Indibulin dampens microtubule dynamics and produces synergistic antiproliferative effect with vinblastine in MCF-7 cells: Implications in cancer chemotherapy

Cell cycle checkpoint revolution: targeted therapies in the fight against malignant tumors

Malignant tumors are among the most important causes of death worldwide. The pathogenesis of a malignant tumor is complex and has not been fully elucidated. Studies have shown that such pathogenesis is related to abnormal cell cycle progression. The expression levels of cyclins, cyclin-dependent kinases (CDKs), and CDK inhibitors as well as functions of the cell cycle checkpoints determine whether the cell cycle progression is smooth. Cell-cycle-targeting drugs have the advantages of high specificity, low toxicity, low side effects, and low drug resistance. Identifying drugs that target the cell cycle and applying them in clinical treatments are expected to promote chemotherapeutic developments against malignant tumors. This article aims to review drugs targeted against the cell cycle and their action mechanisms.

High-Throughput Image-Based Assay for Identifying In Vitro Hepatocyte Microtubule Disruption

Disruption of microtubule stability in mammalian cells may lead to genotoxicity and carcinogenesis. The ability to screen for microtubule destabilization or stabilization is therefore a useful and efficient approach to aid in the design of molecules that are safe for human health. In this study, we developed a high-throughput 384-well assay combining immunocytochemistry with high-content imaging to assess microtubule disruption in the metabolically competent human liver cell line: HepaRG. To enhance analysis throughput, we implemented a supervised machine learning approach using a curated training library of 180 compounds. A majority voting ensemble of eight machine learning classifiers was employed for predicting microtubule disruptions. Our prediction model achieved over 99.0% accuracy and a 98.4% F1 score, which reflects the balance between precision and recall for in-sample validation and 93.5% accuracy and a 94.3% F1 score for out-of-sample validation. This automated image-based testing can provide a simple, high-throughput screening method for early stage discovery compounds to reduce the potential risk of genotoxicity for crop protection product development.

Unveiling the anti-cancer potentiality of phthalimide-based Analogues targeting tubulin polymerization in MCF-7 cancerous Cells: Rational design, chemical Synthesis, and Biological-coupled Computational investigation

The present study deals with an anti-cancer investigation of an array of phthalimide-1,2,3-triazole molecular conjugates with various sulfonamide fragments against human breast MCF-7 and prostate PC3 cancer cell lines. The targeted 1,2,3-triazole derivatives 4a-l and 6a-c were synthesized from focused phthalimide-based alkyne precursors using a facile click synthesis approach and were thoroughly characterized using several spectroscopic techniques (IR, 1H, 13C NMR, and elemental analysis). The hybrid click adducts 4b, 4 h, and 6c displayed cytotoxic potency (IC50 values of 1.49, 1.07, and 0.56 μM, respectively) against MCF-7 cells. On the contrary, none of the synthesized compounds showed apparent cytotoxic efficacy for PC3 cells (IC50 ranging from 9.87- >100 μM). As a part of the mechanism analysis, compound 6c demonstrated a potent inhibitory effect (78.3 % inhibition) of tubulin polymerization in vitro with an IC50 value of 6.53 µM. In addition, biological assays showed that compound 6c could prompt apoptotic cell death and induce G2/M cell cycle arrest in MCF-7 cells. Accordingly, compound 6c can be further developed as an anti-breast cancer agent through apoptosis-induction.

Inhibition of STAT3 by 2-Methoxyestradiol suppresses M2 polarization and protumoral functions of macrophages in breast cancer

BACKGROUND

Breast cancer metastasis remains the leading cause of cancer-related deaths in women worldwide. Infiltration of tumor-associated macrophages (TAMs) in the tumor stroma is known to be correlated with reduced overall survival. The inhibitors of TAMs are sought after for reprogramming the tumor microenvironment. Signal transducer and activator of transcription 3 (STAT3) is well known to contribute in pro-tumoral properties of TAMs. 2-Methoxyestradiol (2ME2), a potent anticancer and antiangiogenic agent, has been in clinical trials for treatment of breast cancer. Here, we investigated the potential of 2ME2 in modulating the pro-tumoral effects of TAMs in breast cancer.

