Targeting the microtubular network as a new antimyeloma strategy

The adaptor protein Miro1 modulates horizontal transfer of mitochondria in mouse melanoma models

Recent research has shown that mtDNA-deficient cancer cells (ρ0 cells) acquire mitochondria from tumor stromal cells to restore respiration, facilitating tumor formation. We investigated the role of Miro1, an adaptor protein involved in movement of mitochondria along microtubules, in this phenomenon. Inducible Miro1 knockout (Miro1KO) mice markedly delayed tumor formation after grafting ρ0 cancer cells. Miro1KO mice with fluorescently labeled mitochondria revealed that this delay was due to hindered mitochondrial transfer from the tumor stromal cells to grafted B16 ρ0 cells, which impeded recovery of mitochondrial respiration and tumor growth. Miro1KO led to the perinuclear accumulation of mitochondria and impaired mobility of the mitochondrial network. In vitro experiments revealed decreased association of mitochondria with microtubules, compromising mitochondrial transfer via tunneling nanotubes (TNTs) in mesenchymal stromal cells. Here we show the role of Miro1 in horizontal mitochondrial transfer in mouse melanoma models in vivo and its involvement with TNTs.

Arrest and Attack: Microtubule-Targeting Agents and Oncolytic Viruses Employ Complementary Mechanisms to Enhance Anti-Tumor Therapy Efficacy

Oncolytic viruses (OVs) are promising cancer immunotherapy agents that stimulate anti-tumor immunity through the preferential infection and killing of tumor cells. OVs are currently under limited clinical usage, due in part to their restricted efficacy as monotherapies. Current efforts for enhancement of the therapeutic potency of OVs involve their combination with other therapy modalities, aiming at the concomitant exploitation of complementary tumor weaknesses. In this context, microtubule-targeting agents (MTAs) pose as an enticing option, as they perturb microtubule dynamics and function, induce cell-cycle arrest, and cause mitotic cell death. MTAs induce therapeutic benefit through cancer-cell-autonomous and non-cell-autonomous mechanisms and are a main component of the standard of care for different malignancies. However, off-target effects and acquired resistance involving distinct cellular and molecular mechanisms may limit the overall efficacy of MTA-based therapy. When combined, OVs and MTAs may enhance therapeutic efficacy through increases in OV infection and immunogenic cell death and a decreased probability of acquired resistance. In this review, we introduce OVs and MTAs, describe molecular features of their activity in cancer cells, and discuss studies and clinical trials in which the combination has been tested.

Anti-cancer effects of fenbendazole on 5-fluorouracil-resistant colorectal cancer cells

Benzimidazole anthelmintic agents have been recently repurposed to overcome cancers resistant to conventional therapies. To evaluate the anti-cancer effects of benzimidazole on resistant cells, various cell death pathways were investigated in 5-fluorouracil-resistant colorectal cancer cells. The viability of wild-type and 5-fluorouracil-resistant SNU-C5 colorectal cancer cells was assayed, followed by Western blotting. Flow cytometry assays for cell death and cell cycle was also performed to analyze the anti-cancer effects of benzimidazole. When compared with albendazole, fenbendazole showed higher susceptibility to 5-fluorouracil-resistant SNU-C5 cells and was used in subsequent experiments. Flow cytometry revealed that fenbendazole significantly induces apoptosis as well as cell cycle arrest at G2/M phase on both cells. When compared with wild-type SNU-C5 cells, 5-fluorouracil-resistant SNU-C5 cells showed reduced autophagy, increased ferroptosis and ferroptosis-augmented apoptosis, and less activation of caspase-8 and p53. These results suggest that fenbendazole may be a potential alternative treatment in 5-fluorouracil-resistant cancer cells, and the anticancer activity of fenbendazole does not require p53 in 5-fluorouracil-resistant SNU-C5 cells.

