In Situ Characterization of Bak Clusters Responsible for Cell Death Using Single Molecule Localization Microscopy

Visualization of BOK pores independent of BAX and BAK reveals a similar mechanism with differing regulation

BOK is a poorly understood member of the BCL-2 family of proteins that has been proposed to function as a pro-apoptotic, BAX-like effector. However, the molecular mechanism and structural properties of BOK pores remain enigmatic. Here, we show that the thermal stability and pore activity of BOK depends on the presence of its C-terminus as well as on the mitochondrial lipid cardiolipin. We directly visualized BOK pores in liposomes by electron microscopy, which appeared similar to those induced by BAX, in line with comparable oligomerization properties quantified by single molecule imaging. In addition, super-resolution STED imaging revealed that BOK organized into dots and ring-shaped assemblies in apoptotic mitochondria, also reminiscent of those found for BAX and BAK. Yet, unlike BAX and BAK, the apoptotic activity of BOK was limited by partial mitochondrial localization and was independent of and unaffected by other BCL-2 proteins. These results suggest that, while BOK activity is kept in check by subcellular localization instead of interaction with BCL-2 family members, the resulting pores are structurally similar to those of BAX and BAK.

Membrane damage and repair: a thin line between life and death

Bilayered membranes separate cells from their surroundings and form boundaries between intracellular organelles and the cytosol. Gated transport of solutes across membranes enables cells to establish vital ion gradients and a sophisticated metabolic network. However, an advanced compartmentalization of biochemical reactions makes cells also particularly vulnerable to membrane damage inflicted by pathogens, chemicals, inflammatory responses or mechanical stress. To avoid potentially lethal consequences of membrane injuries, cells continuously monitor the structural integrity of their membranes and readily activate appropriate pathways to plug, patch, engulf or shed the damaged membrane area. Here, we review recent insights into the cellular mechanisms that underly an effective maintenance of membrane integrity. We discuss how cells respond to membrane lesions caused by bacterial toxins and endogenous pore-forming proteins, with a primary focus on the intimate crosstalk between membrane proteins and lipids during wound formation, detection and elimination. We also discuss how a delicate balance between membrane damage and repair determines cell fate upon bacterial infection or activation of pro-inflammatory cell death pathways.

The interplay between BAX and BAK tunes apoptotic pore growth to control mitochondrial-DNA-mediated inflammation

BAX and BAK are key apoptosis regulators that mediate the decisive step of mitochondrial outer membrane permeabilization. However, the mechanism by which they assemble the apoptotic pore remains obscure. Here, we report that BAX and BAK present distinct oligomerization properties, with BAK organizing into smaller structures with faster kinetics than BAX. BAK recruits and accelerates BAX assembly into oligomers that continue to grow during apoptosis. As a result, BAX and BAK regulate each other as they co-assemble into the same apoptotic pores, which we visualize. The relative availability of BAX and BAK molecules thereby determines the growth rate of the apoptotic pore and the relative kinetics by which mitochondrial contents, most notably mtDNA, are released. This feature of BAX and BAK results in distinct activation kinetics of the cGAS/STING pathway with implications for mtDNA-mediated paracrine inflammatory signaling.

Mechano-chemo-biological Computational Models for Arteries in Health, Disease and Healing: From Tissue Remodelling to Drug-eluting Devices

This review aims to highlight urgent priorities for the computational biomechanics community in the framework of mechano-chemo-biological models. Recent approaches, promising directions and open challenges on the computational modelling of arterial tissues in health and disease are introduced and investigated, together with in silico approaches for the analysis of drug-eluting stents that promote pharmacological-induced healing. The paper addresses a number of chemo-biological phenomena that are generally neglected in biomechanical engineering models but are most likely instrumental for the onset and the progression of arterial diseases. An interdisciplinary effort is thus encouraged for providing the tools for an effective in silico insight into medical problems. An integrated mechano-chemo-biological perspective is believed to be a fundamental missing piece for crossing the bridge between computational engineering and life sciences, and for bringing computational biomechanics into medical research and clinical practice.

Mitochondrial outer membrane permeabilization at the single molecule level

Apoptotic cell death is essential for development, immune function or tissue homeostasis, and its mis-regulation is linked to various diseases. Mitochondrial outer membrane permeabilization (MOMP) is a central event in the intrinsic apoptotic pathway and essential to control the execution of cell death. Here we review current concepts in regulation of MOMP focusing on the interaction network of the Bcl-2 family proteins as well as further regulatory elements influencing MOMP. As MOMP is a complex spatially and temporally controlled process, we point out the importance of single-molecule techniques to unveil processes which would be masked by ensemble measurements. We report key single-molecule studies applied to decipher the composition, assembly mechanism and structure of protein complexes involved in MOMP regulation.

