Annonaceous acetogenin mimic AA005 induces cancer cell death via apoptosis inducing factor through a caspase-3-independent mechanism
© Han et al.; licensee BioMed Central. 2015
Received: 31 December 2014
Accepted: 24 February 2015
Published: 18 March 2015
Annonaceous acetogenins are a family of natural products with antitumor activities. Annonaceous acetogenin mimic AA005 reportedly inhibits mammalian mitochondrial NADH-ubiquinone reductase (Complex I) and induces gastric cancer cell death. However, the mechanisms underlying its cell-death-inducing activity are unclear.
We used SW620 colorectal adenocarcinoma cells to study AA005 cytotoxic activity. Cell deaths were determined by Trypan blue assay and flow cytometry, and related proteins were characterized by western blot. Immunofluorescence and subcellular fractionation were used to evaluate AIF nuclear translocation. Reactive oxygen species were assessed by using redox-sensitive dye DCFDA.
AA005 induces a unique type of cell death in colorectal adenocarcinoma cells, characterized by lack of caspase-3 activation or apoptotic body formation, sensitivity to poly (ADP-ribose) polymerase inhibitor Olaparib (AZD2281) but not pan-caspase inhibitor Z-VAD.fmk, and dependence on apoptosis-inducing factor (AIF). AA005 treatment also reduced expression of mitochondrial Complex I components, and leads to accumulation of intracellular reactive oxygen species (ROS) at the early stage. Blocking ROS formation significantly suppresses AA005-induced cell death in SW620 cells. Moreover, blocking activation of RIP-1 by necroptosis inhibitor necrotatin-1 inhibits AIF translocation and partially suppresses AA005-induced cell death in SW620 cells demonstrating that RIP-1 protein may be essential for cell death.
AA005 may trigger the cell death via mediated by AIF through caspase-3 independent pathway. Our work provided new mechanisms for AA005-induced cancer cell death and novel clues for cancer treatment via AIF dependent cell death.
KeywordsAnnonaceous acetogenins Cancer AIF ROS RIP-1
Biochemical qualities of the Annonaceae (custard-apple) family are not completely known due to its large size (130 genera and 2300 species) . Many Annonaceae species have been used in folk medicine and as insecticides . Products from the Annonaceae family, collectively called annonaceous acetogenins (AAs), are very potent inhibitors of mammalian mitochondria NADH-ubiquinone reductase (Complex I) . To date, over 400 members of this compound family have been found, most of which have been proven to exhibit high cytotoxic and antitumor activities . Over the past few years, we have successfully developed a series of AA mimetics. More interestingly, we found that some of these analogues have significant selectivity between human cancer cells and normal cells . AA005 shows the best inhibitory effect against several human cancer cell lines , although its exact mechanisms are largely unknown.
Mitochondria are the central relay station for apoptotic signal transduction. In response to apoptotic stimulus, permeabilized mitochondria release cytochrome c into the cytoplasm, where cytochrome c forms an apoptosome with Apaf-1 and caspase-9 and triggers the caspase cascade. The most important caspase in this cascade is caspase-3, which is cleaved and activated to transduce the apoptotic signal [6,7]. Mitochondria can also release apoptosis-inducing factor (AIF) to initiate caspase-independent cell death [8,9]. The mitochondrial flavoprotein AIF is a caspase-independent cell-death-inducing factor . During apoptotic signaling without caspase-3 activation, AIF is released from the mitochondria when the mitochondrial membrane is permeabilized, then translocates to the nucleus where it induces cell death by triggering chromatin condensation and large-scale DNA fragmentation into ~50-kilobase strands with the help of other proteins such as Endo G (C. elegans), CypA (mice) or FEN-1 [10-17]. Here we report that AA005 may trigger caspase-3-independent cell death, mediated by AIF. Our work may provide novel therapeutic clues for treating cancers via a non-canonical apoptotic pathway.
Cell culture and treatments
Human colorectal adenocarcinoma cell line SW620, breast cancer cell line BT-549, and U937 acute myelomonocytic leukemic cell line came from the Cell Bank of Shanghai Institutes for Biological Sciences (Shanghai, China); acute promyelocytic leukemia (APL) cell line NB4 were kindly provided by Dr. M. Lanotte in France . These cells were cultured in RPMI-1640 medium (Sigma-Aldrich, St Louis, MO) supplemented with 10% heat-inactivated fetal calf serum (FCS; HyClone, Logan, UT) in a 5% CO2 humidified atmosphere at 37°C. For experiments, cells were seeded at 2–5 × 105 cells/ml and incubated with the indicated concentrations of AA mimic AA005 (kindly provided by Institute of Chemical Biology and Drug Innovation, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, China), MNNG (Sigma-Aldrich), and camptothecin (kindly provided by National Cancer Institute Anticancer Drug Screen standard agent database, Bethesda, MD) with or without caspase inhibitor Z-VAD.fmk (Sigma-Aldrich). AA mimic AA005 were dissolved in 75% ethanol as a 1 mM stock solution and was kept at −80°C. MNNG (100 mM) was freshly prepared in dimethylsulfoxide (DMSO) and diluted in culture media to 0.5 mM. After treatment for 15 min, cells were washed and returned to the normal growth medium. camptothecin was dissolved in double-distilled water as a 1 mM stock solution. Z-VAD.fmk was dissolved in DMSO before use.
