An Image-based Assay for High-throughput Analysis of Cell Proliferation and Cell Death of Adherent Cells

Cellular heterogeneity in TNF/TNFR1 signalling: live cell imaging of cell fate decisions in single cells

Cellular responses to TNF are inherently heterogeneous within an isogenic cell population and across different cell types. TNF promotes cell survival by activating pro-inflammatory NF-κB and MAPK signalling pathways but may also trigger apoptosis and necroptosis. Following TNF stimulation, the fate of individual cells is governed by the balance of pro-survival and pro-apoptotic signalling pathways. To elucidate the molecular mechanisms driving heterogenous responses to TNF, quantifying TNF/TNFR1 signalling at the single-cell level is crucial. Fluorescence live-cell imaging techniques offer real-time, dynamic insights into molecular processes in single cells, allowing for detection of rapid and transient changes, as well as identification of subpopulations, that are likely to be missed with traditional endpoint assays. Whilst fluorescence live-cell imaging has been employed extensively to investigate TNF-induced inflammation and TNF-induced cell death, it has been underutilised in studying the role of TNF/TNFR1 signalling pathway crosstalk in guiding cell-fate decisions in single cells. Here, we outline the various opportunities for pathway crosstalk during TNF/TNFR1 signalling and how these interactions may govern heterogenous responses to TNF. We also advocate for the use of live-cell imaging techniques to elucidate the molecular processes driving cell-to-cell variability in single cells. Understanding and overcoming cellular heterogeneity in response to TNF and modulators of the TNF/TNFR1 signalling pathway could lead to the development of targeted therapies for various diseases associated with aberrant TNF/TNFR1 signalling, such as rheumatoid arthritis, metabolic syndrome, and cancer.

Substitution Kinetics, Albumin and Transferrin Affinities, and Hypoxia All Affect the Biological Activities of Anticancer Vanadium(V) Complexes

Limited stability of most transition-metal complexes in biological media has hampered their medicinal applications but also created a potential for novel cancer treatments, such as intratumoral injections of cytotoxic but short-lived anticancer drugs. Two related V(V) complexes, [VO(Hshed)(dtb)] (1) and [VO(Hshed)(cat)] (2), where H2shed = N-(salicylideneaminato)-N'-(2-hydroxyethyl)-1,2-ethanediamine, H2dtb = 3,5-di-tert-butylcatechol, and H2cat = 1,2-catechol, decomposed within minutes in cell culture medium at 310 K (t1/2 = 43 and 9 s for 1 and 2, respectively). Despite this, both complexes showed high antiproliferative activities in triple-negative human breast cancer (MDA-MB-231) cells, but the mechanisms of their activities were radically different. Complex 1 formed noncovalent adducts with human serum albumin, rapidly entered cells via passive diffusion, and was nearly as active in a short-term treatment (IC50 = 1.9 ± 0.2 μM at 30 min) compared with a long-term treatment (IC50 = 1.3 ± 0.2 μM at 72 h). The activity of 1 decreased about 20-fold after its decomposition in cell culture medium for 30 min at 310 K. Complex 2 showed similar activities (IC50 ≈ 12 μM at 72 h) in both fresh and decomposed solutions and was inactive in a short-term treatment. The activity of 2 was mainly due to the reactions among V(V) decomposition products, free catechol, and O2 in cell culture medium. As a result, the activity of 1 was less sensitive than that of 2 to the effects of hypoxic conditions that are characteristic of solid tumors and to the presence of apo-transferrin that acts as a scavenger of V(V/IV) decomposition products in blood serum. In summary, complex 1, but not 2, is a suitable candidate for further development as an anticancer drug delivered via intratumoral injections. These results demonstrate the importance of fine-tuning the ligand properties for the optimization of biological activities of metal complexes.

p53 isoform expression promotes a stemness phenotype and inhibits doxorubicin sensitivity in breast cancer

