Heterocyclic Organobismuth(III) Compound Targets Tubulin to Induce G2/M Arrest in HeLa Cells

Coordination environment dependence of anticancer activity in cyclometalated bismuth(III) complexes with C,O-chelating ligands

In this paper, a series of cyclometalated bismuth(III) complexes bearing C,O-bidentate ligands were synthesized and characterized by techniques such as UV-vis, NMR, HRMS, and single crystal X-ray diffraction. Meanwhile, their cytotoxicities against various human cell lines, including colon cancer cells (HCT-116), breast cancer cells (MDA-MB-231), lung cancer cells (A549), gastric cancer cells (SGC-7901), and normal embryonic kidney cells (HEK-293) were assessed in vitro. Compared with the clinical cisplatin, most of the synthesized complexes possessed significantly higher degrees of anticancer activity and selectivity, giving a selectivity index of up to 71.3. The structure-activity relationship study revealed that the anticancer performance of these bismuth(III) species depends on the factors of coordination environment surrounding the metal center, such as coordination number, coordination bonding strength, lone 6s2 electron pair stereoactivity. The Annexin V-FITC/PI double staining assay results suggested that the coordination environment-dependent cytotoxicity is ascribable to apoptosis. Western blot analysis confirmed the proposal, as evidenced by the down-regulating level of Bcl-2 and the activation of caspase-3. Furthermore, the representative complexes Bi1, Bi4, Bi6, and Bi8 exhibited relatively lower inhibitory efficiency on human ovarian cancer cells (A2780) than on its cisplatin-resistant daughter cells (A2780/cis), thus demonstrating that such compounds are capable of circumventing the cisplatin-induced resistance. This investigation elucidated the excellent anticancer performance of C,O-coordinated bismuth(III) complexes and established the correlation between cytotoxic activity and coordination chemistry, which provides a practical basis for in-depth designing and developing bismuth-based chemotherapeutics.

Deficiency of copper responsive gene stmn4 induces retinal developmental defects

As part of the central nervous system (CNS), the retina senses light and also conducts and processes visual impulses. The damaged development of the retina not only causes visual damage, but also leads to epilepsy, dementia and other brain diseases. Recently, we have reported that copper (Cu) overload induces retinal developmental defects and down-regulates microtubule (MT) genes during zebrafish embryogenesis, but whether the down-regulation of microtubule genes mediates Cu stress induced retinal developmental defects is still unknown. In this study, we found that microtubule gene stmn4 exhibited obviously reduced expression in the retina of Cu overload embryos. Furthermore, stmn4 deficiency (stmn4-/-) resulted in retinal defects similar to those seen in Cu overload embryos, while overexpression of stmn4 effectively rescued retinal defects and cell apoptosis occurred in the Cu overload embryos and larvae. Meanwhile, stmn4 deficient embryos and larvae exhibited reduced mature retinal cells, the down-regulated expression of microtubules and cell cycle-related genes, and the mitotic cell cycle arrests of the retinal cells, which subsequently tended to apoptosis independent on p53. The results of this study demonstrate that Cu stress might lead to retinal developmental defects via down-regulating expression of microtubule gene stmn4, and stmn4 deficiency leads to impaired cell cycle and the accumulation of retinal progenitor cells (RPCs) and their subsequent apoptosis. The study provides a certain referee for copper overload in regulating the retinal development in fish.

Exploring the Potential of Metal-Based Candidate Drugs as Modulators of the Cytoskeleton

During recent years, accumulating evidence suggested that metal-based candidate drugs are promising modulators of cytoskeletal and cytoskeleton-associated proteins. This was substantiated by the identification and validation of actin, vimentin and plectin as targets of distinct ruthenium(II)- and platinum(II)-based modulators. Despite this, structural information about molecular interaction is scarcely available. Here, we compile the scattered reports about metal-based candidate molecules that influence the cytoskeleton, its associated proteins and explore their potential to interfere in cancer-related processes, including proliferation, invasion and the epithelial-to-mesenchymal transition. Advances in this field depend crucially on determining binding sites and on gaining comprehensive insight into molecular drug-target interactions. These are key steps towards establishing yet elusive structure-activity relationships.

