Inhibition of the autophagic flux by salinomycin in breast cancer stem-like/progenitor cells interferes with their maintenance

Evaluation of growth inhibitory response of Resveratrol and Salinomycin combinations against triple negative breast cancer cells

Resveratrol (RSVL) a dietary phytochemical showed to enhance the efficacy of chemotherapeutic drugs. Recently, Salinomycin (SAL) has gained importance as cancer therapeutic value for breast cancer (BC), however, its superfluxious toxicity delimits the utility. Taking the advantage of RSVL, the therapeutic efficacy of RSVL and SAL combination was studied in vitro and in vivo system. Firstly, the synergistic combination dose of RSVL and SAL was calculated and further, the efficacy was examined by wound healing, and Western blots analysis. Further, in vivo study was performed to confirm the effect of colony formation and apoptosis detection by flow cytometry based assays. Further, the molecular mode of action was determined at both transcript and translational level by quantitative Real Time PCR combination in Ehrlich ascitic carcinoma model.The combination of IC20 (R20) of RSVL and IC10 (S10) dose of SAL showed best synergism (CI<1) with ∼5 fold dose advantage of SAL. Gene expression results at mRNA and protein level revealed that the unique combination of RSVL and SAL significantly inhibited epithelial mesenchymal transition (Fibronectin, Vimentin, N-Cadherin, and Slug); chronic inflammation (Cox2, NF-kB, p53), autophagy (Beclin and LC3) and apoptotic (Bax, Bcl-2) markers. Further, i n vivo study showed that low dose of SAL in combination with RSVL increased life span of Ehrlich ascitic mice. Overall, our study revealed that RSVL synergistically potentiated the anticancer potential of SAL against triple negative BC.

Multidrug-resistant cancer cells and cancer stem cells hijack cellular systems to circumvent systemic therapies, can natural products reverse this?

Chemotherapy is one of the most effective and broadly used approaches for cancer management and many modern regimes can eliminate the bulk of the cancer cells. However, recurrence and metastasis still remain a major obstacle leading to the failure of systemic cancer treatments. Therefore, to improve the long-term eradication of cancer, the cellular and molecular pathways that provide targets which play crucial roles in drug resistance should be identified and characterised. Multidrug resistance (MDR) and the existence of tumor-initiating cells, also referred to as cancer stem cells (CSCs), are two major contributors to the failure of chemotherapy. MDR describes cancer cells that become resistant to structurally and functionally unrelated anti-cancer agents. CSCs are a small population of cells within cancer cells with the capacity of self-renewal, tumor metastasis, and cell differentiation. CSCs are also believed to be associated with chemoresistance. Thus, MDR and CSCs are the greatest challenges for cancer chemotherapy. A significant effort has been made to identify agents that specifically target MDR cells and CSCs. Consequently, some agents derived from nature have been developed with a view that they may overcome MDR and/or target CSCs. In this review, natural products-targeting MDR cancer cells and CSCs are summarized and clustered by their targets in different signaling pathways.

Cationic liposomes promote antigen cross-presentation in dendritic cells by alkalizing the lysosomal pH and limiting the degradation of antigens

Cationic liposomes (CLs) have been widely examined as vaccine delivery nanoparticles since they can form complexes with biomacromolecules, promote delivery of antigens and adjuvant molecules to antigen-presenting cells (APCs), and mediate cellular uptake of vaccine components. CLs are also known to trigger antigen cross-presentation - the process by which APCs internalize extracellular protein antigens, degrade them into minimal CD8⁺ T-cell epitopes, and present them in the context of major histocompatibility complex-I (MHC-I). However, the precise mechanisms behind CL-mediated induction of cross-presentation and cross-priming of CD8⁺ T-cells remain to be elucidated. In this study, we have developed two distinct CL systems and examined their impact on the lysosomal pH in dendritic cells (DCs), antigen degradation, and presentation of peptide:MHC-I complexes to antigen-specific CD8⁺ T-cells. To achieve this, we have used 3β-[N-(N',N'-dimethylaminoethane)-carbamoyl] cholesterol (DC-Chol) and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) as the prototypical components of CLs with tertiary amine groups and compared the effect of CLs and anionic liposomes on lysosomal pH, antigen degradation, and cross-presentation by DCs. Our results showed that CLs, but not anionic liposomes, elevated the lysosomal pH in DCs and reduced antigen degradation, thereby promoting cross-presentation and cross-priming of CD8⁺ T-cell responses. These studies shed new light on CL-mediated cross-presentation and suggest that intracellular fate of vaccine components and subsequent immunological responses can be controlled by rational design of nanomaterials.

Therapeutic implications of tumor interstitial acidification

Interstitial acidification is a hallmark of solid tumor tissues resulting from the combination of different factors, including cellular buffering systems, defective tissue perfusion and high rates of cellular metabolism. Besides contributing to tumor pathogenesis and promoting tumor progression, tumor acidosis constitutes an important intrinsic and extrinsic mechanism modulating therapy sensitivity and drug resistance. In fact, pharmacological properties of anticancer drugs can be affected not only by tissue structure and organization but also by the distribution of the interstitial tumor pH. The acidic tumor environment is believed to create a chemical barrier that limits the effects and activity of many anticancer drugs. In this review article we will discuss the general protumorigenic effects of acidosis, the role of tumor acidosis in the modulation of therapeutic efficacy and potential strategies to overcome pH-dependent therapy-resistance.

Lysosomal Biology in Cancer

Cells depend on the lysosome for sequestration and degradation of macromolecules in order to maintain metabolic homeostasis. These membrane-enclosed organelles can receive intracellular and extracellular cargo through endocytosis, phagocytosis, and autophagy. Lysosomes establish acidic environments to activate enzymes that are able to break down biomolecules engulfed through these various pathways. Recent advances in methods to study the lysosome have allowed the discovery of extended roles for the lysosome in various diseases, including cancer, making it an attractive and targetable node for therapeutic intervention. This review focuses on key aspects of lysosomal biology in the context of cancer and how these properties can be exploited for the development of new therapeutic strategies. This will provide a contextual framework for how advances in methodology could be applied in future translational research.

Defective autophagy leads to the suppression of stem-like features of CD271+ osteosarcoma cells

Background

As an important stress-response mechanism, autophagy plays crucial role in the tumor formation and drug resistance of cancer cells including osteosarcoma (OS). OS cancer stem cells (CSCs) also are considered a key factor of tumorigenesis, drug resistance and tumor recurrence. However, the relationship between autophagy and OS CSCs still remains unclear.

Methods

CD271+ OS CSCs and CD271- OS cells were isolated by magnetic activated cell sorting. The autophagy level was evaluated by the mRNA expression of autophagy genes, the protein level of LC3II and p62, and the mean number of GFP-LC3 dot per cell. Lentivirus-delivered specific shRNA was utilized to inhibit the corresponding gene expression. The cell viability was examined with CCK8 assay. The cell proliferation level was detected with BrdU staining assay. Cell death was determined by Annexin V/PI double staining of fluorescence activated cell sorting, lactate dehydrogenase release and caspase-3 activity. Tumorigenicity ability was evaluated by colony and sphere formation assay, the protein expression of stemness markers and tumor formation in nude mice.

