The role of autophagy in the death of L1210 leukemia cells initiated by the new antitumor agents, XK469 and SH80

Design, synthesis, biological assessment and molecular docking studies of new 2-aminoimidazole-quinoxaline hybrids as potential anticancer agents

In a search for novel antiproliferative agents, a series of quinoxaline derivatives containing 2-aminoimidazole (8a-8x) were designed and synthesized. The structures of synthesized compounds were confirmed by IR, ¹H NMR, ¹³C NMR, Mass Spectroscopy and analyzed using HSQC, COSY, ROESY, HMBC techniques. The anticancer activity of all derivatives were evaluated for colon cancer and breast cancer cell lines by the MTT assay and acridine orange/ethidium bromide double staining method. The anti-cancer effect in human colon cancer (HCT-116) and breast cancer (MCF-7) cell lines exhibited that compounds 8a, 8s, 8t, 8w, 8x appeared as potent antiproliferative agents and especially inhibited the human colon cancer cell proliferation with percentage of inhibition by over 50%. The most active compound was (E)-4-phenyl-1-((quinoxalin-2-ylmethylene)amino)-1H-imidazol-2-amine (8a) with the highest inhibition for MCF-7 (83.3%) and HCT-116 (70%) cell lines after 48 and 24h, respectively. Molecular docking studies of these derivatives within c-kit active site as a validated target might be suggested them as appropriate candidates for further efforts toward more potent anticancer compounds.

Upregulation of the growth arrest-specific-2 in recurrent colorectal cancers, and its susceptibility to chemotherapy in a model cell system

Colorectal cancer (CRC) is one of the most common life-threatening malignances worldwide. CRC relapse markedly decreases the 5-year survival of patients following surgery. Aberrant expression of genes involved in pathways regulating the cell cycle, cell proliferation, or cell death are frequently reported in CRC tumorigenesis. We hypothesized that genes involved in CRC relapse might serve as prognostic indicators. We first evaluated the significance of gene sequences in the feces of patients with CRC relapse by consulting a public database. Tumorigenesis of target tissues was tested through tumor cell growth, cell cycle regulation, and chemotherapeutic efficacy. We found a highly significant correlation between CRC relapse and growth arrest-specific 2 (GAS2) gene expression. Based on cell models, the overexpressed GAS2 was associated with cellular growth rate, cell cycle regulation, and with chemotherapeutic sensitivity. Cell division was impaired by treating cells with 2-[4-(7-chloro-2-quinoxalinyloxy)phenoxy]-propionic acid (XK469), even when the cells were overexpressing GAS2. Thus, downregulation of GAS2 expression might control CRC relapse after curative resection. GAS2 could serve as a noninvasive marker from the feces of patients with prediagnosed CRC. Our findings suggest that GAS2 could have potential clinical applications for predicting early CRC relapse after radical resection, and that XK469 might impair tumor cell division by reducing GAS2 expression or blocking its cellular translocation. This will help in selecting the best therapeutic option, 5-fluorouracil in combination with XK469, for patients overexpressing GAS2 in CRC cells. Thus, GAS2 might act as a prognostic biomolecule and potential therapeutic target in patients with CRC relapse.

Rottlerin-induced autophagy leads to the apoptosis in breast cancer stem cells: molecular mechanisms

Background

Autophagy is an indispensable lysosomal self-digestion process involved in the degradation of aggregated proteins and damaged organelles. Autophagy is associated with the several pathological processes, including cancer. Cancer stem cells (CSCs) play significant roles in cancer initiation, progression and drug resistance. Recent studies have demonstrated the antitumor activities of plant-derived chemopreventive agent rottlerin (Rott). However, the molecular mechanism by which Rott induces autophagy in breast CSCs has not been investigated.

