Biljana Ristic

Graphene quantum dots as autophagy-inducing photodynamic agents

The excellent photoluminescent properties of graphene quantum dots (GQD) makes them suitable candidates for biomedical applications, but their cytotoxicity has not been extensively studied. Here we show that electrochemically produced GQD irradiated with blue light (470 nm, 1W) generate reactive oxygen species, including singlet oxygen, and kill U251 human glioma cells by causing oxidative stress. The cell death induced by photoexcited GQD displayed morphological and/or biochemical characteristics of both apoptosis (phosphatidylserine externalization, caspase activation, DNA fragmentation) and autophagy (formation of autophagic vesicles, LC3-I/LC3-II conversion, degradation of autophagic target p62). Moreover, a genetic inactivation of autophagy-essential LC3B protein partly abrogated the photodynamic cytotoxicity of GQD. These data indicate potential usefulness of GQD in photodynamic therapy, but also raise concerns about their possible toxicity.

Chloroquine-mediated lysosomal dysfunction enhances the anticancer effect of nutrient deprivation.

ABSTRACTPurposeTo investigate the ability of chloroquine, a lysosomotropic autophagy inhibitor, to enhance the anticancer effect of nutrient deprivation.MethodsSerum-deprived U251 glioma, B16 melanoma and L929 fibrosarcoma cells were treated with chloroquine in vitro. Cell viability was measured by crystal violet and MTT assay. Oxidative stress, apoptosis/necrosis and intracellular acidification were analyzed by flow cytometry. Cell morphology was examined by light and electron microscopy. Activation of AMP-activated protein kinase (AMPK) and autophagy were monitored by immunoblotting. RNA interference was used for AMPK and LC3b knockdown. The anticancer efficiency of intraperitoneal chloroquine in calorie-restricted mice was assessed using a B16 mouse melanoma model.ResultsChloroquine rapidly killed serum-starved cancer cells in vitro. This effect was not mimicked by autophagy inhibitors or LC3b shRNA, indicating autophagy-independent mechanism. Chloroquine-induced lysosomal accumulation and oxidative stress, leading to mitochondrial depolarization, caspase activation and mixed apoptotic/necrotic cell death, were prevented by lysosomal acidification inhibitor bafilomycin. AMPK downregulation participated in chloroquine action, as AMPK activation reduced, and AMPK shRNA mimicked chloroquine toxicity. Chloroquine inhibited melanoma growth in calorie-restricted mice, causing lysosomal accumulation, mitochondrial disintegration and selective necrosis of tumor cells.ConclusionCombined treatment with chloroquine and calorie restriction might be useful in cancer therapy.

Autophagy-dependent and -independent involvement of AMP-activated protein kinase in 6-hydroxydopamine toxicity to SH-SY5Y neuroblastoma cells.

The role of the main intracellular energy sensor adenosine monophosphate (AMP)-activated protein kinase (AMPK) in the induction of autophagic response and cell death was investigated in SH-SY5Y human neuroblastoma cells exposed to the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA). The induction of autophagy in SH-SY5Y cells was demonstrated by acridine orange staining of intracellular acidic vesicles, the presence of autophagosome- and autophagolysosome-like vesicles confirmed by transmission electron microscopy, as well as by microtubule-associated protein 1 light-chain 3 (LC3) conversion and p62 degradation detected by immunoblotting. 6-OHDA induced phosphorylation of AMPK and its target Raptor, followed by the dephosphorylation of the major autophagy inhibitor mammalian target of rapamycin (mTOR) and its substrate p70S6 kinase (S6K). 6-OHDA treatment failed to suppress mTOR/S6K phosphorylation and to increase LC3 conversion, p62 degradation and cytoplasmatic acidification in neuroblastoma cells in which AMPK expression was downregulated by RNA interference. Transfection of SH-SY5Y cells with AMPK or LC3β shRNA, as well as treatment with pharmacological autophagy inhibitors suppressed, while mTOR inhibitor rapamycin potentiated 6-OHDA-induced oxidative stress and apoptotic cell death. 6-OHDA induced phosphorylation of p38 mitogen-activated protein (MAP) kinase in an AMPK-dependent manner, and pharmacological inhibition of p38 MAP kinase reduced neurotoxicity, but not AMPK activation and autophagy triggered by 6-OHDA. Finally, the antioxidant N-acetyl cysteine antagonized 6-OHDA-induced activation of AMPK, p38 and autophagy. These data suggest that oxidative stress-mediated AMPK/mTOR-dependent autophagy and AMPK/p38-dependent apoptosis could be valid therapeutic targets for neuroprotection.