In breast cancer, dysregulated TP53 expression signatures are a better predictor of chemotherapy response and survival outcomes than TP53 mutations. Our previous studies have shown that high levels of Δ40p53 are associated with worse disease-free survival and disruption of p53-induced DNA damage response in breast cancers. Here, we further investigated the in vitro and in vivo implications of Δ40p53 expression in breast cancer. We have shown that genes associated with cell differentiation are downregulated while those associated with stem cell regulation are upregulated in invasive ductal carcinomas expressing high levels of Δ40p53. In contrast to p53, endogenous ∆40p53 co-localised with the stem cell markers Sox2, Oct4, and Nanog in MCF-7 and ZR75-1 cell lines. ∆40p53 and Sox2 co-localisation was also detected in breast cancer specimens. Further, in cells expressing a high ∆40p53:p53 ratio, increased expression of stem cell markers, greater mammosphere and colony formation capacities, and downregulation of miR-145 and miR-200 (p53-target microRNAs that repress stemness) were observed compared to the control subline. In vivo, a high ∆40p53:p53 ratio led to increased tumour growth, Ki67 and Sox2 expression, and blood microvessel areas in the vehicle-treated mice. High expression of ∆40p53 also reduced tumour sensitivity to doxorubicin compared to control tumours. Enhanced therapeutic efficacy of doxorubicin was observed when transiently targeting Δ40p53 or when treating cells with OTSSP167 with concomitant chemotherapy. Taken together, high Δ40p53 levels induce tumour growth and may promote chemoresistance by inducing a stemness phenotype in breast cancer; thus, targeting Δ40p53 in tumours that have a high Δ40p53:p53 ratio could enhance the efficacy of standard-of-care therapies such as doxorubicin.

Dual inhibition of thioredoxin reductase and proteasome is required for auranofin-induced paraptosis in breast cancer cells

Auranofin (AF), a gold (I)-containing phosphine compound, is being investigated for oncological application as a repurposed drug. We show here that 4~5 µM AF induces paraptosis, a non-apoptotic cell death mode characterized by dilation of the endoplasmic reticulum (ER) and mitochondria, in breast cancer cells. Although the covalent inhibition of thioredoxin reductase (TrxR), an enzyme that critically controls intracellular redox homeostasis, is considered the primary mechanism of AF's anticancer activity, knockdown of TrxR1 did not induce paraptosis. Instead, both TrxR1 knockdown plus the proteasome inhibitor (PI), bortezomib (Bz), and 2 μM AF plus Bz induced paraptosis, thereby mimicking the effect of 5 μM AF. These results suggest that the paraptosis induced by 5 μM AF requires the inhibition of both TrxR1 and proteasome. We found that TrxR1 knockdown/Bz or subtoxic doses of AF and Bz induced paraptosis selectively in breast cancer cells, sparing non-transformed MCF10A cells, whereas 4~5 μM AF killed both cancer and MCF10A cells. GSH depletion was found to be more critical than ROS generation for the paraptosis induced by dual TrxR1/proteasome inhibition. In this process, the ATF4/CHAC1 (glutathione-specific gamma-glutamylcyclotransferase 1) axis leads to GSH degradation, contributing to proteotoxic stress possibly due to the accumulation of misfolded thiol-containing proteins. These results suggest that the paraptosis-inducing strategy of AF plus a PI may provide an effective therapeutic strategy against pro-apoptotic therapy-resistant cancers and reduce the potential side effects associated with high-dose AF.

p53 regulates lysosomal membrane permeabilization as well as cytoprotective autophagy in response to DNA-damaging drugs

