Mimulone-Induced Autophagy through p53-Mediated AMPK/mTOR Pathway Increases Caspase-Mediated Apoptotic Cell Death in A549 Human Lung Cancer Cells

Anticancer properties and mechanisms of mimulone (MML), C-geranylflavonoid isolated from the Paulownia tomentosa fruits, were firstly elucidated in this study. MML prevented cell proliferation in a dose- and time-dependent way and triggered apoptosis through the extrinsic pathway in A549 human lung adenocarcinoma cells. Furthermore, MML-treated cells displayed autophagic features, such as the formation of autophagic vacuoles, a primary morphological feature of autophagy, and the accumulation of microtubule-associated protein 1 light chain 3 (LC3) puncta, another typical maker of autophagy, as determined by FITC-conjugated immunostaining and monodansylcadaverine (MDC) staining, respectively. The expression levels of LC3-I and LC3-II, specific markers of autophagy, were also augmented by MML treatment. Autophagy inhibition by 3-methyladenine (3-MA), pharmacological autophagy inhibitor, and shRNA knockdown of Beclin-1 reduced apoptotic cell death induced by MML. Autophagic flux was not significantly affected by MML treatment and lysosomal inhibitor, chloroquine (CQ) suppressed MML-induced autophagy and apoptosis. MML-induced autophagy was promoted by decreases in p53 and p-mTOR levels and increase of p-AMPK. Moreover, inhibition of p53 transactivation by pifithrin-α (PFT-α) and knockdown of p53 enhanced induction of autophagy and finally promoted apoptotic cell death. Overall, the results demonstrate that autophagy contributes to the cytotoxicity of MML in cancer cells harboring wild-type p53. This study strongly suggests that MML is a potential candidate for an anticancer agent targeting both autophagy and apoptotic cell death in human lung cancer. Moreover, co-treatment of MML and p53 inhibitor would be more effective in human lung cancer therapy.     

Role of autophagy in cancer: management of metabolic stress

Human breast, ovarian, and prostate tumors display allelic loss of the essential autophagy gene beclin1 with high frequency, and an increase in the incidence of tumor formation is observed in beclin1(+/-) mutant mice. These findings suggest a role for beclin1 and autophagy in tumor suppression; however, the mechanism by which this occurs has been unclear. Autophagy is a bulk degradation process whereby organelles and cytoplasm are engulfed and targeted to lysosomes for proteolysis,(1,2) There is evidence that autophagy sustains cell survival during nutrient deprivation through catabolism, but also that autophagy is a means of achieving cell death when executed to completion. If or how either of these diametrically opposing functions proposed for autophagy may be related to tumor suppression is unknown. We found that metabolic stress is a potent trigger of apoptotic cell death, defects in which enable long-term survival that is dependent on autophagy both in vitro and in tumors in vivo.(3) These findings raise the conundrum whereby inactivation of a survival pathway (autophagy) promotes tumorigenesis. Interestingly, when cells with defects in apoptosis are denied autophagy, this creates the inability to tolerate metabolic stress, reduces cellular fitness, and activates a necrotic pathway to cell death. This necrosis in tumors is associated with inflammation and enhancement of tumor growth, due to the survival of a small population of surviving, but injured, cells in a microenvironment that favors oncogenesis. Thus, by sustaining metabolism through autophagy during periods of metabolic stress, cells can limit energy depletion, cellular damage, and cell death by necrosis, which may explain how autophagy can prevent cancer, and how loss of a survival function can be tumorigenic.     

Anticancer activities of chalcone flavokawain B from Alpinia pricei Hayata in human lung adenocarcinoma (A549) cells via induction of reactive oxygen species-mediated apoptotic and autophagic cell death

Chalcones found in fruits and vegetables have promising cancer chemopreventive properties. This study attempts to identify the anticancer efficacies of chalcone flavokawain B (FKB) in the rhizomes of Alpinia pricei Hayata by examining key molecular events in non-small-cell lung cancer (A549) cells. Our results indicated that in human A549 cells, FKB (0–15 μg/ml) decreases cell viability and colony formation, dysregulates the Bax:B-cell lymphoma 2 ratio and increases apoptotic DNA fragmentation. Mitochondrial (caspase-9/-3 and poly ADP ribose polymerase [PARP]) signaling was found to be involved in FKB-induced apoptosis. In addition, FKB-induced reactive oxygen species (ROS) generation, and N-acetylcysteine attenuated FKB-induced apoptotic cell death. Moreover, FKB triggered autophagy, as evidenced by the improved acidic vesicular organelle formation, lipidated light chain 3 (microtubule-related light chain 3) accumulation, and ATG7 expression and the decreased mammalian target of rapamycin phosphorylation. Furthermore, FKB suppressed ROS-mediated ATG4B expression. Inhibiting autophagy using 3-methyladenine/chloroquine diminished FKB-induced cell death, indicating that autophagy is triggered as a death mechanism by FKB. In summary, FKB has a crucial role in the execution and propagation of ROS-mediated apoptotic and autophagic cell death of lung adenocarcinoma cells.     

