to characterize their in vitro pharmacological parameters. All compounds were more soluble in pHs that reflect the gastrointestinal tract (pH 5 and 6.5) than plasma (pH 7.4). Those compounds that had solubility limits >100 µM were tested in a passive permeability assay, and there was no general trend in the compounds' passive permeability at any pH. Twenty-nine compounds with the best absorption parameters were tested in HEK293 cells to assess potential cytotoxicity; measured toxicities were similar to those in the hepatic HepDES19 cells used for screening (R2 = 0.55). Sixteen representative compounds were tested against five major CYP450 isoforms and there was no substantial inhibition by any compound against any of the enzymes tested (<50%). The t1/2 values of 15 compounds were determined in the microsome stability assay and 12 compounds were evaluated in plasma protein binding assays to assess factors affecting their rate of clearance. All compounds with detectable analyte peaks had t1/2 > 30 min, and while 4 of 12 had statistically significant decreased potency in conditions with increased albumin concentrations, only one compound's potency was biologically significant. These data indicate that the tropolones have pharmacological characteristics that reflect approved drugs and inform future structure activity relationships during drug design.

Tropolones and Thailandepsin B as Lead-like Natural Compounds in the Development of Potent and Selective Histone Deacetylase Inhibitors.

BACKGROUND: Tropolone and thailandepsin B are naturally occurring substances that are primarily isolated from fungi and plants, although they can also be found in certain bacteria. Tropolones belong to an important class of aromatic compounds with a seven-membered nonbenzenoid ring structure. Thailandepsins are a group of natural products that were initially discovered in the culture broth of the Gram-negative bacterium Burkholderia thailandensis. Tropolonebased structures have been identified in over 200 natural compounds, ranging from simple tropolone derivatives to complex multicyclic systems like pycnidione and pyrerubrine A. These natural compounds exhibit a diverse range of pharmacological effects, including antibacterial, antifungal, insecticidal, phytotoxic, anti-inflammatory, antimitotic, anti-diabetic, enzyme inhibitory, anticancer, cytoprotective, and ROS scavenging properties. It is worth noting that thujaplicane, a compound similar to tropolone, displays all of the listed biological activities except for antimitotic action, which has only been observed in one natural tropolone compound, colchicine. Tropolone can be synthesized from commercially available seven-membered rings or derived through various cyclization and cycloaddition reactions. Thailandepsin B, on the other hand, can be synthesized by macro-lactonization of the corresponding secoacid, followed by the formation of internal disulfide bonds. It is important to mention that thailandepsin B exhibits different selective inhibition profiles compared to FK228. OBJECTIVE: We investigated the HDAC inhibitory activity of the Tropolones and Thailandepsin B and discussed the biosynthesis of the naturally occurring compounds and their synthetic scheme. RESULTS AND CONCLUSION: It has been observed that Tropolone derivatives act as isoenzyme-selective inhibitors of proven anticancer drug targets, histone deacetylases (HDACs). Some monosubstituted tropolones show remarkable levels of selectivity for HDAC2 and strongly inhibit the growth of T-lymphocyte cell lines. And Thailandepsins have different selective inhibition profiles than FK228. They exhibit comparable inhibitory activities to FK228 against human HDAC1, HDAC2, HDAC3, HDAC6, HDAC7, and HDAC9, but less potent inhibitory activities than FK228 toward HDAC4 and HDAC8, the latter of which may be useful. Thailandepsins possess potent cytotoxic activities toward some types of cell lines.

Synthesis and antitumoral evaluation of natural product-like compounds based on tropolone and benzotropolone derivatives.

We present the preparation of a series of novel natural product-like homobarrelenones, norcaranes, and dihydrofluorenones through a diversity-oriented synthetic (DOS) strategy that combines Diels-Alder reactions and phototransformations, as well as their biological evaluation against MCF-7, HT-29, and NCI-H460 human tumor cells. Six of these demonstrated activities in the micromolar range against the three cell lines, and none were predicted as cytotoxic against human nontumor cells according to in silico studies. In addition, within the set of active derivatives, three exhibited low unspecific cytotoxicity in a sperm motility assay. The rich functionality of the new compounds makes them ideal candidates for exhaustive structure-activity relationship studies.

