Role of intracellular calcium in proteasome inhibitor-induced endoplasmic reticulum stress, autophagy, and cell death

Proteasome inhibitors induce apoptosis by superoxide anion generation via NADPH oxidase 5 in human neuroblastoma SH-SY5Y cells

The ubiquitin-proteasome system (UPS) is a major proteolytic system that plays an important role in the regulation of various cell processes, such as cell cycle, stress response, and transcriptional regulation, especially in neurons, and dysfunction of UPS is considered to be a cause of neuronal cell death in neurodegenerative diseases. However, the mechanism of neuronal cell death caused by UPS dysfunction has not yet been fully elucidated. In this study, we investigated the mechanism of neuronal cell death induced by proteasome inhibitors using human neuroblastoma SH-SY5Y cells. Z-Leu-D-Leu-Leu-al (MG132), a proteasome inhibitor, induced apoptosis in SH-SY5Y cells in a concentration- and time-dependent manner. Antioxidants N-acetylcysteine and EUK-8 attenuated MG132-induced apoptosis. Apocynin and diphenyleneiodonium, inhibitors of NADPH oxidase (NOX), an enzyme that produces superoxide anions, also attenuated MG132-induced apoptosis. It was also found that MG132 treatment increased the expression of NOX5, a NOX family member, and that siRNA-mediated silencing of NOX5 and BAPTA-AM, which inhibits NOX5 by chelating calcium, suppressed MG132-induced apoptosis and production of reactive oxygen species in SH-SY5Y cells. These results suggest that MG132 induces apoptosis in SH-SY5Y cells through the production of superoxide anion by NOX5.

PDX proteins from Arabidopsis thaliana as novel substrates of cathepsin B: implications for vitamin B6 biosynthesis regulation

Vitamin B6 is a critical molecule for metabolism, development, and stress sensitivity in plants. It is a cofactor for numerous biochemical reactions, can serve as an antioxidant, and has the potential to increase tolerance against both biotic and abiotic stressors. Due to the importance of vitamin B6, its biosynthesis is likely tightly regulated. Plants can synthesize vitamin B6 de novo via the concerted activity of Pyridoxine Biosynthesis Protein 1 (PDX1) and PDX2. Previously, PDX proteins have been identified as targets for ubiquitination, indicating they could be marked for degradation by two highly conserved pathways: the Ubiquitin Proteasome Pathway (UPP) and the autophagy pathway. Initial experiments show that PDXs are in fact degraded, but surprisingly, in a ubiquitin-independent manner. Inhibitor studies pointed toward cathepsin B, a conserved lysosomal cysteine protease, which is implicated in both programed cell death and autophagy in humans and plants. In plants, cathepsin Bs are poorly described, and no confirmed substrates have been identified. Here, we present PDX proteins from Arabidopsis thaliana as interactors and substrates of a plant Cathepsin B. These findings not only describe a novel cathepsin B substrate in plants, but also provide new insights into how plants regulate de novo biosynthesis of vitamin B6.

Molecular phenotypes segregate missense mutations in SLC13A5 Epilepsy

The sodium-coupled citrate transporter (NaCT, SLC13A5) mediates citrate uptake across the plasma membrane via an inward Na + gradient. Mutations in SLC13A5 cause early infantile epileptic encephalopathy type-25 (EIEE25, SLC13A5 Epilepsy) due to impaired citrate uptake in neurons. Despite clinical identification of disease-causing mutations, underlying mechanisms and cures remain elusive. We mechanistically classify the molecular phenotypes of six mutations. C50R, T142M, and T227M exhibit impaired citrate transport despite normal expression at the cell surface. G219R, S427L, and L488P are hampered by low protein expression, ER retention, and reduced transport. Mutants' mRNA levels resemble wildtype, suggesting post-translational defects. Class II mutations display immature core-glycosylation and shortened half-lives, indicating protein folding defects. These experiments provide a comprehensive understanding of the mutation's defects in SLC13A5 Epilepsy at the biochemical and molecular level and shed light into the trafficking pathway(s) of NaCT. The two classes of mutations will require fundamentally different treatment approaches to either restore transport function, or enable correction of protein folding defects.

Summary

Loss-of-function mutations in the SLC13A5 causes SLC13A5-Epilepsy, a devastating disease characterized by neonatal epilepsy. Currently no cure is available. We clarify the molecular-level defects to guide future developments for phenotype-specific treatment of disease-causing mutations.

Cellular effects of BAPTA: Are they only about Ca2+ chelation?

Intracellular Ca2+ signals play a vital role in a broad range of cell biological and physiological processes in all eukaryotic cell types. Dysregulation of Ca2+ signaling has been implicated in numerous human diseases. Over the past four decades, the understanding of how cells use Ca2+ as a messenger has flourished, largely because of the development of reporters that enable visualization of Ca2+ signals in different cellular compartments, and tools that can modulate cellular Ca2+ signaling. One such tool that is frequently used is BAPTA; a fast, high-affinity Ca2+-chelating molecule. By making use of a cell-permeable acetoxymethyl ester (AM) variant, BAPTA can be readily loaded into the cytosol of cells (referred to as BAPTAi), where it is trapped and able to buffer changes in cytosolic Ca2+. Due to the ease of loading of the AM version of BAPTA, this reagent has been used in hundreds of studies to probe the role of Ca2+ signaling in specific processes. As such, for decades, researchers have almost universally attributed changes in biological processes caused by BAPTAi to the involvement of Ca2+ signaling. However, BAPTAi has often been used without any form of control, and in many cases has neither been shown to be retained in cells for the duration of experiments nor to buffer any Ca2+ signals. Moreover, increasing evidence points to off-target cellular effects of BAPTA that are clearly not related to Ca2+ chelation. Here, we briefly introduce Ca2+ signaling and the history of Ca2+ chelators and fluorescent Ca2+ indicators. We highlight Ca2+-independent effects of BAPTAi on a broad range of molecular targets and describe some of BAPTAi's impacts on cell functions that occur independently of its Ca2+-chelating properties. Finally, we propose strategies for determining whether Ca2+ chelation, the binding of other metal ions, or off-target interactions with cell components are responsible for BAPTAi's effect on a particular process and suggest some future research directions.

Exploring the multifaceted impact of lanthanides on physiological pathways in human breast cancer cells

Lanthanum (La) and cerium (Ce), rare earth elements with physical properties similar to calcium (Ca), are generally considered non-toxic when used appropriately. However, their ions possess anti-tumor capabilities. This investigation explores the potential applications and mechanisms of LaCl3 or CeCl3 treatment in triple-negative breast cancer (TNBC) cell lines. TNBC, characterized by the absence of estrogen receptor (ERα), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER-2) expression, is prone to early metastasis and resistant to hormone therapy. Our results demonstrate that La/Ce treatment reduces cell growth, and when combined with cisplatin, it synergistically inhibits cell growth and the PI3K/AKT pathway. La and Ce induce oxidative stress by disrupting mitochondrial function, leading to protein oxidation. Additionally, they interfere with protein homeostasis and induce nucleolar stress. Furthermore, disturbance in F-actin web formation impairs cell migration. This study delves into the mechanism by which calcium-like elements La and Ce inhibit breast cancer cell growth, shedding light on their interference in mitochondrial function, protein homeostasis, and cytoskeleton assembly.

Calcium signaling in chemotherapy-induced neuropathy

Alterations in calcium (Ca²⁺) signaling is a major mechanism in the development of chemotherapy-induced peripheral neuropathy (CIPN), a side effect caused by multiple chemotherapy regimens. CIPN is associated with numbness and incessant tingling in hands and feet which diminishes quality of life during treatment. In up to 50% of survivors, CIPN is essentially irreversible. There are no approved, disease-modifying treatments for CIPN. The only recourse for oncologists is to modify the chemotherapy dose, a situation that can compromise optimal chemotherapy and impact patient outcomes. Here we focus on taxanes and other chemotherapeutic agents that work by altering microtubule assemblies to kill cancer cells, but also have off-target toxicities. There have been many molecular mechanisms proposed to explain the effects of microtubule-disrupting drugs. In neurons, an initiating step in the off-target effects of treatment by taxane is binding to neuronal calcium sensor 1 (NCS1), a sensitive Ca²⁺ sensor protein that maintains the resting Ca²⁺ concentration and dynamically enhances responses to cellular stimuli. The taxane/NCS1 interaction causes a Ca²⁺ surge that starts a pathophysiological cascade of consequences. This same mechanism contributes to other conditions including chemotherapy-induced cognitive impairment. Strategies to prevent the Ca²⁺ surge are the foundation of current work.