Results

Our data indicated that CD271+ OS CSCs had a similar basic autophagy level with CD271- OS cells. Autophagy deficiency had no observable effects on the levels of cell proliferation and death both in CD271+ and CD271- OS cells under normal condition. However, CD271+ OS cells showed a higher autophagy activity than CD271- OS cells under hypoxia and low nutrient (LH) condition. Moreover, autophagy-deficient CD271+ OS cells lost the advantage of tolerance to LH condition compared to CD271- OS cells. Meanwhile, autophagy deficiency enhanced the sensitivity to chemotherapeutics in the CD271+ cells to the comparable level in the CD271- cells. More importantly, deficient-autophagy decreased the protein expression of stemness markers and caused the disappearance of the superiority in tumorigenicity in vitro and vivo in CD271+ OS cells.

Conclusion

The results above demonstrated that autophagy contributes to the stem-like features of CD271+ OS CSCs. Inhibition of autophagy is a promising strategy in the CSCs-targeting OS therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-016-0297-5) contains supplementary material, which is available to authorized users.

Cationic liposomes induce cell necrosis through lysosomal dysfunction and late-stage autophagic flux inhibition

AIM

The application of cationic liposomes (CLs) as nonviral vectors is hampered by their cellular toxicity. Thus we aim to investigate the mechanisms underlying the cellular toxicity of CLs.

MATERIALS & METHODS

The effect of CLs on the autophagic flux, autophagosome-lysosome fusion, lysosome membrane permeabilization and cell necrosis of liver cells was investigated.

RESULTS & CONCLUSION

Our results reveal a novel mechanism of CL-induced cell necrosis involving the induction of lysosome membrane permeabilization and late-stage autophagic flux inhibition that resulted in cytoplasmic release of cathepsin B, mitochondrial dysfunction and reactive oxygen species production, which are the key mediators of cell necrosis. Our study is important for revealing the cellular toxicity of CLs and designing safer gene delivery systems.

Melatonin antagonizes cadmium-induced neurotoxicity by activating the transcription factor EB-dependent autophagy-lysosome machinery in mouse neuroblastoma cells

Cadmium (Cd), a highly ubiquitous heavy metal, induces neurotoxicity. Melatonin, a major secretory product of the pineal gland, protects against Cd-induced neurotoxicity. However, the mechanism that accounts for this protection remains to be elucidated. Herein, we exposed mouse neuroblastoma cells (Neuro-2a cells) to different concentrations of cadmium chloride (CdCl2 ) (12.5, 25, and 50 μ mol L(-1) ) for 24 hours. We showed that Cd inhibits autophagosome-lysosome fusion and impairs lysosomal function, subsequently leading to nerve cell death. In addition, Cd decreases the level of transcription factor EB (TFEB) but induces the nuclear translocation of TFEB, associated with compromised lysosomal function or a compensatory effect after the impairment of the autophagic flux. Moreover, compared to the 50-μ mol L(-1) Cd group, administration of 1 μ mol L(-1) melatonin increased "TFEB-responsive genes" (P<.05) and the levels of lysosomal-associated membrane protein (0.57±0.06 vs 1.00±0.11, P<.05), preserved lysosomal protease activity (0.52±0.01 vs 0.90±0.02, P<.05), maintained the lysosomal pH level (0.50±0.01 vs 0.87±0.05, P<.01), and enhanced autophagosome-lysosome fusion (0.05±0.00 vs 0.21±0.01, P<.01). Notably, melatonin enhanced TFEB expression (0.37±0.04 vs 0.72±0.07, P<.05) and nuclear translocation (2.81±0.08 vs 3.82±0.05, P<.05). Tfeb siRNA blocked the melatonin-mediated elevation in autophagy-lysosome machinery in Cd-induced neurotoxicity (P<.01). Taken together, these results uncover a potent role for TFEB-mediated autophagy in the pathogenesis of Cd-induced neurotoxicity, suggesting that control of the autophagic pathway by melatonin might provide an important clue for exploring potential targets for novel therapeutics of Cd-induced neurotoxicity.

Inhibition of autophagosome-lysosome fusion by ginsenoside Ro via the ESR2-NCF1-ROS pathway sensitizes esophageal cancer cells to 5-fluorouracil-induced cell death via the CHEK1-mediated DNA damage checkpoint

Modulation of autophagy has been increasingly regarded as a promising cancer therapeutic approach. In this study, we screened several ginsenosides extracted from Panax ginseng and identified ginsenoside Ro (Ro) as a novel autophagy inhibitor. Ro blocked the autophagosome-lysosome fusion process by raising lysosomal pH and attenuating lysosomal cathepsin activity, resulting in the accumulation of the autophagosome marker MAP1LC3B/LC3B and SQSTM1/p62 (sequestosome 1) in various esophageal cancer cell lines. More detailed studies demonstrated that Ro activated ESR2 (estrogen receptor 2), which led to the activation of NCF1/p47(PHOX) (neutrophil cytosolic factor 1), a subunit of NADPH oxidase, and subsequent reactive oxygen species (ROS) production. Treatment with siRNAs or inhibitors of the ESR2-NCF1-ROS axis, such as N-acetyl-L-cysteine (NAC), diphenyleneiodonium chloride (DPI), apocynin (ACN), Tiron, and Fulvestrant apparently decreased Ro-induced LC3B-II, GFP-LC3B puncta, and SQSTM1, indicating that ROS instigates autophagic flux inhibition triggered by Ro. More importantly, suppression of autophagy by Ro sensitized 5-fluorouracil (5-Fu)-induced cell death in chemoresistant esophageal cancer cells. 5-Fu induced prosurvival autophagy, and by inhibiting such autophagy, siRNAs against BECN1/beclin 1, ATG5, ATG7, and LC3B enhanced 5-Fu-induced autophagy-associated and apoptosis-independent cell death. We observed that Ro potentiates 5-Fu cytotoxicity via delaying CHEK1 (checkpoint kinase 1) degradation and downregulating DNA replication process, resulting in the delayed DNA repair and the accumulation of DNA damage. In summary, these data suggest that Ro is a novel autophagy inhibitor and could function as a potent anticancer agent in combination therapy to overcome chemoresistance.

Combination of salinomycin and silver nanoparticles enhances apoptosis and autophagy in human ovarian cancer cells: an effective anticancer therapy

Ovarian cancer is one of the most important malignancies, and the origin, detection, and pathogenesis of epithelial ovarian cancer remain elusive. Although many cancer drugs have been developed to dramatically reduce the size of tumors, most cancers eventually relapse, posing a critical problem to overcome. Hence, it is necessary to identify possible alternative therapeutic approaches to reduce the mortality rate of this devastating disease. To identify alternative approaches, we first synthesized silver nanoparticles (AgNPs) using a novel bacterium called Bacillus clausii. The synthesized AgNPs were homogenous and spherical in shape, with an average size of 16–20 nm, which are known to cause cytotoxicity in various types of human cancer cells, whereas salinomycin (Sal) is able to kill cancer stem cells. Therefore, we selected both Sal and AgNPs to study their combined effect on apoptosis and autophagy in ovarian cancer cells. The cells treated with either Sal or AgNPs showed a dose-dependent effect with inhibitory concentration (IC)-50 values of 6.0 µM and 8 µg/mL for Sal and AgNPs, respectively. To determine the combination effect, we measured the IC₂₅ values of both Sal and AgNPs (3.0 µM and 4 µg/mL), which showed a more dramatic inhibitory effect on cell viability and cell morphology than either Sal or AgNPs alone. The combination of Sal and AgNPs had more pronounced effect on cytotoxicity and expression of apoptotic genes and also significantly induced the accumulation of autophagolysosomes, which was associated with mitochondrial dysfunction and loss of cell viability. Our data show a strong synergistic interaction between Sal and AgNPs in tested cancer cells. The combination treatment increased the therapeutic potential and demonstrated the relevant targeted therapy for the treatment of ovarian cancer. Furthermore, we provide, for the first time, a mode of action for Sal and AgNPs in ovarian cancer cells: enhanced apoptosis and autophagy.