Biological Activities of Bismuth Compounds: An Overview of the New Findings and the Old Challenges Not Yet Overcome

Bismuth-based drugs have been used primarily to treat ulcers caused by Helicobacter pylori and other gastrointestinal ailments. Combined with antibiotics, these drugs also possess synergistic activity, making them ideal for multiple therapy regimens and overcoming bacterial resistance. Compounds based on bismuth have a low cost, are safe for human use, and some of them are also effective against tumoral cells, leishmaniasis, fungi, and viruses. However, these compounds have limited bioavailability in physiological environments. As a result, there is a growing interest in developing new bismuth compounds and approaches to overcome this challenge. Considering the beneficial properties of bismuth and the importance of discovering new drugs, this review focused on the last decade's updates involving bismuth compounds, especially those with potent activity and low toxicity, desirable characteristics for developing new drugs. In addition, bismuth-based compounds with dual activity were also highlighted, as well as their modes of action and structure-activity relationship, among other relevant discoveries. In this way, we hope this review provides a fertile ground for rationalizing new bismuth-based drugs.

Asymmetric bismuth-rhodamines as an activatable fluorogenic photosensitizer

Bismuth-rhodamine compounds stand out for their unique excitable photosensitizing properties and concomitant fluorescence; however, further knowledge of the structure-property relationship is required to expand the scope of their practical application. With this aim, this study describes the first examples of asymmetric bismuth-incorporated rhodamines, BiRNH and BiRAc, including their synthesis, photophysical properties, and photosensitizing abilities. Upon red light excitation, BiRNH exhibits detectable emission and photosensitizing properties, while the N-acetylated derivative BiRAc shows a hypsochromic shift in the absorption wavelength and attenuation of emission and photosensitizing ability. These significantly different photophysical properties enabled us to design an activatable fluorogenic photosensitizer, BiRGlu, which bears a γ-glutamyl group instead of the acetyl group in BiRAc. The γ-glutamyl group can be cleaved by γ-glutamyl transpeptidase (GGT) to produce BiRNH, which acts as a red-light-excitable fluorophore and photosensitizer. A cell study revealed that the phototoxicity and fluorescence of BiRGlu could be simultaneously and selectively activated in the cells with high GGT activity. Thus, we established that BiRNH could be envisaged as a versatile scaffold for activatable fluorogenic photosensitizers.

Heterocyclic organobismuth(III) compound induces nonapoptotic cell death via lipid peroxidation

Heterocyclic organobismuth compounds, such as N-tert-butyl-bi-chlorodibenzo[c,f][1,5]azabismocine (compound 1) and bi-chlorodibenzo[c,f ][1,5]thiabismocine (compound 3), exert potent antiproliferative activities in vitro in human cancer cell lines. We showed that compound 3 induced both apoptotic and nonapoptotic cell death via reactive oxygen species production and mitotic arrest in a dose-dependent manner. The mechanisms underlying the dose-dependent effect of these organobismuth compounds were not clear. In the present study, we examined the dose-dependent mechanism underlying cell death induced by compound 1 in a human pancreatic cancer cell line, SUIT-2, and a human colorectal cancer cell line, DLD-1. Compound 1 inhibited cell growth in a dose-dependent manner and induced cell death. Treatment with the pan-caspase inhibitor zVAD-fmk reduced cell death induced by compound 1, whereas the inhibitory effect of zVAD-fmk was limited. Moreover, compound 1 significantly induced lipid peroxidation with concomitant induction of caspase-independent cell death. Our results suggested that eight-membered ring organobismuth compounds induce nonapoptotic cell death via lipid peroxidation.