Cytotoxic Effects of the Schweinfurthin Analog 5′-Methylschweinfurthin G in Malignant Plasma Cells

<b><i>Introduction:</i></b> Multiple myeloma (MM) is a B plasma cell malignancy currently incurable, and novel therapeutics are needed. Evidences regarding the effect of natural compound schweinfurthins suggest that hematological cancers showed growth inhibitory effects to this family of compounds at single nanomolar concentrations. In this study, we evaluated the cytotoxicity of the schweinfurthin synthetic analog 5′-methylschweinfurthin G (MeSG) in MM cell lines, to better understand the validity of this compound as a therapeutic candidate for further studies in MM. <b><i>Methods:</i></b> MeSG toxicity against MM cell lines RPMI-8226, MM.1S, and H-929 was evaluated. Trypan blue exclusion and MTT assays measured cell viability and mitochondrial activity, respectively. Flow cytometry was performed to detect apoptotic mitochondria. Flow cytometry and Western blotting techniques were used to investigate apoptosis and to examine the cell cycle. Western blotting was used to determine AKT activation upon MeSG treatment. <b><i>Results:</i></b> We provide evidence that in all MM cells analyzed, MeSG exerts diverse cytotoxic effects. MeSG treatment of MM.1S and H-929, but not in RPMI-8226, causes a loss of mitochondria membrane potential. MeSG causes an arrest in G₂/M, especially in RPMI-8226, supported by decreased levels of cyclin-B1 and early increased levels of p21. Finally, there is a diverse response to the MeSG treatment for AKT phosphorylation. MM.1S and H-929 showed a marked decrease in AKT phosphorylation at earlier time points compared to the RPMI-8226 line. <b><i>Conclusions:</i></b> MeSG cytotoxicity has been confirmed in all of 3 cell lines studied. Results suggest an early event of increased reactive oxygen species, and/or involvement of cholesterol homeostasis via decreased AKT activation, both of which are currently under investigation.

Medicinal Chemistry Strategies for the Development of Bruton's Tyrosine Kinase Inhibitors against Resistance

Despite significant efficacy, one of the major limitations of small-molecule Bruton's tyrosine kinase (BTK) agents is the presence of clinically acquired resistance, which remains a major clinical challenge. This Perspective focuses on medicinal chemistry strategies for the development of BTK small-molecule inhibitors against resistance, including the structure-based design of BTK inhibitors targeting point mutations, e.g., (i) developing noncovalent inhibitors from covalent inhibitors, (ii) avoiding steric hindrance from mutated residues, (iii) making interactions with the mutated residue, (iv) modifying the solvent-accessible region, and (v) developing new scaffolds. Additionally, a comparative analysis of multi-inhibitions of BTK is presented based on cross-comparisons between 2916 unique BTK ligands and 283 other kinases that cover 7108 dual/multiple inhibitions. Finally, targeting the BTK allosteric site and uding proteolysis-targeting chimera (PROTAC) as two potential strategies are addressed briefly, while also illustrating the possibilities and challenges to find novel ligands of BTK.

The evolving role and utility of off-label drug use in multiple myeloma

The treatment landscape for multiple myeloma (MM) has dramatically changed over the last three decades, moving from no US Food and Drug Administration approvals and two active drug classes to over 19 drug approvals and at least eight different active classes. The advances seen in MM therapy have relied on both a structured approach to obtaining new labels and cautious off-label drug use. Although there are country and regional differences in drug approval processes, many of the basic principles behind off-label drug use in MM can be summarized into four main categories: 1) use of a therapy prior to the current approval regulations; 2) widespread use of a therapy following the release of promising clinical trial results but prior to drug approval; 3) use of a cheap therapy supported by clinical safety and efficacy data but without commercial backing; and 4) niche therapies for small well-defined patient populations where large clinical trials with sufficient power may be difficult to perform. This review takes a historical approach to discuss how off-label drug use has helped to shape the current treatment approach for MM.