Small molecule '4ab' induced autophagy and endoplasmic reticulum stress-mediated death of aggressive cancer cells grown under adherent and floating conditions

Metastasis is the leading cause of death in cancer patients and a major challenging aspect of cancer biology. Various adaptive molecular signaling pathways play a crucial role in cancer metastasis and later in the formation of secondary tumors. Aggressive cancer cells like triple negative breast cancer (TNBCs) are more inclined to undergo metastasis hence having a high recurrence rate and potential of micro-metastasis. Tumor cells in circulation known as circulating tumor cells (CTCs) offer an attractive drug target to treat metastatic disease. Cell cycle regulation and stress response of CTCs in blood has a crucial role in their survival and progression and thus may be considered therapeutically active hotspots. The cyclin D/cyclin-dependent kinase (CDK) pathway regulates cell cycle checkpoints, a process that is frequently dysregulated in cancer cells. Selective CDK inhibitors can limit the phosphorylation of cell cycle regulatory proteins by inducing cell cycle phase arrest, and thus may be an effective therapeutic strategy for aggressive cancer cells in their dividing phase at the primary or secondary site. However, during the floating condition, cancer cells halt their multiplication process and proceed through the various steps of metastasis. Current study showed that a novel CDK inhibitor 4ab induced autophagy and endoplasmic reticulum (ER) stress in agressive cancer cells grown under adherent and floating conditions resulting in paraptosis. Further, our results showed that 4ab efficiently induced cell death in aggressive cancer cells through ER stress-mediated activation of JNK signaling. Additionally, was observed that treatment of 4ab in tumor-bearing mice displayed a significant reduction in tumor burden and micro-metastasis. The outcome of these studies showed that 4ab can be a potential anti-tumor and anti-metastatic agent. Graphical representation of 4ab: image representing the effect of 4ab on death-inducing pathways in aggressive cancer cells. 4ab induces ER stress and activates autophagy leading to vacuolation of there by causing apoptosis in aggressive cancer cells.

The ERAD Pathway Participates in Fungal Growth and Cellulase Secretion in Trichoderma reesei

Trichoderma reesei is a powerful fungal cell factory for the production of cellulolytic enzymes due to its outstanding protein secretion capacity. Endoplasmic reticulum-associated degradation (ERAD) plays an integral role in protein secretion that responds to secretion pressure and removes misfolded proteins. However, the role of ERAD in fungal growth and endogenous protein secretion, particularly cellulase secretion, remains poorly understood in T. reesei. Here, we investigated the ability of T. reesei to grow under different stresses and to secrete cellulases by disrupting three major genes (hrd1, hrd3 and der1) involved in the critical parts of the ERAD pathway. Under the ER stress induced by high concentrations of DTT, knockout of hrd1, hrd3 and der1 resulted in severely impaired growth, and the mutants Δhrd1 and Δhrd3 exhibited high sensitivity to the cell wall-disturbing agents, CFW and CR. In addition, the absence of either hrd3 or der1 led to the decreased heat tolerance of this fungus. These mutants showed significant differences in the secretion of cellulases compared to the parental strain QM9414. During fermentation, the secretion of endoglucanase in the mutants was essentially consistent with that of the parental strain, while cellobiohydrolase and β-glucosidase were declined. It was further discovered that the transcription levels of the endoglucanase-encoding genes (eg1 and eg2) and the cellobiohydrolase-encoding gene (cbh1) were not remarkedly changed. However, the β-glucosidase-encoding gene (bgl1) was significantly downregulated in the ERAD-deficient mutants, which was presumably due to the activation of a proposed feedback mechanism, repression under secretion stress (RESS). Taken together, our results indicate that a defective ERAD pathway negatively affects fungal growth and cellulase secretion, which provides a novel insight into the cellulase secretion mechanism in T. reesei.

The CRTC-CREB axis functions as a transcriptional sensor to protect against proteotoxic stress in Drosophila

cAMP Responsible Element Binding Protein (CREB) is an evolutionarily conserved transcriptional factor that regulates cell growth, synaptic plasticity and so on. In this study, we unexpectedly found proteasome inhibitors, such as MLN2238, robustly increase CREB activity in adult flies through a large-scale compound screening. Mechanistically, reactive oxidative species (ROS) generated by proteasome inhibition are required and sufficient to promote CREB activity through c-Jun N-terminal kinase (JNK). In 293 T cells, JNK activation by MLN2238 is also required for increase of CREB phosphorylation at Ser¹³³. Meanwhile, transcriptome analysis in fly intestine identified a group of genes involved in redox and proteostatic regulation are augmented by overexpressing CRTC (CREB-regulated transcriptional coactivator). Intriguingly, CRTC overexpression in muscles robustly restores protein folding and proteasomal activity in a fly Huntington's disease (HD) model, and ameliorates HD related pathogenesis, such as protein aggregates, motility, and lifespan. Moreover, CREB activity increases during aging, and further enhances its activity can suppress protein aggregates in aged muscles. Together, our results identified CRTC/CREB downstream ROS/JNK signaling as a conserved sensor to tackle oxidative and proteotoxic stresses. Boosting CRTC/CREB activity is a potential therapeutic strategy to treat aging related protein aggregation diseases.

Physiological Overview of the Potential Link between the UPS and Ca2+ Signaling

The ubiquitin–proteasome system (UPS) is the main proteolytic pathway by which damaged target proteins are degraded after ubiquitination and the recruit of ubiquitinated proteins, thus regulating diverse physiological functions and the maintenance in various tissues and cells. Ca²⁺ signaling is raised by oxidative or ER stress. Although the basic function of the UPS has been extensively elucidated and has been continued to define its mechanism, the precise relationship between the UPS and Ca²⁺ signaling remains unclear. In the present review, we describe the relationship between the UPS and Ca²⁺ signaling, including Ca²⁺-associated proteins, to understand the end point of oxidative stress. The UPS modulates Ca²⁺ signaling via the degradation of Ca²⁺-related proteins, including Ca²⁺ channels and transporters. Conversely, the modulation of UPS is driven by increases in the intracellular Ca²⁺ concentration. The multifaceted relationship between the UPS and Ca²⁺ plays critical roles in different tissue systems. Thus, we highlight the potential crosstalk between the UPS and Ca²⁺ signaling by providing an overview of the UPS in different organ systems and illuminating the relationship between the UPS and autophagy.

New sight into interaction between endoplasmic reticulum stress and autophagy induced by vanadium in duck renal tubule epithelial cells

Vanadium (V) is a common environmental and industrial pollutant that can cause nephrotoxicity in animals in excess. The purpose of this research was to explore the interaction between endoplasmic reticulum (ER) stress and autophagy induced by V in the kidney of ducks. Duck renal tubule epithelial cells were exposed to different concentrations of sodium metavanadate (NaVO₃) (0, 100 and 200 μM) and PERK inhibitor (GSK, 1 μM), or autophagy inhibitor (chloroquine, 50 μM) alone for 24 h (chloroquine for the last 4 h). The results showed that exposure to V caused the dilatation and swelling of the ER and intracellular calcium overload, and upregulated PERK, eIF2α, ATF4 and CHOP mRNA levels and p-PERK and CHOP protein levels associated with ER stress in cells. Additionally, V markedly increased the number of autophagosomes, acidic vesicular organelles (AVOs) and LC3 puncta, as well as the mRNA levels of Beclin1, Atg5, Atg12, LC3A and LC3B and protein levels of Beclin1, Atg5 and LC3B-II/LC3B-I, but decreased the imRNA and protein levels of p62. Moreover, treatment with the PERK inhibitor ameliorated the changed factors above induced by V, but the V-induced variation of ER-stress related factors were aggravated after treatment with the autophagy inhibitor. Together, our data suggested that excessive V could induce ER stress and autophagy in duck renal tubular epithelial cells. ER stress might promote V-induced autophagy via the PERK/ATF4/CHOP signaling pathway, and autophagy may play a role in alleviating ER stress induced by V.

Acute catabolism of leukocyte lipid bodies: Characterization of a nordihydroguaiaretic acid (NDGA)-induced proteasomal-dependent model

Cytoplasmic availability of leukocyte lipid bodies is controlled by a highly regulated cycle of opposing biogenesis- and catabolism-related events. While leukocyte biogenic machinery is well-characterized, lipid body catabolic mechanisms are yet mostly unknown. Here, we demonstrated that nordihydroguaiaretic acid (NDGA) very rapidly decreases the numbers of pre-formed lipid bodies within lipid body-enriched cytoplasm of mouse leukocytes - macrophages, neutrophils and eosinophils. NDGA mechanisms driving leukocyte lipid body disappearance were not related to loss of cell viability, 5-lipoxygenase inhibition, ATP autocrine/paracrine activity, or biogenesis inhibition. Proteasomal-dependent breakdown of lipid bodies appears to control NDGA-driven leukocyte lipid body reduction, since it was Bortezomib-sensitive in macrophages, neutrophils and eosinophils. Our findings unveil an acute NDGA-triggered lipid body catabolic event - a novel experimental model for the still neglected research area on leukocyte lipid body catabolism, additionally favoring further insights on proteasomal contribution to lipid body breakdown.