Tropolone-Conjugated DNA: A Fluorescent Thymidine Analogue Exhibits Solvatochromism, Enzymatic Incorporation into DNA and HeLa Cell Internalization.

Tropolone is a non-benzenoid aromatic scaffold with unique photophysical and metal-chelating properties. Recently, it has been conjugated with DNA, and the photophysical properties of this conjugate have been explored. Tropolonyl-deoxyuridine (tr-dU) is a synthetic fluorescent DNA nucleoside analogue that exhibits pH-dependent emissions. However, its solvent-dependent fluorescence properties are unexplored owing to its poor solubility in most organic solvents. It would be interesting to incorporate it into DNA primer enzymatically. This report describes the solvent-dependent fluorescence properties of the silyl-derivative, and enzymatic incorporation of its triphosphate analogue. For practical use, its cell-internalization and cytotoxicity are also explored. tr-dU nucleoside was found to be a potential analogue to design DNA probes and can be explored for various therapeutic applications in the future.

Isatropolone/isarubrolone Cm from Streptomyces with biological activity of inducing incomplete autophagy.

Isatropolones/isarubrolones are Streptomyces secondary metabolites featuring a tropolone ring in the pentacyclic scaffolds of these molecules. They are able to induce complete autophagy in human HepG2 cells. Here, methyl isatropolones (1-2) and isarubrolone (3) are identified from Streptomyces CPCC 204095. They all have a methyl tropolone ring in the pentacyclic scaffold of these molecules resolved by MS and NMR spectra. Biological activity assay indicates that isatropolone Cm (1) and isarubrolone Cm (3) induce incomplete autophagy in human HepG2 cells.

Discovery of Tropolone Stipitaldehyde as a Potential Agent for Controlling Phytophthora Blight and Its Action Mechanism Research.

The fermentation of endophytic Nigrospora chinensis GGY-3 resulted in the isolation of tropolone stipitaldehyde (1), which exhibited broad-spectrum inhibition activity against fungi and bacteria, especially against Phytophthora capsici, with an EC50 value of 0.83 µg/mL and Xanthomonas oryzae pv. oryzicola, with a minimum inhibitory concentration value of 4.0 µg/mL. The in vitro and in vivo assays demonstrated that 1 had a significant protective effect on P. capsici. Furthermore, 1 inhibited the spore germination of P. capsici and damaged the plasma membrane structure. As observed by SEM and TEM, after exposure to 1, mycelia exhibited swelling, shrunken, branch-increasing phenomena, cell wall and membrane damage, and disordered content. Transcriptome analysis revealed that 1 might affect starch and sucrose metabolism and fatty acid biosynthesis by suppressing the expression of genes relevant to cell wall synthetases and cell membrane-associated genes. These findings indicate that 1 may be a potential agrochemical fungicide for controlling phytophthora blight.

Tropolone derivative hinokitiol ameliorates cerulein-induced acute pancreatitis in mice.

Hinokitiol is a natural bio-active tropolone derivative with promising antioxidant and anti-inflammatory properties. This study was conducted to evaluate the ameliorative effects of hinokitiol against acute pancreatitis induced by cerulein. Mice were pre-treated with hinokitiol intraperitoneally for 7 days (50 and 100 mg/kg), and on the final day of study, cerulein (6 × 50 µg/kg) was injected every hour for six times. Six hours after the last dose of cerulein, blood was collected from the mice through retro-orbital plexus for biochemical analysis. After blood collection, mice were euthanized and the pancreas was harvested for studying effects on oxidative stress, pro-inflammatory cytokines, immunohistochemistry and histopathology of tissue sections. Hinokitiol treatment significantly reduced edema of the pancreas and reduced the plasma levels of lipase and amylase in mice with cerulein-induced acute pancreatitis. It also attenuated the oxidative and nitrosative stress related damage as evident from the reduced malondialdehyde (MDA) and nitrite levels, which were significantly increased in the mice with acute pancreatitis. Furthermore, hinokitiol administration significantly reduced the pancreatitis-evoked decrease in the activity of catalase, glutathione (GSH) and superoxide dismutase (SOD) in the pancreatic tissue. Pre-treatment with hinokitiol significantly reduced the elevated levels of pro-inflammatory cytokines like interleukin-6 (IL-6), interleukin-1ß (IL-1ß), tumor necrosis factor-alpha (TNF-α) as well as increased the levels of anti-inflammatory cytokine interleukin-10 (IL-10) in the pancreatic tissue of mice with acute pancreatitis. The immunohistochemical expression of nuclear factor kappa light chain enhancer of activated B cells (NF-κB), cyclooxygenase (COX-2) and TNF-α were significantly decreased by hinokitiol in mice with cerulein-induced acute pancreatitis. In conclusion, the results of the present study demonstrate that hinokitiol has significant potential to prevent cerulein-induced acute pancreatitis.