Inhibition of AMPK-dependent autophagy enhances in vitro antiglioma effect of simvastatin

The role of autophagy, a process in which the cell self-digests its own components, was investigated in glioma cell death induced by the hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase-inhibiting drug simvastatin. Induction of autophagy and activation of autophagy-regulating signalling pathways were analyzed by immunoblotting. Flow cytometry/fluorescent microscopy was used to assess autophagy-associated intracellular acidification and apoptotic markers (phosphatidylserine exposure, DNA fragmentation and caspase activation). Cell viability was determined by crystal violet, MTT or LDH release assay. Simvastatin treatment of U251 and C6 glioma cell lines caused the appearance of autophagolysosome-like intracytoplasmic acidic vesicles. The induction of autophagy in U251 cells was confirmed by the upregulation of autophagosome-associated LC3-II and pro-autophagic beclin-1, as well as by the downregulation of the selective autophagic target p62. Simvastatin induced the activation of AMP-activated protein kinase (AMPK) and its target Raptor, while simultaneously downregulating activation of Akt. Mammalian target of rapamycin (mTOR), a major AMPK/Akt downstream target and a major negative autophagy regulator, and its substrate p70 S6 kinase 1 were also inhibited by simvastatin. Mevalonate, the product of HMG-CoA reductase enzymatic activity, AMPK siRNA or pharmacological inactivation of AMPK with compound C suppressed, while the inhibitors of Akt (10-DEBC hydrochloride) and mTOR (rapamycin) mimicked autophagy induction by simvastatin. Inhibition of autophagy with bafilomycin A1, 3-methyladenine and LC3β shRNA, as well as AMPK inhibition with compound C or AMPK siRNA, markedly increased apoptotic death of simvastatin-treated U251 cells. These data suggest that inhibition of AMPK-dependent autophagic response might sensitize glioma cells to statin-induced apoptotic death.

mTOR-independent autophagy counteracts apoptosis in herpes simplex virus type 1-infected U251 glioma cells.

We investigated the role of autophagy, a stress-inducible lysosomal self-digestion of cellular components, in modulation of herpes simplex virus type 1 (HSV-1)-triggered death of U251 human glioma cells. HSV-1 caused apoptotic death in U251 cells, characterized by phosphatidylserine externalization, caspase activation and DNA fragmentation. HSV-1-induced apoptosis was associated with the induction of autophagic response, as confirmed by the conversion of cytosolic LC3-I to autophagosome-associated LC3-II, increase in intracellular acidification, presence of autophagic vesicles, and increase in proteolysis of the selective autophagic target p62. HSV-1-triggered autophagy was not associated with the significant increase in the expression of proautophagic protein beclin-1 or downregulation of the major autophagy suppressor mammalian target of rapamycin (mTOR). Moreover, the phosphorylation of mTOR and its direct substrate p70 S6 kinase was augmented by HSV-1 infection, while the mTOR stimulator Akt and inhibitor AMPK-activated protein kinase (AMPK) were accordingly activated and suppressed, respectively. An shRNA-mediated knockdown of the autophagy-essential LC3β, as well as pharmacological inhibition of autophagy with bafilomycin A1 or 3-methyladenine, markedly accelerated apoptotic changes and ensuing cell death in HSV-1-infected glioma cells. These data indicate that AMPK/Akt/mTOR-independent autophagy could prolong survival of HSV-1-infected U251 glioma cells by counteracting the coinciding apoptotic response.

Inhibition of mTOR-Dependent Autophagy Sensitizes Leukemic Cells to Cytarabine-Induced Apoptotic Death

The present study investigated the role of autophagy, a cellular self-digestion process, in the cytotoxicity of antileukemic drug cytarabine towards human leukemic cell lines (REH, HL-60, MOLT-4) and peripheral blood mononuclear cells from leukemic patients. The induction of autophagy was confirmed by acridine orange staining of intracellular acidic vesicles, electron microscopy visualization of autophagic vacuoles, as well as by the increase in autophagic proteolysis and autophagic flux, demonstrated by immunoblot analysis of p62 downregulation and LC3-I conversion to autophagosome-associated LC3-II in the presence of proteolysis inhibitors, respectively. Moreover, the expression of autophagy-related genes Atg4, Atg5 and Atg7 was stimulated by cytarabine in REH cells. Cytarabine reduced the phosphorylation of the major negative regulator of autophagy, mammalian target of rapamycin (mTOR), and its downstream target p70S6 kinase in REH cells, which was associated with downregulation of mTOR activator Akt and activation of extracellular signal- regulated kinase. Cytarabine had no effect on the activation of mTOR inhibitor AMP-activated protein kinase. Leucine, an mTOR activator, reduced both cytarabine-induced autophagy and cytotoxicity. Accordingly, pharmacological downregulation of autophagy with bafilomycin A1 and chloroquine, or RNA interference-mediated knockdown of LC3β or p62, markedly increased oxidative stress, mitochondrial depolarization, caspase activation and subsequent DNA fragmentation and apoptotic death in cytarabine-treated REH cells. Cytarabine also induced mTOR-dependent cytoprotective autophagy in HL-60 and MOLT-4 leukemic cell lines, as well as primary leukemic cells, but not normal leukocytes. These data suggest that the therapeutic efficiency of cytarabine in leukemic patients could be increased by the inhibition of the mTOR-dependent autophagic response.