Membrane-dependent amyloid aggregation of human BAX α9 (173-192)

Mitochondrial outer membrane permeabilization, which is a critical step in apoptosis, is initiated upon transmembrane insertion of the C-terminal α-helix (α9) of the proapoptotic Bcl-2 family protein BAX. The isolated α9 fragment (residues 173-192) is also competent to disrupt model membranes, and the structures of its membrane-associated oligomers are of interest in understanding the potential roles of this sequence in apoptosis. Here, we used ultrafast two-dimensional infrared (2D IR) spectroscopy, thioflavin T binding, and transmission electron microscopy to show that the synthetic BAX α9 peptide (α9p) forms amyloid aggregates in aqueous environments and on the surfaces of anionic small unilamellar vesicles. Its inherent amyloidogenicity was predicted by sequence analysis, and 2D IR spectra reveal that vesicles modulate the β-sheet structures of insoluble aggregates, motivating further examination of the formation or suppression of BAX amyloids in apoptosis.

SRT2183 impairs ovarian cancer by facilitating autophagy

The 5-year survival rate of ovarian cancer patients is only 47%, and developing novel drugs for ovarian cancer is needed. Herein, we evaluated if and how SRT2183, a sirtuin-1 activator, impairs the ovarian cancer cells. OVCAR-3 and A2780 cells were treated with SRT2183. Cell viability was measured by cell counting kit-8 assay and clonogenic assay. Apoptosis was determined by flow cytometry with Annexin V and propidium iodide. The level of autophagy was evaluated by western blot and immunofluorescence. The activities of AKT/mTOR/70s6k and MAPK signaling pathway were measured by immunoblot. SRT2183 inhibited the growth of ovarian cancer cells, increased the accumulation of BAX, cleaved-caspase 3 and cleaved-PARP, and decreased the level of anti-apoptotic Bcl-2 and Mcl-1. SRT2183 increased the LC3II level, and enhanced the degradation of p62/SQSTM1. SRT2183 increased the formation of GFP-LC3 puncta and induced the maturation of autophagosome. Interestingly, knockdown of autophagy related 5 and 7 significantly impaired the anti-carcinoma activity of SRT2183, implying that SRT2183 impaired the ovarian cancer cells by inducing autophagy. SRT2183 decreased the accumulation of p-Akt, p-mTOR and p-70s6k, and activated the p38 MAPK signaling pathway. This indicated that Akt/mTOR/70s6k and p38 MAPK signaling pathway might be involved in the SRT2183-mediated autophagy and apoptosis.

Oncogenic seRNA functional activation: a novel mechanism of tumorigenesis

seRNA is a noncoding RNA (ncRNA) transcribed from active super-enhancer (SE), through which SE exerts biological functions and participates in various physiological and pathological processes. seRNA recruits cofactor, RNA polymerase II and mediator to constitute and stabilize chromatin loop SE and promoter region, which regulates target genes transcription. In tumorigenesis, DNA insertion, deletion, translocation, focal amplification and carcinogen factor mediate oncogenic SE generation, meanwhile, oncogenic SE transcribes into tumor-related seRNA, termed as oncogenic seRNA. Oncogenic seRNA participates in tumorigenesis through activating various signal-pathways. The recent reports showed that oncogenic seRNA implicates in a widespread range of cytopathological processes in cancer progression including cell proliferation, apoptosis, autophagy, epithelial-mesenchymal transition, extracellular matrix stiffness and angiogenesis. In this article, we comprehensively summarized seRNA’s characteristics and functions, and emphatically introduced inducible formation of oncogenic seRNA and its functional mechanisms. Lastly, some research strategies on oncogenic seRNA were introduced, and the perspectives on cancer therapy that targets oncogenic seRNA were also discussed.

The anti-carcinogenesis properties of erianin in the modulation of oxidative stress-mediated apoptosis and immune response in liver cancer

In this study, erianin was found to reduce the viability of cancer cells, inhibit their proliferation and migration, induce G2/M phase arrest, enhance cancer cell apoptosis, promote an increase in levels of intracellular reactive oxygen species and a decrease in mitochondrial membrane potential, and regulate the expression levels of anti- and pro-apoptosis-related proteins in HepG2 and SMMC-7721 cells. Erianin inhibited tumor growth in HepG2- and SMMC-7721-xenograft tumor nude mouse models, reduced the expression levels of anti-apoptosis proteins and enhanced the expression levels of pro-apoptosis proteins in tumor tissues. Erianin inhibited tumor growth in immunosuppressed BALB/c mice bearing heterotopic tumors. Among 111 types of cytokines detected in proteome profiling of tumor tissues, erianin substantially influenced levels of 38 types of cytokines in HepG2-xenografted tumors and of 15 types of cytokines in SMMC-7721-xenografted tumors, most of which are related to immune functions. Erianin strongly affected the serum levels of cytokines, and regulated the activation of nuclear factor-kappa B (NF-κB), and the expression levels of nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream proteins in spleen. The anti-liver cancer properties of erianin were found to be related mostly to its modulation of oxidative stress-mediated mitochondrial apoptosis and immune response.