KML001 Induces Apoptosis and Autophagic Cell Death in Prostate Cancer Cells via Oxidative Stress Pathway

We investigated the effects of KML001 (NaAsO₂, sodium metaarsenite, Kominox), an orally bioavailable arsenic compound, on the growth and death of human prostate cancer cells and its mechanism of action. Growth inhibition was assessed by cytotoxicity assays in the presence or absence of inhibitor of apoptosis, inhibitor of autophagy or antioxidant N-Acetyl-_L-cysteine to study mechanism of cell death induced by KML001 in PC3, DU145 and LNCaP prostate cancer cell lines. Electron microscopy, flow cytometry and Western blotting were used to study apoptotic and autophagic mechanisms. The DU145 xenograft model was used to determine the efficacy of KML001 in vivo. KML001 decreased the viability of cells and increased the percentage of annexin V-positive cells dose-dependently in prostate cancer cells, and LNCaP cells were more sensitive to KML001 than PC3 or DU145 cells. Electron microscopy revealed typical apoptotic characters and autophagic vacuoles in cells treated with KML001. Exposure to KML001 in prostate cancer cells induced apoptosis and autophagy in a time- and dose-dependent manner. KML001 induced dose-dependent accumulation of reactive oxygen species, and scavenging the reactive oxygen species with N-Acetyl-_L-cysteine reduced LC3 and cleaved poly (ADP-ribose) polymerase. KML001 significantly inhibited tumor growth in the DU145 xenograft model. In addition, significant decrease of proliferation and significant increases of apoptosis and autophagy were observed in KML001-treated tumors than in vehicle-treated tumors. Exposure of human prostate cancer cells to KML001 induced both apoptosis and autophagic cell death via oxidative stress pathway. And KML001 had an antiproliferative effect on DU145 cells in xenograft mice.     

Apoptotic Cell Death Induced by Resveratrol Is Partially Mediated by the Autophagy Pathway in Human Ovarian Cancer Cells

Resveratrol (trans-3,4,5’ –trihydroxystilbene) is an active compound in food, such as red grapes, peanuts, and berries. Resveratrol exhibits an anticancer effect on various human cancer cells. However, the mechanism of resveratrol-induced anti-cancer effect at the molecular level remains to be elucidated. In this study, the mechanism underlying the anti-cancer effect of resveratrol in human ovarian cancer cells (OVCAR-3 and Caov-3) was investigated using various molecular biology techniques, such as flow cytometry, western blotting, and RNA interference, with a major focus on the potential role of autophagy in resveratrol-induced apoptotic cell death. We demonstrated that resveratrol induced reactive oxygen species (ROS) generation, which triggers autophagy and subsequent apoptotic cell death. Resveratrol induced ATG5 expression and promoted LC3 cleavage. The apoptotic cell death induced by resveratrol was attenuated by both pharmacological and genetic inhibition of autophagy. The autophagy inhibitor chloroquine, which functions at the late stage of autophagy, significantly reduced resveratrol-induced cell death and caspase 3 activity in human ovarian cancer cells. We also demonstrated that targeting ATG5 by siRNA also suppressed resveratrol-induced apoptotic cell death. Thus, we concluded that a common pathway between autophagy and apoptosis exists in resveratrol-induced cell death in OVCAR-3 human ovarian cancer cells.     