An Autocrine Negative Feedback Loop Inhibits Dictyostelium discoideum Proliferation through Pathways Including IP3/Ca2

Little is known about how eukaryotic cells can sense their number or spatial density and stop proliferating when the local density reaches a set value. We previously found that Dictyostelium discoideum accumulates extracellular polyphosphate to inhibit its proliferation, and this requires the G protein-coupled receptor GrlD and the small GTPase RasC. Here, we show that cells lacking the G protein component Gβ, the Ras guanine nucleotide exchange factor GefA, phosphatase and tensin homolog (PTEN), phospholipase C (PLC), inositol 1,4,5-trisphosphate (IP3) receptor-like protein A (IplA), polyphosphate kinase 1 (Ppk1), or the TOR complex 2 component PiaA have significantly reduced sensitivity to polyphosphate-induced proliferation inhibition. Polyphosphate upregulates IP3, and this requires GrlD, GefA, PTEN, PLC, and PiaA. Polyphosphate also upregulates cytosolic Ca²⁺, and this requires GrlD, Gβ, GefA, RasC, PLC, IplA, Ppk1, and PiaA. Together, these data suggest that polyphosphate uses signal transduction pathways including IP3/Ca²⁺ to inhibit the proliferation of D. discoideum. IMPORTANCE Many mammalian tissues such as the liver have the remarkable ability to regulate their size and have their cells stop proliferating when the tissue reaches the correct size. One possible mechanism involves the cells secreting a signal that they all sense, and a high level of the signal tells the cells that there are enough of them and to stop proliferating. Although regulating such mechanisms could be useful to regulate tissue size to control cancer or birth defects, little is known about such systems. Here, we use a microbial system to study such a mechanism, and we find that key elements of the mechanism have similarities to human proteins. This then suggests the possibility that we may eventually be able to regulate the proliferation of selected cell types in humans and animals.

The parkinsonian LRRK2 R1441G mutation shows macroautophagy-mitophagy dysregulation concomitant with endoplasmic reticulum stress

Autophagy is a mechanism responsible for the degradation of cellular components to maintain their homeostasis. However, autophagy is commonly altered and compromised in several diseases, including neurodegenerative disorders. Parkinson's disease (PD) can be considered a multifactorial disease because environmental factors, genetic factors, and aging are involved. Several genes are involved in PD pathology, among which the LRRK2 gene and its mutations, inherited in an autosomal dominant manner, are responsible for most genetic PD cases. The R1441G LRRK2 mutation is, after G2019S, the most important in PD pathogenesis. Our results demonstrate a relationship between the R1441G LRRK2 mutation and a mechanistic dysregulation of autophagy that compromises cell viability. This altered autophagy mechanism is associated with organellar stress including mitochondrial (which induces mitophagy) and endoplasmic reticulum (ER) stress, consistent with the fact that patients with this mutation are more vulnerable to toxins related to PD, such as MPP⁺.

Substituted meso-vinyl-BODIPY as thiol-selective fluorogenic probes for sensing unfolded proteins in the endoplasmic reticulum

A new type of thiol probes based on the meso-vinyl-BODIPY (VB) scaffold were developed. The monochloro-substituted VB1Cl exhibited the largest fluorescence enhancement (>200-fold) as well as high selectivity upon biological thiol sensing. VB1Cl was successfully applied for reporting the protein unfolding process under ER stress in living cells.

Post-treatment with Posiphen Reduces Endoplasmic Reticulum Stress and Neurodegeneration in Stroke Brain

Acetylcholinesterase (AChE) inhibitors have protective and anti-inflammatory actions against brain injury, mediated by nicotinic α7 cholinergic receptor activation. The use of AChE inhibitors in patients is limited by systemic cholinergic side effects. Posiphen, a stereoisomer of the AChE inhibitor Phenserine, lacks AChE inhibitor activity. The purpose of this study is to determine the protective effect of Posiphen in cellular and animal models of stroke. Both Posiphen and Phenserine reduced glutamate-mediated neuronal loss in co-cultures of primary cortical cells and microglia. Phenserine-, but not Posiphen-, mediated neuroprotection was diminished by the nicotinic α7 receptor antagonist methyllycaconitine. Posiphen antagonized NMDA-mediated Ca⁺⁺ influx, thapsigargin-mediated neuronal loss and ER stress in cultured cells. Early post-treatment with Posiphen reduced ER stress signals, IBA1 immunoreactivity, TUNEL and infarction in the ischemic cortex, as well as neurological deficits in stroke rats. These findings indicate that Posiphen is neuroprotective against stroke through regulating Ca⁺⁺i and ER stress.

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Posiphen induces protection in cell culture through noncholinergic mechanism

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Posiphen attenuates glutamate-mediated Ca⁺⁺i and ER stress in neuronal culture

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Posiphen mitigates ER stress in stroke brain

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Posiphen reduces neurodegeneration in stroke rats

Combination Lenalidomide/Bortezomib Treatment Synergistically Induces Calpain-Dependent Ikaros Cleavage and Apoptosis in Myeloma Cells

Multiple myeloma had been successfully treated by combining lenalidomide and bortezomib with reports suggesting benefits of such a combination even in relapsed/refractory cases. Recently, it was demonstrated that Ikaros degradation by lenalidomide happens via proteasome-dependent pathway and this process is critical for the eradication of myeloma cells. On the basis of this, an antagonistic effect should be observed if a combination of both these agents were used, which however is not the observation seen in the clinical setting. Our study demonstrates that when these agents are combined they exhibit a synergistic activity against myeloma cells and degradation of Ikaros happens by a proteasome-independent calcium-induced calpain pathway. Our study identifies the crucial role of calcium-induced calpain pathway in inducing apoptosis of myeloma cells when this combination or lenalidomide and bortezomib is used. We also report that this combination enhanced the expression of CD38 compared with lenalidomide alone. Thus, data from our study would establish the rationale for the addition of daratumumab along with this combination to further enhance therapeutic activity against multiple myeloma. IMPLICATIONS: Lenalidomide and bortezomib combination degrades IKZF1 in multiple myeloma through a calcium-dependent calpain and caspase pathway. VISUAL OVERVIEW: http://mcr.aacrjournals.org/content/molcanres/18/4/529/F1.large.jpg.

Astrocyte elevated gene 1 (AEG-1) promotes anoikis resistance and metastasis by inducing autophagy in hepatocellular carcinoma

Astrocyte elevated gene 1 (AEG-1) is overexpressed in hepatocellular carcinoma (HCC) and is strongly associated with tumor metastasis. Anoikis resistance and autophagy may play an important role in the survival of circulating tumor cells. However, the relationship among AEG-1, anoikis resistance, autophagy, and metastasis in HCC is still not clear. The results of this study indicate that AEG-1 expression is increased in HCC cell lines grown in suspension culture. AEG-1 could enhance anoikis resistance to promote the survival of detached HCC cells. Moreover, the anoikis resistance appears to be partly dependent on autophagy. Regulating AEG-1 expression changed the autophagy levels to modulate anoikis resistance, likely acting via the protein kinase RNA-like ER kinase (PERK)-eIF2α-ATF4-CHOP signaling axis. Finally, inhibiting autophagy by RNA interference prevented the AEG-1-promoted metastasis of HCC xenografts to the liver and lungs of nude mice. Taken together, AEG-1 is a key contributor to anoikis resistance and metastasis by inducing autophagy in vitro and in vivo, and it may be a potential target for therapeutic intervention in HCC.