Epigallocatechin gallate exerts protective effects against myocardial ischemia/reperfusion injury through the PI3K/Akt pathway-mediated inhibition of apoptosis and the restoration of the autophagic flux

Epigallocatechin gallate (EGCG), a polyphenol derived from green tea, exhibits a wide range of biological activities, including antioxidant, atherosclerosis and antitumor activities. In this study, the cardioprotective effects of EGCG on myocardial ischemia/reperfusion (I/R) injury in rats and the underlying mechanisms were investigated. A rat model of I/R injury was established by ligating the left anterior descending coronary artery for 30 min, followed by reperfusion for 2 h. The levels of I/R-induced creatine kinase-MB (CK-MB) and the release of lactate dehydrogenase (LDH), as well as the infarct size, cardiomyocyte apoptosis and cardiac functional impairment were examined and compared. Western blot analysis was carried out to elucidate the potential molecular mechanisms of action of EGCG. The results revealed that EGCG post-conditioning significantly decreased the levels of CK-MB and the release of LDH, reduced the myocardial infarct size, decreased the apoptotic rate and partially preserved heart function. Furthermore, EGCG decreased the expression of cleaved caspase-3 concomitantly with the upregulation of PI3K, and the phosphorylation of Akt and endothelial nitric oxide synthase (eNOS). It also inhibited I/R-induced overautophagy and promoted the clearance of autophagosomes, as evidenced by a decrease in the ratio of microtubule-associated protein 1 light chain 3 (LC3)-II/LC3-I, the downregulation of Beclin1, Atg5 and p62, and the upregulation of active cathepsin D. Additionally, we observed an increase in the phosphorylation levels of the mammalian target of rapamycin (mTOR) following treatment with EGCG. Taken together, the findings of this study demonstrate that, EGCG post-conditioning alleviates myocardial I/R injury by inhibiting apoptosis and restoring the autophagic flux, which is associated with several targets of the PI3K/Akt signaling pathway.

Codelivery of salinomycin and chloroquine by liposomes enables synergistic antitumor activity in vitro

Aim: To improve the suboptimal therapeutic efficacy of salinomycin (SAL) toward liver cancer cells using chloroquine (CQ) combination by the liposomes co-delivering SAL and CQ (SCNL). Materials & methods: The synergy of these two drugs was evaluated in liver cancer cells (HepG2) and liver cancer stem cells (LCSCs) by median-effect analysis. SCNL with optimized ratio were developed. The cytotoxic effect and basal autophagy flux (measure of autophagic degradation activity) of various formulations were evaluated. Results & conclusion: CQ could significantly increase the cytotoxic effect of SAL in HepG2 cells, but not in HepG2-LCSCs, due to the greater basal autophagy flux in HepG2 cells. This combination therapy is promising for liver cancer treatment by eradicating liver cancer cells and LCSCs.

Inhibition of Dopamine Receptor D4 Impedes Autophagic Flux, Proliferation, and Survival of Glioblastoma Stem Cells

Glioblastomas (GBM) grow in a rich neurochemical milieu, but the impact of neurochemicals on GBM growth is largely unexplored. We interrogated 680 neurochemical compounds in patient-derived GBM neural stem cells (GNS) to determine the effects on proliferation and survival. Compounds that modulate dopaminergic, serotonergic, and cholinergic signaling pathways selectively affected GNS growth. In particular, dopamine receptor D4 (DRD4) antagonists selectively inhibited GNS growth and promoted differentiation of normal neural stem cells. DRD4 antagonists inhibited the downstream effectors PDGFRβ, ERK1/2, and mTOR and disrupted the autophagy-lysosomal pathway, leading to accumulation of autophagic vacuoles followed by G0/G1 arrest and apoptosis. These results demonstrate a role for neurochemical pathways in governing GBM stem cell proliferation and suggest therapeutic approaches for GBM.

Endoplasmic reticulum stress-inducing drugs sensitize glioma cells to temozolomide through downregulation of MGMT, MPG, and Rad51

BACKGROUND

Endoplasmic reticulum (ER) stress results from protein misfolding imbalance and has been postulated as a therapeutic strategy. ER stress activates the unfolded protein response which leads to a complex cellular response, including the upregulation of aberrant protein degradation in the ER, with the goal of resolving that stress. O(6)-methylguanine DNA methyltransferase (MGMT), N-methylpurine DNA glycosylase (MPG), and Rad51 are DNA damage repair proteins that mediate resistance to temozolomide in glioblastoma. In this work we sought to evaluate whether ER stress-inducing drugs were able to downmodulate DNA damage repair proteins and become candidates to combine with temozolomide.

METHODS

MTT assays were performed to evaluate the cytotoxicity of the treatments. The expression of proteins was evaluated using western blot and immunofluorescence. In vivo studies were performed using 2 orthotopic glioblastoma models in nude mice to evaluate the efficacy of the treatments. All statistical tests were 2-sided.

RESULTS

Treatment of glioblastoma cells with ER stress-inducing drugs leads to downregulation of MGMT, MPG, and Rad51. Inhibition of ER stress through pharmacological treatment resulted in rescue of MGMT, MPG, and Rad51 protein levels. Moreover, treatment of glioblastoma cells with salinomycin, an ER stress-inducing drug, and temozolomide resulted in enhanced DNA damage and a synergistic antitumor effect in vitro. Of importance, treatment with salinomycin/temozolomide resulted in a significant antiglioma effect in 2 aggressive orthotopic intracranial brain tumor models.

CONCLUSIONS

These findings provide a strong rationale for combining temozolomide with ER stress-inducing drugs as an alternative therapeutic strategy for glioblastoma.

Clinical relevance of autophagic therapy in cancer: Investigating the current trends, challenges, and future prospects

Oncophagy (cancer-related autophagy) has a complex dual character at different stages of tumor progression. It remains an important clinical problem to unravel the reasons that propel the shift in the role of oncophagy from tumor inhibition to a protective mechanism that shields full-blown malignancy. Most treatment strategies emphasize curbing protective oncophagy while triggering the oncophagy that is lethal to tumor cells. In this review, we focus on the trends in current therapeutics as well as various challenges in clinical trials to address the oncophagic dilemma and evaluate the potential of these developing therapies. A detailed analysis of the clinical and pre-clinical scenario of the anticancer medicines highlights the various inducers and inhibitors of autophagy. The ways in which tumor stage, the microenvironment and combination drug treatment continue to play an important tactical role are discussed. Moreover, autophagy targets also play a crucial role in developing the best possible solution to this oncophagy paradox. In this review, we provide a comprehensive update on the current clinical impact of autophagy-based cancer therapeutic drugs and try to lessen the gap between translational medicine and clinical science.