Hypoxia induces autophagic cell death in apoptosis-competent cells through a mechanism involving BNIP3

Hypoxia (lack of oxygen) is a physiological stress often associated with solid tumors. Hypoxia correlates with poor prognosis since hypoxic regions within tumors are considered apoptosisresistant. Autophagy (cellular "self digestion") has been associated with hypoxia during cardiac ischemia and metabolic stress as a survival mechanism. However, although autophagy is best characterized as a survival response, it can also function as a mechanism of programmed cell death. Our results show that autophagic cell death is induced by hypoxia in cancer cells with intact apoptotic machinery. We have analyzed two glioma cell lines (U87, U373), two breast cancer cell lines (MDA-MB-231, ZR75) and one embryonic cell line (HEK293) for cell death response in hypoxia (<1% O(2)). Under normoxic conditions, all five cell lines undergo etoposide-induced apoptosis whereas hypoxia fails to induce these apoptotic responses. All five cell lines induce an autophagic response and undergo cell death in hypoxia. Hypoxia-induced cell death was reduced upon treatment with the autophagy inhibitor 3-methyladenine, but not with the caspase inhibitor z-VAD-fmk. By knocking down the autophagy proteins Beclin-1 or ATG5, hypoxia-induced cell death was also reduced. The pro-cell death Bcl-2 family member BNIP3 (Bcl-2/adenovirus E1B 19kDainteracting protein 3) is upregulated during hypoxia and is known to induce autophagy and cell death. We found that BNIP3 overexpression induced autophagy, while expression of BNIP3 siRNA or a dominant-negative form of BNIP3 reduced hypoxia-induced autophagy. Taken together, these results suggest that prolonged hypoxia induces autophagic cell death in apoptosis-competent cells, through a mechanism involving BNIP3.     

Silibinin triggers apoptotic signaling pathways and autophagic survival response in human colon adenocarcinoma cells and their derived metastatic cells

Silibinin, a flavonolignan isolated from the milk thistle plant (Silybum marianum), possesses anti-neoplastic properties. In vitro and in vivo studies have recently shown that silibinin inhibits the growth of colorectal cancer (CRC). The present study investigates the mechanisms of silibinin-induced cell death using an in vitro model of human colon cancer progression, consisting of primary tumor cells (SW480) and their derived metastatic cells (SW620) isolated from a metastasis of the same patient. Silibinin induced apoptotic cell death evidenced by DNA fragmentation and activation of caspase-3 in both cell lines. Silibinin enhanced the expression (protein and mRNA) of TNF-related apoptosis-inducing ligand (TRAIL) death receptors (DR4/DR5) at the cell surface in SW480 cells, and induced their expression in TRAIL-resistant SW620 cells normally not expressing DR4/DR5. Caspase-8 and -10 were activated demonstrating the involvement of the extrinsic apoptotic pathway in silibinin-treated SW480 and SW620 cells. The protein Bid was cleaved in SW480 cells indicating a cross-talk between extrinsic and intrinsic apoptotic pathway. We demonstrated that silibinin activated also the intrinsic apoptotic pathway in both cell lines, including the perturbation of the mitochondrial membrane potential, the release of cytochrome c into the cytosol and the activation of caspase-9. Simultaneously to apoptosis, silibinin triggered an autophagic response. The inhibition of autophagy with a specific inhibitor enhanced cell death, suggesting a cytoprotective function for autophagy in silibinin-treated cells. Taken together, our data show that silibinin initiated in SW480 and SW620 cells an autophagic-mediated survival response overwhelmed by the activation of both the extrinsic and intrinsic apoptotic pathways.     

赖氨酰氧化酶样蛋白4在人类恶性肿瘤发生与发展中的作用

赖氨酰氧化酶样蛋白4(lysyl oxidase like 4, LOXL4)是一种属于赖氨酰氧化酶(lysyl oxidase, LOX)蛋白质家族的分泌型铜依赖性胺氧化酶,参与细胞外基质(extracellular matrix, ECM)的组装和维持。LOXL4蛋白在人类肝癌、胃癌、乳腺癌、宫颈癌、头颈鳞癌、食管癌和结直肠癌中表达上调,而在人类膀胱癌和肺癌中表达下调并抑制肿瘤的生长,表明LOXL4蛋白在不同类型的人类恶性肿瘤中具有促癌或抑癌的双向作用。肿瘤细胞外泌体中的LOXL4蛋白通过催化作用产生过氧化氢,后者直接激活FAK/Src信号通路,并促进细胞基质粘附和细胞迁移。外泌体介导的LOXL4还可以通过激活PI3K/Akt信号通路来促进肿瘤细胞的增殖和免疫逃逸。肿瘤细胞中的 LOXL4可以经外泌体转运至巨噬细胞,进一步通过STAT1和STAT3介导的信号通路激活细胞免疫抑制功能和激活程序性死亡配体 1(programmed death ligand 1, PD-L1)表达,触发巨噬细胞的免疫抑制功能,促进肿瘤细胞的免疫逃逸。此外,LOXL4蛋白还能通过激活p53蛋白和抑制Ras/ERK信号转导通路发挥抑癌功能。本文主要总结了LOXL4蛋白的结构、功能及其在人类恶性肿瘤发生发展的作用机制,进一步探讨LOXL4蛋白在恶性肿瘤研究中的应用前景,为恶性肿瘤的临床诊断、治疗和筛选预后标志物提供理论基础和参考依据。     