Synthesis, characterization and cytotoxicity of a new palladium(II) complex with a coumarine-derived ligand

The new coumarine derivative, 3-(1-(2-hydroxyethylamino)ethylidene)chroman-2,4--dione, and corresponding palladium(II) complex have been synthesized and characterized by microanalysis, infrared, (1)H and (13)C NMR spectroscopy. The proposed structure of the complex was confirmed on the basis of the X-ray structural study. The palladium(II) complex decreased viability of L929 mouse fibrosarcoma, U251 human glioma and B16 mouse melanoma cell lines in a dose dependent manner, while its ligand exhibited no significant cytotoxicity. The cytotoxic effect of the complex was comparable to that of cisplatin, and mediated by apoptosis associated with oxidative stress, mitochondrial depolarization and caspase activation. Therefore, our results indicate that newly synthesized palladium(II) complex might be a potential candidate for anticancer therapy.

Idarubicin induces mTOR-dependent cytotoxic autophagy in leukemic cells.

We investigated if the antileukemic drug idarubicin induces autophagy, a process of programmed cellular self-digestion, in leukemic cell lines and primary leukemic cells. Transmission electron microscopy and acridine orange staining demonstrated the presence of autophagic vesicles and intracellular acidification, respectively, in idarubicin-treated REH leukemic cell line. Idarubicin increased punctuation/aggregation of microtubule-associated light chain 3B (LC3B), enhanced the conversion of LC3B-I to autophagosome-associated LC3B-II in the presence of proteolysis inhibitors, and promoted the degradation of the selective autophagic target p62, thus indicating the increase in autophagic flux. Idarubicin inhibited the phosphorylation of the main autophagy repressor mammalian target of rapamycin (mTOR) and its downstream target p70S6 kinase. The treatment with the mTOR activator leucine prevented idarubicin-mediated autophagy induction. Idarubicin-induced mTOR repression was associated with the activation of the mTOR inhibitor AMP-activated protein kinase and down-regulation of the mTOR activator Akt. The suppression of autophagy by pharmacological inhibitors or LC3B and beclin-1 genetic knockdown rescued REH cells from idarubicin-mediated oxidative stress, mitochondrial depolarization, caspase activation and apoptotic DNA fragmentation. Idarubicin also caused mTOR inhibition and cytotoxic autophagy in K562 leukemic cell line and leukocytes from chronic myeloid leukemia patients, but not healthy controls. By demonstrating mTOR-dependent cytotoxic autophagy in idarubicin-treated leukemic cells, our results warrant caution when considering combining idarubicin with autophagy inhibitors in leukemia therapy.

The role and therapeutic potential of autophagy modulation in controlling virus-induced cell death.

Macroautophagy (hereafter referred to as autophagy) is an evolutionary conserved catabolic process in which the cytoplasmic content is sequestered and degraded by the lysosomal machinery in order to maintain cellular homeostasis or provide energy during metabolic and hypoxic stress. It also represents an important component of the host response against infectious agents, performing surveillance and effector functions involved in detection and clearance of pathogens, including viruses. Moreover, it appears that autophagy plays a major role in determining the fate of both virally infected and uninfected cells by blocking or promoting their death in a virus‐ and cell‐type‐dependent manner. We here review the current knowledge on the complex involvement of autophagy in survival and death of the host cells during viral infection, focusing on the molecular mechanisms underlying viral modulation of autophagic response and its interference with the cell death pathways. We also discuss a possible significance of the autophagy‐dependent modulation of cell death for the outcome and therapy of viral infections, emphasizing the need for a time‐ and cell‐type‐dependent fine‐tuning of the autophagic response in achieving an optimal balance between beneficial and adverse effects.

A novel C,D-spirolactone analogue of paclitaxel: autophagy instead of apoptosis as a previously unknown mechanism of cytotoxic action for taxoids

The design, synthesis and biological evaluation of a novel C,D-spirolactone analogue of paclitaxel is described. This is the first paclitaxel analogue without an oxetane D-ring that shows a significant cytotoxic effect (activity one order of magnitude lower than paclitaxel). More importantly, its cytotoxicity is a result of a different mechanism of action, involving mTOR inhibition-dependent autophagy instead of G(2)/M cell cycle arrest-dependent apoptosis.