Decreased expression of autophagy protein LC3 and stemness (CD44+/CD24-/low) indicate poor prognosis in triple-negative breast cancer

This study evaluated the prognostic value of expression of autophagy protein light chain 3 (LC3) and the prognostic value of coexpression of LC3 and stemness markers CD44+/CD24-/low in triple-negative breast cancer (TNBC). LC3 and LC3/CD44+/CD24-/low immunophenotypes in tumor tissues were evaluated by immunohistochemistry in 67 TNBC patients. LC3- was expressed in 30 (44.78%) cases. The LC3- phenotype revealed a significant negative association with overall survival in both univariate (P = .0006) and multivariate (P = .0153) analyses. LC3-/CD44+/CD24-/low phenotype was observed in 24 (35.82%) of 67 TNBC patients. According to Kaplan-Meier analysis, prognosis was significantly worse in tumors with LC3-/CD44+/CD24-/low phenotype (P = .0280). Multivariate analysis indicated that LC3-/CD44+/CD24-/low phenotype was a significant independent prognostic indicator of overall survival. These results suggest that LC3 suppresses TNBC in mature tumor cells and cancer stem cells (CSCs). In conclusion, this study suggests that CSCs are linked to progression of autophagy in TNBC. During the progression and development of TNBC, autophagy of CSCs/progenitor cells is low. LC3-/CD44+/CD24-/low immunophenotype indicates a highly aggressive TNBC subgroup associated with a poor prognosis. This study investigated that LC3 deficiency may restrain TNBC in mature tumor cells and CSCs. Therefore, a reasonable inference is that inducing autophagy may be an effective therapeutic strategy in TNBC.

Early effects of the antineoplastic agent salinomycin on mitochondrial function

Salinomycin, isolated from Streptomyces albus, displays antimicrobial activity. Recently, a large-scale screening approach identified salinomycin and nigericin as selective apoptosis inducers of cancer stem cells. Growing evidence suggests that salinomycin is able to kill different types of non-stem tumor cells that usually display resistance to common therapeutic approaches, but the mechanism of action of this molecule is still poorly understood. Since salinomycin has been suggested to act as a K(+) ionophore, we explored its impact on mitochondrial bioenergetic performance at an early time point following drug application. In contrast to the K(+) ionophore valinomycin, salinomycin induced a rapid hyperpolarization. In addition, mitochondrial matrix acidification and a significant decrease of respiration were observed in intact mouse embryonic fibroblasts (MEFs) and in cancer stem cell-like HMLE cells within tens of minutes, while increased production of reactive oxygen species was not detected. By comparing the chemical structures and cellular effects of this drug with those of valinomycin (K(+) ionophore) and nigericin (K(+)/H(+) exchanger), we conclude that salinomycin mediates K(+)/H(+) exchange across the inner mitochondrial membrane. Compatible with its direct modulation of mitochondrial function, salinomycin was able to induce cell death also in Bax/Bak-less double-knockout MEF cells. Since at the concentration range used in most studies (around 10 μM) salinomycin exerts its effect at the level of mitochondria and alters bioenergetic performance, the specificity of its action on pathologic B cells isolated from patients with chronic lymphocytic leukemia (CLL) versus B cells from healthy subjects was investigated. Mesenchymal stromal cells (MSCs), proposed to mimic the tumor environment, attenuated the apoptotic effect of salinomycin on B-CLL cells. Apoptosis occurred to a significant extent in healthy B cells as well as in MSCs and human primary fibroblasts. The results indicate that salinomycin, when used above μM concentrations, exerts direct, mitochondrial effects, thus compromising cell survival.

Blockade of autophagy reduces pancreatic cancer stem cell activity and potentiates the tumoricidal effect of gemcitabine

Background

Cancer stem cells (CSCs) are considered responsible for the recurrence and chemoresistance of cancer. Dysregulated autophagy is highly prevalent in many types of cancer including pancreatic cancer and has been implicated in cytoprotection and tumor promotion. This study aimed to investigate the role of autophagy in regulating cancer stemness and chemoresistance of pancreatic cancer.

Methods

The correlation between autophagy and CSCs and its clinical significance were analyzed using pancreatic cancer tissue microarrays. Genetic and pharmacological approaches were applied to explore the function of autophagy on CSC activity and gemcitabine resistance of pancreatic cancer cells in vitro and in vivo.

Results

LC3 expression positively correlated with the expression of CSC markers aldehyde dehydrogenase 1 (ALDH1), CD44, and CD133 in pancreatic cancer tissues. High coexpression of LC3/ALDH1 was associated with both poor overall survival and progression-free survival. In pancreatic cancer cell lines, higher LC3-II expression was observed in the sphere-forming cells than in the bulk cells. Blockade of autophagy by silencing ATG5, ATG7, and BECN1 or the administration of autophagy inhibitor chloroquine markedly reduced the CSC populations, ALDH1 activity, sphere formation, and resistance to gemcitabine in vitro and in vivo. Furthermore, osteopontin (OPN) was found to stimulate LC3-II, ALDH1, CD44, and CD133 expression in PANC-1 cells, whereas this effect could be prevented by OPN knockdown and autophagy blockade. After treatment with various inhibitors against the major signaling pathways downstream of OPN, only the inhibitor of NF-κB activation, BAY 1170–82, could effectively counteract OPN-induced autophagy and CSC activity. According to the histochemical results, pancreatic cancer patients manifesting high levels of OPN/LC3/ALDH1 and OPN/CD44/CD133 had poor survival.

Conclusions

Induction of autophagy mediated by OPN/NF-κB signaling is required for maintenance of pancreatic CSC activity. Combination of gemcitabine with pharmacological autophagy inhibitors is a promising therapeutic strategy for pancreatic cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0449-3) contains supplementary material, which is available to authorized users.

Lithium Modulates Autophagy in Esophageal and Colorectal Cancer Cells and Enhances the Efficacy of Therapeutic Agents In Vitro and In Vivo

Many epithelial cancers, particularly gastrointestinal tract cancers, remain poor prognosis diseases, due to resistance to cytotoxic therapy and local or metastatic recurrence. We have previously shown that apoptosis incompetent esophageal cancer cells induce autophagy in response to chemotherapeutic agents and this can facilitate their recovery. However, known pharmacological inhibitors of autophagy could not enhance cytotoxicity. In this study, we have examined two well known, clinically approved autophagy inducers, rapamycin and lithium, for their effects on chemosensitivity in apoptosis incompetent cancer cells. Both lithium and rapamycin were shown to induce autophagosomes in esophageal and colorectal cancer cells by western blot analysis of LC3 isoforms, morphology and FACS quantitation of Cyto-ID or mCherry-GFP-LC3. Analysis of autophagic flux indicates inefficient autophagosome processing in lithium treated cells, whereas rapamycin treated cells showed efficient flux. Viability and recovery was assessed by clonogenic assays. When combined with the chemotherapeutic agent 5-fluorouracil, rapamycin was protective. In contrast, lithium showed strong enhancement of non-apoptotic cell death. The combination of lithium with 5-fluorouracil or oxaliplatin was then tested in the syngenic mouse (balb/c) colorectal cancer model—CT26. When either chemotherapeutic agent was combined with lithium a significant reduction in tumor volume was achieved. In addition, survival was dramatically increased in the combination group (p < 0.0001), with > 50% of animals achieving long term cure without re-occurrence (> 1 year tumor free). Thus, combination treatment with lithium can substantially improve the efficacy of chemotherapeutic agents in apoptosis deficient cancer cells. Induction of compromised autophagy may contribute to this cytotoxicity.