GMI, an immunomodulatory protein from Ganoderma microsporum, induces autophagy in non-small cell lung cancer cells

Autophagy is a self-digestive process that degrades the cytoplasmic constituents. Immunomodulatory protein, one major bioactive component of Ganoderma, has antitumor activity. In this study, recombinant fungal immunomodulatory protein, GMI, was cloned from Ganoderma microsporum and purified. We demonstrated that GMI induces lung cancer cell death by activating autophagy, but does not induce apoptotic cell death. On western blot, GMI increased LC3 conversion and decreased p53 expression in a time- and concentration-dependent manner. Cytoplasmic calcium chelator BAPTA-AM was used to prove that GMI promotes autophagy via a calcium-mediated signaling pathway. 3-methyladenine (3-MA), an autophagy inhibitor, enhanced the cytotoxicity of GMI on cell viability assay. Using VZV-G pseudotyped lentivirus-shRNA system for autophagy-related genes silencing, the capabilities of GMI to reduce cell viability and colony formation were abolished in autophagy-defective cells. Furthermore, GMI did not stimulate apoptosis after blocking of autophagy by 3-MA or shRNA knockdown system. In xenograft studies, oral administration of GMI inhibited the tumor growth and induced autophagy significantly in nude mice that had received a subcutaneous injection of A549 cells. This is the first study to reveal the novel function of GMI in activating autophagy. GMI may be a potential chemopreventive agent against non-small cell lung cancer.     