Thiophene derivative-loaded nanoparticles mediate anticancer activity through the inhibition of kinases and microtubule assembly

Different tetrahydrobenzo[b]thiophene derivatives were explored as new tubulin polymerization destabilizers to arrest tumor cell mitosis. A series of compounds incorporating the tetrahydrobenzo[b]thiophene scaffold were synthesized, and their biological activities were investigated. The cytotoxicity of each of the synthesized compounds was assessed against a range of cell lines. Specifically, the benzyl urea tetrahydrobenzo[b]thiophene derivative, 1-benzyl-3-(3-cyano-4,5,6,7-tetrahydrobenzo[b]thiophen-2-yl)urea (BU17), was identified as the most potent compound with broad-spectrum antitumor activity against several cancer cell lines. The potential mechanism(s) of action were investigated where dose-dependent G2/M accumulation and A549 cell cycle arrest were detected. Additionally, A549 cells treated with BU17 expressed enhanced levels of caspase 3 and 9, indicating the induction of apoptosis. Furthermore, it was found that BU17 inhibits WEE1 kinase and targets tubulin by blocking its polymerization. BU17 was also formulated into PLGA nanoparticles, and it was demonstrated that BU17-loaded nanoparticles could significantly enhance antitumor activity compared to the soluble counterpart.

Cyclohepta[b]thiophenes as Potential Antiproliferative Agents: Design, Synthesis, In Vitro, and In Vivo Anticancer Evaluation

Several thiophene featuring compounds are known for their promising antiproliferative activity. Prompted by the urgent need to identify new potent anticancer agents, 16 compounds of benzamides, benzylamines, and urea analogues incorporating a cyclohepta[b]thiophene scaffold were synthesized and biologically evaluated with a cell proliferation assay using the A549 nonsmall cell lung cancer cell line. Compound 17 demonstrated both potent and broad-spectrum anticancer activity with submicromolar 50% growth inhibition (GI₅₀) values. It also showed superior antiproliferative activity (vs nocodazole) in OVACAR-4, OVACAR-5, CAKI-1, and T47D cell lines with GI₅₀ values of 2.01 (vs 22.28), 2.27 (vs 20.75), 0.69 (vs 1.11), and 0.362 (vs 81.283) μM, respectively. Additionally, compound 17 displayed minimal cytotoxicity based on 50% lethal concentration (LC₅₀) values toward all tested cell lines. Further cell-based mechanistic studies of compound 17 revealed its ability to induce cell cycle arrest of A549 cells as evidenced by dose dependent G₂/M accumulation. Furthermore, induction of early apoptosis along with activation of caspase 3, 8, and 9 were confirmed in A549 cells treated with compound 17. Targeting tubulin polymerization may explain the mechanism of the antiproliferative activity of compound 17 based on cell cycle analysis, detected apoptosis, and in vitro inhibition of tubulin polymerization. In vitro data were further supported by in vivo antitumor efficacy studies of compound 17 in a CT26 murine model for which the results showed a reduction in the tumor growth compared to untreated mice. Overall, compound 17 has the potential to function as a promising candidate for further development of potent anticancer chemotherapeutics.

Cell Adhesiveness Serves as a Biophysical Marker for Metastatic Potential

Tumors are heterogeneous and composed of cells with different dissemination abilities. Despite significant effort, there is no universal biological marker that serves as a metric for metastatic potential of solid tumors. Common to disseminating cells from such tumors, however, is the need to modulate their adhesion as they detach from the tumor and migrate through stroma to intravasate. Adhesion strength is heterogeneous even among cancer cells within a given population, and using a parallel plate flow chamber, we separated and sorted these populations into weakly and strongly adherent groups; when cultured under stromal conditions, this adhesion phenotype was stable over multiple days, sorting cycles, and common across all epithelial tumor lines investigated. Weakly adherent cells displayed increased migration in both two-dimensional and three-dimensional migration assays; this was maintained for several days in culture. Subpopulations did not show differences in expression of proteins involved in the focal adhesion complex but did exhibit intrinsic focal adhesion assembly as well as contractile differences that resulted from differential expression of genes involved in microtubules, cytoskeleton linkages, and motor activity. In human breast tumors, expression of genes associated with the weakly adherent population resulted in worse progression-free and disease-free intervals. These data suggest that adhesion strength could potentially serve as a stable marker for migration and metastatic potential within a given tumor population and that the fraction of weakly adherent cells present within a tumor could act as a physical marker for metastatic potential. SIGNIFICANCE: Cancer cells exhibit heterogeneity in adhesivity, which can be used to predict metastatic potential.