The radiosensitizing effect of ß-Thujaplicin, a tropolone derivative inducing S-phase cell cycle arrest, in head and neck squamous cell carcinoma-derived cell lines.

BACKGROUND: Resistance to radiotherapy is a common cause of treatment failure in advanced head and neck squamous cell carcinoma (HNSCC). ß-Thujaplicin, a natural tropolone derivative, acts as an anti-cancer agent and has recently been shown to radiosensitize non-HNSCC cancer cells. However, no data is currently available on its radiosensitizing potential in HNSCC. METHODS: To investigate the effect of ß-Thujaplicin and irradiation in HNSCC cell lines CAL27 and FADU, we performed a cell viability assay, colony forming assay, flow cytometry for cell cycle analysis and a wound healing assay. Drug-irradiation interaction was analyzed using a zero-interaction potency model. RESULTS: Treatment with ß-Thujaplicin led to a dose-dependent decrease in cell viability and enhanced the effect of irradiation. Clonogenic survival was inhibited with synergistic drug-irradiation interaction. ß-Thujaplicin further led to S-phase arrest and increased the sub-G1 population. Moreover, combined ß-Thujaplicin and irradiation treatment had a higher anti-migratory effect compared to irradiation alone. CONCLUSIONS: ß-Thujaplicin acts as a radiosensitizer in HNSCC cell lines. Further evaluation of its use in HNSCC therapy is warranted.

Different Cell Responses to Hinokitiol Treatment Result in Senescence or Apoptosis in Human Osteosarcoma Cell Lines.

Hinokitiol is a tropolone-related compound isolated from the heartwood of cupressaceous plants. It is known to exhibit various biological functions including antibacterial, antifungal, and antioxidant activities. In the study, we investigated the antitumor activities of hinokitiol against human osteosarcoma cells. The results revealed that hinokitiol treatment inhibited cell viability of human osteosarcoma U-2 OS and MG-63 cells in the MTT assay. Further study revealed that hinokitiol exposure caused cell cycle arrest at the S phase and a DNA damage response with the induction of γ-H2AX foci in both osteosarcoma cell lines. In U-2 OS cells with wild-type tumor suppressor p53, we found that hinokitiol exposure induced p53 expression and cellular senescence, and knockdown of p53 suppressed the senescence. However, in MG-63 cells with mutated p53, a high percentage of cells underwent apoptosis with cleaved-PARP expression and Annexin V staining after hinokitiol treatment. In addition, up-regulated autophagy was observed both in hinokitiol-exposed U-2 OS and MG-63 cells. As the autophagy was suppressed through the autophagy inhibitor chloroquine, hinokitiol-induced senescence in U-2 OS cells was significantly enhanced accompanying more abundant p53 expression. In MG-63 cells, co-treatment of chloroquine increased hinokitiol-induced apoptosis and decreased cell viability of the treated cells. Our data revealed that hinokitiol treatment could result in different cell responses, senescence or apoptosis in osteosarcoma cell lines, and suppression of autophagy could promote these effects. We hypothesize that the analysis of p53 status and co-administration of autophagy inhibitors might provide more precise and efficacious therapies in hinokitiol-related trials for treating osteosarcoma.

Novel RNase H Inhibitors Blocking RNA-directed Strand Displacement DNA Synthesis by HIV-1 Reverse Transcriptase.