Upregulation of nuclear factor (erythroid-derived 2)-like 2 protein level in the human colorectal adenocarcinoma cell line DLD-1 by a heterocyclic organobismuth(III) compound: Effect of organobismuth(III) compound on NRF2 signaling

An increasing number of metal-based compounds, including arsenic trioxide, auranofin, and cisplatin, have been reported to have antitumor activity. Their beneficial effects are controlled by a transcription factor, nuclear factor (erythroid-derived 2)-like 2 (NRF2). In response to oxidative stress, NRF2 induces the expression of cytoprotective genes. NRF2 protein levels are regulated by Kelch-like ECH-associated protein 1 (KEAP1) via ubiquitination. Bi-chlorodibenzo[c,f][1,5]thiabismocine (compound 3), a bismuth compound, is known for its potent anti-proliferative activity against various cancer cell lines. In the present study, we investigated the effect of compound 3 on NRF2 signaling in the human colorectal adenocarcinoma cell line DLD-1 in terms of cell viability as well as mRNA and protein expression levels of NRF2. Compound 3 upregulated NRF2 protein levels in a time- and concentration-dependent manner, accompanied by a marked increase in heme-oxygenase-1 (HO-1) mRNA and protein levels. We observed that brusatol, an NRF2 inhibitor, as well as small interfering RNA (siRNA)-mediated knockdown of NRF2 in DLD-1 cells suppressed compound 3-induced HO-1 expression. The anticancer activity of compound 3 was enhanced by compounds that downregulate NRF2. These results suggest that compound 3 upregulates HO-1 via NRF2 activation and that the NRF2-HO-1 pathway is the cellular response to compound 3. We also discovered that compound 3 slightly downregulated KEAP1; thus, NRF2 activation may be associated with KEAP1 modification. Collectively, our results indicate that compound 3 simultaneously activates an anti-oxidative stress pathway, such as NRF2 and HO-1, and a pro-cell death signal in DLD-1 cells. Our findings may provide useful information for the development of a potent anticancer organobismuth(III) compound.

Heterocyclic organobismuth (III) compounds containing an eight-membered ring: Inhibitory effects on cell cycle progression

We previously showed that heterocyclic organobismuth compounds have excellent antimicrobial and antitumor potential. These compounds structurally consist of either six- or eight-membered rings. Previous research has shown that bi-chlorodibenzo[c,f][1,5]thiabismocine (Compound 3), an eight-membered ring, induced G₂/M arrest via inhibition of tubulin polymerization in HeLa cells. Additionally, N-tert-butyl-bi-chlorodi-benzo[c,f][1,5]azabismocine (Compound 1), another eight-membered ring, exhibited higher cytotoxicity than Compound 3 against several cancer cell lines, including HeLa and K562. Finally, bi-chlorophenothiabismin-S,S-dioxide (Compound 5), a six-membered ring, exhibited lower antitumor activity than eight-membered ring compounds. In this study, we investigated the antimitotic activity of Compounds 1 and 5 in HeLa cells. At low concentrations, (0.1 and 0.25 μM), Compound 1 inhibited cell growth and arrested the cell cycle in mitosis. However, 0.5 μM Compound 1 exhibited no antimitotic activity. Conversely, Compound 5 weakly inhibited cell growth and did not markedly arrest the cell cycle. Flow cytometry showed that Compound 1 arrested the cell cycle at G₂/M, resulting in apoptosis. Compound 1 inhibited tubulin polymerization as revealed by a cell-free assay, and both Compounds 1 and 3 inhibited microtubule spindle formation and chromosome alignment during prometaphase. These results suggest that eight-membered ring-containing organobismuth compounds can induce mitotic arrest by perturbing spindle dynamics.

Studies on the cytotoxicity and anticancer performance of heterocyclic hypervalent organobismuth(III) compounds

Novel organobismuth(III) complex of 5H-dibenzo[c,f][1,5]oxabismocin-12(7H)-yl nitrate (C2) was synthesized and characterized by spectral and elemental analysis. It was compared with other five C,E,C-chelating (E = N, O, S) organobismuth(III) complexes against human adenocarcinoma alveolar basal epithelial cells (A549), human liver cancer cell line (SMCC7721), human gastric cancer cell line (SGC-7901), human colon adenocarcinoma cell line (SW480) and healthy human bronchial cell line (16HBE14o-) in vitro. It was found that C2 exhibited the best anticancer activity. Further mechanistic investigation indicated that toxicological activity of C2 was ascribable to apoptosis rather than anti-proliferative activity. Apoptosis was induced through up-regulating the level of Bcl-2/Bax as well as the activation of caspase-3. The results demonstrate that heterocyclic organobismuth(III) complexes of this type have great potential in the treatment of cancer.