The novel compound STK405759 is a microtubule-targeting agent with potent and selective cytotoxicity against multiple myeloma in vitro and in vivo

Despite advances in treatment, multiple myeloma (MM) remains incurable. Here we propose the use of STK405759, a novel microtubule targeting agent (MTA) and member of the furan metotica family for MM therapy.

STK405759 inhibited tubulin polymerization in a cell-free system and in myeloma cells. This molecule had potent cytotoxic activity against several MM cell lines and patient-derived MM cells. Moreover, STK405759 demonstrated cytotoxicity against drug-resistant myeloma cells that overexpressed the P-glycoprotein drug-efflux pump. STK405759 was not cytotoxic to peripheral blood mononuclear cells, including activated B and T lymphocytes. This compound caused mitotic arrest and apoptosis of myeloma cells characterized by cleavage of poly (ADP-ribose) polymerase-1 and caspase-8, as well as decreased protein expression of mcl-1. The combination of STK405759 with bortezomib, lenalidomide or dexamethasone had synergistic cytotoxic activity. In in vivo studies, STK405759-treated mice had significantly decreased MM tumor burden and prolonged survival compared to vehicle treated- mice.

These results provide a rationale for further evaluation of STK405759 as monotherapy or part of combination therapy for treating patients with MM.

Multilevel induction of apoptosis by microtubule-interfering inhibitors 4β-S-aromatic heterocyclic podophyllum derivatives causing multi-fold mitochondrial depolarization and PKA signaling pathways in HeLa cells

Herein is a first effort to study effect of carbon-sulfur (C-S) and carbon-nitrogen (C-N) bonds modification on the antitumor activity of the podophyllum derivatives in HeLa cells. Compared with the derivative modified by the C-N bond, the C-S bond modification exhibited superior antitumor activity by further causing significant mitochondria depolarization from three signaling pathway. First, a large number of microtubules were depolymerized by 4β-S-heterocyclic substituted podophyllum derivatives. The increasing free tubulin bond with voltage-dependent anion-selective channel (VDAC). Second, cAMP-dependent protein kinase A (PKA) was activated by 4β-S-heterocyclic substituted podophyllum derivatives. And then the activated PKA further caused significantly mitochondria depolarization. Third, the activated PKA also activated c-Jun N-terminal kinase (JNK) and further deceased MMP by improving the level of reactive oxygen species. Understanding the molecular events that contribute to drug-induced tumors apoptosis should provide a paradigm for a more rational approach to antitumor drug design.

Elevated Translation Initiation Factor eIF4E Is an Attractive Therapeutic Target in Multiple Myeloma