METHODS

THP-1-derived macrophages were polarized to macrophages with or without 2ME2. The effect of 2ME2 on macrophage surface markers and anti-inflammatory genes was determined by Western blotting, flow cytometry, immunofluorescence, qRT‒PCR. The concentration of cytokines secreted by cells was monitored by ELISA. The effect of M2 macrophages on malignant properties of breast cancer cells was determined using colony formation, wound healing, transwell, and gelatin zymography assays. An orthotopic model of breast cancer was used to determine the effect of 2ME2 on macrophage polarization and metastasis in vivo.

RESULTS

First, our study found that polarization of monocytes to alternatively activated M2 macrophages is associated with the reorganization of the microtubule cytoskeleton. At lower concentrations, 2ME2 treatment depolymerized microtubules and reduced the expression of CD206 and CD163, suggesting that it inhibits the polarization of macrophages to M2 phenotype. However, the M1 polarization was not significantly affected at these concentrations. Importantly, 2ME2 inhibited the expression of several anti-inflammatory cytokines and growth factors, including CCL18, TGF-β, IL-10, FNT, arginase, CXCL12, MMP9, and VEGF-A, and hindered the metastasis-promoting effects of M2 macrophages. Concurrently, 2ME2 treatment reduced the expression of CD163 in tumors and inhibited lung metastasis in the orthotopic breast cancer model. Mechanistically, 2ME2 treatment reduced the phosphorylation and nuclear translocation of STAT3, an effect which was abrogated by colivelin.

CONCLUSIONS

Our study presents novel findings on mechanism of 2ME2 from the perspective of its effects on the polarization of the TAMs via the STAT3 signaling in breast cancer. Altogether, the data supports further clinical investigation of 2ME2 and its derivatives as therapeutic agents to modulate the tumor microenvironment and immune response in breast carcinoma.

A Novel Pyrazole Exhibits Potent Anticancer Cytotoxicity via Apoptosis, Cell Cycle Arrest, and the Inhibition of Tubulin Polymerization in Triple-Negative Breast Cancer Cells

In this study, we screened a chemical library to find potent anticancer compounds that are less cytotoxic to non-cancerous cells. This study revealed that pyrazole PTA-1 is a potent anticancer compound. Additionally, we sought to elucidate its mechanism of action (MOA) in triple-negative breast cancer cells. Cytotoxicity was analyzed with the differential nuclear staining assay (DNS). Additional secondary assays were performed to determine the MOA of the compound. The potential MOA of PTA-1 was assessed using whole RNA sequencing, Connectivity Map (CMap) analysis, in silico docking, confocal microscopy, and biochemical assays. PTA-1 is cytotoxic at a low micromolar range in 17 human cancer cell lines, demonstrating less cytotoxicity to non-cancerous human cells, indicating a favorable selective cytotoxicity index (SCI) for the killing of cancer cells. PTA-1 induced phosphatidylserine externalization, caspase-3/7 activation, and DNA fragmentation in triple-negative breast MDA-MB-231 cells, indicating that it induces apoptosis. Additionally, PTA-1 arrests cells in the S and G2/M phases. Furthermore, gene expression analysis revealed that PTA-1 altered the expression of 730 genes at 24 h (198 upregulated and 532 downregulated). A comparison of these gene signatures with those within CMap indicated a profile similar to that of tubulin inhibitors. Subsequent studies revealed that PTA-1 disrupts microtubule organization and inhibits tubulin polymerization. Our results suggest that PTA-1 is a potent drug with cytotoxicity to various cancer cells, induces apoptosis and cell cycle arrest, and inhibits tubulin polymerization, indicating that PTA-1 is an attractive drug for future clinical cancer treatment.