DNA binding activity of Ku during chemotherapeutic agent-induced early apoptosis

Ku protein is a heterodimer composed of two subunits, and is capable of both sequence-independent and sequence-specific DNA binding. The former mode of DNA binding plays a crucial role in DNA repair. The biological role of Ku protein during apoptosis remains unclear. Here, we show characterization of Ku protein during apoptosis. In order to study the DNA binding properties of Ku, we used two methods for the electrophoresis mobility shift assay (EMSA). One method, RI-EMSA, which is commonly used, employed radiolabeled DNA probes. The other method, WB-EMSA, employed unlabeled DNA followed by western blot and detection with anti-Ku antiserum. In this study, Ku-DNA probe binding activity was found to dramatically decrease upon etoposide treatment, when examined by the RI-EMSA method. In addition, pre-treatment with apoptotic cell extracts inhibited Ku-DNA probe binding activity in the non-treated cell extract. The inhibitory effect of the apoptotic cell extract was reduced by DNase I treatment. WB-EMSA showed that the Ku in the apoptotic cell extract bound to fragmented endogenous DNA. Interestingly, Ku in the apoptotic cell extract purified by the Resource Q column bound 15-bp DNA in both RI-EMSA and WB-EMSA, whereas Ku in unpurified apoptotic cell extracts did not bind additional DNA. These results suggest that Ku binds cleaved chromosomal DNA and/or nucleosomes in apoptotic cells. In conclusion, Ku is intact and retains DNA binding activity in early apoptotic cells.

Oxidative stress induced by copper and iron complexes with 8-hydroxyquinoline derivatives causes paraptotic death of HeLa cancer cells

Here, we report the antiproliferative/cytotoxic properties of 8-hydroxyquinoline (8-HQ) derivatives on HeLa cells in the presence of transition metal ions (Cu(2+), Fe(3+), Co(2+), Ni(2+)). Two series of ligands were tested, the arylvinylquinolinic L1-L8 and the arylethylenequinolinic L9-L16, which can all interact with metal ions by virtue of the N,O donor set of 8-HQ; however, only L9-L16 are flexible enough to bind the metal in a multidentate fashion, thus exploiting the additional donor functions. L1-L16 were tested for their cytotoxicity on HeLa cancer cells, both in the absence and in the presence of copper. Among them, the symmetric L14 exhibits the highest differential activity between the ligand alone (IC50 = 23.7 μM) and its copper complex (IC50 = 1.8 μM). This latter, besides causing a significant reduction of cell viability, is associated with a considerable accumulation of the metal inside the cells. Metal accumulation is also observed when the cells are incubated with L14 complexed with other late transition metal ions (Fe(3+), Co(2+), Ni(2+)), although the biological response of HeLa cells is different. In fact, while Ni/L14 and Co/L14 exert a cytostatic effect, both Cu/L14 and Fe/L14 trigger a caspase-independent paraptotic process, which results from the induction of a severe oxidative stress and the unfolded protein response.

Bismuth compounds in medicinal chemistry

In recent years, the chemical potential of bismuth and bismuth compounds has been actively exploited. Bismuth salts are known for their low toxicity, making them potential valuable reagents for large-scale synthesis, which becomes more obvious when dealing with products such as active pharmaceutical ingredients or synthetic intermediates. Conversely, bismuth compounds have been widely used in medicine. After extensive use in the treatments of syphilis and other bacterial infections before the advent of modern antibiotics, bismuth compounds remain important for the treatment of several gastrointestinal disorders and also exhibit antimicrobial properties and cytotoxic activity, among others. This review updates relevant advances in the past few years, concerning the application of bismuth reagents and catalysts in innovative synthetic processes for the preparation of compounds of medicinal interest, as well as the preparation, biological evaluation and potential medicinal uses of bismuth compounds.