Trypan blue assay
After treatments cells were harvested, resuspended in cell growth medium, and diluted 1:1 with 0.4% trypan blue stain (Sigma-Aldrich). Stained and unstained cells were counted using a hemocytometer.
Cells were seeded in 6-well plates 1 day prior to treatments. Fragmented DNA was assessed using terminal deoxynucleotidyl transferase (TdT)-dUTP nick end-labeling (TUNEL) kit (Roche) according to the manufacturer’s protocol.
DNA gel electrophoresis
Appropriate 106 cells were harvested, and pellets were suspended in lysis buffer (0.1 M NaCl, 50 mM Tris–HCl, pH 7.5, 10 mM EDTA (ethylenedia-minetetraacetic acid), 0.5% sodium dodecyl sulfate [SDS], 500 μg/ml protease K). After a 30 minutes, incubation on ice, samples were centrifuged at 14,000 g for 30 minutes, and cellular DNA was extracted. The samples were electrophoresed in 2% agarose gel at 100 V in 40 mM Tris-acetate buffer (pH 7.4) and visualized by ethidium bromide staining.
Flow cytometric assays for Annexin-V
Briefly, about 106 cells were rinsed with phosphate-buffered saline (PBS), and Annexin-V assay was performed on a flow cytometry (Beckman Coulter) according to instructions provided by the ApoAlertAnnexin-V kit (Clontech, PaloAlto, CA) as well as stained with 50 μg/ml propidium iodide (PI; Sigma).
RNA interference and transfection
For siRNA in SW620 cells, the following oligonucleotides were inserted into RNAi-Ready pSIREN-RetroQ vector (Clontech, Palo Alto, CA): 5′-TAGCGGTCGCCGAAATGTT-3′ (A3) and 5′-CTGGTATCCGATCAGAGAG-3′ (A5) for AIF, and 5′-ACTACCGTTGTTATAGGTG-3′ for scrambled negative control. Retrovirus with these shRNA produced in 293 T cells were used to infect SW620 cells. Stable retroviral transduction was achieved by infection for 48 h, after which selection with puromycin was initiated. Selection was stopped as soon as the non-infected control cell died off. Media were then replaced with normal growing medium.
The protein lysates were mixed with equal volume of Laemmli buffer (62.5 mM Tris–HCl, pH 6.8, 2% sodium dodecyl sulfate, 50 mM Dithiothreitol, 10% glycerol, 0.01% bromophenol blue), boiled for 3 min at 100°C and then resolved by sodium dodecyl sulfate–polyacrylamide gel electrophoresis on a 10%–12% gel using a mini gel apparatus (Bio-Rad, Hercules, CA). Subsequently, the proteins were electrophoretically transferred to a nitrocellulose membrane (Bio-Rad). The membranes were blocked with 5% nonfat dry milk solution in Tris-buffered saline with 0.1% Tween-20 for 1 h at room temperature and then incubated in primary antibody dissolved in block solution at 4°C overnight. The proteins were probed by antibodies against AIF (Cell Signaling, Beverly, MA), with mouse anti-β-actin mAb (Merck, Darmstadt, Germany) to confirm equal loading. After washing, the blots were incubated with horseradish peroxidase-conjugated secondary antibody (Dako Cytomation, Glostrup, Denmark) corresponding to the primary antibody in blocking buffer for 1 h at room temperature, and detections were performed by Super Signal West Pico Chemiluminescent Substrate kit (Pierce, Rockford, IL) according to the manufacture’s instructions.
Colorectal adenocarcinoma cells were crawled onto cover slides, fixed with 4% paraformaldehyde and permeabilized with 0.3% Triton X-100 for 10 min. Slides were blocked with 1% bovine serum albumin and incubated with rabbit anti-AIF monoclonal antibody (1:100) overnight at 4°C. After washing in PBS, the cells were stained with secondary antibodies (FITC-conjugated bovine antirabbit; 1:200; Santa Cruz Biotech) and incubated for 1 h in room temperature, followed by nuclear counterstaining with DAPI. Fluorescence signals were detected on a Olympus BX-51 fluorescence microscope (Tokyo, Japan).