Results

The objectives of this study were to examine the molecular mechanism by which Rott induces autophagy which leads to apoptosis in breast CSCs. Treatment of breast CSCs with Rott for 24 h resulted in a concentration dependent induction of autophagy, followed by apoptosis as measured by flow cytometry. Electron microscopy confirmed the presence of autophagosomes in Rott treated breast CSCs. Western blot analysis showed that Rott treatment increased the expression of LC3, Beclin-1 and Atg12 that are accumulated during autophagy. Prolonged exposure of breast CSCs to Rott caused apoptosis which was associated with the suppression of phosphorylated Akt and mTOR, upregulation of phosphorylated AMPK, and downregulation of anti-apoptosis Bcl-2, Bcl-X_L, XIAP and cIAP-1. Knock-down of Atg7 or Beclin-1 by shRNA inhibited Rott-induced autophagy at 24 h. Our study also demonstrates that pre-treatment of breast CSCs with autophagosome inhibitors 3-methyladenine and Bafilomycin, as well as protein synthesis inhibitor cycloheximide inhibited Rott-induced autophagy and apoptosis. Rott induces autophagy via extensive cytoplasmic vacuolization in breast CSCs. Molecular docking results between C2-domain of protein kinase C-delta and Rott indicated that both hydrogen bonding and hydrophobic interactions contributed significantly for ligand binding with minimum binding affinity of ≈ 7.5 Kcal/mol. Although, autophagy inhibitors suppress the formation of cytoplasmic vacuolization and autophagy in breast CSCs, the potency of Rott to induce autophagy and apoptosis might be based on its capability to activate several pathways such as AMPK and proteasome inhibition.

Conclusions

A better understanding of the relationship between autophagy and apoptosis would eventually allow us to discover novel drugs for the treatment of breast cancer by eliminating CSCs.

Roles of autophagy-related genes Beclin-1 and LC3 in the development and progression of prostate cancer and benign prostatic hyperplasia

Prostate cancer (PCa) is common in Western populations and the second leading cause of cancer-related mortality among males in North America, with an increasing morbidity in China and other Asian countries. The aim of this study was to evaluate the protein expression of autophagy-related genes Beclin-1 and LC3 in patients with prostate cancer (PCa) and benign prostatic hyperplasia (BPH) and elucidate their association with p53 and Bcl-2. The total protein of 34 PCa and 50 BPH samples was extracted and the expression of Beclin-1 and LC3 was analyzed by western blotting assay. Subsequently, a total of 96 paraffin-embedded BPH tissue samples was subdivided into 2 groups, one group in which patients had received 5α-reductase inhibitor, due to its effect of androgen ablation, and the control group, in which patients had not received the 5α-reductase inhibitor. The samples were randomly collected and examined using immunohistochemical (IHC) analysis. The western blot analysis demonstrated that Beclin-l and LC3 expression was higher in BPH tissues compared to PCa tissues (P<0.001). There was no statistically significant difference between PCas of different Gleason scores (P>0.05). The result of IHC revealed that Beclin-l and LC3 expression in the group of patients who had received the 5α-reductase inhibitor was significantly higher compared to that in the control group; however, the expression of Bcl-2 and p53 was lower (P<0.05). Beclin-1 expression exhibited a negative correlation with Bcl-2 (r=-0.402, P<0.001), whereas LC3 expression exhibited a positive correlation with Beclin-1 (r=0.345, P=0.001) and a negative correlation with Bcl-2 (r=-0.216, P=0.035). It was suggested that autophagy-related genes Beclin-l and LC3 may be involved in the development and progression of PCa. In addition, the expression of these genes was higher in patients with BPH who had received a 5α-reductase inhibitor, due to androgen reduction. As a result, the induced autophagy may reduce the risk of PCa.