Lysosomes are single-membraned organelles that mediate the intracellular degradation of macromolecules. Various stress can induce lysosomal membrane permeabilization (LMP), translocating intralysosomal components, such as cathepsins, to the cytoplasm, which induces lysosomal-dependent cell death (LDCD). This study reports that p53 regulates LMP in response to DNA-damaging drugs. Treating wild-type TP53 A549 cells with DNA-damaging drugs (namely, doxorubicin, carboplatin, and etoposide) induced LMP and accelerated cell death more rapidly than treating TP53-knockout (KO) A549 cells. This suggested p53-dependent LMP and LDCD induction in response to DNA damage. LMP was induced by p53-dependent BID upregulation and activation, followed by translocation of truncated BID to lysosomes. Simultaneously, autophagy for damaged lysosome elimination (lysophagy) was activated via the p53-mTOR-TEFB/TFE3 pathways in response to DNA damage. These data suggested the dichotomous nature of p53 for LMP regulation; LMP induction and repression via the p53-BID axis and p53-mTOR-TFEB/TFE3 pathway, respectively. Blocking autophagy with hydroxychloroquine or azithromycin as well as ATG5 KO enhanced LMP and LDCD induction after exposure to DNA-damaging drugs. Furthermore, lysosomal membrane stabilization using U18666A, a cholesterol transporter Niemann-Pick disease C1 (NPC1) inhibitor, suppressed LMP as well as LDCD in wild-type TP53, but not in TP53-KO, A549 cells. Thus, LMP is finely regulated by TP53 after exposure to DNA-damaging drugs.

Ricolinostat enhances adavosertib‑induced mitotic catastrophe in TP53‑mutated head and neck squamous cell carcinoma cells

TP53 mutation is one of the most frequent gene mutations in head and neck squamous cell carcinoma (HNSCC) and could be a potential therapeutic target. Recently, the WEE1 G2 checkpoint kinase (WEE1) inhibitor adavosertib (Adv) has attracted attention because of its selective cytotoxicity against TP53-mutated cells and has shown promising activity in early phase clinical trials. In the present study, it was demonstrated that combined treatment with Adv and a selective histone deacetylase 6 (HDAC6) inhibitor, ricolinostat (RCS), synergistically enhanced cell death induction in four out of five HNSCC cell lines with TP53 mutation (CAL27, SAS, HSC-3, and OSC-19), one HNSCC cell line with impaired TP53 function by HPV-infection (UPCI-SCC154), and TP53-knockout human lung cancer cell line (A549 TP53-KO), but not in TP53 wild-type A549 cells. Time-lapse imaging showed that RCS enhanced the Adv-induced mitotic catastrophe. Consistent with this, RCS treatment suppressed checkpoint kinase 1 (Chk1) (Ser345) phosphorylation and co-administration of RCS with Adv suppressed cyclin-dependent kinase 1 (Tyr15) phosphorylation along with increased expression of γ-H2A.X, a marker of DNA double-strand breaks in CAL27 cells. These data showed that RCS enhanced Adv-induced premature mitotic entry and cell death induction in the mitotic phase. However, although HDAC6 knockdown enhanced Adv-induced cell death with γ-H2A.X elevation, HDAC6 knockdown did not repress Chk1 phosphorylation in CAL27 cells. Our data demonstrated that the co-administration of RCS with Adv in HNSCC cells resulted in the suppression of Chk1 activity, leading to synergistically enhanced apoptosis via mitotic catastrophe in a p53-dependent manner. This enhanced cell death appeared to be partially mediated by the inhibition of HDAC6 activity by RCS.

Cell death induced by the ER stressor thapsigargin involves death receptor 5, a non-autophagic function of MAP1LC3B, and distinct contributions from unfolded protein response components

BACKGROUND

Cell death triggered by unmitigated endoplasmic reticulum (ER) stress plays an important role in physiology and disease, but the death-inducing signaling mechanisms are incompletely understood. To gain more insight into these mechanisms, the ER stressor thapsigargin (Tg) is an instrumental experimental tool. Additionally, Tg forms the basis for analog prodrugs designed for cell killing in targeted cancer therapy. Tg induces apoptosis via the unfolded protein response (UPR), but how apoptosis is initiated, and how individual effects of the various UPR components are integrated, is unclear. Furthermore, the role of autophagy and autophagy-related (ATG) proteins remains elusive.