Detection of ROS by flow cytometry
Cells were washed with phosphate-buffered saline (PBS), re-suspended in pre-warmed PBS (37°C), and incubated with 10 mM 5-(and 6)-chloromethyl-2′,7′-dichlorodihydrofluorescein diacetate, acetyl ester (C-6827, CM-H2-DCFDA, Invitrogen, Carlsbad, CA, USA) for 30 min at 37°C. Cells were then washed with PBS twice and scraped into 0.3 ml of ice-cold PBS. CM-H2-DCFDA fluorescence was determined by measuring 10,000 events per sample following excitation with a 488-nm wavelength laser and reading through a 530/30 filter (FACSCalibur, BD Bioscience, San Jose, CA, USA).
Each experiment was done independently at least 3 times with similar results. Results are expressed as mean ± S.D. Significant differences were assessed with the Student’s t test (2-tailed). P < 0.05 was considered to be significant.
AA005 induces cell death in various cancer cell lines
AA005 induces non-canonical apoptosis
AA005 induces cell death through a caspase-3 independent pathway
AIF potentially contributes to AA005-induced cell death
ROS mediates AA005-induced cell death of SW620 cells
RIP1 is required for AA005-induced cell death
Directed induction of cell death could provide therapeutic benefits for cancer treatment. Such treatments mainly target caspase pathways to induce apoptosis. However, caspase activation may be dispensable for some kinds of apoptosis, and increasing attention has been drawn to key molecules involved in non-apoptotic cell death or caspase-independent apoptosis [10,33]. The mitochondrial protein AIF is a new therapeutic target involved in most of the caspase-independent apoptosis systems, including programmed necrosis .
In this work, we found that AA005 could induce cell death of SW620 cells and NB4 cells with evidence that implies a caspase-independent mechanism; we also found that AIF might be involved in this process. We also found that AA005-induced death of cancer cells provides a morphology similar to that of MNNG treated cells. Previous studies indicated that MNNG-mediated cell death was AIF-mediated and PARP-1-dependent, resulting in a caspase-independent type of apoptosis, called parthanatos [20,25]. Our findings indicate that AA005 targets AIF signaling by promoting its nuclear translocation. The fact that AA005-induced cell death is mostly blocked by AIF knockdown suggests that AIF is crucial to AA005-induced cell death.
It has previously been reported that AAs were very potent inhibitors of the mitochondrial NADH-ubiquinone reductase (Complex I) and induced apoptosis by reducing intracellular cAMP and cGMP levels in human cancer [3,35]. Recently, Liu et al. revealed that AA005 co-localized with mitochondria in colon cancer cells. In their view, AA005 could activate AMP-activated protein kinase (AMPK) and inhibit the mTOR complex 1 (mTORC1) signal pathway, leading to growth inhibition and autophagy of colon cancer cells. However, AMPK inhibitors compound C and inosine can only partially attenuate AA005-caused proliferation suppression of colon cancer cells , indicating that other mechanisms affect the cancer suppression activity of AA005. In addition, the mechanism of Complex I inhibition by AA005 is unclear, although AIF is known to affect Complex I activity . In fact, here we find that AA005 treatment decreased mitochondrial Complex I subunits, and significantly increased intracellular concentrations of ROS. AA005, as a novel ROS-inducing agent, may be an effective chemical probe to examine the mechanisms of tumor cells that are more sensitive and vulnerable to toxic oxidative stress . Our research here shows RIP-1 activation is required for translocation of AIF from the mitochondria to the nucleus; and AIF is necessary for AA005-induced cell death, which is prevented by PARP inhibitors, RIP-1 inhibitors or knockdown of AIF, but is caspase independent. We speculate that AA005 may disrupt mitochondrial function by reducing mitochondrial Complex I expression, thus triggering ROS, RIP and AIF-dependent pathway. Thus provides a new clue to the action of AA005.
As a core executor in caspase-independent cell death, AIF is intensively studied . However, many studies’ results are highly controversial. We suggest that AA005 is an effective chemical probe to examine the role of AIF. Furthermore, AA005 may be the basis of a novel treatment for cancers that are resistant to classical apoptotic reagents.
AA005 can induce an AIF-dependent but caspase-independent cell death, which is mediated through ROS and RIP1. Our work shows new mechanisms for AA005-induced cancer cell death and implies a novel cancer treatment via AIF dependent cell death.
Apoptosis inducing factor
- Complex I:
This work is supported in part by grants from National Natural Science Foundation of China (81472758, 31170783, U1302225) and Ministry of Science and Technology of China (2013CB910903).
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