Rottlerin induces autophagy which leads to apoptotic cell death through inhibition of PI3K/Akt/mTOR pathway in human pancreatic cancer stem cells

Multiple lines of evidence support the idea that autophagy plays an essential role in the development of drug resistance, self-renewal, differentiation, and tumorigenic potentials of cancer stem cells (CSCs). Rottlerin (ROT) is widely used as a protein kinase C-delta (PKC-δ) inhibitor. Recent studies revealed that ROT induces apoptosis through engagement of mitochondria. However, it is not known whether ROT-induced apoptosis is associated with other mechanisms such as autophagy. Here we found that ROT induced autophagy followed by induction of apoptosis via inhibition of PI3K/Akt/mTOR pathway and activation of caspase cascade in human pancreatic CSCs. ROT induced a dose- and time-dependent inhibition of cell survival and induction of cytoplasmic vacuolations. The conversion of microtubule-associated protein LC3-I to LC3-II, and increased accumulations of Atg7 and Beclin-1 were also observed in CSCs treated with ROT. Prolonged exposure of CSCs to ROT eventually caused apoptosis which was associated with the suppression of phosphorylated Akt (Ser473) and mTOR (Ser2448), downregulation of XIAP, cIAP-1, Bcl-2 and Bcl-X(L), induction of Bax, activation of caspase-3 and -9, and concomitant degradation of PARP. ROT-induced apoptosis was enhanced by dominant negative AKT, Akt1/2 inhibitor, and rapamycin. Our study also demonstrates that gene silencing of Atg7 and Beclin1, or cotreatment of the autophagosome inhibitor, 3-methyladenine, inhibited ROT-induced autophagy and accelerated ROT-induced apoptosis. The knockdown of PKC-δ did not block ROT-induced autophagy and cell death, suggesting these effects of ROT were exerted through PKC-δ-independent pathway. In summary, our data demonstrate that ROT can induce autophagy which leads to cell death in pancreatic CSCs.

New insights into the mechanisms for photodynamic therapy-induced cancer cell death

Photodynamic therapy (PDT) is a promising therapeutic modality for cancer treatment; however, a more detailed understanding is needed to improve the clinical use of this therapy. PDT induces cancer cell death by apoptosis, necrosis, and autophagy, and these mechanisms can be concurrently occurred. PDT destroys cancer cells by inducing apoptosis through diverse signaling pathways coupled with Bcl-2 family members, caspases, and apopotosis-inducing factor. When the apoptotic pathway is unavailable, PDT can cause cancer cell death through induction of a necrotic or autophagic mechanism. Autophagy is occurred in a Bax-independent manner and can be stimulated in parallel with apoptosis. PDT directly destroys cancer cells by inducing either apoptotic or necrotic death. PDT also can induce autophagy as a death or a survival mechanism. These mechanisms are dependent on a variety of parameters including the nature of the photosensitizer, PDT dose, and cell genotype. Understanding the complex cross talk between these pathways may improve the effectiveness of PDT. Here, we discuss the interplay between these mechanisms based on recent evidence and suggest prospects with regard to advances in PDT.

ATG7 deficiency suppresses apoptosis and cell death induced by lysosomal photodamage

Photodynamic therapy (PDT) involves photosensitizing agents that, in the presence of oxygen and light, initiate formation of cytotoxic reactive oxygen species (ROS). PDT commonly induces both apoptosis and autophagy. Previous studies with murine hepatoma 1c1c7 cells indicated that loss of autophagy-related protein 7 (ATG7) inhibited autophagy and enhanced the cytotoxicity of photosensitizers that mediate photodamage to mitochondria or the endoplasmic reticulum. In this study, we examined two photosensitizing agents that target lysosomes: the chlorin NPe6 and the palladium bacteriopheophorbide WST11. Irradiation of wild-type 1c1c7 cultures loaded with either photosensitizer induced apoptosis and autophagy, with a blockage of autophagic flux. An ATG7- or ATG5-deficiency suppressed the induction of autophagy in PDT protocols using either photosensitizer. Whereas ATG5-deficient cells were quantitatively similar to wild-type cultures in their response to NPe6 and WST11 PDT, an ATG7-deficiency suppressed the apoptotic response (as monitored by analyses of chromatin condensation and procaspase-3/7 activation) and increased the LD(50) light dose by > 5-fold (as monitored by colony-forming assays). An ATG7-deficiency did not prevent immediate lysosomal photodamage, as indicated by loss of the lysosomal pH gradient. However, unlike wild-type and ATG5-deficient cells, the lysosomes of ATG7-deficient cells recovered this gradient within 4 h of irradiation, and never underwent permeabilization (monitored as release of endocytosed 10-kDa dextran polymers). We propose that the efficacy of lysosomal photosensitizers is in part due to both promotion of autophagic stress and suppression of autophagic prosurvival functions. In addition, an effect of ATG7 unrelated to autophagy appears to modulate lysosomal photodamage.