METHODS

To systematically address these key questions, we analyzed the effects of Tg and therapeutically relevant Tg analogs in two human cancer cell lines of different origin (LNCaP prostate- and HCT116 colon cancer cells), using RNAi and inhibitory drugs to target death receptors, UPR components and ATG proteins, in combination with measurements of cell death by fluorescence imaging and propidium iodide staining, as well as real-time RT-PCR and western blotting to monitor caspase activity, expression of ATG proteins, UPR components, and downstream ER stress signaling.

RESULTS

In both cell lines, Tg-induced cell death depended on death receptor 5 and caspase-8. Optimal cytotoxicity involved a non-autophagic function of MAP1LC3B upstream of procaspase-8 cleavage. PERK, ATF4 and CHOP were required for Tg-induced cell death, but surprisingly acted in parallel rather than as a linear pathway; ATF4 and CHOP were independently required for Tg-mediated upregulation of death receptor 5 and MAP1LC3B proteins, whereas PERK acted via other pathways. Interestingly, IRE1 contributed to Tg-induced cell death in a cell type-specific manner. This was linked to an XBP1-dependent activation of c-Jun N-terminal kinase, which was pro-apoptotic in LNCaP but not HCT116 cells. Molecular requirements for cell death induction by therapy-relevant Tg analogs were identical to those observed with Tg.

CONCLUSIONS

Together, our results provide a new, integrated understanding of UPR signaling mechanisms and downstream mediators that induce cell death upon Tg-triggered, unmitigated ER stress. Video Abstract.

Autophagy Inhibition Induces the Secretion of Macrophage Migration Inhibitory Factor (MIF) with Autocrine and Paracrine Effects on the Promotion of Malignancy in Breast Cancer

Breast cancer is the main cause of cancer-related death in women in the world. Because autophagy is a known survival pathway for cancer cells, its inhibition is currently being explored in clinical trials for treating several types of malignancies. In breast cancer, autophagy has been shown to be necessary for the survival of cancer cells from the triple negative subtype (TNBC), which has the worst prognosis among breast cancers and currently has limited therapeutic options. Autophagy has also been involved in the regulation of protein secretion and, of importance for this work, the inhibition of autophagy is known to promote the secretion of proinflammatory cytokines from distinct cell types. We found that the inhibition of autophagy in TNBC cell lines induced the secretion of the macrophage migration inhibitory factor (MIF), a pro-tumorigenic cytokine involved in breast cancer invasion and immunomodulation. MIF secretion was dependent on an increase in reactive oxygen species (ROS) induced by the inhibition of autophagy. Importantly, MIF secreted from autophagy-deficient cells increased the migration of cells not treated with autophagy inhibitors, indicating that autophagy inhibition in cancer cells promoted malignancy in neighboring cells through the release of secreted factors, and that a combinatorial approach should be evaluated for cancer therapy.

The kinase PERK and the transcription factor ATF4 play distinct and essential roles in autophagy resulting from tunicamycin-induced ER stress

Endoplasmic reticulum (ER) stress is thought to activate autophagy via unfolded protein response (UPR)-mediated transcriptional up-regulation of autophagy machinery components and modulation of microtubule-associated protein 1 light chain 3 (LC3). The upstream UPR constituents pancreatic EIF2-α kinase (PERK) and inositol-requiring enzyme 1 (IRE1) have been reported to mediate these effects, suggesting that UPR may stimulate autophagy via PERK and IRE1. However, how the UPR and its components affect autophagic activity has not been thoroughly examined. By analyzing the flux of LC3 through the autophagic pathway, as well as the sequestration and degradation of autophagic cargo, we here conclusively show that the classical ER stressor tunicamycin (TM) enhances autophagic activity in mammalian cells. PERK and its downstream factor, activating transcription factor 4 (ATF4), were crucial for this induction, but surprisingly, IRE1 constitutively suppressed autophagic activity. TM-induced autophagy required autophagy-related 13 (ATG13), Unc-51-like autophagy-activating kinases 1/2 (ULK1/ULK2), and GABA type A receptor-associated proteins (GABARAPs), but interestingly, LC3 proteins appeared to be redundant. Strikingly, ATF4 was activated independently of PERK in both LNCaP and HeLa cells, and our further examination revealed that ATF4 and PERK regulated autophagy through separate mechanisms. Specifically, whereas ATF4 controlled transcription and was essential for autophagosome formation, PERK acted in a transcription-independent manner and was required at a post-sequestration step in the autophagic pathway. In conclusion, our results indicate that TM-induced UPR activates functional autophagy, and whereas IRE1 is a negative regulator, PERK and ATF4 are required at distinct steps in the autophagic pathway.