Autophagy in pancreatic cancer: An emerging mechanism of cell death

Pancreatic cancer, the fourth leading cause of cancer-related death in the United States, is resistant to current chemotherapies. Therefore, identification of different pathways of cell death is important to develop novel therapeutics. Our previous study has shown that triptolide, a diterpene triepoxide, inhibits the growth of pancreatic cancer cells in vitro and prevents tumor growth in vivo. However, the mechanism by which triptolide kills pancreatic cancer cells was not known, hence, this study aimed at elucidating it. Our study reveals that triptolide kills diverse types of pancreatic cancer cells by two different pathways; it induces caspase-dependent apoptotic death in some cell lines and death via a caspase-independent autophagic pathway in the other cell lines tested. Triptolide-induced autophagy requires autophagy-specific genes, atg5 or beclin 1 and its inhibition results in cell death via the apoptotic pathway, whereas inhibition of both autophagy and apoptosis rescues triptolide-mediated cell death. Our study shows for the first time that induction of autophagy by triptolide has a pro-death role in pancreatic cancer cells. Since triptolide kills diverse pancreatic cancer cells by different mechanisms, it makes an attractive chemotherapeutic agent for future use against a broad spectrum of pancreatic cancers.Key words: pancreatic cancer, triptolide, apoptosis, caspase-3Pancreatic adenocarcinoma is one of the most lethal human malignancies. It is the fourth leading cause of cancer-related death in the United States. The five-year survival rate for pancreatic cancer is estimated to be <5% due to its aggressive growth, metastasis and resistance to radiation and most systemic chemotherapies. Hence, efforts are ongoing to understand the pathobiology of pancreatic cancer to develop innovative and effective therapies against it. A promising candidate for future therapeutic use against pancreatic cancer is a diterpene triepoxide, triptolide. Our previous studies show that triptolide inhibits the growth of pancreatic cancer cells in vitro and prevents tumor growth in vivo. Since the mechanism by which triptolide kills pancreatic cancer cells was not known, we decided to elucidate it.The K-ras, p53, p16 and DPC4 genes are the most frequently altered genes in pancreatic adenocarcinoma. In this study we have used diverse pancreatic cancer cell lines, MiaPaCa-2, Capan-1, S2-013 and S2-VP10 cells, which have mutations in all the above-mentioned genes and BxPC-3 and Hs766T cells, which have mutations in the p53, p16 and DPC4 genes, but have a wild-type K-ras gene. The treatment of all the cell lines with triptolide results in a significant time- and dose-dependent decrease in cell viability, independent of cell cycle arrest. After treatment with triptolide, only MiaPaCa-2, Capan-1 and BxPC-3 cells show an increase in the apoptosis parameters: cytochrome c release from mitochondria into the cytosol, caspase-3 activation and phosphatidylserine externalization. In contrast to this, S2-013, S2-VP10 and Hs766T cells show an induction of autophagy: an increase in LC3-II levels (by immunoblotting and immufluorescence), increase in acridine orange-positive cells, inhibition of the PtdIns3K/Akt/mTOR pathway and induction of the ERK1/2 pathway. Also, none of the cell lines tested show necrosis as evidenced by the absence of the release of lactate dehydrogenase. These results indicate that triptolide induces apoptosis in MiaPaCa-2, Capan-1 and BxPC-3 cells, whereas it induces autophagy in S2-013, S2-VP10 and Hs766T cells.Since the role of autophagy in cancer was controversial we investigated whether triptolide-induced autophagy has a prosurvival or a pro-death role. As autophagy-associated cell death is independent of caspase-3, we tested the effect of triptolide on pancreatic cancer cells in the absence of caspase-3. Treatment of cells with triptolide post-caspase-3 knockdown shows a significant rescue of cell viability only in MiaPaCa-2, but not S2-013 or S2-VP10 cells. This indicates that in contrast to MiaPaCa-2, triptolide-mediated cell death in S2-013 and S2-VP10 cells is independent of caspase-3. Next, we tested the role of autophagy in triptolide-mediated cell death in pancreatic cancer cells. In spite of a knockdown of autophagy-specific genes (atg5 and beclin 1), treatment of S2-013 and S2-VP10 cells with triptolide show a significant decline in cell viability, which is comparable to the cells treated with triptolide in the presence of autophagy genes. Subsequently we show that death in the absence of autophagy-specific genes is due to the utilization of an alternate cell death pathway, apoptosis. Furthermore, in the absence of both autophagy-specific and apoptosis-specific genes, triptolide-mediated cell death is rescued in S2-013 and S2-VP10 cells. Thus, these results confirm that triptolide-induced autophagy has a pro-death role in S2-013 and S2-VP10 cells and that these cells do not have a defect in the apoptotic machinery; however, they respond to triptolide by activating the autophagic pathway instead of the apoptotic pathway. Our studies also reveal the presence of a crosstalk between the two cell death pathways, apoptosis and autophagy, in pancreatic cancer cells.In conclusion, our study shows for the first time that triptolide induces autophagy in pancreatic cancer cells. It sheds light on the fundamental question as to whether autophagy is protective or causes cell death, proving convincingly that induction of autophagy causes cell death of some pancreatic cancer cells. Although a basal level of autophagy is necessary to maintain cellular homeostasis, its prosurvival role can be switched into a cell death mechanism if the amplitude of autophagy increases above a threshold level which is incompatible with viability, as seen in S2-013, S2-VP10 and Hs766T cells after triptolide treatment. Furthermore, there exists a crosstalk between apoptosis and autophagy in S2-013 and S2-VP10 cells; either both pathways function independently to kill the cells, with autophagy being the preferred pathway or autophagy antagonizes apoptosis and hence apoptosis is seen only after inhibiting autophagy. Although there is no direct correlation between the selection of cell death pathway in response to triptolide and the genotype of the cell lines, the choice of autophagic cell death pathway could depend on the metastatic potential of the cells; S2-013, S2-VP10 and Hs766T cell lines being more metastatic than the others, which merits further investigation. In conclusion, the ability of triptolide to induce cell death in diverse pancreatic cancer cells by either mechanism makes it an attractive chemotherapeutic agent against a broad spectrum of pancreatic cancers.     