Golgi stress response and organelle zones

Organelles have been studied traditionally as single units, but a novel concept is now emerging: each organelle has distinct functional zones that regulate specific functions. The Golgi apparatus seems to have various zones, including zones for: glycosylphosphatidylinositol-anchored proteins; proteoglycan, mucin and lipid glycosylation; transport of cholesterol and ceramides; protein degradation (Golgi membrane-associated degradation); and signalling for apoptosis. The capacity for these specific functions and the size of the corresponding zones appear to be tightly regulated by the Golgi stress response to accommodate cellular demands. For instance, the proteoglycan and mucin zones seem to be separately augmented during the differentiation of chondrocytes and goblet cells, respectively. The mammalian Golgi stress response consists of several response pathways. The TFE3 pathway regulates the general function of the Golgi, such as structural maintenance, N-glycosylation and vesicular transport, whereas the proteoglycan pathway increases the expression of glycosylation enzymes for proteoglycans. The CREB3 and HSP47 pathways regulate pro- and anti-apoptotic functions, respectively. These observations indicate that the Golgi is a dynamic organelle, the capacity of which is upregulated according to cellular needs.

Checkpoint kinase inhibitor AZD7762 enhance cisplatin-induced apoptosis in osteosarcoma cells

Background

AZD7762 is a checkpoint kinase 1 (Chk 1) inhibitor, which has been reported to sensitize many tumor cells to DNA damage. However, whether AZD7762 could sensitize osteosarcoma cells to chemotherapy cisplatin has not been defined.

Methods

We used a variety of methods such as cell viability assays, flow cytometry, western blotting, and immunohistochemistry analysis to determine AZD7762 enhancing cisplatin-induced apoptosis on osteosarcoma cell lines in vitro and in vivo.

Results

In the present study, we demonstrated that AZD7762 could enhance cisplatin-mediated apoptosis and mitotic catastrophe of osteosarcoma cells in vitro, and promote the inhibition of xenograft growth induced by cisplatin in vivo. The mechanistic study indicated that AZD7762 enhance the effect of cisplatin through abrogating cisplatin-mediated G2/M arrest and inhibiting the cisplatin damage repair as demonstrated by increasing cisplatin-induced γH2AX expression.

Conclusion

These results suggest that AZD7762 could effectively promote cisplatin-induced apoptosis and mitotic catastrophe in osteosarcoma cells. The clinical application of AZD7762 as an adjuvant in the chemotherapy of osteosarcoma should be further explored.

Electronic supplementary material

The online version of this article (10.1186/s12935-019-0896-9) contains supplementary material, which is available to authorized users.

Contribution of Mitochondrial Ion Channels to Chemo-Resistance in Cancer Cells

Mitochondrial ion channels are emerging oncological targets, as modulation of these ion-transporting proteins may impact on mitochondrial membrane potential, efficiency of oxidative phosphorylation and reactive oxygen production. In turn, these factors affect the release of cytochrome c, which is the point of no return during mitochondrial apoptosis. Many of the currently used chemotherapeutics induce programmed cell death causing damage to DNA and subsequent activation of p53-dependent pathways that finally leads to cytochrome c release from the mitochondrial inter-membrane space. The view is emerging, as summarized in the present review, that ion channels located in this organelle may account in several cases for the resistance that cancer cells can develop against classical chemotherapeutics, by preventing drug-induced apoptosis. Thus, pharmacological modulation of these channel activities might be beneficial to fight chemo-resistance of different types of cancer cells.

Synthetic and genetic dimers as quantification ruler for single-molecule counting with PALM

How membrane proteins oligomerize determines their function. Superresolution microscopy can report on protein clustering and extract quantitative molecular information. Here, we evaluate the blinking kinetics of four photoactivatable fluorescent proteins for quantitative single-molecule microscopy. We identified mEos3.2 and mMaple3 to be suitable for molecular quantification through blinking histogram analysis. We designed synthetic and genetic dimers of mEos3.2 as well as fusion proteins of monomeric and dimeric membrane proteins as reference structures, and we demonstrate their versatile use for quantitative superresolution imaging in vitro and in situ. We further found that the blinking behavior of mEos3.2 and mMaple3 is modified by a reducing agent, offering the possibility to adjust blinking parameters according to experimental needs.