Activation of ERK-p53 and ERK-mediated phosphorylation of Bcl-2 are involved in autophagic cell death induced by the c-Met inhibitor SU11274 in human lung cancer A549 cells

SU11274, a small molecule inhibitor of c-Met, was reported to induce apoptosis in human non-small-cell lung cancer (NSCLC) cells. However, SU11274-mediated autophagy in NSCLC cells has rarely been reported. The aim of this study was to elucidate the molecular mechanisms mediating SU11274-induced autophagy in NSCLC A549 cells. Here we reported that SU11274-induced autophagy was accompanied with an increase in the conversion of LC3-I to LC3-II and up-regulation of Beclin-1 expression. Subsequently, we also found that small interfering RNA against c-Met induced A549 cell autophagy while promotion of c-Met by hepatocyte growth factor (HGF) suppressed A549 cell autophagy. Inhibition of autophagy by 3-methyladenine (3-MA) suppressed SU11274-induced cell death, suggesting that SU11274-induced autophagy caused cell death. Further study showed that ERK and p53 were activated after SU11274 treatment. Interruption of ERK and p53 activities decreased SU11274-induced autophagy, and blocking of ERK by the specific inhibitor PD98059 suppressed SU11274-induced p53 activation. Moreover, ERK activation upregulated Beclin-1 expression through induction of Bcl-2 phosphorylation, but p53 did not induce Bcl-2 phosphorylation. In conclusion, inhibition of c-Met induced autophagic cell death, which was associated with ERK-p53 activation and ERK-mediated Bcl-2 phosphorylation in A549 cells.

Aborted autophagy and nonapoptotic death induced by farnesyl transferase inhibitor and lovastatin

Exposure of the human malignant peripheral nerve sheath tumor cell lines STS-26T, ST88-14, and NF90-8 to nanomolar concentrations of both lovastatin and farnesyl transferase inhibitor (FTI)-1 but not to either drug alone induced cell death. ST88-14 and NF90-8 cells underwent apoptosis, yet dying STS-26T cells did not. FTI-1 cotreatment induced a strong and sustained autophagic response as indicated by analyses of microtubule-associated protein-1 light chain 3 (LC3)-II accumulation in STS-26T cultures. Extensive colocalization of LC3-positive punctate spots was observed with both lysosome-associated membrane protein (LAMP)-1 and LAMP-2 (markers of late endosomes/lysosomes) in solvent or FTI-1 or lovastatin-treated STS-26T cultures but very little colocalization in lovastatin/FTI-1-cotreated cultures. The absence of colocalization in the cotreatment protocol correlated with loss of LAMP-2 expression. Autophagic flux studies indicated that lovastatin/FTI-1 cotreatment inhibited the completion of the autophagic program. In contrast, rapamycin induced an autophagic response that was associated with cytostasis but maintenance of viability. These studies indicate that cotreatment of STS-26T cells with lovastatin and FTI-1 induces an abortive autophagic program and nonapoptotic cell death.

The quinoxaline anti-tumor agent (R+)XK469 inhibits neuroblastoma tumor growth

The quinoxaline anti-tumor agent (R+)XK469 mediates its effects by topoisomerase IIB inhibition. This report describes a 14-year old with relapsed neuroblastoma who experienced disease stabilization for 14 months while receiving (R+)XK469 monotherapy. Due to this favorable response, laboratory studies were undertaken to determine efficacy in the preclinical setting. (R+)XK469 inhibited proliferation, caused G(2) cell cycle arrest of neuroblastoma cells in vitro, and inhibited growth of neuroblastoma xenograft tumors. These preclinical results, coupled with the favorable clinical response, demonstrate that (R+)XK469 and similar anti-tumor agents may be effective in the treatment of high-risk neuroblastoma and warrant further testing.