A New Histone Deacetylase Inhibitor Enhances Radiation Sensitivity through the Induction of Misfolded Protein Aggregation and Autophagy in Triple-Negative Breast Cancer

Radiation therapy (RT) is one of the main treatments for triple-negative breast cancer (TNBC). However, many patients experience RT failure due to the metastatic potential of RT and the radiation resistance of several cancers. Histone deacetylase inhibitors (HDACis) can serve as radiosensitizers. In this study, we investigated whether a novel HDACi, TMU-35435, could reinforce radiosensitivity through the induction of misfolded protein aggregation and autophagy in TNBC. Significantly enhanced toxicity was found for the combination treatment compared with TMU-35435 or irradiation (IR) treatment alone in TNBC cells. The combination treatment induced misfolded protein aggregation and TMU-35435 inhibited the interaction of HDAC6 with dynein. Furthermore, the combined treatment induced endoplasmic reticulum (ER) stress but did not trigger apoptosis. In addition, the combination treatment caused autophagic cell death. Tumor growth in the mouse of model orthotopic breast cancer was suppressed by the combination treatment through the induction of ER stress and autophagy. These findings support the future evaluation of the novel HDACi TMU-35435, as a potent radiosensitizer in TNBC.

Kalantuboside B induced apoptosis and cytoprotective autophagy in human melanoma A2058 cells: An in vitro and in vivo study

Kalantuboside B (KB), a natural bufadienolide derivative extracted from the succulent plant Kalanchoe tubiflora, is well-known for its cardiotonic, immunomodulatory, and anti-inflammatory properties. In this study, we tested in vitro and in vivo anti-cancer efficacy with low concentrations of KB (5-30 ng/mL; 8.7-52.2 nM) on A2058 melanoma cells; and for the molecular mechanisms that underlie them. KB significantly inhibited the cell viability and colony formation via arresting the cell cycle at G2/M phase. There was an association with a decrease in Cyclin A/B1, Cdc25C, and Cdc2 expressions. Further, this treatment indicated the induction of apoptosis, DNA fragmentation, cytochrome c release, and caspase-3, -8, -9, and -12 activation, and PARP cleavage, which shows that mitochondrial, death-receptor, and ER-stress signaling pathways are involved. KB-induced autophagy was apparent from enhanced LC3-II accumulation, GFP-LC3 puncta, and AVO formation. Surprisingly, KB-mediated cell death was potentiated by 3-MA and CQ to suggest the role of autophagy as a cytoprotective mechanism. Moreover, KB-treated A2058 cells enhanced intracellular ROS generation and antioxidant NAC prevented apoptosis and reversed cytoprotective autophagy. Interestingly, KB-induced apoptosis (PARP cleavage) and cytoprotective autophagy (LC3-II accumulation) were mediated by the up-regulation of the ERK signaling pathway. It was also shown that KB promoted cytoprotective autophagy by a calcium dependent-p53 downregulation pathway. In vivo data showed that KB suppressed tumor growth significantly in A2058-xenografted nude mice. A Western blot indicated cell-cycle inhibition (cyclin A reduction), apoptosis induction (PARP cleavage and Bcl-2 inhibition), and cytoprotective autophagy (LC3-II upregulation and p53 downregulation) in KB-treated A2058-xenografted mice. Our findings suggested that KB-induced ROS pathway plays a role in mediating the apoptosis and cytoprotective autophagy in human melanoma cells. Thus, KB is considered to be a putative anti-tumor agent.

Butylated Hydroxyanisole Exerts Neurotoxic Effects by Promoting Cytosolic Calcium Accumulation and Endoplasmic Reticulum Stress in Astrocytes

Astrocytes provide nutritional support, regulate inflammation, and perform synaptic functions in the human brain. Although butylated hydroxyanisole (BHA) is a well-known antioxidant, several studies in animals have indicated BHA-mediated liver toxicity, retardation in reproductive organ development and learning, and sleep deficit. However, the specific effects of BHA on human astrocytes and the underlying mechanisms are yet unclear. Here, we investigated the antigrowth effects of BHA through cell cycle arrest and downregulation of regulatory protein expression. The typical cell proliferative signaling pathways, phosphoinositide 3-kinase/protein kinase B and extracellular signal-regulated kinase 1/2, were downregulated in astrocytes after BHA treatment. BHA increased the levels of pro-apoptotic proteins, such as BAX, cytochrome c, cleaved caspase 3, and cleaved caspase 9, and decreased the level of anti-apoptotic protein BCL-XL. It also increased the cytosolic calcium level and the expression of endoplasmic reticulum stress proteins. Treatment with BAPTA-AM, a calcium chelator, attenuated the increased levels of ER stress proteins and cleaved members of the caspase family. We further performed an in vivo evaluation of the neurotoxic effect of BHA on zebrafish embryos and glial fibrillary acidic protein, a representative astrocyte biomarker, in a gfap:eGFP zebrafish transgenic model. Our results provide clear evidence of the potent cytotoxic effects of BHA on human astrocytes, which lead to disruption of the brain and nerve development.

Anti-Cancer Agents in Proliferation and Cell Death: The Calcium Connection

Calcium (Ca2+) signaling and the modulation of intracellular calcium ([Ca2+]i) levels play critical roles in several key processes that regulate cellular survival, growth, differentiation, metabolism, and death in normal cells. On the other hand, aberrant Ca2+-signaling and loss of [Ca2+]i homeostasis contributes to tumor initiation proliferation, angiogenesis, and other key processes that support tumor progression in several different cancers. Currently, chemically and functionally distinct drugs are used as chemotherapeutic agents in the treatment and management of cancer among which certain anti-cancer drugs reportedly suppress pro-survival signals and activate pro-apoptotic signaling through modulation of Ca2+-signaling-dependent mechanisms. Most importantly, the modulation of [Ca2+]i levels via the endoplasmic reticulum-mitochondrial axis and corresponding action of channels and pumps within the plasma membrane play an important role in the survival and death of cancer cells. The endoplasmic reticulum-mitochondrial axis is of prime importance when considering Ca2+-signaling-dependent anti-cancer drug targets. This review discusses how calcium signaling is targeted by anti-cancer drugs and highlights the role of calcium signaling in epigenetic modification and the Warburg effect in tumorigenesis.

Upregulation of PSMB8 and cathepsins in the human brains of dementia with Lewy bodies

Proteasome and lysosome are responsible for the homeostasis of proteins, lipids and carbohydrates in cells. Numerous reports indicate the proteolytic pathways have altered functions during neurodegeneration and aging. Dementia with Lewy bodies (DLB) is one of the leading forms of dementia, and the proteolytic alteration in DLB has not yet been fully investigated. This study shows that the components of proteasome and lysosome had selectively altered gene expression and enzymatic functions. Specifically, PSMB8, an inducible proteasomal β subunit, had elevated mRNA level and protein level in DLB brain compared with age-matched controls. The proteasomal caspase-like peptidase showed significant decreased activity in DLB brains and the trypsin-like/chemotrypsin-like activities did not reach statistical significance. Lysosomal cathepsin B and D had elevated mRNA levels while only cathepsin B showed elevated enzymatic activity in DLB brains. This data indicate that the alteration of proteolytic pathways is highly selective and comprehensive. Further study to elucidate the correlation between neurodegenerative development and the alteration of proteolytic pathways would be important for therapeutic development.

Reactive oxygen species (ROS) and the calcium-(Ca2+) mediated extrinsic and intrinsic pathways underlying BDE-47-induced apoptosis in rainbow trout (Oncorhynchus mykiss) gonadal cells

The brominated flame retardant, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), is well documented to exert potential negative impacts on different marine biota. However, the responsible mechanism remains unknown. The rainbow trout gonadal cell line RTG-2 was used as a model, and the mechanism and pathway underlying BDE-47 (6, 12.5 and 25 μM)-induced apoptosis and toxicity were examined in vitro. Apoptosis occurred in the RTG-2 cells exposed to BDE-47 in a clear concentration-dependent manner. The morphology of the mitochondrial alterations was observed using transmission electron microscopy. BDE-47 exposure decreased the cellular mitochondrial membrane potential, increased the cytochrome c released into the cytoplasm and elevated Fas protein expression. The mRNA expressions of Fas-associated death domain-containing protein (FADD), CHOP and GRP78 were also elevated, and similar increases were found in the activities of intracellular caspase-8, caspase-12, caspase-9 and caspase-3. These results indicated that the mitochondrial, endoplasmic reticulum and death-receptor pathways were involved in apoptosis in RTG-2 cells following BDE-47 exposure. ROS and Ca²⁺ were responsible for these changes because their overproduction was detected prior to apoptosis. However, the addition of the ROS scavenger N-acetyl-l-cysteine (NAC) and the intracellular calcium chelator (acetoxymethyl)-1,2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) did not significantly alleviate the apoptosis rate. The results of the present study show that BDE-47 exposure induced apoptosis in RTG-2 cells via ROS- and Ca²-mediated mitochondrial, endoplasmic reticulum and death-receptor apoptotic pathways.