The ER Stress Inducer l-Azetidine-2-Carboxylic Acid Elevates the Levels of Phospho-eIF2α and of LC3-II in a Ca2+-Dependent Manner

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) activates the unfolded protein response (UPR) to reduce protein load and restore homeostasis, including via induction of autophagy. We used the proline analogue l-azetidine-2-carboxylic acid (AZC) to induce ER stress, and assessed its effect on autophagy and Ca2+ homeostasis. Treatment with 5 mM AZC did not induce poly adenosine diphosphate ribose polymerase (PARP) cleavage while levels of binding immunoglobulin protein (BiP) and phosphorylated eukaryotic translation initiation factor 2α (eIF2α) increased and those of activating transcription factor 6 (ATF6) decreased, indicating activation of the protein kinase RNA-like ER kinase (PERK) and the ATF6 arms of the UPR but not of apoptosis. AZC treatment in combination with bafilomycin A1 (Baf A1) led to elevated levels of the lipidated form of the autophagy marker microtubule-associated protein light chain 3 (LC3), pointing to activation of autophagy. Using the specific PERK inhibitor AMG PERK 44, we could deduce that activation of the PERK branch is required for the AZC-induced lipidation of LC3. Moreover, both the levels of phospho-eIF2α and of lipidated LC3 were strongly reduced when cells were co-treated with the intracellular Ca2+ chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraaceticacid tetra(acetoxy-methyl) ester (BAPTA-AM) but not when co-treated with the Na⁺/K⁺ ATPase inhibitor ouabain, suggesting an essential role of Ca2+ in AZC-induced activation of the PERK arm of the UPR and LC3 lipidation. Finally, AZC did not trigger Ca2+ release from the ER though appeared to decrease the cytosolic Ca2+ rise induced by thapsigargin while also decreasing the time constant for Ca2+ clearance. The ER Ca2+ store content and mitochondrial Ca2+ uptake however remained unaffected.

Nonlinear relationship between ER Ca2+ depletion versus induction of the unfolded protein response, autophagy inhibition, and cell death

Endoplasmic reticulum (ER) Ca²⁺ depletion activates the unfolded protein response (UPR), inhibits bulk autophagy and eventually induces cell death in mammalian cells. However, the extent and duration of ER Ca²⁺ depletion required is unknown. We instigated a detailed study in two different cell lines, using sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) inhibitors to gradually reduce ER Ca²⁺ levels in a controlled manner. Remarkably, UPR induction (as assessed by expression analyses of UPR-regulated proteins) and autophagy inhibition (as assessed by analyses of effects on starvation-induced bulk autophagy) required substantially higher drug concentrations than those needed to strongly decrease total ER Ca²⁺ levels. In fact, even when ER Ca²⁺ levels were so low that we could hardly detect any release of Ca²⁺ upon challenge with ER Ca²⁺ purging agents, UPR was not induced, and starvation-induced bulk autophagy was still fully supported. Moreover, although we observed reduced cell proliferation at this very low level of ER Ca²⁺, cells could tolerate prolonged periods (days) without succumbing to cell death. Addition of increasing concentrations of extracellular EGTA also gradually depleted the ER of Ca²⁺, and, as with the SERCA inhibitors, EGTA-induced activation of UPR and cell death required higher EGTA concentrations than those needed to strongly reduce ER Ca²⁺ levels. We conclude that ER Ca²⁺ depletion-induced effects on UPR, autophagy and cell death require either an extreme general depletion of ER Ca²⁺ levels, or Ca²⁺ depletion in areas of the ER that have a higher resistance to Ca²⁺ drainage than the bulk of the ER.