Autophagy blockade enhances HDAC inhibitors' pro-apoptotic effects: potential implications for the treatment of a therapeutic-resistant malignancy

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive, highly metastatic, poor prognosis tumors for which effective therapeutic strategies are currently lacking. We summarize recent work focusing on preclinical evaluation of histone deacetylase inhibitors (HDACis) for the treatment of MPNST. HDACis are a novel drug class with anti-cancer therapeutic promise. Using human MPNST cell lines and xenograft models we found that a MPNST subset is highly sensitive to HDACis, whereas a fraction is relatively resistant. HDACis were found to induce autophagy in all MPNST cells in vitro and in vivo; in "sensitive" MPNST cells autophagy occurs in concert with apoptosis, whereas unopposed autophagy develops in "resistant" cells. Genetic and chemical autophagy blockade significantly enhances HDACi-induced apoptotic cell death in both resistant and sensitive cells. Combined chloroquine and HDACi treatment abrogates growth of human MPNST xenografts and lung metastases. The potential role of autophagy in cancer therapeutic response remains controversial; however, our study supports HDACi-induced autophagy as a MPNST survival mechanism. These data also imply that the consequences of drug-induced autophagy may be compound-type, tumor-type, or even molecular context-dependent, suggesting a complex crosstalk between autophagy and apoptosis. Clinical trials evaluating HDACis with autophagy blockade for therapy of MPNST therefore merit consideration.     

Stimulation of autophagic activity in human glioma cells by anti-proliferative ardipusilloside I isolated from Ardisia pusilla

Aims

Ardipusilloside I (ADS-I), a triterpenoid saponin isolated from Ardisia pusilla A.DC (Myrsinaceae), has been recently tested for cancer treatment including brain cancer. However, the mechanism of its action remains elusive. The present study was to investigate the role of autophagy activation in the anti-tumor activities of ADS-I in human glioma cells.

Main methods

The tetrazolium dye (MTT) colorimetric assay was used for the measurement of cell proliferation in cultured glioma cells, transmission electron microscopy (TEM) for the examination of autophagic activity, flow cytometric analysis for the determination of cell cycle and apoptotic cells, and immunocytochemistry and Western blot for protein expression of microtubule-associated protein light-chain 3 (LC3) and Beclin 1.

Key findings

ADS-I significantly inhibited the proliferation of both U373 and T98G glioma cells in cultures in a dose-dependent manner. The cytotoxic activity of ADS-I against glioma cell growth was associated not only with the induction of cell cycle arrest at G₂/M phase and cell apoptosis in flow cytometric analysis, but also with the activation of autophagy, indicated by the formation of autophagosomes and up-regulated expression of both autophagic protein Beclin 1 and LC3 in glioma cells. Additionally, the treatment with chloroquine, an autophagy inhibitor, reduced ADS-1-mediated cell death.

Significance

These data suggest that the anti-proliferative activity of ADS-I in human glioma cells is associated with the activation of autophagy in addition to cell cycle arrest and apoptosis, and the antagonistic effect of chloroquine suggests an important role of autophagy in ADS-I-mediated cell death against tumor growth.     

Three dimensions of autophagy in regulating tumor growth: cell survival/death,cell proliferation,and tumor dormancy

Autophagy is an intracellular lysosomal degradation process involved in multiple facets of cancer biology. Various dimensions of autophagy are associated with tumor growth and cancer progression, and here we focus on the dimensions involved in regulation of cell survival/cell death, cell proliferation and tumor dormancy. The first dimension of autophagy supports cell survival under stress within tumors and under certain contexts drives cell death, impacting tumor growth. The second dimension of autophagy promotes proliferation through directly regulating cell cycle or indirectly maintaining metabolism, increasing tumor growth. The third dimension of autophagy facilitates tumor cell dormancy, contributing to cancer treatment resistance and cancer recurrence. The intricate relationship between these three dimensions of autophagy influences the extent of tumor growth and cancer progression. In this review, we summarize the roles of the three dimensions of autophagy in tumor growth and cancer progression, and discuss unanswered questions in these fields.     

Autophagy blockade enhances HDAC inhibitors’ pro-apoptotic effects

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive, highly metastatic, poor prognosis tumors for which effective therapeutic strategies are currently lacking. We summarize recent work focusing on preclinical evaluation of histone deacetylase inhibitors (HDACis) for the treatment of MPNST. HDACis are a novel drug class with anti-cancer therapeutic promise. Using human MPNST cell lines and xenograft models we found that a MPNST subset is highly sensitive to HDACis, whereas a fraction is relatively resistant. HDACis were found to induce autophagy in all MPNST cells in vitro and in vivo; in “sensitive” MPNST cells autophagy occurs in concert with apoptosis, whereas unopposed autophagy develops in “resistant” cells. Genetic and chemical autophagy blockade significantly enhances HDACi-induced apoptotic cell death in both resistant and sensitive cells. Combined chloroquine and HDACi treatment abrogates growth of human MPNST xenografts and lung metastases. The potential role of autophagy in cancer therapeutic response remains controversial; however, our study supports HDACi-induced autophagy as a MPNST survival mechanism. These data also imply that the consequences of drug-induced autophagy may be compound-type, tumor-type, or even molecular context-dependent, suggesting a complex crosstalk between autophagy and apoptosis. Clinical trials evaluating HDACis with autophagy blockade for therapy of MPNST therefore merit consideration.     