ER Stress is Involved in Epithelial-To-Mesenchymal Transition of Alveolar Epithelial Cells Exposed to a Hypoxic Microenvironment

Background: Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive and fatal interstitial lung disease of unknown origin. Alveolar epithelial cells (AECs) play an important role in the fibrotic process as they undergo sustained endoplasmic reticulum (ER) stress, and may acquire a mesenchymal phenotype through epithelial-to-mesenchymal transition (EMT), two phenomena that could be induced by localized alveolar hypoxia. Here we investigated the potential links between hypoxia, ER stress and EMT in AECs. Methods: ER stress and EMT markers were assessed by immunohistochemistry, western blot and qPCR analysis, both in vivo in rat lungs exposed to normoxia or hypoxia (equivalent to 8% O₂) for 48 h, and in vitro in primary rat AECs exposed to normoxia or hypoxia (1.5% O₂) for 2–6 days. Results: Hypoxia induced expression of mesenchymal markers, pro-EMT transcription factors, and the activation of ER stress markers both in vivo in rat lungs, and in vitro in AECs. In vitro, pharmacological inhibition of ER stress by 4-PBA limited hypoxia-induced EMT. Calcium chelation or hypoxia-inducible factor (HIF) inhibition also prevented EMT induction under hypoxic condition. Conclusions: Hypoxia and intracellular calcium are both involved in EMT induction of AECs, mainly through the activation of ER stress and HIF signaling pathways.

Eeyarestatin Compounds Selectively Enhance Sec61-Mediated Ca2+ Leakage from the Endoplasmic Reticulum

Eeyarestatin 1 (ES1) inhibits p97-dependent protein degradation, Sec61-dependent protein translocation into the endoplasmic reticulum (ER), and vesicular transport within the endomembrane system. Here, we show that ES1 impairs Ca²⁺ homeostasis by enhancing the Ca²⁺ leakage from mammalian ER. A comparison of various ES1 analogs suggested that the 5-nitrofuran (5-NF) ring of ES1 is crucial for this effect. Accordingly, the analog ES24, which conserves the 5-NF domain of ES1, selectively inhibited protein translocation into the ER, displayed the highest potency on ER Ca²⁺ leakage of ES1 analogs studied and induced Ca²⁺-dependent cell death. Using small interfering RNA-mediated knockdown of Sec61α, we identified Sec61 complexes as the targets that mediate the gain of Ca²⁺ leakage induced by ES1 and ES24. By interacting with the lateral gate of Sec61α, ES1 and ES24 likely capture Sec61 complexes in a Ca²⁺-permeable, open state, in which Sec61 complexes allow Ca²⁺ leakage but are translocation incompetent.

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ES1, ES2, and ES24 deplete Ca²⁺ in ER

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ESR35 and ES47 do not affect cellular Ca²⁺ homeostasis

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The most potent eeyarestatin, ES24, comprises only the 5-nitrofuran domain

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ES1 and ES24 target Sec61 complexes in ER

Inhibition of acid‑sensing ion channel 1a attenuates acid‑induced activation of autophagy via a calcium signaling pathway in articular chondrocytes

Acid-sensing ion channel 1a (ASIC1a), member of the degenerin/epithelial sodium channel protein superfamily, serves a critical role in various physiological and pathological processes. The aim of the present study was to examine the role of ASIC1a in the autophagy of rat articular chondrocytes. Autophagy was induced by acidic stimulation in rat articular chondrocytes and the extent of autophagy was evaluated via the expression levels of microtubule-associated protein 1 light chain 3II, Beclin1 and uncoordinated-51 like kinase1. Suppression of ASIC1a was achieved using small interfering RNA technology and/or inhibitor psalmotoxin-1. The expression levels of autophagy markers were measured by western blot analysis and reverse transcription-quantitative polymerase chain reaction methods. Intracellular calcium ([Ca²⁺]_i) was analyzed using a Ca²⁺-imaging method. Additionally, protein expression levels of the Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ)/5′-monophosphate-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) pathway were measured by western blot analysis. The results showed that autophagy was increased in a pH-and time-dependent manner with exposure to an acidic environment. In addition, silencing ASIC1a significantly decreased the expression levels of autophagy makers, accompanied by abrogation of the acid-induced [Ca²⁺]_i increase. Furthermore, silencing of ASIC1a downregulated the levels of CaMKKβ/β-actin and phosphorylated (p-) AMPK/AMPK, and upregulated the levels of p-mTOR/mTOR. These results indicated that ASIC1a is a potent regulator of autophagy in chondrocytes, which may be associated with decreased Ca²⁺ influx and the CaMKKβ/AMPK/mTOR pathway.

Longitudinal monoaminergic PET imaging of chronic proteasome inhibition in minipigs

Impairment of the ubiquitin proteasome system has been implicated in Parkinson’s disease. We used positron emission tomography to investigate longitudinal effects of chronic intracerebroventricular exposure to the proteasome inhibitor lactacystin on monoaminergic projections and neuroinflammation. Göttingen minipigs were implanted in the cisterna magna with a catheter connected to a subcutaneous injection port. Minipigs were imaged at baseline and after cumulative doses of 200 and 400 μg lactacystin, respectively. Main radioligands included [¹¹C]-DTBZ (vesicular monoamine transporter type 2) and [¹¹C]-yohimbine (α2-adrenoceptor). [¹¹C]-DASB (serotonin transporter) and [¹¹C]-PK11195 (activated microglia) became available later in the study and we present their results in a smaller subset of animals for information purposes only. Striatal [¹¹C]-DTBZ binding potentials decreased significantly by 16% after 200 μg compared to baseline, but the decrease was not sustained after 400 μg (n = 6). [¹¹C]-yohimbine volume of distribution increased by 18–25% in the pons, grey matter and the thalamus after 200 μg, which persisted at 400 μg (n = 6). In the later subset of minipigs, we observed decreased [¹¹C]-DASB (n = 5) and increased [¹¹C]-PK11195 (n = 3) uptake after 200 μg. These changes may mimic monoaminergic changes and compensatory responses in early Parkinson’s disease.

Proteasome inhibition promotes autophagy and protects from endoplasmic reticulum stress in rat alveolar macrophages exposed to hypoxia-reoxygenation injury

Alveolar macrophages play vital roles in acute lung injury, and macrophage response to hypoxia play relevant roles to disease mechanisms. There is growing evidence that cell death pathways play crucial roles in physiological and pathological settings and that the ubiquitin-proteasome system is involved in the regulation of these processes. However, the functional role of proteasome in alveolar macrophages exposed to hypoxia-reoxygenation (H/R) injury is unknown. We aimed to investigate the function of proteasome on alveolar macrophages exposed to H/R and the underlying mechanisms. NR8383 cells were pretreated with proteasome activator sulforaphane (SFN) or inhibitor MG-132 for 1 hr, and then submitted to 2/6 hr, 4/6 hr, and 6/6 hr H/R treatment. Cell viability was assessed with MTT assay. Autophagy was monitored using electron transmission microscope and flow cytometry and western blotting. The endoplasmic reticulum (ER) stress and unfolded protein response (UPR) pathways were equally analyzed by western blotting. Cell apoptosis was detected by immunohistochemistry, caspase3/7 activity, and western blotting. The viability of NR8383 cells exposed to H/R was affected by proteasome activity and proteasome inhibition significantly inhibited cell death. Treatment with MG-132 led to autophagy activation and induced the survival of NR8383 cells exposed to H/R. Pretreatment with SFN significantly decreased cell autophagy and induced cell death. ER stress was activated in H/R-treated NR8383 cells, and SFN further promoted ER stress whereas proteasome inhibition led to contrary results. Proteasome inhibtion hindered cell apoptosis as demonstrated by decreased caspase-3/7 activity, immunolabelling, and western blot results. Proteasome inhibition might be a promising approach for treating H/R injury-related lung diseases.

2-Aminoethoxydiphenylborane sensitizes anti-tumor effect of bortezomib via suppression of calcium-mediated autophagy

Non-small-cell lung cancer (NSCLC) accounts for most lung cancer cases. Therapeutic interventions integrating the use of different agents that focus on different targets are needed to overcome this set of diseases. The proteasome system has been demonstrated clinically as a potent therapeutic target for haematological cancers. However, promising preclinical data in solid tumors are yet to be confirmed in clinics. Herein, the combinational use of Bortezomib (BZM) and 2-aminoethoxydiphenylborane (2-APB) toward NSCLC cells was studied. We confirmed that BZM-triggered cytoprotective autophagy that may counteract with the cytotoxic effects of the drug per se. 2-APB was selected from screening of a commercial natural compounds library, which potentiated BZM-induced cytotoxicity. Such an enhancement effect was associated with 2-APB-mediated autophagy inhibition. In addition, we revealed that 2-APB suppressed calcium-induced autophagy in H1975 and A549 NSCLC cells. Interestingly, BZM [0.3 mg/kg/3 days] combined with 2-APB [2 mg/kg/day] significantly inhibited both primary (around 47% tumor growth) and metastatic Lewis lung carcinoma after a 20-day treatment. Our results suggested that BZM and 2-APB combination therapy can potentially be developed as a novel formulation for lung cancer treatment.