Endophytic fungi: A future prospect for breast cancer therapeutics and drug development

Globally, breast cancer is a primary contributor to cancer-related fatalities and illnesses among women. Consequently, there is a pressing need for safe and effective treatments for breast cancer. Bioactive compounds from endophytic fungi that live in symbiosis with medicinal plants have garnered significant interest in pharmaceutical research due to their extensive chemical composition and prospective medicinal attributes. This review underscores the potentiality of fungal endophytes as a promising resource for the development of innovative anticancer agents specifically tailored for breast cancer therapy. The diversity of endophytic fungi residing in medicinal plants, success stories of key endophytic bioactive metabolites tested against breast cancer and the current progress with regards to in vivo studies and clinical trials on endophytic fungal metabolites in breast cancer research forms the underlying theme of this article. A thorough compilation of putative anticancer compounds sourced from endophytic fungi that have demonstrated therapeutic potential against breast cancer, spanning the period from 1990 to 2022, has been presented. This review article also outlines the latest trends in endophyte-based drug discovery, including the use of artificial intelligence, machine learning, multi-omics approaches, and high-throughput strategies. The challenges and future prospects associated with fungal endophytes as substitutive sources for developing anticancer drugs targeting breast cancer are also being highlighted.

Comparative safety, pharmacokinetics, and off-target assessment of 1,1-bis(3'-indolyl)-1-(p-chlorophenyl) methane in mouse and dog: implications for therapeutic development

The modified phytochemical derivative, 1,1-bis(3'-indolyl)-1-(p-chlorophenyl) methane (C-DIM12), has been identified as a potential therapeutic platform based on its capacity to improve disease outcomes in models of neurodegeneration and cancer. However, comprehensive safety studies investigating pathology and off-target binding have not been conducted. To address this, we administered C-DIM12 orogastrically to outbred male CD-1 mice for 7 days (50 mg/kg/day, 200 mg/kg/day, and 300 mg/kg/day) and investigated changes in hematology, clinical chemistry, and whole-body tissue pathology. We also delivered a single dose of C-DIM12 (1 mg/kg, 5 mg/kg, 25 mg/kg, 100 mg/kg, 300 mg/kg, 1,000 mg/kg) orogastrically to male and female beagle dogs and investigated hematology and clinical chemistry, as well as plasma pharmacokinetics over 48-h. Consecutive in-vitro off-target binding through inhibition was performed with 10 μM C-DIM12 against 68 targets in tandem with predictive off-target structural binding capacity. These data show that the highest dose C-DIM12 administered in each species caused modest liver pathology in mouse and dog, whereas lower doses were unremarkable. Off-target screening and predictive modeling of C-DIM12 show inhibition of serine/threonine kinases, calcium signaling, G-protein coupled receptors, extracellular matrix degradation, and vascular and transcriptional regulation pathways. Collectively, these data demonstrate that low doses of C-DIM12 do not induce pathology and are capable of modulating targets relevant to neurodegeneration and cancer.

A network-based drug prioritization and combination analysis for the MEK5/ERK5 pathway in breast cancer

BACKGROUND

Prioritizing candidate drugs based on genome-wide expression data is an emerging approach in systems pharmacology due to its holistic perspective for preclinical drug evaluation. In the current study, a network-based approach was proposed and applied to prioritize plant polyphenols and identify potential drug combinations in breast cancer. We focused on MEK5/ERK5 signalling pathway genes, a recently identified potential drug target in cancer with roles spanning major carcinogenesis processes.