Drug Transporter-Mediated Protection of Cancer Stem Cells From Ionophore Antibiotics

Ionophore antibiotics were reported to selectively kill cancer stem cells and to overcome multidrug resistance, but mechanistic studies of the significance of drug transporters for treatment with these compounds are lacking. We applied chemosensitivity testing of well-characterized human cancer cell lines to elaborate on whether drug transporters are involved in protection from the cytotoxic effects of the ionophore antibiotics salinomycin and nigericin. Our experiments demonstrated that ionophore antibiotics were ineffective against both stem-like ovarian cancer side population cells (expressing either ABCB1 or ABCG2) and K562/Dox-H1 cells, which constitute a genetically defined model system for ABCB1 expression. Considering that cancer stem cells often express high levels of drug transporters, we deduced from our results that ionophore antibiotics are less suited to cancer stem cell-targeted treatment than previously thought.

SIGNIFICANCE

Ionophore antibiotics such as salinomycin have repeatedly been shown to target cancer stem and progenitor cells from various tumor entities. Meanwhile, cancer stem cell (CSC)-selective toxicity of ionophore antibiotics seems to be a commonly accepted concept that is about to encourage their clinical testing. This study provides data that challenge the concept of targeted elimination of CSC by ionophore antibiotics. Stem-like ovarian cancer side population (SP) cells expressing high levels of ABC drug transporters are shown to largely resist the cytotoxic effects of salinomycin and nigericin. Furthermore, using a small interfering RNA-based knockdown model specific for ABCB1, this study demonstrates that ABC drug transporters are indeed causally involved in mediating protection from ionophore antibiotics. Considering that it is a hallmark of CSCs to exhibit drug resistance conferred by ABC drug transporters, it must be deduced from these results that CSCs may also be protected from ionophore antibiotics by means of drug-transporter mediated efflux.

Autophagy and cell reprogramming

Autophagy is an evolutionarily conserved process that degrades cytoplasmic components, thus contributing to cell survival and tissue homeostasis. Recent studies have demonstrated that autophagy maintains stem cells in relatively undifferentiated states (stemness) and also contributes to differentiation processes. Autophagy likewise plays a crucial role in somatic cell reprogramming, a finely regulated process that resets differentiated cells to a pluripotent state and that requires comprehensive alterations in transcriptional activities and epigenetic signatures. Autophagy assists in manifesting the functional consequences that arise from these alterations by modifying cellular protein expression profiles. The role of autophagy appears to be particularly relevant for early phases of cell reprogramming during the generation of induced pluripotent stems cells (iPSCs). In this review, we provide an overview of the core molecular machinery that constitutes the autophagic degradation system, describe the roles of autophagy in maintenance, self-renewal, and differentiation of stem cells, and discuss the autophagic process and its regulation during cell reprogramming.

YAP enhances autophagic flux to promote breast cancer cell survival in response to nutrient deprivation

The Yes-associated protein (YAP), a transcriptional coactivator inactivated by the Hippo tumor suppressor pathway, functions as an oncoprotein in a variety of cancers. However, its contribution to breast cancer remains controversial. This study investigated the role of YAP in breast cancer cells under nutrient deprivation (ND). Here, we show that YAP knockdown sensitized MCF7 breast cancer cells to nutrient deprivation-induced apoptosis. Furthermore, in response to ND, YAP increased the autolysosome degradation, thereby enhancing the cellular autophagic flux in breast cancer cells. Of note, autophagy is crucial for YAP to protect MCF7 cells from apoptosis under ND conditions. In addition, the TEA domain (TEAD) family of growth-promoting transcription factors was indispensable for YAP-mediated regulation of autophagy. Collectively, our data reveal a role for YAP in promoting breast cancer cell survival upon ND stress and uncover an unappreciated function of YAP/TEAD in the regulation of autophagy.

Up-regulation of lysosomal TRPML1 channels is essential for lysosomal adaptation to nutrient starvation

Upon nutrient starvation, autophagy digests unwanted cellular components to generate catabolites that are required for housekeeping biosynthesis processes. A complete execution of autophagy demands an enhancement in lysosome function and biogenesis to match the increase in autophagosome formation. Here, we report that mucolipin-1 (also known as TRPML1 or ML1), a Ca(2+) channel in the lysosome that regulates many aspects of lysosomal trafficking, plays a central role in this quality-control process. By using Ca(2+) imaging and whole-lysosome patch clamping, lysosomal Ca(2+) release and ML1 currents were detected within hours of nutrient starvation and were potently up-regulated. In contrast, lysosomal Na(+)-selective currents were not up-regulated. Inhibition of mammalian target of rapamycin (mTOR) or activation of transcription factor EB (TFEB) mimicked a starvation effect in fed cells. The starvation effect also included an increase in lysosomal proteostasis and enhanced clearance of lysosomal storage, including cholesterol accumulation in Niemann-Pick disease type C (NPC) cells. However, this effect was not observed when ML1 was pharmacologically inhibited or genetically deleted. Furthermore, overexpression of ML1 mimicked the starvation effect. Hence, lysosomal adaptation to environmental cues such as nutrient levels requires mTOR/TFEB-dependent, lysosome-to-nucleus regulation of lysosomal ML1 channels and Ca(2+) signaling.

Impaired Autophagosome Clearance Contributes to Local Anesthetic Bupivacaine-induced Myotoxicity in Mouse Myoblasts

Background:

The current study examined the role(s) of autophagy in myotoxicity induced by bupivacaine in mouse myoblast C2c12 cells.

Methods:

C2c12 cells were treated with bupivacaine. Myotoxicity was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (n = 3 to 30), live/dead assay (n = 3 to 4), and morphological alterations (n = 3). Autophagosome formation was reflected by microtubule-associated protein light chain 3 conversion (n = 4 to 12) and light chain 3 punctation (n = 4 to 5). Autophagosome clearance was evaluated by p62 protein level (n = 4) and autolysosomes generation (n = 3).

Results:

Bupivacaine induced significant cell damage. Notably, there was a significant increase in autophagosome generation as evidenced by light chain 3 puncta formation (72.7 ± 6.9 vs. 2.1 ± 1.2) and light chain 3 conversion (2.16 ± 0.15 vs. 0.33 ± 0.04) in bupivacaine-treated cells. Bupivacaine inactivated the protein kinase B/mammalian target of rapamycin/p70 ribosomal protein S6 kinase signaling. However, cellular levels of p62 protein were significantly increased upon bupivacaine treatment (1.29 ± 0.15 vs. 1.00 ± 0.15), suggesting that the drug impaired autophagosome clearance. Further examination revealed that bupivacaine interrupted autophagosome–lysosome fusion (10.87% ± 1.48% vs. 32.94% ± 4.22%). Administration of rapamycin increased autophagosome clearance and, most importantly, improved the survival in bupivacaine-treated cells. However, knockdown of autophagy-related protein 5 (atg5) exacerbated bupivacaine-induced impairment of autophagosome clearance and myotoxicity.

Conclusions:

The data suggest that autophagosome formation was induced as a stress response mechanism after bupivacaine challenge; however, autophagosome clearance was impaired due to inadequate autophagosome–lysosome fusion. Therefore, impairment of autophagosome clearance appears to be a novel mechanism underlying bupivacaine-induced myotoxicity.

Untangling knots between autophagic targets and candidate drugs, in cancer therapy

Autophagy is an evolutionarily conserved lysosomal mechanism implicated in a wide variety of pathological processes, such as cancer. Autophagy can be regulated by a limited number of autophagy-related genes (Atgs) such as oncogenic Bcl-2/Bcl-XL , mTORC1, Akt and PI3KCI, and tumour suppressive proteins PI3KCIII, Beclin-1, Bif-1, p53, DAPKs, PTEN and UVRAG, which play their crucial roles in regulating autophagy-related cancer. As autophagy has a dual role in cancer cells, with tumour-promoting and tumour-suppressing properties, it has become an attractive target for a series of emerging small molecule drugs. In this review, we reveal new discoveries of related small molecules or chemical compounds that can regulate autophagic pathways and lead to pro-death or pro-survival autophagy, in different types of cancer. We discuss the knots between autophagic targets and candidate drugs, in the hope of shedding new light on exploiting new anti-tumour small molecule drugs for future cancer therapy.