Organelle zones in mitochondria

The recent development of scientific imaging techniques have lead to the identification of various novel organellar functions. Most of these functions are perfomred in limited areas within organelles. These local functional organelle regions are referred to as 'zones'. In mitochondria, there are various types of zones exist, including the apoptosis zone, necrosis zone, mitophagy zone, mitochondrial-derived vesicle budding zone, innate immunity zone, etc. The membrane contact site between mitochondria and the endoplasmic reticulum, which is referred to as MAM, is also considered a zone. This contact site is crucial for various biological events, such as lipid transfer, sterol exchange, Ca2+ transfer, autophagy and the suppression of neurological disorders. Therefore, the elucidation of organelle zones is essential towards a deeper understanding of cell biology.

Molecular and topological reorganizations in mitochondrial architecture interplay during Bax-mediated steps of apoptosis

During apoptosis, Bcl-2 proteins such as Bax and Bak mediate the release of pro-apoptotic proteins from the mitochondria by clustering on the outer mitochondrial membrane and thereby permeabilizing it. However, it remains unclear how outer membrane openings form. Here, we combined different correlative microscopy and electron cryo-tomography approaches to visualize the effects of Bax activity on mitochondria in human cells. Our data show that Bax clusters localize near outer membrane ruptures of highly variable size. Bax clusters contain structural elements suggesting a higher order organization of their components. Furthermore, unfolding of inner membrane cristae is coupled to changes in the supramolecular assembly of ATP synthases, particularly pronounced at membrane segments exposed to the cytosol by ruptures. Based on our results, we propose a comprehensive model in which molecular reorganizations of the inner membrane and sequestration of outer membrane components into Bax clusters interplay in the formation of outer membrane ruptures.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Endoplasmic reticulum stress triggers Xanthoangelol-induced protective autophagy via activation of JNK/c-Jun Axis in hepatocellular carcinoma

Background

Xanthoangelol (XAG) was reported to exhibit antitumor properties in several cancer. However, the specific anti-tumor activity of XAG in human hepatocellular carcinoma (HCC) and the relevant mechanisms are not known.

Methods

The effects of XAG on HCC cell proliferation and apoptosis were respectively examined by CCK-8 assay and Annexin V-FITC/PI apoptosis kit. Western blotting was conducted to detect the expression of proteins. The effect of XAG on the development of acidic vesicle organelles was assessed using acridine orange staining. mRFP-GFP-LC3 adenovirus was used to transfect HCC cells and the formation of autolysosome was detected using a confocal microscope.

Results

Mechanistically, XAG promotes HCC cell death through triggering intrinsic apoptosis pathway, not extrinsic apoptotic pathway. Furthermore, XAG treatment induced autophagy in Bel 7402 and SMMC 7721 cells, as evidenced by an increase in autophagy-associated proteins, including LC3B-II, Beclin-1, and Atg5. Interestingly, inhibition of autophagy with 3-MA, Bafilomycin A1 (Baf A1), or siRNA targeting Atg5 effectively enhanced the apoptotic cell ratio in XAG-treated cells, indicating that protective effect of autophagy induced by XAG in HCC. Moreover, autophagy induced by XAG was mediated by activating endoplasmic reticulum stress (ERS), along with administration of XAG, the expression levels of ERS-associated proteins, including CHOP, GRP78, ATF6, p-eIF2α, IRE1α, and cleaved caspase-12 were significantly increased in HCC cells. Meanwhile, suppressing ERS with chemical chaperones (TUDCA) or CHOP shRNA could effectively abrogate the autophagy-inducing effect of XAG, and increase the apoptotic cell death. Further mechanistic studies showed that ERS-induced autophagy in XAG-treated cells was mediated by activation of JNK/c-jun pathway. XAG treatment resulted in the increase of p-JNK and p-c-jun, while suppressing ERS with TUDCA or CHOP shRNA could effectively reverse it. Meanwhile, SP600125, a JNK inhibitor, effectively reversed XAG-induced protective autophagy and enhanced cell apoptosis in XAG-treated HCC cells. In vivo results demonstrated that XAG exerts potent antitumor properties with low toxicity.

Conclusions

Collectively, these results suggested that XAG could be served as a promising candidate for the treatment and prevention of HCC.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-1012-z) contains supplementary material, which is available to authorized users.