RESULTS

By constructing and identifying perturbed protein-protein interaction networks for luminal A breast cancer, plant polyphenols and drugs from transcriptome data, we first demonstrated their systemic effects on the MEK5/ERK5 signalling pathway. Subsequently, we applied a pathway-specific network pharmacology pipeline to prioritize plant polyphenols and potential drug combinations for use in breast cancer. Our analysis prioritized genistein among plant polyphenols. Drug combination simulations predicted several FDA-approved drugs in breast cancer with well-established pharmacology as candidates for target network synergistic combination with genistein. This study also highlights the concept of target network enhancer drugs, with drugs previously not well characterised in breast cancer being prioritized for use in the MEK5/ERK5 pathway in breast cancer.

CONCLUSION

This study proposes a computational framework for drug prioritization and combination with the MEK5/ERK5 signaling pathway in breast cancer. The method is flexible and provides the scientific community with a robust method that can be applied to other complex diseases.

β-III Tubulin Levels Determine the Neurotoxicity Induced by Colchicine-Site Binding Agent Indibulin

Indibulin, a microtubule-depolymerizing agent, produces minimal neurotoxicity in animals. It is also less cytotoxic toward differentiated neuronal cells than undifferentiated cells. We found that the levels of β-III tubulin, acetylated tubulin, and polyglutamylated tubulin were significantly increased in differentiated neuroblastoma cells (SH-SY5Y). Since neuronal cells express β-tubulin isotypes differently from other cell types, we explored the binding of indibulin to different β-tubulin isotypes. Our molecular docking analysis suggested that indibulin binds to β-III tubulin with lower affinity than to other β-tubulin isotypes. We therefore studied the implications of different β-tubulin isotypes on the cytotoxic effects of indibulin, colchicine, and vinblastine in differentiated SH-SY5Y cells. Upon depletion of β-III tubulin in the differentiated cells, the toxicity of indibulin and colchicine significantly increased, while sensitivity to vinblastine was unaffected. Using biochemical, bioinformatics, and fluorescence spectroscopic techniques, we have identified the binding site of indibulin on tubulin, which had not previously been established. Indibulin inhibited the binding of colchicine and C12 (a colchicine-site binder) to tubulin and also increased the dissociation constant of the interaction between tubulin and colchicine. Indibulin did not inhibit the binding of vinblastine or taxol to tubulin. Interestingly, indibulin antagonized colchicine treatment but synergized with vinblastine treatment in a combination study performed in MDA-MB-231 cells. The results indicate that indibulin is a colchicine-site binder and that the efficacy of colchicine-site binders is affected by the β-III tubulin levels in the cells.

Naturally derived indole alkaloids targeting regulated cell death (RCD) for cancer therapy: from molecular mechanisms to potential therapeutic targets

Regulated cell death (RCD) is a critical and active process that is controlled by specific signal transduction pathways and can be regulated by genetic signals or drug interventions. Meanwhile, RCD is closely related to the occurrence and therapy of multiple human cancers. Generally, RCD subroutines are the key signals of tumorigenesis, which are contributed to our better understanding of cancer pathogenesis and therapeutics. Indole alkaloids derived from natural sources are well defined for their outstanding biological and pharmacological properties, like vincristine, vinblastine, staurosporine, indirubin, and 3,3′-diindolylmethane, which are currently used in the clinic or under clinical assessment. Moreover, such compounds play a significant role in discovering novel anticancer agents. Thus, here we systemically summarized recent advances in indole alkaloids as anticancer agents by targeting different RCD subroutines, including the classical apoptosis and autophagic cell death signaling pathways as well as the crucial signaling pathways of other RCD subroutines, such as ferroptosis, mitotic catastrophe, necroptosis, and anoikis, in cancer. Moreover, we further discussed the cross talk between different RCD subroutines mediated by indole alkaloids and the combined strategies of multiple agents (e.g., 3,10-dibromofascaplysin combined with olaparib) to exhibit therapeutic potential against various cancers by regulating RCD subroutines. In short, the information provided in this review on the regulation of cell death by indole alkaloids against different targets is expected to be beneficial for the design of novel molecules with greater targeting and biological properties, thereby facilitating the development of new strategies for cancer therapy.