Gypenoside L inhibits autophagic flux and induces cell death in human esophageal cancer cells through endoplasm reticulum stress-mediated Ca2+ release

Esophageal cancer is one of the leading cause of cancer mortality in the world. Due to the increased drug and radiation tolerance, it is urgent to develop novel anticancer agent that triggers nonapoptotic cell death to compensate for apoptosis resistance. In this study, we show that treatment with gypenoside L (Gyp-L), a saponin isolated from Gynostemma pentaphyllum, induced nonapoptotic, lysosome-associated cell death in human esophageal cancer cells. Gyp-L-induced cell death was associated with lysosomal swelling and autophagic flux inhibition. Mechanistic investigations revealed that through increasing the levels of intracellular reactive oxygen species (ROS), Gyp-L triggered protein ubiquitination and endoplasm reticulum (ER) stress response, leading to Ca2+ release from ER inositol trisphosphate receptor (IP3R)-operated stores and finally cell death. Interestingly, there existed a reciprocal positive-regulatory loop between Ca2+ release and ER stress in response to Gyp-L. In addition, protein synthesis was critical for Gyp-L-mediated ER stress and cell death. Taken together, this work suggested a novel therapeutic option by Gyp-L through the induction of an unconventional ROS-ER-Ca2+-mediated cell death in human esophageal cancer.

Hydrogen sulfide attenuates myocardial fibrosis in diabetic rats through the JAK/STAT signaling pathway

The aim of the present study was to determine the role of hydrogen sulfide (H₂S) in improving myocardial fibrosis and its effects on oxidative stress, endoplasmic reticulum (ER) stress and cell apoptosis in diabetic rats, by regulating the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway. A total of 40 male Sprague-Dawley rats were randomly divided into four groups (n=10) as follows: Normal (control group), diabetes mellitus [streptozotocin (STZ) group], diabetes mellitus treated with H₂S (STZ + H₂S group), and normal rats treated with H₂S (H₂S group). Diabetes in rats was induced by intra-peritoneal (i.p.) injection of STZ at a dose of 40 mg/kg. NaHS (100 µmol/kg, i.p.), which was used as an exogenous donor of H₂S, was administered to rats in the STZ + H₂S and H₂S groups. After 8 weeks, the pathological morphological changes in myocardial fibers were observed following hematoxylin and eosin and Masson's trichrome staining. Apoptosis of myocardial tissue was analyzed by the terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Oxidative stress was evaluated through detecting the content of malondialdehyde (MDA), 4-hydroxynonenal (4-HNE), glutathione (GSH) and superoxide dismutase (SOD) in the myocardial cells by ELISA. The expression of collagen III, matrix metalloproteinase (MMP)8, MMP14, tissue inhibitor of metalloproteinase (TIMP)2, transforming growth factor (TGF)-β, cystathionine-γ-lyase (CSE), eukaryotic initiation factor 2α (eIF2α), GRP94, Bcl-2, caspase-3, tumor necrosis factor (TNF)-α, nuclear factor-κB (NF-κB) and proteins related to the JAK/STAT pathway, was detected by western blot analysis. The results indicated that the array of myocardial cells was markedly disordered in STZ group rats; compared with the control group, both myocardial interstitial fibrosis and the deposition of collagen III were increased. Furthermore, the expression ratio of MMPs/TIMPs was dysregulated, while the expression levels of TGF-β, eIF2α, GRP94, caspase-3, TNF-α, NF-κB, MDA and 4-HNE were significantly increased. Furthermore, the expressions of JAK-1/2 and STAT1/3/5/6 were also markedly upregulated, while those of CSE, SOD, GSH and Bcl-2 were downregulated. Compared with the STZ group, these changes were reversed in the STZ + H₂S group. The results of the present study demonstrated that H₂S can improve myocardial fibrosis in diabetic rats, and the underlying mechanism may be associated with the downregulation of the JAK/STAT signaling pathway, thereby suppressing oxidative stress and ER stress, inflammatory reaction and cell apoptosis.

Proteolytic activities in cortex of apical parts of Vicia faba ssp. minor seedling roots during kinetin-induced programmed cell death

Programmed cell death (PCD) is a crucial process in plant development. In this paper, proteolytically related aspects of kinetin-induced PCD in cortex cells of Vicia faba ssp. minor seedlings were examined using morphological, fluorometric, spectrophotometric, and fluorescence microscopic analyses. Cell viability estimation after 46 μM kinetin treatment of seedling roots showed that the number of dying cortex cells increased with treatment duration, reaching maximum after 72 h. Weight of the apical root segments increased with time and was about 2.5-fold greater after 96 h, while the protein content remained unchanged, compared to the control. The total and cysteine-dependent proteolytic activities fluctuated during 1-96-h treatment, which was not accompanied by the changes in the protein amount, indicating that the absolute protein amounts decreased during kinetin-induced PCD. N-ethylmaleimide (NEM), phenylmethylsulfonyl fluoride (PMSF), and Z-Leu-Leu-Nva-H (MG115), the respective cysteine, serine, and proteasome inhibitors, suppressed kinetin-induced PCD. PMSF significantly decreased serine-dependent proteolytic activities without changing the amount of proteins, unlike NEM and MG115. More pronounced effect of PMSF over NEM indicated that in the root apical segments, the most important proteolytic activity during kinetin-induced PCD was that of serine proteases, while that of cysteine proteases may be important for protein degradation in the last phase of the process. Both NEM and PMSF inhibited apoptotic-like structure formation during kinetin-induced PCD. The level of caspase-3-like activity of β1 proteasome subunit increased after kinetin treatment. Addition of proteasome inhibitor MG-115 reduced the number of dying cells, suggesting that proteasomes might play an important role during kinetin-induced PCD.

Inhibition of Sp1 prevents ER homeostasis and causes cell death by lysosomal membrane permeabilization in pancreatic cancer

Endoplasmic reticulum (ER) stress initiates an important mechanism for cell adaptation and survival, named the unfolded protein response (UPR). Severe or chronic/prolonged UPR can breach the threshold for survival and lead to cell death. There is a fundamental gap in knowledge on the molecular mechanism of how chronic ER stress is stimulated and leads to cell death in pancreatic ductal adenocarcinoma (PDAC). Our study shows that downregulating specificity protein 1 (Sp1), a transcription factor that is overexpressed in pancreatic cancer, activates UPR and results in chronic ER stress. In addition, downregulation of Sp1 results in its decreased binding to the ER stress response element present in the promoter region of Grp78, the master regulator of ER stress, thereby preventing homeostasis. We further show that inhibition of Sp1, as well as induction of ER stress, leads to lysosomal membrane permeabilization (LMP), a sustained accumulation of cytosolic calcium, and eventually cell death in pancreatic cancer.

Role of the unfolded protein response in topography-induced osteogenic differentiation in rat bone marrow mesenchymal stem cells

The topography of biomaterials can significantly influence the osteogenic differentiation of cells. Understanding topographical signal transduction is critical for developing biofunctional surfaces, but the current knowledge is insufficient. Recently, numerous reports have suggested that the unfolded protein response (UPR) and osteogenic differentiation are inter-linked. Therefore, we hypothesize that the UPR pathway may be involved in the topography-induced osteogenesis. In the present study, different surface topographies were fabricated on pure titanium foils and the endoplasmic reticulum (ER) stress and UPR pathway were systematically investigated. We found that ER stress and the PERK-eIF2α-ATF4 pathway were activated in a time- and topography-dependent manner. Additionally, the activation of the PERK-eIF2α-ATF4 pathway by different topographies was in line with their osteogenic induction capability. More specifically, the osteogenic differentiation could be enhanced or weakened when the PERK-eIF2α-ATF4 pathway was promoted or inhibited, respectively. Furthermore, tuning of the degree of ER stress with different concentrations of thapsigargin revealed that mild ER stress promotes osteogenic differentiation, whereas excessive ER stress inhibits osteogenic differentiation and causes apoptosis. Taken together, our findings suggest that the UPR may play a critical role in topography-induced osteogenic differentiation, which may help to provide new insights into topographical signal transduction.