Synthesis of 4H-chromene, coumarin, 12H-chromeno[2,3-d]pyrimidine derivatives and some of their antimicrobial and cytotoxicity activities

Condensation of 3-N,N-diethylaminophenol (1) with α-cyanocinnamonitriles (2a-c) and ethyl α-cyanocinnamates (2d-f) provided compounds 3a-f and 4a-c. 12H-Chromeno[2,3-d]pyrimidine derivatives 6, 11-13 and 16 were obtained by treatment of 4H-chromene compounds (3) with different electrophiles followed by nucleophilic reagents. Structures of these compounds were established on the basis of IR, UV, 1H NMR, 13C NMR and MS data. Some of the new compounds were evaluated for antimicrobial and cytotoxicity activities.

Synthesis and antiproliferative activity of novel symmetrical alkylthio- and alkylseleno-imidocarbamates

The study described here concerns the synthesis of a series of thirty new symmetrically substituted imidothiocarbamate and imidoselenocarbamate derivatives and their evaluation for antitumoral activity in vitro against a panel of five human tumor cell lines: breast adenocarcinoma (MCF-7), colon carcinoma (HT-29), lymphocytic leukemia (K-562), hepatocarcinoma (Hep-G2), prostate cancer (PC-3) and one non-malignant mammary gland-derived cell line (MCF-10A). The GI(50) values for eighteen of the compounds were below 10 μM in at least one cell line. Two cancer cells (MCF-7 and HT-29) proved to be the most sensitive to five compounds (1b, 2b, 3b, 4b and 5b), with growth inhibition in the nanomolar range, and compounds 1b, 3b, 7b, 8b and 9b gave values of less than 1 μM. In addition, all of the aforementioned compounds exhibited lower GI(50) values than some of the standard chemotherapeutic drugs used as references. The results also reveal that the nature of the aliphatic chain (methyl is better than benzyl) at the selenium position and the nature of the heteroatom (Se better than S) have a marked influence on the antiproliferative activity of the compounds. These findings reinforce our earlier hypothesis concerning the determinant role of the selenomethyl group as a scaffold for the biological activity of this type of compound. Considering both the cytotoxic parameters and the selectivity index (which was compared in MCF-7 and MCF-10A cells), compounds 2b and 8b (with a selenomethyl moiety) displayed the best profiles, with GI(50) values ranging from 0.34 nM to 6.07 μM in the five cell lines tested. Therefore, compounds 2b and 8b were evaluated by flow cytometric analysis for their effects on cell cycle distribution and apoptosis in MCF-7 cells. 2b was the most active, with an apoptogenic effect similar to camptothecin, which was used as a positive control. Both of them provoked cell cycle arrest leading to the accumulation of cells in either G(2)/M and S phase. These two compounds can therefore be considered as the most promising candidates for the development of novel generations of antitumor agents.

Assessing autophagy in the context of photodynamic therapy

Photodynamic therapy (PDT) is a procedure that has applications in the selective eradication of neoplasia where sites of malignant lesions are clearly delineated. It is a two-step process whereby cells are first sensitized to light and then photoirradiated. This results in the formation of singlet molecular oxygen and other reactive oxygen species that can cause photodamage at sites where the photosensitizing agent has localized. Photosensitizers found to be clinically useful show affinity for the endoplasmic reticulum (ER), mitochondria, lysosomes, or combinations of these sites. The induction of apoptosis and/or autophagy in photosensitized cells is a common outcome of PDT. This report explores the following issues: (1) Does the induction of autophagy in PDT protocols occur independent of, or in association with, apoptosis? (2) Does the resulting autophagy play a prosurvival or prodeath role? (3) Do photosensitizers damage/inactivate specific proteins that are components of, or that modulate the autophagic process? (4) Can an autophagic response be mounted in cells in which lysosomes are specifically photodamaged? In brief, autophagy can occur independently of apoptosis in PDT protocols, and appears to play a prosurvival role in apoptosis competent cells, and a prodeath role in apoptosis incompetent cells. Mitochondrial and ER-localized sensitizers cause selective photodamage to some (i.e., Bcl-2, Bcl-x(L), mTOR) proteins involved in the apoptotic/autophagic process. Finally, an aborted autophagic response occurs in cells with photodamaged lysosomes. Whereas autophagosomes form, digestion of their cargo is compromised because of the absence of functional lysosomes.