Monitoring of autophagy is complicated--salinomycin as an example

Monitoring of autophagy is challenging because of its multiple steps and lack of single befitting technique for a complete mechanistic understanding, which makes the task complicated. Here, we evaluate the functionality of autophagy triggered by salinomycin (anti-cancer stem cell agent) using flow cytometry and advanced microscopy. We show that salinomycin does induce functional autophagy at lower concentrations and such a dose is cell type-dependent. For example, PC3 cells show active autophagic flux at 10 μM concentration of salinomycin while murine embryonic fibroblasts already show an inhibition of flux at such doses. A higher concentration of salinomycin (i.e. 30 μM) inhibits autophagic flux in both cell types. The data confirms our previous findings that salinomycin is an inducer of autophagy, whereas autophagic flux inhibition is a secondary response.

Salinomycin inhibits hepatocellular carcinoma cell invasion and migration through JNK/JunD pathway-mediated MMP9 expression

The antibiotic salinomycin (Salin) was recently identified as an antitumor drug for the treatment of several types of solid tumors. However, the effects of Salin on the migratory and invasive properties of hepatocellular carcinoma (HCC) cells are unclear. The present study aimed to determine the antitumor efficacy and mechanism of Salin in HCC cells. Human HCC cells (HCCLM3) treated with Salin showed a concentration-dependent reduction in cell migration and invasion, and this was associated with reduced MMP9 expression. The MMP9 promoter and enhancer in a luciferase reporter assay revealed that Salin can regulate MMP9 expression through an activator protein (AP-1) site within the MMP9 enhancer. JunD, one of the AP-1 components, was significantly decreased by Salin in a concentration- and time-dependent manner. Salin was able to induce c-Jun NH2-kinase (JNK) phosphorylation and to block both JunD and MMP9 expression. Our results showed that JNK phosphorylation and JunD may be involved in the Salin-regulated MMP9 signaling pathway in HCCLM3 cells and may mediate HCC cell biological characteristics. Our studies provide new insight into the antitumor effects of Salin.

Autophagy in cancer stem/progenitor cells

Macroautophagy is widely accepted as a cytoprotective mechanism against various environmental stresses. While inhibition of autophagy is generally considered to increase the susceptibility of cancer cells to therapeutic agents, whether it also plays a similar role in tumor stem cells is unclear and still controversial. With increased attention and efforts focused on the cytoprotective feature of autophagy in cancer, it is also essential to understand its role in the biology of cancer stem cells, including self-renewal, differentiation, and tumorigenicity. Although there are very few studies that evaluate autophagy in cancer stem/progenitor cells, understanding the mechanisms governing autophagic responses in various cancer stem cells could provide support for the future development of clinical therapeutics. The present review summarizes current studies that assess the role of autophagy in various types of cancer stem cells and those that evaluate the application of inhibitors of key components within the autophagy pathway in cancer stem/progenitor cells.

Targeting breast cancer initiating cells: advances in breast cancer research and therapy

Over the past 10 years there have been significant advances in our understanding of breast cancer and the important roles that breast cancer initiating cells (CICs) play in the development and resistance of breast cancer. Breast CICs endowed with self-renewing and tumor-initiating capacities are believed to be responsible for the relapses which often occur after various breast cancer therapies. In this review, we will summarize some of the key developments in breast CICs which will include discussion of some of the key genes implicated: estrogen receptor (ER), HER2, BRCA1, TP53, PIK3CA, RB, P16INK1 and various miRs as well some drugs which are showing promise in targeting CICs. In addition, the concept of combined therapies will be discussed. Basic and clinical research is resulting in novel approaches to improve breast cancer therapy by targeting the breast CICs.

Effect of autophagy inhibition on cell viability and cell cycle progression in MDA‑MB‑231 human breast cancer cells

Atg7 is an autophagy‑related gene, and is involved in two ubiquitin‑like conjugation systems in the process of autophagy. It is well established that 3‑methyladenine (3Ma) is an autophagy inhibitor. The present study aimed to investigate the effect of autophagy inhibition on the cell viability and cell cycle progression of human breast cancer cells. MDA‑MB‑231 human breast cancer cells were cultured in Dulbecco's modified Eagle's medium (DMEM) with high glucose, then divided into six groups. The six groups included the three fundamental groups as follows: The control group (untreated); the starvation group (high‑glucose DMEM replaced with glucose‑free minimal essential medium); and the starvation 3Ma group (maintained in glucose‑free culture medium and treated with the autophagy inhibitor 3Ma). The three fundamental groups were further divided into Atg7 siRNA‑transfected and non‑transfected groups. The cell viability and apoptosis of each group was determined by MTT assay and flow cytometry. The results of the current study demonstrated that Atg7 deficiency alone had no statically significant effect on the cell viability of MDA‑MB‑231 human breast cancer cells, while 3Ma reduced the cell viability and its effect was potentiated by Atg7 deficiency. Atg7 deficiency was more intense than 3Ma in the promotion of apoptosis and cell arrest in G0/G1‑phase in the absence of glucose and its effect was reduced by 3Ma. In conclusion, 3Ma and Atg7 may be involved in different pathways in the process of autophagy. Inhibition of autophagy may influence the cell viability and cell cycle through different pathways in MDA‑MB‑231 human breast cancer cells.

Restoration of autophagic flux in myocardial tissues is required for cardioprotection of sevoflurane postconditioning in rats

AIM

Sevoflurane postconditioning (SpostC) has been shown to protect the heart from ischemia-reperfusion (I/R) injury. In this study, we examined whether SpostC affected autophagic flux in myocardial tissues that contributed to its cardioprotective effects in rats following acute I/R injury.

METHODS

SD rats underwent 30 min of left anterior descending coronary artery ligation followed by 120 min of reperfusion. The rats were subjected to inhalation of 2.4% (v/v) sevoflurane during the first 5 min of reperfusion, and chloroquine (10 mg/kg, ip) was injected 1 h before I/R. Myocardial infarct size was estimated using TTC staining. Autophagosomes in myocardial tissues were detected under TEM. Expression of LC3B-II, beclin-1, p62/SQSTM1, cathepsin B, caspase-3 and cleaved PARP was assessed using Western blot analysis. Plasma cardiac troponin I was measured using ELISA. Cardiomyocyte apoptosis was evaluated with TUNEL staining.

RESULTS

I/R procedure produced severe myocardium infarct and apoptosis accompanied by markedly increased number of autophagosomes, as well as increased levels of LC3B-II, beclin-1 and p62 in myocardial tissues. SpostC significantly reduced infarct size, attenuated myocardial apoptosis, restored intact autophagic flux and improved the lysosomal function in myocardial tissues. Administration of chloroquine that blocked autophagic flux abrogated the cardioprotective effects of SpostC.

CONCLUSION

SpostC exerts its cardioprotective effects in rats following I/R injury via restoring autophagic flux in myocardial tissues.