STATEMENT OF SIGNIFICANCE

Suitable implant surface topography can effectively improve bioactivity and eventual bone affinity. However, the mechanism of topographical signaling transduction is unclear and criteria for designation of an appropriate implant surface topography is lacking. This study shows that the ER stress and PERK-eIF2α-ATF4 pathway were activated by micro- and micro/nano-topographies, which is corresponding to the osteogenic induction abilities of these topographies. Furthermore, we have found that mild ER stress improves osteogenic differentiation, whereas excessive ER stress inhibits osteogenic differentiation and causes apoptosis. Our findings demonstrate that the UPR plays a critical role in the topography induced osteogenic differentiation, which may help to provide new insights into the topographical signaling transduction.

Increased hepatic receptor interacting protein kinase 3 expression due to impaired proteasomal functions contributes to alcohol-induced steatosis and liver injury

Chronic alcohol exposure increased hepatic receptor-interacting protein kinase (RIP) 3 expression and necroptosis in the liver but its mechanisms are unclear. In the present study, we demonstrated that chronic alcohol feeding plus binge (Gao-binge) increased RIP3 but not RIP1 protein levels in mouse livers. RIP3 knockout mice had decreased serum alanine amino transferase activity and hepatic steatosis but had no effect on hepatic neutrophil infiltration compared with wild type mice after Gao-binge alcohol treatment. The hepatic mRNA levels of RIP3 did not change between Gao-binge and control mice, suggesting that alcohol-induced hepatic RIP3 proteins are regulated at the posttranslational level. We found that Gao-binge treatment decreased the levels of proteasome subunit alpha type-2 (PSMA2) and proteasome 26S subunit, ATPase 1 (PSMC1) and impaired hepatic proteasome function. Pharmacological or genetic inhibition of proteasome resulted in the accumulation of RIP3 in mouse livers. More importantly, human alcoholics had decreased expression of PSMA2 and PSMC1 but increased protein levels of RIP3 compared with healthy human livers. Moreover, pharmacological inhibition of RIP1 decreased Gao-binge-induced hepatic inflammation, neutrophil infiltration and NF-κB subunit (p65) nuclear translocation but failed to protect against steatosis and liver injury induced by Gao-binge alcohol. In conclusion, results from this study suggest that impaired hepatic proteasome function by alcohol exposure may contribute to hepatic accumulation of RIP3 resulting in necroptosis and steatosis while RIP1 kinase activity is important for alcohol-induced inflammation.

Soluble bioactive microbial mediators regulate proteasomal degradation and autophagy to protect against inflammation-induced stress

Bifidobacterium breve and other Gram-positive gut commensal microbes protect the gastrointestinal epithelium against inflammation-induced stress. However, the mechanisms whereby these bacteria accomplish this protection are poorly understood. In this study, we examined soluble factors derived from Bifidobacterium breve and their impact on the two major protein degradation systems within intestinal epithelial cells, proteasomes and autophagy. Conditioned media from gastrointestinal Gram-positive, but not Gram-negative, bacteria activated autophagy and increased expression of the autophagy proteins Atg5 and Atg7 along with the stress response protein heat shock protein 27. Specific examination of media conditioned by the Gram-positive bacterium Bifidobacterium breve (Bb-CM) showed that this microbe produces small molecules (<3 kDa) that increase expression of the autophagy proteins Atg5 and Atg7, activate autophagy, and inhibit proteasomal enzyme activity. Upregulation of autophagy by Bb-CM was mediated through MAP kinase signaling. In vitro studies using C2BBe1 cells silenced for Atg7 and in vivo studies using mice conditionally deficient in intestinal epithelial cell Atg7 showed that Bb-CM-induced cytoprotection is dependent on autophagy. Therefore, this work demonstrates that Gram-positive bacteria modify protein degradation programs within intestinal epithelial cells to promote their survival during stress. It also reveals the therapeutic potential of soluble molecules produced by these microbes for prevention and treatment of gastrointestinal disease.

Nrf2 but not autophagy inhibition is associated with the survival of wild-type epidermal growth factor receptor non-small cell lung cancer cells

Non-small cell lung cancer (NSCLC) is one of the most common malignancies in the world. Icotinib and Gefitinib are two epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) that have been used to treat NSCLC. While it is well known that mutations of EGFR can affect the sensitivity of NSCLC to the EGFR-TKI, other mechanisms may also be adopted by lung cancer cells to develop resistance to EGFR-TKI treatment. Cancer cells can use multiple adaptive mechanisms such as activation of autophagy and Nrf2 to protect against various stresses and chemotherapeutic drugs. Whether autophagy or Nrf2 activation contributes to the resistance of NSCLC to EGFR-TKI treatment in wild-type EGFR NSCLC cells remains elusive. In the present study, we confirmed that Icotinib and Gefitinib induced apoptosis in EGFR mutant HCC827 but not in EGFR wild-type A549 NSCLC cells. Icotinib and Gefitinib did not induce autophagic flux or inhibit mTOR in A549 cells. Moreover, suppression of autophagy by chloroquine, a lysosomal inhibitor, did not affect Icotinib- or Gefitinib-induced cell death in A549 cells. In contrast, Brusatol, an Nrf2 inhibitor, significantly suppressed the cell survival of A549 cells. However, Brusatol did not further sensitize A549 cells to EGFR TKI-induced cell death. Results from this study suggest that inhibition of Nrf2 can decrease cell vitality of EGFR wild-type A549 cells independent of autophagy.

Hydrogen sulfide exhibits cardioprotective effects by decreasing endoplasmic reticulum stress in a diabetic cardiomyopathy rat model

Endoplasmic reticulum (ER) stress is critical in the occurrence and development of diabetic cardiomyopathy (DC). Hydrogen sulfide (H2S) has been found to be the third gaseous signaling molecule with anti‑ER stress effects. Previous studies have shown that H2S acts as a potent inhibitor of fibrosis in the heart of diabetic rats. This study aimed to demonstrate whether H2S exhibits protective effects on the myocardium of streptozotocin (STZ)‑induced diabetic rats by suppressing ER stress. In this study, diabetic models were established by intraperitoneal (i.p.) injection of 40 mg/kg STZ. The STZ‑treated mice were divided into three groups, and subsequently treated with normal saline, 30 µmol/kg or 100 µmol/kg NaHS, i.p., respectively, for 8 weeks. The extent of myocyte hypertrophy was measured using hematoxylin and eosin‑stained sections and collagen components were investigated using immunostaining. The expression of glucose-regulated protein (Grp78), C/EBP‑homologous protein (CHOP) and caspase‑12 in the heart tissue of each group was detected by western blot analysis. It was demonstrated that H2S could improve myocardial hypertrophy and myocardial collagen deposition in diabetic rats. In addition, it could reduce the expression of Grp78, caspase-12 and CHOP. In conclusion, these findings demonstrate that H2S suppresses STZ‑induced ER stress in the hearts of rats, and it may serve as a novel cardioprotective agent for DC.

ER stress: Autophagy induction, inhibition and selection

An accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) leads to stress conditions. To mitigate such circumstances, stressed cells activate a homeostatic intracellular signaling network cumulatively called the unfolded protein response (UPR), which orchestrates the recuperation of ER function. Macroautophagy (hereafter autophagy), an intracellular lysosome-mediated bulk degradation pathway for recycling and eliminating wornout proteins, protein aggregates, and damaged organelles, has also emerged as an essential protective mechanism during ER stress. These 2 systems are dynamically interconnected, and recent investigations have revealed that ER stress can either stimulate or inhibit autophagy. However, the stress-associated molecular cues that control the changeover switch between induction and inhibition of autophagy are largely obscure. This review summarizes the crosstalk between ER stress and autophagy and their signaling networks mainly in mammalian-based systems. Additionally, we highlight current knowledge on selective autophagy and its connection to ER stress.

Basal Autophagy and Feedback Activation of Akt Are Associated with Resistance to Metformin-Induced Inhibition of Hepatic Tumor Cell Growth

While accumulating evidence has shown that the use of the diabetic drug metformin may be beneficial against various tumors in some epidemiological studies, a few studies failed to show the same beneficial effects. The molecular and cellular mechanisms for these conflicting observations are not clear. In this study, we compared the inhibitory effects of cell growth by metformin on several hepatic tumor cell lines: SMMC-7721, HCC-97L, HCC-LM3 and HepG2. While metformin inhibited cell growth in all these cells, we found that SMMC-7721, HCC-97L and HCC-LM3 cells were more resistant than HepG2 cells. Mechanistically, we found that metformin inhibited mTOR in all these hepatic tumor cells. However, SMMC-7721 cells had higher levels of basal autophagy and mTORC2-mediated feedback activation of Akt than HepG2 cells, which may render SMMC-7721 cells to be more resistant to metformin-induced inhibition of cell growth. Similarly, HCC-97L and HCC-LM3 cells also had higher feedback activation of AKT than HepG2 cells, which may also account for their resistance to metformin-induced inhibition of cell growth. Therefore, the various basal autophagy and mTOR activity in different cancer cells may contribute to the controversial findings on the use of metformin in inhibition of cancers in humans.