What we talk about when we talk about nanoclusters

Superresolution microscopy results have sparked the idea that many membrane proteins are not randomly distributed across the plasma membrane but are instead arranged in nanoclusters. Frequently, these new results seemed to confirm older data based on biochemical and electron microscopy experiments. Recently, however, it was recognized that multiple countings of the very same fluorescently labeled protein molecule can be easily confused with true protein clusters. Various strategies have been developed, which are intended to solve the problem of discriminating true protein clusters from imaging artifacts. We believe that there is currently no perfect algorithm for this problem; instead, different approaches have different strengths and weaknesses. In this review, we discuss single molecule localization microscopy in view of its ability to detect nanoclusters of membrane proteins. To capture the different views on nanoclustering, we chose an unconventional style for this article: we placed its scientific content in the setting of a fictive conference, where five researchers from different fields discuss the problem of detecting and quantifying nanoclusters. Using this style, we feel that the different approaches common for different research areas can be well illustrated. Similarities to a short story by Raymond Carver are not unintentional.

Programmed Cell Death, from a Cancer Perspective: An Overview

Programmed cell death (PCD) is probably the most widely discussed subject among the topics of cancer therapy. Over the last 2 decades an astonishing boost in our perception of cell death has been seen, and its role in cancer and cancer therapy has been thoroughly investigated. A number of discoveries have clarified the molecular mechanism of PCD, thus expounding the link between PCD and therapeutic tools. Even though PCD is assumed to play a major role in anticancer therapy, the clinical relevance of its induction remains uncertain. Since PCD involves multiple death programs including programmed necrosis and autophagic cell death, it has contributed to our better understanding of cancer pathogenesis and therapeutics. In this review, we discuss a brief outline of PCD types as well as their role in cancer therapeutics. Since irregularities in the cell death process are frequently found in various cancers, key proteins governing cell death type could be used as therapeutic targets for a wide range of cancer.

Regulation of tNOX expression through the ROS-p53-POU3F2 axis contributes to cellular responses against oxaliplatin in human colon cancer cells

BACKGROUND

Oxaliplatin belongs to the platinum-based drug family and has shown promise in treating cancer by binding to DNA to induce cytotoxicity. However, individual patients show diverse therapeutic responses toward oxaliplatin due to yet-unknown underlying mechanisms. We recently established that oxaliplatin also exert its anti-cancer activity in gastric cancer cell lines by targeting tumor-associated NADH oxidase (tNOX), attenuate NAD⁺ generation and reduce NAD⁺-dependent sirtuin 1 (SIRT1) deacetylase activity, which in turn enhances p53 acetylation and apoptosis.

METHODS

In this study, differential cellular outcomes in response to oxaliplatin exposure of p53-wild-type versus p53-null HCT116 human colon cancer cells were examined. Cell growth profile was determined by cell impedance measurements and apoptosis was analyzed by flow cytometry. The engagement between oxaliplatin and tNOX protein was studied by cellular thermal shift assay. Furthermore, western blot analysis revealed that p53 was important in regulating tNOX expression in these cell lines.

RESULTS

In p53-wild-type cells, we found that oxaliplatin inhibited cell growth by inducing apoptosis and concurrently down-regulating tNOX at both the transcriptional and translational levels. In p53-null cells, in contrast, oxaliplatin moderately up-regulated tNOX expression and yielded no apoptosis and much less cytotoxicity. Further experiments revealed that in p53-wild-type cells, oxaliplatin enhanced ROS generation and p53 transcriptional activation, leading to down-regulation of the transcriptional factor, POU3F2, which enhances the expression of tNOX. Moreover, the addition of a ROS scavenger reversed the p53 activation, POU3F2 down-regulation, and apoptosis induced by oxaliplatin in p53-wild-type cells. In the p53-null line, on the other hand, oxaliplatin treatment triggered less ROS generation and no p53 protein, such that POU3F2 and tNOX were not down-regulated and oxaliplatin-mediated cytotoxicity was attenuated.

CONCLUSION

Our results show that oxaliplatin mediates differential cellular responses in colon cancer cells depending on their p53 status, and demonstrate that the ROS-p53 axis is important for regulating POU3F2 and its downstream target, tNOX. Notably, the depletion of tNOX sensitizes p53-null cells to both spontaneous and oxaliplatin-induced apoptosis. Our work thus clearly shows a scenario in which targeting of tNOX may be a potential strategy for cancer therapy in a p53-inactivated system.

Activated gastric cancer-associated fibroblasts contribute to the malignant phenotype and 5-FU resistance via paracrine action in gastric cancer

Background

Cancer-associated fibroblasts (CAFs) play important roles in tumor progression. However, the behaviors of activated CAFs in gastric cancer remain to be determined. The aim of the present study was to investigate the correlations between activated gastric CAFs and the prognosis of patients with gastric cancer, and to determine the effects of activated CAFs on the malignant phenotype and 5-fluorouracil resistance in this cancer.