In retroviruses, strand displacement DNA-dependent DNA polymerization catalyzed by the viral reverse transcriptase (RT) is required to synthesize double-stranded proviral DNA. In addition, strand displacement during RNA-dependent DNA synthesis is critical to generate high-quality cDNA for use in molecular biology and biotechnology. In this work, we show that the loss of RNase H activity due to inactivating mutations in HIV-1 RT (e.g. D443N or E478Q) has no significant effect on strand displacement while copying DNA templates, but has a large impact on DNA polymerization in reactions carried out with RNA templates. Similar effects were observed with ß-thujaplicinol and other RNase H active site inhibitors, including compounds with dual activity (i.e., characterized also as inhibitors of HIV-1 integrase and/or the RT DNA polymerase). Among them, dual inhibitors of HIV-1 RT DNA polymerase/RNase H activities, containing a 7-hydroxy-6-nitro-2H-chromen-2-one pharmacophore were found to be very potent and effective strand displacement inhibitors in RNA-dependent DNA polymerization reactions. These findings might be helpful in the development of transcriptomics technologies to obtain more uniform read coverages when copying long RNAs and for the construction of more representative libraries avoiding biases towards 5' and 3' ends, while providing valuable information for the development of novel antiretroviral agents.

Synthesis of Polyoxygenated Tropolones and their Antiviral Activity against Hepatitis B Virus and Herpes Simplex Virus-1.

Polyoxygenated tropolones possess a broad range of biological activity, and as a result are promising lead structures or fragments for drug development. However, structure-function studies and subsequent optimization have been challenging, in part due to the limited number of readily available tropolones and the obstacles to their synthesis. Oxidopyrylium [5+2] cycloaddition can effectively generate a diverse array of seven-membered ring carbocycles, and as a result can provide a highly general strategy for tropolone synthesis. Here, we describe the use of 3-hydroxy-4-pyrone-based oxidopyrylium cycloaddition chemistry in the synthesis of functionalized 3,7-dimethoxytropolones, 3,7-dihydroxytropolones, and isomeric 3-hydroxy-7-methoxytropolones through complementary benzyl alcohol-incorporating procedures. The antiviral activity of these molecules against herpes simplex virus-1 and hepatitis B virus is also described, highlighting the value of this approach and providing new structure-function insights relevant to their antiviral activity.

Regioselective approach to colchiceine tropolone ring functionalization at C(9) and C(10) yielding new anticancer hybrid derivatives containing heterocyclic structural motifs.

The influence of base type, temperature, and solvent on regioselective C(9)/C(10) "click" modifications within the tropolone ring of colchiceine (2) is investigated. New ether derivatives of 2, bearing alkyne, azide, vinyl, or halide aryl groups enable assembly of the alkaloid part with heterocycles or important biomolecules such as saccharides, geldanamycin or AZT into hybrid scaffolds by dipolar cycloaddition (CuAAC) or Heck reaction. Compared to colchicine (1) or colchiceine (2), ether congeners, as e.g. 3e [IC50s(3e) ∼ 0.9 nM], show improved or similar anticancer effects, whereby the bulkiness of the substituents and the substitution pattern of the tropolone proved to be essential. Biological studies reveal that expanding the ether arms by terminal basic heterocycles as quinoline or pyridine, decreases the toxicity in HDF cells at high anticancer potency (IC50s ∼ 1-2 nM). Docking of ether and hybrid derivatives into the colchicine pocket of αGTP/ß tubulin dimers reveals a relationship between the favourable binding mode and the attractive anticancer potency.

Role of melatonin in TLR4-mediated inflammatory pathway in the MTPT-induced mouse model.