eIF4E is the key regulator of protein translation and critical for translation. The oncogenic potential of tumorigenesis, which is highly contingent on cap-dependent eIF4E, also arises from the critical role in the nuclear export and cytosolic translation of oncogenic transcripts. Inhibition of Exportin1 (XPO1), which is the major nuclear export protein for eIF4E-bound oncoprotein mRNAs, results in decreased tumor cell growth in vitro and in vivo, suggesting that eIF4E is critical in multiple myeloma. Indeed, we found that eIF4E is overexpressed in myeloma cell lines and primary myeloma cells compared with normal plasma cells. Although stable overexpression of eIF4E in multiple myeloma cells significantly increases tumorigenesis, knockdown of eIF4E impairs multiple myeloma tumor progression in a human xenograft mouse model. Using a tet-on-inducible eIF4E-knockdown system, eIF4E downregulation blocks multiple myeloma tumor growth in vivo, correlating with decreased eIF4E expression. Further overexpression and knockdown of eIF4E revealed that eIF4E regulates translation of mRNAs with highly complex 5'-untranslated regions, such as c-MYC and C/EBPβ, and subsequently proliferation in multiple myeloma cells, but not in nonmalignant bone marrow stromal cells. Because many transcription factors that are critical for multiple myeloma proliferation exhibit a higher dependency on protein translation, eIF4E is an ideal and selective tool to target multiple myeloma cell growth. Mol Cancer Ther; 15(4); 711-9. ©2016 AACR.

Mitotic Transcriptional Activation: Clearance of Actively Engaged Pol II via Transcriptional Elongation Control in Mitosis

Although it is established that some general transcription factors are inactivated at mitosis, many details of mitotic transcription inhibition (MTI) and its underlying mechanisms are largely unknown. We have identified mitotic transcriptional activation (MTA) as a key regulatory step to control transcription in mitosis for genes with transcriptionally engaged RNA polymerase II (Pol II) to activate and transcribe until the end of the gene to clear Pol II from mitotic chromatin, followed by global impairment of transcription reinitiation through MTI. Global nascent RNA sequencing and RNA fluorescence in situ hybridization demonstrate the existence of transcriptionally engaged Pol II in early mitosis. Both genetic and chemical inhibition of P-TEFb in mitosis lead to delays in the progression of cell division. Together, our study reveals a mechanism for MTA and MTI whereby transcriptionally engaged Pol II can progress into productive elongation and finish transcription to allow proper cellular division.

A novel ER-microtubule-binding protein, ERLIN2, stabilizes Cyclin B1 and regulates cell cycle progression

The gene encoding endoplasmic reticulum (ER) lipid raft-associated protein 2 (ERLIN2) is amplified in human breast cancers. ERLIN2 gene mutations were also found to be associated with human childhood progressive motor neuron diseases. Yet, an understanding of the physiological function and mechanism for ERLIN2 remains elusive. In this study, we reveal that ERLIN2 is a spatially and temporally regulated ER–microtubule-binding protein that has an important role in cell cycle progression by interacting with and stabilizing the mitosis-promoting factors. Whereas ERLIN2 is highly expressed in aggressive human breast cancers, during normal development ERLIN2 is expressed at the postnatal stage and becomes undetectable in adulthood. ERLIN2 interacts with the microtubule component α-tubulin, and this interaction is maximal during the cell cycle G2/M phase where ERLIN2 simultaneously interacts with the mitosis-promoting complex Cyclin B1/Cdk1. ERLIN2 facilitates K63-linked ubiquitination and stabilization of Cyclin B1 protein in G2/M phase. Downregulation of ERLIN2 results in cell cycle arrest, represses breast cancer proliferation and malignancy and increases sensitivity of breast cancer cells to anticancer drugs. In summary, our study revealed a novel ER–microtubule-binding protein, ERLIN2, which interacts with and stabilizes mitosis-promoting factors to regulate cell cycle progression associated with human breast cancer malignancy.

Targeting cannabinoid receptor-2 pathway by phenylacetylamide suppresses the proliferation of human myeloma cells through mitotic dysregulation and cytoskeleton disruption