Combination of microtubule targeting agents with other antineoplastics for cancer treatment

Microtubule targeting agents (MTAs) have attracted extensive attention for cancer treatment. However, their clinical efficacies are limited by intolerable toxicities, inadequate efficacy and acquired multidrug resistance. The combination of MTAs with other antineoplastics has become an efficient strategy to lower the toxicities, overcome resistance and improve the efficacies for cancer treatment. In this article, we review the combinations of MTAs with some other anticancer drugs, such as cytotoxic agents, kinases inhibitors, histone deacetylase inhibitors, immune checkpoints inhibitors, to overcome these obstacles. We strongly believe that this review will provide helpful information for combination therapy based on MTAs.

Exploring the anticancer activities of novel bioactive compounds derived from endophytic fungi: mechanisms of action, current challenges and future perspectives

Cancer is the second leading cause of death all around the world. The natural compounds derived from the endophytic flora of fungi are possible solutions to cancer treatment because they are safe for health, cost-effective, biocompatible and have fewer toxicity issues. The active ingredients in endophytic fungi that are responsible for anti-cancer activities are alkaloids, terpenoids, glycosides, saponin, peptides, steroids, phenols, quinones, and flavonoids. This review highlights the anti-cancer activities of entophytic fungus against human papillary thyroid carcinoma (IHH4), human pancreatic (PANC-1), ovarian (OVCAR-3), hepatic (HepG2), lung (A-549), human lymphoma (U937), human skin carcinoma (A431), breast (MCF-7), and Kaposi's sarcoma. The emerging evidence suggested that bioactive compounds isolated from endophytic fungi showed their anti-cancer activities by revealing the disturbance of the microtubule network caused by increased levels of Bax and Bcl-2 proteins that triggers cell cycle arrest at the G2-M phase, by inhibiting the DNA replication via binding with topoisomerase II, by regulating the activity of extracellular signal-regulated kinase and NF-kB, by evaluating the levels of p21, p27, and cyclins B/D1/E that led to cell death by apoptosis and cell cycle arrest. This review will assist readers in better comprehending bioactive chemicals and the beneficial interaction between the fungal endophytes and medicinal plants.

Discovery of new quinolines as potent colchicine binding site inhibitors: design, synthesis, docking studies, and anti-proliferative evaluation

Discovering of new anticancer agents with potential activity against tubulin polymerisation is still a promising approach. Colchicine binding site inhibitors are the most relevant anti-tubulin polymerisation agents. Thus, new quinoline derivatives have been designed and synthesised to possess the same essential pharmacophoric features of colchicine binding site inhibitors. The synthesised compounds were tested in vitro against a panel of three human cancer cell lines (HepG-2, HCT-116, and MCF-7) using colchicine as a positive control. Comparing to colchicine (IC50 = 7.40, 9.32, and 10.41 µM against HepG-2, HCT-116, and MCF-7, respectively), compounds 20, 21, 22, 23, 24, 25, 26, and 28 exhibited superior cytotoxic activities with IC50 values ranging from 1.78 to 9.19 µM. In order to sightsee the proposed mechanism of anti-proliferative activity, the most active members were further evaluated in vitro for their inhibitory activities against tubulin polymerisation. Compounds 21 and 32 exhibited the highest tubulin polymerisation inhibitory effect with IC50 values of 9.11 and 10.5 nM, respectively. Such members showed activities higher than that of colchicine (IC50 = 10.6 nM) and CA-4 (IC50 = 13.2 nM). The impact of the most promising compound 25 on cell cycle distribution was assessed. The results revealed that compound 25 can arrest the cell cycle at G2/M phase. Annexin V and PI double staining assay was carried out to explore the apoptotic effect of the synthesised compounds. Compound 25 induced apoptotic effect on HepG-2 thirteen times more than the control cells. To examine the binding pattern of the target compounds against the tubulin heterodimers active site, molecular docking studies were carried out.