Neuroinflammation has an essential role in various neurodegenerative diseases including Parkinson's disease (PD). Microglial activation as a result of neuroinflammation exacerbates the pathological consequences of the disease. The toxic effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes alpha-synuclein (α-synuclein) accumulation, which leads to dopaminergic neuron death in the MPTP-induced mouse model. Toll-like receptor 4 (TLR4) stimulates release of cytokine through NF-kB by activating glial cells, thus resulting in the death of dopaminergic neurons. Melatonin has the ability to cross the blood-brain barrier and protect neurons through anti-inflammatory properties. We hypothesized that melatonin could suppress TLR4-mediated neuroinflammation, decrease cytokine release due to the inflammatory response, and reduce dopaminergic neuron loss in the MPTP-induced mouse model. In the MPTP-induced mouse model, we aimed to assess the neuroinflammatory responses caused by TLR4 activation as well as the effect of melatonin on these responses. Three-month-old male C57BL/6 mice were randomly divided into five groups; Control (Group-C), Sham (Group-S), Melatonin-treated (Group-M), MPTP-injected (Group-P), and MPTP + melatonin-injected (Group-P + M). MPTP toxin (20 mg/kg) was dissolved in saline and intraperitoneally (i.p.) injected to mice for two days with 12 h intervals. The total dose per mouse was 80 mg/kg. Melatonin was administered (20 mg/kg) intraperitoneally to Group-M and Group-P + M twice a day for five days. Eight days after starting the experiment, the motor activities of mice were evaluated by locomotor activity tests. The effects on dopamine neurons in the SNPc was determined by tyrosine hydroxylase (TH) immunohistochemistry. TLR4, α-synuclein, and p65 expression was evaluated by immunostaining as well. The amount of TNF-alpha in the total brain was evaluated by western blot analysis. In our results seen that locomotor activity was lower in Group-P compared to Group-C. However, melatonin administration was improved this impairment. MPTPcaused decrease in TH immuno-expression in dopaminergic neurons in Group-P. TLR4 (p < 0.001), α-synuclein (p < 0.001), and p65 (p < 0.01) immuno-expressions were also decreased in Group-P+M compared to Group-P (using MPTP). TNF-α expression was lower in Group-C, Group-S, Group-M, and Group-P+M, when compared to Group-P (p < 0.0001) due to the absence of inflammatory response. In conclusion, our study revealed that melatonin administration reduced α-synuclein aggregation and TLR4-mediated inflammatory response in the MPTP-induced mouse model.

Effects of Troponoids on Mitochondrial Function and Cytotoxicity.

The α-hydroxytropolones (αHTs) are troponoid inhibitors of hepatitis B virus (HBV) replication that can target HBV RNase H with submicromolar efficacies. αHTs and related troponoids (tropones and tropolones) can be cytotoxic in cell lines as measured by 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assays that assess mitochondrial function. Previous studies suggest that tropolones induce cytotoxicity through inhibition of mitochondrial respiration. Therefore, we screened 35 diverse troponoids for effects on mitochondrial function, mitochondrial/nuclear genome ratios, cytotoxicity, and reactive oxygen species (ROS) production. Troponoids as a class did not inhibit respiration or glycolysis, although the α-ketotropolone subclass interfered with these processes. The troponoids had no impact on the mitochondrial DNA/nuclear DNA ratio after 3 days of compound exposure. The patterns of troponoid-induced cytotoxicity among three hepatic cell lines were similar for all compounds, but three potent HBV RNase H inhibitors were not cytotoxic in primary human hepatocytes. Tropolones and αHTs increased ROS production in cells at cytotoxic concentrations but had no effect at lower concentrations that efficiently inhibit HBV replication. Troponoid-mediated cytotoxicity was significantly decreased upon the addition of the ROS scavenger N-acetylcysteine. These studies show that troponoids can increase ROS production at high concentrations within cell lines, leading to cytotoxicity, but are not cytotoxic in primary hepatocytes. Future development of αHTs as potential therapeutics against HBV may need to mitigate ROS production by altering compound design and/or by coadministering ROS antagonists to ameliorate increased ROS levels.

Hinokitiol-induced decreases of tyrosinase and microphthalmia-associated transcription factor are mediated by the endoplasmic reticulum-associated degradation pathway in human melanoma cells.

Tyrosinase (TYR) is a key enzyme for melanin production. We previously showed that hinokitiol, a naturally occurring seven-membered ring terpenoid, potently inhibits human TYR activity. Interestingly, hinokitiol was recently reported to decrease expression of TYR and microphthalmia-associated transcription factor (MITF), which is a main transcription factor of the TYR gene, in murine melanoma cells. However, the mechanisms by which hinokitiol decreases the intracellular levels of TYR and MITF have not been fully elucidated. Here, we investigated the underlying mechanisms of the decreases using cultured human melanoma cells. As a result, hinokitiol treatment decreased TYR protein level in a time- and dose-dependent manner in G361 human melanoma cells, while MITF protein level was decreased only at higher concentrations after 3 days treatment. Notably, the mRNA levels of TYR and MITF were slightly increased by hinokitiol treatment. Therefore, we focused on the degradation of TYR and MITF in endoplasmic reticulum (ER)-associated protein degradation (ERAD) pathway. Importantly, co-treatment of ERAD inhibitor with hinokitiol restored the protein levels of TYR and MITF to approximately 30% and 20% of total those in untreated control cells, respectively. Hinokitiol affected the ER homeostasis as well as degradation of TYR and MITF in two human melanoma cell lines, G361 and HT-144, but the changes of ER-stress markers under the hinokitiol treatment were different in the two human melanoma cell lines. Taken together, these observations indicate that hinokitiol may induce ER stress and trigger the degradation of unfolded newly synthesizing TYR and MITF via the ERAD pathway.