Methods

Ninety-five patients with primary gastric cancer were enrolled in this study. Activation states of gastric CAFs were evaluated by immunohistochemistry. A modified method for the primary culture of gastric CAFs was employed. Types of CAFs and activation states were identified by immunocytochemical and immunofluorescent staining. Cell co-culture and gastric CAF conditioned medium transfer models were established to investigate the paracrine effects of activated CAFs on the migration and invasion of gastric cell lines. The half maximal inhibitory concentration of 5-fluorouracil and levels of cell apoptosis were examined using cell viability assay and flow cytometry, respectively. Protein expression levels of associated molecules were measured by Western blotting.

Results

Kaplan–Meier survival curves showed that activated gastric CAFs identified via fibroblast activation protein were significantly related to poorer cumulative survival in gastric cancer patients. Five strains of CAFs were successfully cultured via the modified culture method, and three gastric CAFs strains were identified as activated gastric CAFs. The migration and invasion abilities of gastric cells were significantly enhanced in both the co-culture group and the conditioned medium group. The half maximal inhibitory concentration for 5-fluorouracil in BGC-823 cells was elevated after treatment with conditioned medium, and early apoptosis was inhibited. Additionally, an obvious elevation of epithelial–mesenchymal transition level was observed in the conditioned medium group.

Conclusions

Activated gastric CAFs correlate with a poor prognosis of cancer patients and may contribute to the malignant phenotype and the development of resistance to 5-fluorouracil via paracrine action in gastric cancer. Gastric CAFs with a specific activation state might be used as a tumor biomarker within the microenvironment for prognosis and as a new therapeutic target for chemoresistant gastric cancer.

Electronic supplementary material

The online version of this article (10.1186/s12935-018-0599-7) contains supplementary material, which is available to authorized users.

Regorafenib suppresses colon tumorigenesis and the generation of drug resistant cancer stem-like cells via modulation of miR-34a associated signaling

Background

Colorectal cancer (CRC) is one of the most prevalent malignancies in the world and developed drug resistance has represented one of the most challenging tasks for management. The current therapeutic regimens may select and enrich cancer stem-like cells (CSCs) resulting in the increased resistance against treatment, metastatic potential and mortality. Regorafenib is a multi-kinase inhibitor, an FDA-approved last-of-line treatment for patients with chemo-refractory metastatic CRC. However, regorafenib’s potential effects on CSCs have not been fully elucidated.

Methods

Here, we developed two 5-FU resistant CRC cell lines, HCT-116R and DLD-1R and showed the increased CSCs characteristics such as increased side-population cells, tumor sphere formation and expression of stemness markers. These cell lines and CSCs properties were used for evaluating the potential of regorafenib in suppressing CSCs.

Results

We showed that regorafenib treatment decreased the stemness phenotypes including tumor sphere formation, and side-population, of both HCT-116R and DLD-1R cells. Additionally, regorafenib suppressed the cell viability in both cell lines synergistically with 5-FU. In vivo, the combination of regorafenib and 5-FU significantly suppressed the tumorigenesis and stemness markers of 5-FU resistant DLD-1R. Mechanistically, regorafenib-mediated effects were associated with the induction of tumor suppressor miR-34a and suppression of WNT/β-catenin signaling. Our findings demonstrated that regorafenib treatment was associated with the increased level of miR-34a, resulting in reversing drug resistance and cancer-initiating cell phenotypes by degrading WNT/β-catenin in CRC.

Conclusion

Regorafenib might be a potential drug for colon cancer stem-like cells and it should be investigated in future clinical trials.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0836-x) contains supplementary material, which is available to authorized users.

3D super-resolution microscopy reflects mitochondrial cristae alternations and mtDNA nucleoid size and distribution

3D super-resolution microscopy based on the direct stochastic optical reconstruction microscopy (dSTORM) with primary Alexa-Fluor-647-conjugated antibodies is a powerful method for accessing changes of objects that could be normally resolved only by electron microscopy. Despite the fact that mitochondrial cristae yet to become resolved, we have indicated changes in cristae width and/or morphology by dSTORM of ATP-synthase F₁ subunit α (F₁α). Obtained 3D images were analyzed with the help of Ripley's K-function modeling spatial patterns or transferring them into distance distribution function. Resulting histograms of distances frequency distribution provide most frequent distances (MFD) between the localized single antibody molecules. In fasting state of model pancreatic β-cells, INS-1E, MFD between F₁α were ~80 nm at 0 and 3 mM glucose, whereas decreased to 61 nm and 57 nm upon glucose-stimulated insulin secretion (GSIS) at 11 mM and 20 mM glucose, respectively. Shorter F₁α interdistances reflected cristae width decrease upon GSIS, since such repositioning of F₁α correlated to average 20 nm and 15 nm cristae width at 0 and 3 mM glucose, and 9 nm or 8 nm after higher glucose simulating GSIS (11, 20 mM glucose, respectively). Also, submitochondrial entities such as nucleoids of mtDNA were resolved e.g. after bromo-deoxyuridine (BrDU) pretreatment using anti-BrDU dSTORM. MFD in distances distribution histograms reflected an average nucleoid diameter (<100 nm) and average distances between nucleoids (~1000 nm). Double channel PALM/dSTORM with Eos-lactamase-β plus anti-TFAM dSTORM confirmed the latter average inter-nucleoid distance. In conclusion, 3D single molecule (dSTORM) microscopy is a reasonable tool for studying mitochondrion.