Discovery of novel quinoline-based analogues of combretastatin A-4 as tubulin polymerisation inhibitors with apoptosis inducing activity and potent anticancer effect

A new series of quinoline derivatives of combretastatin A-4 have been designed, synthesised and demonstrated as tubulin polymerisation inhibitors. These novel compounds showed significant antiproliferative activities, among them, 12c exhibited the most potent inhibitory activity against different cancer cell lines (MCF-7, HL-60, HCT-116 and HeLa) with IC50 ranging from 0.010 to 0.042 µM, and with selectivity profile against MCF-10A non-cancer cells. Further mechanistic studies suggest that 12c can inhibit tubulin polymerisation and cell migration, leading to G2/M phase arrest. Besides, 12c induces apoptosis via a mitochondrial-dependant apoptosis pathway and caused reactive oxygen stress generation in MCF-7 cells. These results provide guidance for further rational development of potent tubulin polymerisation inhibitors for the treatment of cancer.HighlightsA novel series of quinoline derivatives of combretastatin A-4 have been designed and synthesised.Compound 12c showed significant antiproliferative activities against different cancer cell lines.Compound 12c effectively inhibited tubulin polymerisation and competed with [3H] colchicine in binding to tubulin.Compound 12c arrested the cell cycle at G2/M phase, effectively inducing apoptosis and inhibition of cell migration.

Design, Synthesis, and Antipoliferative Activities of Novel Substituted Imidazole-Thione Linked Benzotriazole Derivatives

A new series of benzotriazole moiety bearing substituted imidazol-2-thiones at N1 has been designed, synthesized and evaluated for in vitro anticancer activity against the different cancer cell lines MCF-7(breast cancer), HL-60 (Human promyelocytic leukemia), and HCT-116 (colon cancer). Most of the benzotriazole analogues exhibited promising antiproliferative activity against tested cancer cell lines. Among all the synthesized compounds, BI9 showed potent activity against the cancer cell lines such as MCF-7, HL-60 and HCT-116 with IC50 3.57, 0.40 and 2.63 µM, respectively. Compound BI9 was taken up for elaborate biological studies and the HL-60 cells in the cell cycle were arrested in G2/M phase. Compound BI9 showed remarkable inhibition of tubulin polymerization with the colchicine binding site of tubulin. In addition, compound BI9 promoted apoptosis by regulating the expression of pro-apoptotic protein BAX and anti-apoptotic proteins Bcl-2. These results provide guidance for further rational development of potent tubulin polymerization inhibitors for the treatment of cancer.

Synthesis and biological evaluation of indolylglyoxylamide bisphosphonates, antimitotic microtubule-targeting derivatives of indibulin with improved aqueous solubility

Indibulin (D-24851) derivatives with bisphosphonate fragment connected to the N1 atom of imidazole ring were synthesized by alkylation of (indolyl-3)methylglyoxylates with ethylenebisphosphonate. Biological evaluation of targeted compounds 4a-d using the phenotypic sea urchin embryo assay provided evidence that replacing of p-chlorobenzene ring in indibulin by bisphosphonate group did not eliminate antimitotic microtubule destabilizing activity. The most active molecule, tetraacid 5a, at physiological pH formed tetrasodium salt 6a with aqueous solubility value of at least 10 mg/mL. Molecule 5a was more potent in the sea urchin embryo assay than the parent indibulin. This compound also exhibited pronounced cytotoxicity against A549 lung carcinoma and A375 melanoma cell lines.