Biochemical variations in cytolytic activity of ortho- and paramyxoviruses in human lung tumor cell culture

Human lung cancer cells (Calu-3 line) were studied for the development of apoptosis, necrosis, and autophagy in response to infection with orthoand paramyxoviruses. Biochemical pathways underlying various mechanisms of cell death differed for different viruses. When infected with murine Sendai paramyxovirus, Calu-3 cells demonstrated typical necrotic features such as cell swelling (but not shrinkage), lack of chromatin DNA laddering, of caspase 3 and 8 activation, and of apoptotic cleavage of poly(ADP-ribose) polymerase (PARP) protein; an activation of antiapoptotic protein kinase Akt was also revealed. In contrast, infection with avian influenza virus A/FPV/Rostock/34 (H7N1 subtype) or Newcastle disease virus (NDV, avian paramyxovirus) caused the development of typical apoptotic markers such as cell shrinkage, ladder-type chromosomal DNA fragmentation, caspase 3 and 8 activation, and proteolytic cleavage of PARP in the absence of Akt activation. Notably, no upregulation of p53 protein phosphorylation was observed in all infected cells, which indicates that p53 is not involved in the virus-induced death of Calu-3 cells. Cell death caused by the influenza virus was accompanied by overstimulation of autophagy, whereas no stimulation of autophagy was observed in the NDV-infected cells. Infection with Sendai virus caused moderate stimulation of autophagy, which suggests that the mechanism of the virus-induced cell death and the balance between autophagy and cell death in infected cancer cells depend on the virus type and might significantly differ even for closely related viruses. Therefore, an optimal strategy for oncolytic virus-mediated destruction of tumor cells in cancer patients requires selection of the most appropriate oncolytic virus based on the mechanism of its cytolytic action in a particular type of tumor.     

GMI,an immunomodulatory protein from Ganoderma microsporum,induces autophagy in non-small cell lung cancer cells

Autophagy is a self-digestive process that degrades the cytoplasmic constituents. Immunomodulatory protein, one major bioactive component of Ganoderma, has antitumor activity. In this study, recombinant fungal immunomodulatory protein, GMI, was cloned from Ganoderma microsporum and purified. We demonstrated that GMI induces lung cancer cell death by activating autophagy, but does not induce apoptotic cell death. On western blot, GMI increased LC3 conversion and decreased p53 expression in a time- and concentration-dependent manner. Cytoplasmic calcium chelator BAPTA-AM was used to prove that GMI promotes autophagy via a calcium-mediated signaling pathway. 3-methyladenine (3-MA), an autophagy inhibitor, enhanced the cytotoxicity of GMI on cell viability assay. Using VZV-G pseudotyped lentivirus-shRNA system for autophagy-related genes silencing, the capabilities of GMI to reduce cell viability and colony formation were abolished in autophagy-defective cells. Furthermore, GMI did not stimulate apoptosis after blocking of autophagy by 3-MA or shRNA knockdown system. In xenograft studies, oral administration of GMI inhibited the tumor growth and induced autophagy significantly in nude mice that had received a subcutaneous injection of A549 cells. This is the first study to reveal the novel function of GMI in activating autophagy. GMI may be a potential chemopreventive agent against non-small cell lung cancer.     

Involvement of protective autophagy in TRAIL resistance of apoptosis-defective tumor cells

Targeting TRAIL receptors with either recombinant TRAIL or agonistic DR4- or DR5-specific antibodies has been considered a promising treatment for cancer, particularly due to the preferential apoptotic susceptibility of tumor cells over normal cells to TRAIL. However, the realization that many tumors are unresponsive to TRAIL treatment has stimulated interest in identifying apoptotic agents that when used in combination with TRAIL can sensitize tumor cells to TRAIL-mediated apoptosis. Our studies suggest that various apoptosis defects that block TRAIL-mediated cell death at different points along the apoptotic signaling pathway shift the signaling cascade from default apoptosis toward cytoprotective autophagy. We also obtained evidence that inhibition of such a TRAIL-mediated autophagic response by specific knockdown of autophagic genes initiates an effective mitochondrial apoptotic response that is caspase-8-dependent. Currently, the molecular mechanisms linking disabled autophagy to mitochondrial apoptosis are not known. Our analysis of the molecular mechanisms involved in the shift from protective autophagy to apoptosis in response to TRAIL sheds new light on the negative regulation of apoptosis by the autophagic process and by some of its individual components.     