Inhibition of autophagic flux by salinomycin results in anti-cancer effect in hepatocellular carcinoma cells

Salinomycin raised hope to be effective in anti-cancer therapies due to its capability to overcome apoptosis-resistance in several types of cancer cells. Recently, its effectiveness against human hepatocellular carcinoma (HCC) cells both in vitro and in vivo was demonstrated. However, the mechanism of action remained unclear. Latest studies implicated interference with the degradation pathway of autophagy. This study aimed to determine the impact of Salinomycin on HCC-autophagy and whether primary human hepatocytes (PHH) likewise are affected. Following exposure of HCC cell lines HepG2 and Huh7 to varying concentrations of Salinomycin (0-10 µM), comprehensive analysis of autophagic activity using western-blotting and flow-cytometry was performed. Drug effects were analyzed in the settings of autophagy stimulation by starvation or PP242-treatment and correlated with cell viability, proliferation, apoptosis induction, mitochondrial mass accumulation and reactive oxygen species (ROS) formation. Impact on apoptosis induction and cell function of PHH was analyzed. Constitutive and stimulated autophagic activities both were effectively suppressed in HCC by Salinomycin. This inhibition was associated with dysfunctional mitochondria accumulation, increased apoptosis and decreased proliferation and cell viability. Effects of Salinomycin were dose and time dependent and could readily be replicated by pharmacological and genetic inhibition of HCC-autophagy alone. Salinomycin exposure to PHH resulted in transient impairment of synthesis function and cell viability without apoptosis induction. In conclusion, our data suggest that Salinomycin suppresses late stages of HCC-autophagy, leading to impaired recycling and accumulation of dysfunctional mitochondria with increased ROS-production all of which are associated with induction of apoptosis.

Triggering of erythrocyte cell membrane scrambling by salinomycin

Salinomycin, a polyether ionophore antibiotic effective against a variety of pathogens, has been shown to trigger apoptosis of cancer cells and cancer stem cells. The substance is thus considered for the treatment of malignancy. Salinomycin compromises tumour cell survival at least in part by interference with mitochondrial function. Erythrocytes lack mitochondria but may undergo apoptosis-like suicidal cell death or eryptosis, which is characterized by scrambling of the cell membrane with phosphatidylserine exposure at the erythrocyte surface. Signalling involved in the triggering of eryptosis includes activation of oxidant-sensitive Ca(2+) permeable cation channels with subsequent increase in cytosolic Ca(2+) activity ([Ca(2+)]i). This study explored whether salinomycin stimulates eryptosis. Phosphatidylserine-exposing erythrocytes were identified by measurement of annexin-V binding, cell volume was estimated from forward scatter, haemolysis determined from haemoglobin release, [Ca(2+)]i quantified utilizing Fluo3-fluorescence and oxidative stress from 2',7' dichlorodihydrofluorescein diacetate (DCFDA) fluorescence in flow cytometry. A 48-hr exposure to salinomycin (5-100 nM) was followed by a significant increase in Fluo3-fluorescence, DCFDA fluorescence and annexin-V binding, as well as a significant decrease in forward scatter (at 5-10 nM, but not at 50 and 100 nM). The annexin-V binding after salinomycin treatment was significantly blunted but not abrogated in the nominal absence of extracellular Ca(2+) or in the presence of antioxidant n-acetyl cysteine (1 mM). Salinomycin triggers cell membrane scrambling, an effect at least partially due to oxidative stress and entry of extracellular Ca(2+).

Control of autophagy maturation by acid sphingomyelinase in mouse coronary arterial smooth muscle cells: protective role in atherosclerosis

Recent studies have indicated a protective role of autophagy in regulating vascular smooth muscle cells homeostasis in atherogenesis, but the mechanisms controlling autophagy, particularly autophagy maturation, are poorly understood. Here, we investigated whether acid sphingomyelinase (ASM)-regulated lysosome function is involved in autophagy maturation in coronary arterial smooth muscle cells (CASMCs) in the pathogenesis of atherosclerosis. In coronary arterial wall of ASM-deficient (Smpd1⁻/⁻) mice on Western diet, there were high expression levels of both LC3B, a robust marker of autophagosomes (APs), and p62, a selective autophagy substrate, compared with those in wild-type (Smpd1⁺/⁺) mice. By Western blotting and flow cytometry, atherogenic stimulation of Smpd1⁺/⁺ CASMCs with 7-ketocholesterol was found to significantly enhance LC3B expression and increase the content of both APs and autophagolysosomes (APLs). In Smpd1⁻/⁻ CASMCs, such 7-ketocholesterol-induced increases in LC3B and p62 expression and APs were further augmented, but APLs formation was abolished. Analysis of fluorescence resonance energy transfer between fluorescence-labeled LC3B and Lamp1 (lysosome marker) showed that 7-ketocholesterol markedly induced fusion of APs with lysosomes in Smpd1⁺/⁺ CASMCs, which was abolished in Smpd1⁻/⁻ CASMCs. Moreover, 7-ketocholesterol-induced expression of cell dedifferentiation marker vimentin and proliferation was enhanced in Smpd1⁻/⁻ CASMCs compared with those in Smpd1⁺/⁺ CASMCs. Lastly, overexpression of ASM further increased APLs formation in Smpd1⁺/⁺ CASMCs and restored APLs formation in Smpd1⁻/⁻ CASMCs indicating that increased ASM expression is highly correlated with enhanced APLs formation. Taken together, our data suggest that the control of lysosome trafficking and fusion by ASM is essential to a normal autophagic flux in CASMCs, which implicates that the deficiency of ASM-mediated regulation of autophagy maturation may result in imbalance of arterial smooth muscle cell homeostasis and thus serve as an important atherogenic mechanism in coronary arteries.

KEY MESSAGES

Acid sphingomyelinase (ASM) controls autophagy maturation in smooth muscle cells. ASM maintains smooth muscle cell homeostasis and its contractile phenotype. ASM plays a protective role in smooth muscle dysfunction and atherosclerosis.

Autophagy promotes resistance to photodynamic therapy-induced apoptosis selectively in colorectal cancer stem-like cells

Recent studies have indicated that cancer stem-like cells (CSCs) exhibit a high resistance to current therapeutic strategies, including photodynamic therapy (PDT), leading to the recurrence and progression of colorectal cancer (CRC). In cancer, autophagy acts as both a tumor suppressor and a tumor promoter. However, the role of autophagy in the resistance of CSCs to PDT has not been reported. In this study, CSCs were isolated from colorectal cancer cells using PROM1/CD133 (prominin 1) expression, which is a surface marker commonly found on stem cells of various tissues. We demonstrated that PpIX-mediated PDT induced the formation of autophagosomes in PROM1/CD133⁺ cells, accompanied by the upregulation of autophagy-related proteins ATG3, ATG5, ATG7, and ATG12. The inhibition of PDT-induced autophagy by pharmacological inhibitors and silencing of the ATG5 gene substantially triggered apoptosis of PROM1/CD133⁺ cells and decreased the ability of colonosphere formation in vitro and tumorigenicity in vivo. In conclusion, our results revealed a protective role played by autophagy against PDT in CSCs and indicated that targeting autophagy could be used to elevate the PDT sensitivity of CSCs. These findings would aid in the development of novel therapeutic approaches for CSC treatment.

Promoter hypermethylation of KLF4 inactivates its tumor suppressor function in cervical carcinogenesis

OBJECTIVE

The KLF4 gene has been shown to be inactivated in cervical carcinogenesis as a tumor suppressor. However, the mechanism of KLF4 silencing in cervical carcinomas has not yet been identified. DNA methylation plays a key role in stable suppression of gene expression.