Biotechnological approaches to the production of plant-derived promising anticancer agents: An update and overview

The plant kingdom is a rich source of bioactive compounds, many of which have been used since pre-history for their therapeutic properties to treat a range of illnesses. These metabolites have recently attracted attention to their antineoplastic activities to treat various cancers relying on different mechanisms. Some of these molecules are glycosides, which have proven useful as anti-cancer agents, namely podophyllotoxin (PPT) anaryltetralin lignan or alkaloids. There are three primary forms of alkaloids, such as indole alkaloids (vincristine and vinblastine from Catharanthus roseus), quinoline alkaloid (camptothecin from Camptotheca acuminata), and diterpenoid alkaloid (taxol and it's analogous from Taxus and Corylus species). This review considers various plant biotechnology approaches used to enhance the production of these anticancer molecules in different species. In this regard, many in vitro culture techniques such as stimulation of suspension culture and hairy roots are being used to investigate the effects of plant growth regulators and elicitors on various explants.

Intrinsic and Extrinsic Factors Affecting Microtubule Dynamics in Normal and Cancer Cells

Microtubules (MTs), highly dynamic structures composed of α- and β-tubulin heterodimers, are involved in cell movement and intracellular traffic and are essential for cell division. Within the cell, MTs are not uniform as they can be composed of different tubulin isotypes that are post-translationally modified and interact with different microtubule-associated proteins (MAPs). These diverse intrinsic factors influence the dynamics of MTs. Extrinsic factors such as microtubule-targeting agents (MTAs) can also affect MT dynamics. MTAs can be divided into two main categories: microtubule-stabilizing agents (MSAs) and microtubule-destabilizing agents (MDAs). Thus, the MT skeleton is an important target for anticancer therapy. This review discusses factors that determine the microtubule dynamics in normal and cancer cells and describes microtubule-MTA interactions, highlighting the importance of tubulin isoform diversity and post-translational modifications in MTA responses and the consequences of such a phenomenon, including drug resistance development.

Design, eco-friendly synthesis, molecular modeling and anticancer evaluation of thiazol-5(4H)-ones as potential tubulin polymerization inhibitors targeting the colchicine binding site

In recent years, suppressing tubulin polymerization has been developed as a therapeutic approach for cancer treatment. Thus, new derivatives based on thiazol-5(4H)-ones have been designed and synthesized in an eco-friendly manner. The synthesized derivatives have the same essential pharmacophoric features of colchicine binding site inhibitors. The anti-proliferative activity of the new derivatives was evaluated on three human cancer cell lines (HCT-116, HepG-2, and MCF-7) using MTT assay procedure and colchicine was used as a positive control. Compounds 4f, 5a, 8f, 8g, and 8k showed superior antiproliferative activities against the three tested cell lines with IC50 values ranging from 2.89 to 9.29 μM. Further investigation for the most active cytotoxic agents as tubulin polymerization inhibitors was also performed in order to explore the mechanism of their anti-proliferative activity. Tubulin polymerization assay results were found to be comperable with the cytotoxicity results. Compounds 4f and 5a were the most potent tubulin polymerization inhibitors with an IC50 value of 9.33 and 9.52 nM, respectively. Further studies revealed the ability of 5a to induce apoptosis and arrest cell cycle growth at the G2/M phase. Molecular docking studies were also conducted to investigate possible binding interactions between the target compounds and the tubulin heterodimer active site. From these studies, it was concluded that inhibition of tubulin polymerization yields the reported cytotoxic activity.

Methyl-β-cyclodextrin, an actin depolymerizer augments the antiproliferative potential of microtubule-targeting agents