MiR-144-3p promotes the tumor growth and metastasis of papillary thyroid carcinoma by targeting paired box gene 8

Background

Paired box gene 8 (PAX8) is expressed in and indispensable to thyroid development. MiR-144-3p is found dys-regulated in cancers, and it can block the expression of target gens. This study sought to understand the effect of MiR-144-3p in papillary thyroid carcinoma (PTC) as well as the associated mechanisms.

Materials and methods

Real-time PCR, immunohistochemical and Western blot assays were performed to examine the expression of target miRNA and/or genes. CCK-8 and flow cytometry analysis was used to respectively test cell growth, cell cycle progression and apoptosis. Luciferase reporter assay was performed to find out whether miR-144-3p could bind to the 3′ untranslated region of PAX8 or not.

Results

We found that PAX8 decreased in PTC, while miR-144-3p increased in PTC. Over-expression of miR-144-3p promoted the cell viability and cell cycle progression. The expressions of cell-cycle-related genes, cyclin D1, cyclin-dependent kinase 2 and CDC25A were modulated by miR-144-3p. Meanwhile, the presence or absence of miR-144-3p both affected epithelial-mesenchymal transition of PTC by regulating the expression of E-cadherin, N-cadherin and vimentin. Moreover, PAX8 may be a potential direct target of miR-144-3p. Mechanically, the activation of extracellular signal–regulated kinases 1/2, Akt and c-Jun N-terminal kinases may be associated with the tumor-promoting effect of miR-144-3p. In addition, the blockage of miR-144-3p forced the anti-tumor effect delivered by X-ray exposure or paclitaxel.

Conclusion

MiR-144-3p promoted the growth of tumor and the metastasis of PTC by targeting PAX 8. The study provided promising prognosis markers and valuable treatment strategy for PTC.

Pore formation by dimeric Bak and Bax: an unusual pore?

Apoptotic cell death via the mitochondrial pathway occurs in all vertebrate cells and requires the formation of pores in the mitochondrial outer membrane. Two Bcl-2 protein family members, Bak and Bax, form these pores during apoptosis, and how they do so has been investigated for the last two decades. Many of the conformation changes that occur during their transition to pore-forming proteins have now been delineated. Notably, biochemical, biophysical and structural studies indicate that symmetric homodimers are the basic unit of pore formation. Each dimer contains an extended hydrophobic surface that lies on the outer membrane, and is anchored at either end by a transmembrane domain. Membrane-remodelling events such as positive membrane curvature have been reported to accompany apoptotic pore formation, suggesting Bak and Bax form lipidic pores rather than proteinaceous pores. However, it remains unclear how symmetric dimers assemble to porate the membrane. Here, we review how clusters of dimers and their lipid-mediated interactions provide a molecular explanation for the heterogeneous assemblies of Bak and Bax observed during apoptosis.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

ROS-mediated oligomerization of VDAC2 is associated with quinocetone-induced apoptotic cell death

Quinocetone (QCT) has been approved and widely used as an animal feed additive in China since 2003. However, investigations indicate that QCT shows potential toxicity both in vitro and in vivo. Although voltage dependent anion channel 1 (VDAC1) involved in regulating QCT-induced apoptotic cell death has been established, the role of voltage dependent anion channel 2 (VDAC2) in QCT-induced toxicity remains unclear. In this study, we showed that QCT-induced cell death was coupled to VDAC2 oligomerization. Moreover, VDAC inhibitor 4, 4'-diisothiocyano stilbene-2, 2'-disulfonic acid (DIDS) alleviated QCT-induced cell death and VDAC2 oligomerization. Meanwhile, overexpression VDAC2 aggravated QCT-induced VDAC2 oligomerization. In addition, caspase inhibitor Z-VAD-FMK and reactive oxidative species (ROS) scavenger N-acetyl-l-cysteine (NAC) apparently blocked QCT-induced cell death and VDAC2 oligomerization. Finally, overexpression N-terminal truncated VDAC2 attenuated QCT-induced VDAC2 oligomerization but had no influence on its localization to mitochondria when comparing to the full length of VDAC2. Taken together, our results reveal that ROS-mediated VDAC2 oligomerization is associated with QCT-induced apoptotic cell death. The N-terminal region of VDAC2 is required for QCT-induced VDAC2 oligomerization.