Sensitizing effect of 3-methyladenine on radiation-induced cytotoxicity in radio-resistant HepG2 cells in vitro and in tumor xenografts

Many recent efforts have focused on targeting cell death pathways for discovering new cancer therapies. The relative resistance of liver cancer cells to ionizing radiation (IR) and chemotherapeutic agents due to autophagic response limits the available treatment options for this type of cancer. In this study, 3-methyladenine (3-MA), an autophagy inhibitor, was investigated for its potential to enhance radio-sensitivity under radio-resistant conditions both in vitro and in vivo. Hep3B and HepG2 cells were used to examine the radio-resistance of liver cancer cells. The results show that Hep3B cells respond to irradiation with increased apoptotic cell death and that HepG2 is radio-resistant due to the IR-induced autophagy, as verified by DNA fragmentation, electron microscopy, acidic vesicular organelle formation, and Western blot analysis. Application of IR with 3-MA to inhibit autophagy simultaneously suppressed the expression of LC3 and enhanced cell death. The tumor xenograft model in nude mice verified the synergistic cytotoxic effect of 3-MA and IR, which resulted in significant repression of tumor growth. The results demonstrate that IR-induced autophagy provides a self-protective mechanism against radiotherapy in HepG2 cells. In addition, 3-MA enhances the cytotoxicity of IR in cell models and suppresses tumor growth in animal models. Based on the results, application of 3-MA, or other autophagy inhibitors, could be used as an adjuvant for radiotherapy when radio-resistance develops as a result of autophagy response.     

Fenretinide induces autophagic cell death in caspase-defective breast cancer cells

The elimination of tumor cells by apoptosis is the main mechanism of action of chemotherapeutic drugs. More recently, autophagic cell death has been shown to trigger a nonapoptotic cell death program in cancer cells displaying functional defects of caspases. Fenretinide (FenR), a synthetic derivative of retinoic acid, promotes growth inhibition and induces apoptosis in a wide range of tumor cell types. The present study was designed to evaluate the ability of fenretinide to induce caspase-independent cell death and to this aim we used the human mammary carcinoma cell line MCF-7, lacking functional caspase-3 activity. We demonstrated that in these cells fenretinide is able to trigger an autophagic cell death pathway. In particular we found that fenretinide treatment resulted in the increase in Beclin 1 expression, the conversion of the soluble form of LC3 to the autophagic vesicle-associated form LC3-II and its shift from diffuse to punctate staining and finally the increase in lysosomes/autophagosomes. By contrast, caspase-3 reconstituted MCF-7 cell line showed apoptotic cell death features in response to fenretinide treatment. These data strongly suggest that fenretinide does not invariably elicit an apoptotic response but it is able to induce autophagy when apoptotic pathway is deregulated. The understanding of the molecular mechanisms involved in fenretinide action is important for the future design of therapies employing this retinoid in breast cancer treatment.     

Autophagy in pancreatic cancer

Pancreatic cancer, the fourth leading cause of cancer-related death in the United States, is resistant to current chemotherapies. Therefore, identification of different pathways of cell death is important to develop novel therapeutics. Our previous study has shown that triptolide, a diterpene triepoxide, inhibits the growth of pancreatic cancer cells in vitro and prevents tumor growth in vivo. However, the mechanism by which triptolide kills pancreatic cancer cells was not known, hence, this study aimed at elucidating it. Our study reveals that triptolide kills diverse types of pancreatic cancer cells by two different pathways; it induces caspase-dependent apoptotic death in some cell lines and death via a caspase-independent autophagic pathway in the other cell lines tested. Triptolide-induced autophagy requires autophagy-specific genes, atg5 or beclin 1, and its inhibition results in cell death via the apoptotic pathway, whereas inhibition of both autophagy and apoptosis rescues triptolide-mediated cell death. Our study shows for the first time that induction of autophagy by triptolide has a pro-death role in pancreatic cancer cells. Since triptolide kills diverse pancreatic cancer cells by different mechanisms, it makes an attractive chemotherapeutic agent for future use against a broad spectrum of pancreatic cancers.