Methyl-β-cyclodextrin (MCD), an established pharmacological excipient, depolymerizes the actin cytoskeleton. In this work, we investigated the effect of MCD-mediated actin depolymerization on various cellular phenotypes including traction force, cell stiffness, focal adhesions, and intracellular drug accumulation. In addition to a reduction in the contractile cellular traction, MCD acutely inhibits the maturation of focal adhesions. Alteration of contractile forces and focal adhesions affects the trypsin-mediated detachment kinetics of cells. Moreover, MCD-mediated actin depolymerization increases the intracellular accumulation of microtubule-targeting agents (MTAs) by ~50% with respect to the untreated cells. As MCD treatment enhances the intracellular concentration of drugs, we hypothesized that the MCD-sensitized cancer cells could be effectively killed by low doses of MTAs. Our results in cervical, breast, hepatocellular, prostate cancer and multidrug-resistant breast cancer cells confirmed the above hypothesis. Further, the combined use of MCD and MTAs synergistically inhibits the proliferation of tumor cells. These results indicate the potential use of MCD in combination with MTAs for cancer chemotherapy and suggest that targeting both actin and microtubules simultaneously may be useful for cancer therapy. Importantly, the results provide significant insight into the crosstalk between actin and microtubules in regulating the traction force and dynamics of cell deadhesion.

An acetylation mimicking mutation, K274Q, in tau imparts neurotoxicity by enhancing tau aggregation and inhibiting tubulin polymerization

In Alzheimer's disease, tau is predominantly acetylated at K174, K274, K280, and K281 residues. The acetylation of K274-tau is linked with memory loss and dementia. In this study, we have examined the molecular mechanism of the toxicity of acetylated K274-tau. We incorporated an acetylation mimicking mutation at K274 (K→Q) residue of tau. The mutation (K274Q) strongly reduced the ability of tau to bind to tubulin and also to polymerize tubulin while K274R mutation did not reduce the ability of tau either to bind or polymerize tubulin. In addition, K274Q-tau displayed a higher aggregation propensity than wild-type tau as evident from thioflavin S fluorescence, tryptophan fluorescence, and electron microscopic images. Furthermore, dynamic light scattering, atomic force microscopy, and dot blot analysis using an oligomer-specific antibody suggested that K274Q mutation enhanced the oligomerization of tau. The K274Q mutation also strongly decreased the critical concentration for the liquid-liquid phase separation of tau. The oligomeric forms of K274Q-tau were found to be more toxic than wild tau to neuroblastoma cells. Using circular dichroism and fluorescence spectroscopy, we provide evidence indicating that the acetylation mimicking mutation (K274Q) induced conformational changes in tau. The results suggested that the acetylation of tau at 274 residues can increase tau aggregation and enhance the cytotoxicity of tau oligomers.

Crocin, a carotenoid, suppresses spindle microtubule dynamics and activates the mitotic checkpoint by binding to tubulin

Crocin, a constituent of the saffron spice, exhibits promising antitumor activity in animal models and also inhibits the proliferation of several types of cancer cells in culture. Recently, we have shown that crocin binds to purified tubulin at the vinblastine site, depolymerizes microtubules and induces a mitotic block in cultured cells. Here, we extend our previous suggestion and explore the cellular effects of crocin to further understand its mechanism of action. In a kinetic study, we observed that the crocin-induced depolymerization of microtubules preceded both DNA damage and reactive oxygen species generation indicating that depolymerizing microtubules is the primary action of crocin. Crocin also inhibited the growth of cold-depolymerized microtubules in HeLa cells indicating that it can inhibit microtubule dynamics. Using fluorescence recovery after photobleaching, crocin was found to suppress the spindle microtubule dynamics in live HeLa cells. Further, crocin treatment resulted in activation of spindle assembly checkpoint proteins, BubR1 and Mad2. Similar to other microtubule-targeting agents, crocin also perturbed the localization of end-binding protein EB1 from the growing microtubule ends and enhanced the acetylation of remaining microtubules. Further, crocin was found to bind to purified tubulin with a dissociation constant of 12 ± 1.5 μM. The results suggested that crocin exerted its antiproliferative effect primarily by inhibiting the assembly and dynamics of microtubules. Importantly, the combination of crocin with known anticancer agents like combretastatin A-4, cisplatin, doxorubicin or sorafenib, exerted a strong synergistic cytotoxic effect in HeLa cells indicating that crocin may enhance the effectiveness of other anticancer agents.