Disordered clusters of Bak dimers rupture mitochondria during apoptosis

During apoptosis, Bak and Bax undergo major conformational change and form symmetric dimers that coalesce to perforate the mitochondrial outer membrane via an unknown mechanism. We have employed cysteine labelling and linkage analysis to the full length of Bak in mitochondria. This comprehensive survey showed that in each Bak dimer the N-termini are fully solvent-exposed and mobile, the core is highly structured, and the C-termini are flexible but restrained by their contact with the membrane. Dimer-dimer interactions were more labile than the BH3:groove interaction within dimers, suggesting there is no extensive protein interface between dimers. In addition, linkage in the mobile Bak N-terminus (V61C) specifically quantified association between dimers, allowing mathematical simulations of dimer arrangement. Together, our data show that Bak dimers form disordered clusters to generate lipidic pores. These findings provide a molecular explanation for the observed structural heterogeneity of the apoptotic pore.

DOI:http://dx.doi.org/10.7554/eLife.19944.001

A healthy organism must carefully remove unwanted, diseased or damaged cells. These unwanted cells can bring about their own death in a controlled process known as apoptosis. Maintaining an appropriate level of apoptosis is crucial to good health: excessive cell death can contribute to neurodegenerative disorders, whereas too little can result in cancer.

All cells contain powerhouses called mitochondria, which produce energy. Mitochondria are the scene of a critical ‘point of no return’ in apoptosis. When a cell receives a death signal, a ‘killer’ protein known as Bak punches holes (or pores) in the membrane of the mitochondria. These pores allow toxic molecules to leak out from the mitochondria into the interior of the cell, where they trigger a series of events that dismantles the cell from the inside out.

To create the pores, Bak undergoes extensive shape changes that allow the proteins to form dimers that then cluster and perforate the membrane. To investigate how Bak clusters assemble on the mitochondrial membrane, Uren et al. used cultured cells and biochemical techniques to show where the Bak dimers contacted each other before and after the pore formed; these findings were complemented with mathematical modelling. The results show that during apoptosis, Bak dimers contact each other at several different places (rather than at one or two places) to assemble into disorderly, ever-changing clusters. Based on these observations, Uren et al. suggest that the enlarging clusters stress the membrane and cause pores to form.

The next step is to investigate whether physical forces that act within the mitochondrial membrane could drive the clustering of Bak proteins. This knowledge could ultimately enable us to learn how to manipulate apoptosis in cells, potentially as part of treatments for the diseases in which this cell death process occurs inappropriately.

DOI:http://dx.doi.org/10.7554/eLife.19944.002

Bax and Bak Pores: Are We Closing the Circle?

Bax and its homolog Bak are key regulators of the mitochondrial pathway of apoptosis. On cell stress Bax and Bak accumulate at distinct foci on the mitochondrial surface where they undergo a conformational change, oligomerize, and mediate cytochrome c release, leading to cell death. The molecular mechanisms of Bax and Bak assembly and mitochondrial permeabilization have remained a longstanding question in the field. Recent structural and biophysical studies at several length scales have shed light on key aspects of Bax and Bak function that have shifted how we think this process occurs. These discoveries reveal an unexpected molecular mechanism in which Bax (and likely Bak) dimers assemble into oligomers with an even number of molecules that fully or partially delineate pores of different sizes to permeabilize the mitochondrial outer membrane (MOM) during apoptosis.

Assembly of Bak homodimers into higher order homooligomers in the mitochondrial apoptotic pore

In mitochondrial apoptosis, Bak is activated by death signals to form pores of unknown structure on the mitochondrial outer membrane via homooligomerization. Cytochrome c and other apoptotic factors are released from the intermembrane space through these pores, initiating downstream apoptosis events. Using chemical crosslinking and double electron electron resonance (DEER)-derived distance measurements between specific structural elements in Bak, here we clarify how the Bak pore is assembled. We propose that previously described BH3-in-groove homodimers (BGH) are juxtaposed via the 'α3/α5' interface, in which the C-termini of helices α3 and α5 are in close proximity between two neighboring Bak homodimers. This interface is observed concomitantly with the well-known 'α6:α6' interface. We also mapped the contacts between Bak homodimers and the lipid bilayer based on EPR spectroscopy topology studies. Our results suggest a model for the lipidic Bak pore, whereby the mitochondrial targeting C-terminal helix does not change topology to accommodate the lining of the pore lumen by BGH.