Biosynthesis of Ditropolonyl Sulfide, an Antibacterial Compound Produced by Burkholderia cepacia Complex Strain R-12632.

Burkholderia cepacia complex strain R-12632 produces ditropolonyl sulfide, an unusual sulfur-containing tropone, via a yet-unknown biosynthetic pathway. Ditropolonyl sulfide purified from a culture of strain R-12632 inhibits the growth of various Gram-positive and Gram-negative resistant bacteria, with MIC values as low as 16 µg/ml. In the present study, we used a transposon mutagenesis approach combined with metabolite analyses to identify the genetic basis for antibacterial activity of strain R-12632 against Gram-negative bacterial pathogens. Fifteen of the 8304 transposon mutants investigated completely lost antibacterial activity against Klebsiella pneumoniae LMG 2095. In these loss-of-activity mutants, nine genes were interrupted. Four of those genes were involved in assimilatory sulfate reduction, two were involved in phenylacetic acid (PAA) catabolism, and one was involved in glutathione metabolism. Via semipreparative fractionation and metabolite identification, it was confirmed that inactivation of the PAA degradation pathway or glutathione metabolism led to loss of ditropolonyl sulfide production. Based on earlier studies on the biosynthesis of tropolone compounds, the requirement for a functional PAA catabolic pathway for antibacterial activity in strain R-12632 indicated that this pathway likely provides the tropolone backbone for ditropolonyl sulfide. Loss of activity observed in mutants defective in assimilatory sulfate reduction and glutathione biosynthesis suggested that cysteine and glutathione are potential sources of the sulfur atom linking the two tropolone moieties. The demonstrated antibacterial activity of the unusual antibacterial compound ditropolonyl sulfide warrants further studies into its biosynthesis and biological role. IMPORTANCEBurkholderia bacteria are historically known for their biocontrol properties and have been proposed as a promising and underexplored source of bioactive specialized metabolites. Burkholderia cepacia complex strain R-12632 inhibits various Gram-positive and Gram-negative resistant pathogens and produces numerous specialized metabolites, among which is ditropolonyl sulfide. This unusual antimicrobial has been poorly studied and its biosynthetic pathway remains unknown. In the present study, we performed transposon mutagenesis of strain R-12632 and performed genome and metabolite analyses of loss-of-activity mutants to study the genetic basis for antibacterial activity. Our results indicate that phenylacetic acid catabolism, assimilatory sulfate reduction, and glutathione metabolism are necessary for ditropolonyl sulfide production. These findings contribute to understanding of the biosynthesis and biological role of this unusual antimicrobial.

Zinc thiotropolone combinations as inhibitors of the SARS-CoV-2 main protease.

Numerous organic molecules are known to inhibit the main protease of SARS-CoV-2, (SC2Mpro), a key component in viral replication of the 2019 novel coronavirus. We explore the hypothesis that zinc ions, long used as a medicinal supplement and known to support immune function, bind to the SC2Mpro enzyme in combination with lipophilic tropolone and thiotropolone ligands, L, block substrate docking, and inhibit function. This study combines synthetic inorganic chemistry, in vitro protease activity assays, and computational modeling. While the ligands themselves have half maximal inhibition concentrations, IC50, for SC2Mpro in the 8-34 µM range, the IC50 values are ca. 100 nM for Zn(NO3)2 which are further enhanced in Zn-L combinations (59-97 nM). Isolation of the Zn(L)2 binary complexes and characterization of their ability to undergo ligand displacement is the basis for computational modeling of the chemical features of the enzyme inhibition. Blind docking onto the SC2Mpro enzyme surface using a modified Autodock4 protocol found preferential binding into the active site pocket. Such Zn-L combinations orient so as to permit dative bonding of Zn(L)+ to basic active site residues.