METHODS

The methylation status of the KLF4 promoter CpG islands was analyzed by bisulfite sequencing (BSQ) in tissues of normal cervix and cervical cancer. KLF4 gene expression was detected by RT-PCR, immunohistochemistry and western blot. KLF4 promoter methylation in cervical cancer cell line was determined by BSQ and methylation-specific polymerase chain reaction (MS-PCR). Cell proliferation ability was detected by cell growth curve and MTT assay.

RESULTS

The methylated allele was found in 41.90% of 24 cervical cancer tissues but only in 11.11% of 11 normal cervix tissues (P<0.005). KLF4 mRNA levels were significantly reduced in cervical cancer tissues compared with normal cervix tissues (P<0.01) and KLF4 mRNA expression showed a significant negative correlation with the promoter hypermethylation (r = -0.486, P = 0.003). Cervical cancer cell lines also showed a significant negative correlation between KLF4 expression and hypermethylation. After treatment with the demethylating agent 5-Azacytidine (5-Aza), the expression of KLF4 in the cervical cancer cell lines at both mRNA and protein levels was drastically increased, the cell proliferation ability was inhibited and the chemosensitivity for cisplatin was significantly increased.

CONCLUSION

KLF4 gene is inactivated by methylation-induced silencing mechanisms in a large subset of cervical carcinomas and KLF4 promoter hypermethylation inactivates the gene's function as a tumor suppressor in cervical carcinogenesis.

The natural compound oblongifolin C inhibits autophagic flux and enhances antitumor efficacy of nutrient deprivation

Metabolic stress induces autophagy as an alternative source of energy and metabolites. Insufficient autophagy in nutrient-deprived cancer cells would be beneficial for cancer therapy. Here, we performed a functional screen in search of novel autophagy regulators from natural products. We showed that oblongifolin C (OC), a natural small molecule compound extracted from Garcinia yunnanensis Hu, is a potent autophagic flux inhibitor. Exposure to OC results in an increased number of autophagosomes and impaired degradation of SQSTM1/p62. Costaining of GFP-LC3B with LysoTracker Red or LAMP1 antibody demonstrates that autophagosome-lysosome fusion is blocked by OC treatment. Furthermore, OC inhibits lysosomal proteolytic activity by altering lysosomal acidification and downregulating the expression of lysosomal cathepsins. Importantly, OC can eliminate the tolerance of cancer cells to nutrient starvation. Starvation dramatically increases the susceptibility of cancer cells to OC-induced CASP3-dependent apoptosis in vitro. Subsequent studies in xenograft mouse model showed that OC has anticancer potency as revealed by increased staining of cleaved CASP3, LC3 puncta, and SQSTM1, as well as reduced expression of lysosomal cathepsins. Combined treatment with OC and caloric restriction potentiates anticancer efficacy of OC in vivo. Collectively, these data demonstrated that OC is a novel autophagic flux inhibitor and might be useful in anticancer therapy.

Acidic extracellular pH neutralizes the autophagy-inhibiting activity of chloroquine: implications for cancer therapies

Acidic pH is an important feature of tumor microenvironment and a major determinant of tumor progression. We reported that cancer cells upregulate autophagy as a survival mechanism to acidic stress. Inhibition of autophagy by administration of chloroquine (CQ) in combination anticancer therapies is currently evaluated in clinical trials. We observed in 3 different human cancer cell lines cultured at acidic pH that autophagic flux is not blocked by CQ. This was consistent with a complete resistance to CQ toxicity in cells cultured in acidic conditions. Conversely, the autophagy-inhibiting activity of Lys-01, a novel CQ derivative, was still detectable at low pH. The lack of CQ activity was likely dependent on a dramatically reduced cellular uptake at acidic pH. Using cell lines stably adapted to chronic acidosis we could confirm that CQ lack of activity was merely caused by acidic pH. Moreover, unlike CQ, Lys-01 was able to kill low pH-adapted cell lines, although higher concentrations were required as compared with cells cultured at normal pH conditions. Notably, buffering medium pH in low pH-adapted cell lines reverted CQ resistance. In vivo analysis of tumors treated with CQ showed that accumulation of strong LC3 signals was observed only in normoxic areas but not in hypoxic/acidic regions. Our observations suggest that targeting autophagy in the tumor environment by CQ may be limited to well-perfused regions but not achieved in acidic regions, predicting possible limitations in efficacy of CQ in antitumor therapies.

Defective autophagosome trafficking contributes to impaired autophagic flux in coronary arterial myocytes lacking CD38 gene

AIM

Autophagic flux is an important process during autophagy maturation in smooth muscle cells. However, the molecular mechanisms underlying autophagic flux in these cells are largely unknown. Here, we revealed a previously undefined role of CD38, an enzyme that metabolizes NADP(+) into NAADP, in the regulation of autophagic flux in coronary arterial myocytes (CAMs).

METHODS AND RESULTS

In vivo CD38 gene knockout mice (CD38(-/-)) fed the high-fat Western diet showed increased accumulation of autophagosomes in coronary arterial media compared with that in wild-type (CD38(+/+)) mice, suggesting that CD38 gene deletion results in a defective autophagic process in CAMs of coronary arteries. In primary cultured CAMs, CD38 gene deletion markedly enhanced 7-ketocholesterol (7-Ket, an atherogenic stimulus and autophagy inducer)-induced accumulation of autophagosomes and increased expression of an autophagic marker, LC3B. However, no difference in autophagosome formation was observed between CD38(+/+) and CD38(-/-) CAMs when autophagic flux was blocked, which indicates that CD38 regulates autophagic flux rather than induction of autophagosome formation. Further, 7-Ket-induced formation of autophagolysosomes was markedly attenuated in CD38(-/-) CAMs compared with CD38(+/+) CAMs. Mechanistically, CD38 gene deletion markedly inhibited 7-Ket-induced dynein activation and autophagosome trafficking, which were associated with attenuated lysosomal Ca(2+) release. Importantly, coronary arterial smooth muscle from CD38(-/-) mice fed the Western diet exhibited phenotypic changes towards a more dedifferentiated state with abnormal extracellular matrix metabolism.

CONCLUSION

Taken together, these results suggest that CD38 plays a critical role in autophagosome trafficking and fusion with lysosomes, thus controlling autophagic flux in CAMs under atherogenic stimulation.

Paradoxical roles of autophagy in different stages of tumorigenesis: protector for normal or cancer cells

Autophagy serves as a dynamic degradation and recycling system that provides biological materials and energy in response to stress. The role of autophagy in tumor development is complex. Various studies suggest that autophagy mainly contributes to tumor suppression during the early stage of tumorigenesis and tumor promotion during the late stage of tumorigenesis. During the tumorization of normal cells, autophagy protects genomic stability by retarding stem cells-involved damage/repair cycle, and inhibits the formation of chronic inflammatory microenvironment, thus protecting normal cell homeostasis and preventing tumor generation. On the other hand, autophagy also protects tumor cells survival during malignant progression by supporting cellular metabolic demands, decreasing metabolic damage and supporting anoikis resistance and dormancy. Taken together, autophagy appears to play a role as a protector for either normal or tumor cells during the early or late stage of tumorigenesis, respectively. The process of tumorigenesis perhaps needs to undergo twice autophagy-associated screening. The normal cells that have lower autophagy capacity are prone to tumorization, and the incipient tumor cells that have higher autophagy capacity possibly are easier to survival in the hash microenvironment and accumulate more mutations to promote malignant progression.