Synergistic antitumor effects of radiation and proteasome inhibitor treatment in pancreatic cancer through the induction of autophagy and the downregulation of TRAF6

Ninety percent of human pancreatic cancer is characterized by activating K-RAS mutations. TRAF6 is an oncogene that plays a vital role in K-RAS-mediated oncogenesis. We investigated the synergistic effect of combining ionizing radiation (IR) and proteasome inhibitor (MG132). Furthermore, following combined treatment with IR and MG132, we analyzed the expression of TRAF6 and the mechanism of human pancreatic cancer cell death in vitro and in an orthotopic pancreatic cancer mouse model. The combined treatment groups displayed synergistic cell killing effects and induced endoplasmic reticulum stress in human pancreatic cancer cells. The combined treatment groups were characterized by enhanced cytotoxicity, which resulted from increased autophagy induction through the inhibition of TRAF6. Significantly reduced cytotoxicity was observed following MG132 and IR treatment of MIA PaCa-2 cells pre-treated with 3-MA (an autophagy inhibitor). Down-regulation of TRAF6 led to a significant increase in apoptosis and autophagy. In an orthotopic xenograft model of SCID mice, combination MG132 and IR therapy resulted in a significant increase in the tumor growth delay time and a decreased tumor tissue expression of TRAF6. IR combined with a proteasome inhibitor or TRAF6 inhibition could represent a new therapeutic strategy for human pancreatic cancer.

Shikonin selectively induces apoptosis in human prostate cancer cells through the endoplasmic reticulum stress and mitochondrial apoptotic pathway

Background

Despite the recent progress in screening and therapy, a majority of prostate cancer cases eventually attain hormone refractory and chemo-resistant attributes. Conventional chemotherapeutic strategies are effective at very high doses for only palliative management of these prostate cancers. Therefore chemo-sensitization of prostate cancer cells could be a promising strategy for increasing efficacy of the conventional chemotherapeutic agents in prostate cancer patients. Recent studies have indicated that the chemo-preventive natural agents restore the pro-apoptotic protein expression and induce endoplasmic reticulum stress (ER stress) leading to the inhibition of cellular proliferation and activation of the mitochondrial apoptosis in prostate cancer cells. Therefore reprogramming ER stress-mitochondrial dependent apoptosis could be a potential approach for management of hormone refractory chemoresistant prostate cancers. We aimed to study the effects of the natural naphthoquinone Shikonin in human prostate cancer cells.

Results

The results indicated that Shikonin induces apoptosis in prostate cancer cells through the dual induction of the endoplasmic reticulum stress and mitochondrial dysfunction. Shikonin induced ROS generation and activated ER stress and calpain activity. Moreover, addition of antioxidants attenuated these effects. Shikonin also induced the mitochondrial apoptotic pathway mediated through the enhanced expression of the pro-apoptotic Bax and inhibition of Bcl-2, disruption of the mitochondrial membrane potential (MMP) followed by the activation of caspase-9, caspase-3, and PARP cleavage.

Conclusion

The results suggest that shikonin could be useful in the therapeutic management of hormone refractory prostate cancers due to its modulation of the pro-apoptotic ER stress and mitochondrial apoptotic pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-015-0127-1) contains supplementary material, which is available to authorized users.

Chronic Deletion and Acute Knockdown of Parkin Have Differential Responses to Acetaminophen-induced Mitophagy and Liver Injury in Mice

We previously demonstrated that pharmacological induction of autophagy protected against acetaminophen (APAP)-induced liver injury in mice by clearing damaged mitochondria. However, the mechanism for removal of mitochondria by autophagy is unknown. Parkin, an E3 ubiquitin ligase, has been shown to be required for mitophagy induction in cultured mammalian cells following mitochondrial depolarization, but its role in vivo is not clear. The purpose of this study was to investigate the role of Parkin-mediated mitophagy in protection against APAP-induced liver injury. We found that Parkin translocated to mitochondria in mouse livers after APAP treatment followed by mitochondrial protein ubiquitination and mitophagy induction. To our surprise, we found that mitophagy still occurred in Parkin knock-out (KO) mice after APAP treatment based on electron microscopy analysis and Western blot analysis for some mitochondrial proteins, and Parkin KO mice were protected against APAP-induced liver injury compared with wild type mice. Mechanistically, we found that Parkin KO mice had decreased activated c-Jun N-terminal kinase (JNK), increased induction of myeloid leukemia cell differentiation protein (Mcl-1) expression, and increased hepatocyte proliferation after APAP treatment in their livers compared with WT mice. In contrast to chronic deletion of Parkin, acute knockdown of Parkin in mouse livers using adenovirus shRNA reduced mitophagy and Mcl-1 expression but increased JNK activation after APAP administration, which exacerbated APAP-induced liver injury. Therefore, chronic deletion (KO) and acute knockdown of Parkin have differential responses to APAP-induced mitophagy and liver injury in mice.

Calcium homeostasis and ER stress in control of autophagy in cancer cells

Autophagy is a basic catabolic process, serving as an internal engine during responses to various cellular stresses. As regards cancer, autophagy may play a tumor suppressive role by preserving cellular integrity during tumor development and by possible contribution to cell death. However, autophagy may also exert oncogenic effects by promoting tumor cell survival and preventing cell death, for example, upon anticancer treatment. The major factors influencing autophagy are Ca²⁺ homeostasis perturbation and starvation. Several Ca²⁺ channels like voltage-gated T- and L-type channels, IP3 receptors, or CRAC are involved in autophagy regulation. Glucose transporters, mainly from GLUT family, which are often upregulated in cancer, are also prominent targets for autophagy induction. Signals from both Ca²⁺ perturbations and glucose transport blockage might be integrated at UPR and ER stress activation. Molecular pathways such as IRE 1-JNK-Bcl-2, PERK-eIF2α-ATF4, or ATF6-XBP 1-ATG are related to autophagy induced through ER stress. Moreover ER molecular chaperones such as GRP78/BiP and transcription factors like CHOP participate in regulation of ER stress-mediated autophagy. Autophagy modulation might be promising in anticancer therapies; however, it is a context-dependent matter whether inhibition or activation of autophagy leads to tumor cell death.

Ampelopsin-induced autophagy protects breast cancer cells from apoptosis through Akt-mTOR pathway via endoplasmic reticulum stress

Our previous study has shown that ampelopsin (AMP), a flavonol mainly found in Ampelopsis grossedentata, could induce cell death in human breast cancer cells via reactive oxygen species generation and endoplasmic reticulum (ER) stress pathway. Here, we examined whether autophagy is activated in AMP-treated breast cancer cells and, if so, sought to find the exact role and underlying molecular profile of autophagy in AMP-induced cell death. Our results showed that AMP treatment activated autophagy in MDA-MB-231 and MCF-7 breast cancer cells, as evidenced by the accumulation of autophagosomes, an increase of microtubule-associated protein 1 light chain 3 beta-2 (LC3B-II) and the conversion of LC3B-I to LC3B-II, the degradation of the selective autophagic target p62/SQSTM1, and the formation of green fluorescent protein (GFP)-LC3 puncta. Blockage of autophagy augmented AMP-induced cell death, suggesting that autophagy has cytoprotective effects. Meanwhile, AMP treatment suppressed Akt-mammalian target of rapamycin (mTOR) pathway as evidenced by dose- and time-dependent decrease of the phosphorylation of Akt, mTOR and ribosomal protein S6 kinase (p70S6K), whereas Akt activator insulin-like growth factor-1 (IGF-1) pretreatment partially restored Akt-mTOR pathway inhibited by AMP and decreased AMP-inuduced autophagy, signifying that AMP activated autophagy via inhibition of the Akt-mTOR pathway. Additionally, blocking ER stress not only reduced autophagy induction, but also alleviated inhibition of the Akt-mTOR pathway induced by AMP, suggesting that activation of ER stress was involved in induction of autophagy and inhibition of the Akt-mTOR pathway. Taken together, these findings indicate that AMP induces protective autophagy in human breast cancer cells through Akt-mTOR pathway via ER stress.