Recent advances in bioinorganic chemistry of bismuth

Bismuth has been used in medicine for over two centuries for the treatment of various diseases, in particular for gastrointestinal disorders, owing to its antimicrobial activity. Recent structural characterization of bismuth drugs provides an insight into assembly and pharmacokinetic pathway of the drugs. Mining potential protein targets inside the pathogen via metallomic/metalloproteomic approach and further characterization on the interactions of bismuth drugs with these targets laid foundation in understanding the mechanism of action of bismuth drugs. Such studies would be beneficial in rational design of new potential drugs.

A proteomic analysis of differential cellular responses to the short-chain fatty acids butyrate, valerate and propionate in colon epithelial cancer cells

The short chain fatty acids (SCFAs) are inhibitors of histone deacetylases (HDACi); they are produced naturally in the colon by fermentation. They affect cellular processes at a molecular and transcriptional level, the mechanisms of which may involve large numbers of proteins and integrated pathways. Butyrate is the most biologically potent of the SCFAs in colon epithelial cells, inhibiting human colon carcinoma cell proliferation and inducing apoptosis in vitro. In order to investigate the hypothesis that propionate and valerate possess unique and independent actions from butyrate, we combined proteomic and cellomic approaches for large-scale comparative analysis. Proteomic evaluation was undertaken using an iTRAQ tandem mass-spectrometry workflow and high-throughput High-content Analysis microscopy (HCA) was applied to generate cellomic information on the cell cycle and the cytoskeletal structure. Our results show that these SCFAs possess specific effects. Butyrate was shown to have more pronounced effects on the keratins and intermediate filaments (IFs); while valerate altered the β-tubulin isotypes' expression and the microtubules (MTs); propionate was involved in both mechanisms, displaying intermediate effects. These data suggest distinct physiological roles for SCFAs in colon epithelial function, offering new possibilities for cancer therapeutics.

Alpha-santalol, a chemopreventive agent against skin cancer, causes G2/M cell cycle arrest in both p53-mutated human epidermoid carcinoma A431 cells and p53 wild-type human melanoma UACC-62 cells

Background

α-Santalol, an active component of sandalwood oil, has shown chemopreventive effects on skin cancer in different murine models. However, effects of α-santalol on cell cycle have not been studied. Thus, the objective of this study was to investigate effects of α-santalol on cell cycle progression in both p53 mutated human epidermoid carcinoma A431 cells and p53 wild-type human melanoma UACC-62 cells to elucidate the mechanism(s) of action.

Methods

MTT assay was used to determine cell viability in A431 cells and UACC-62; fluorescence-activated cell sorting (FACS) analysis of propidium iodide staining was used for determining cell cycle distribution in A431 cells and UACC-62 cells; immunoblotting was used for determining the expression of various proteins and protein complexes involved in the cell cycle progression; siRNA were used to knockdown of p21 or p53 in A431 and UACC-62 cells and immunofluorescence microscopy was used to investigate microtubules in UACC-62 cells.

Results

α-Santalol at 50-100 μM decreased cell viability from 24 h treatment and α-santalol at 50 μM-75 μM induced G₂/M phase cell cycle arrest from 6 h treatment in both A431 and UACC-62 cells. α-Santalol altered expressions of cell cycle proteins such as cyclin A, cyclin B1, Cdc2, Cdc25c, p-Cdc25c and Cdk2. All of these proteins are critical for G₂/M transition. α-Santalol treatment up-regulated the expression of p21 and suppressed expressions of mutated p53 in A431 cells; whereas, α-santalol treatment increased expressions of wild-type p53 in UACC-62 cells. Knockdown of p21 in A431 cells, knockdown of p21 and p53 in UACC-62 cells did not affect cell cycle arrest caused by α-santalol. Furthermore, α-santalol caused depolymerization of microtubules similar to vinblastine in UACC-62 cells.

Conclusions

This study for the first time identifies effects of α-santalol in G₂/M phase arrest and describes detailed mechanisms of G₂/M phase arrest by this agent, which might be contributing to its overall cancer preventive efficacy in various mouse skin cancer models.