Cannabinoid receptor-2 (CB2) is expressed dominantly in the immune system, especially on plasma cells. Cannabinergic ligands with CB2 selectivity emerge as a class of promising agents to treat CB2-expressing malignancies without psychotropic concerns. In this study, we found that CB2 but not CB1 was highly expressed in human multiple myeloma (MM) and primary CD138+ cells. A novel inverse agonist of CB2, phenylacetylamide but not CB1 inverse agonist SR141716, inhibited the proliferation of human MM cells (IC50 : 0.62 ∼ 2.5 μM) mediated by apoptosis induction, but exhibited minor cytotoxic effects on human normal mononuclear cells. CB2 gene silencing or pharmacological antagonism markedly attenuated phenylacetylamide's anti-MM effects. Phenylacetylamide triggered the expression of C/EBP homologous protein at the early treatment stage, followed by death receptor-5 upregulation, caspase activation, and β-actin/tubulin degradation. Cell cycle related protein cdc25C and mitotic regulator Aurora A kinase were inactivated by phenylacetylamide treatment, leading to an increase in the ratio inactive/active cdc2 kinase. As a result, phosphorylation of CDK substrates was decreased, and the MM cell mitotic division was largely blocked by treatment. Importantly, phenylacetylamide could overcome the chemoresistance of MM cells against dexamethasone or melphalan. Thus, targeting CB2 may represent an attractive approach to treat cancers of immune origin.

Antagonism of cannabinoid receptor 2 pathway suppresses IL-6-induced immunoglobulin IgM secretion

Background

Cannabinoid receptor 2 (CB2) is expressed predominantly in the immune system, particularly in plasma cells, raising the possibility that targeting the CB2 pathway could yield an immunomodulatory effect. Although the role of CB2 in mediating immunoglobulin class switching has been reported, the effects of targeting the CB2 pathway on immunoglobulin secretion per se remain unclear.

Methods

Human B cell line SKW 6.4, which is capable of differentiating into IgM-secreting cells once treated with human IL-6, was employed as the cell model. SKW 6.4 cells were incubated for 4 days with CB2 ligands plus IL-6 (100 U/ml). The amount of secreted IgM was determined by an ELISA. Cell proliferation was determined by the ³H-Thymidine incorporation assay. Signal molecules involved in the modulation of IgM secretion were examined by real-time RT-PCR and Western blot analyses or by using their specific inhibitors.

Results

We demonstrated that CB2 inverse agonists SR144528 and AM630, but not CB2 agonist HU308 or CB1 antagonist SR141716, effectively inhibited IL-6-induced secretion of soluble IgM without affecting cell proliferation as measured by thymidine uptake. SR144528 alone had no effects on the basal levels of IgM in the resting cells. These effects were receptor mediated, as pretreatment with CB2 agonist abrogated SR144528-mediated inhibition of IL-6 stimulated IgM secretion. Transcription factors relevant to B cell differentiation, Bcl-6 and PAX5, as well as the protein kinase STAT3 pathway were involved in the inhibition of IL-6-induced IgM by SR144528.

Conclusions

These results uncover a novel function of CB2 antagonists and suggest that CB2 ligands may be potential modulators of immunoglobulin secretion.

CXI-benzo-84 reversibly binds to tubulin at colchicine site and induces apoptosis in cancer cells

Here, we have discovered CXI-benzo-84 as a potential anticancer agent from a library of benzimidazole derivatives using cell based screening strategy. CXI-benzo-84 inhibited cell cycle progression in metaphase stage of mitosis and accumulated spindle assembly checkpoint proteins Mad2 and BubR1 on kinetochores, which subsequently activated apoptotic cell death in cancer cells. CXI-benzo-84 depolymerized both interphase and mitotic microtubules, perturbed EB1 binding to microtubules and inhibited the assembly and GTPase activity of tubulin in vitro. CXI-benzo-84 bound to tubulin at a single binding site with a dissociation constant of 1.2±0.2μM. Competition experiments and molecular docking suggested that CXI-benzo-84 binds to tubulin at the colchicine-site. Further, computational analysis provided a significant insight on the binding site of CXI-benzo-84 on tubulin. In addition to its potential use in cancer chemotherapy, CXI-benzo-84 may also be useful to screen colchicine-site agents and to understand the colchicine binding site on tubulin.