Regulation of autophagy by reactive oxygen species (ROS): implications for cancer progression and treatment

Reactive oxygen species (ROS) have been identified as signaling molecules in various pathways regulating both cell survival and cell death. Autophagy, a self-digestion process that degrades intracellular structures in response to stress, such as nutrient starvation, is also involved in both cell survival and cell death. Alterations in both ROS and autophagy regulation contribute to cancer initiation and progression, and both are targets for developing therapies to induce cell death selectively in cancer cells. Many stimuli that induce ROS generation also induce autophagy, including nutrient starvation, mitochondrial toxins, hypoxia, and oxidative stress. Some of these stimuli are under clinical investigation as cancer treatments, such as 2-methoxyestrodial and arsenic trioxide. Recently, it was demonstrated that ROS can induce autophagy through several distinct mechanisms involving Atg4, catalase, and the mitochondrial electron transport chain (mETC). This leads to both cell-survival and cell-death responses and could be selective toward cancer cells. In this review, we give an overview of the roles ROS and autophagy play in cell survival and cell death, and their importance to cancer. Furthermore, we describe how autophagy is mediated by ROS and the implications of this regulation to cancer treatments.

Down-regulation of the HGF/MET autocrine loop induced by celecoxib and mediated by P-gp in MDR-positive human hepatocellular carcinoma cell line

Many tumors are resistant to drug-induced cell-cycle arrest and apoptosis. We have reported that apoptosis can be restored in human multidrug-resistant (MDR) hepatocellular carcinoma cell lines by celecoxib. Here we show that P-glycoprotein (P-gp) mediates cell-cycle arrest and autophagy induced by celecoxib in human MDR overexpressing hepatocellular carcinoma cell line by down-regulation of the HGF/MET autocrine loop and Bcl-2 expression. Exposure of cells to a low concentration of celecoxib down-regulated the expression of mTOR and caused G1 arrest and autophagy, while higher concentration triggered apoptosis. Cell growth inhibition and autophagy were associated with up-regulation of the expression of TGFbeta1, p16(INK4b), p21(Cip1) and p27(Kip1) and down-regulation of cyclin D1, cyclin E, pRb and E2F. The role of P-glycoprotein expression in resistance of MDR cell clone to cell-cycle arrest, autophagy and apoptosis was shown in cells transfected with MDR1 small interfering RNA. These findings demonstrate that the constitutive expression of P-gp is involved in the HGF/MET autocrine loop that leads to increased expression of Bcl-2 and mTor, inhibition of eIF2alpha expression, resistance to autophagy/apoptosis and progression in the cell-cycle. Since mTor inhibitors have been proposed in treatment of "drug resistant" cancer, these data may help explain the reversing effect of mTor inhibitors.

The chemotherapeutic agents XK469 (2-{4-[(7-chloro-2-quinoxalinyl)oxy]phenoxy}propionic acid) and SH80 (2-{4-[(7-bromo-2-quinolinyl)oxy]phenoxy}propionic acid) inhibit cytokinesis and promote polyploidy and induce senescence

The therapeutic usefulness of the quinoxaline derivatives XK469 (2-{4-[(7-chloro-2-quinoxalinyl)oxy]phenoxy}propionic acid) and SH80 (2-{4-[(7-bromo-2-quinolinyl)oxy]phenoxy}propionic acid) has been attributed to their abilities to induce G(2)/M arrest and apoptotic or autophagic cell death. Concentrations of XK469 or SH80 > or = 5 microM were cytostatic to cultures of the normal murine melanocyte cell line Melan-a. Higher concentrations caused dose-dependent cytotoxicity. Concentrations > or =10 microM provoked dramatic morphological changes typified by marked increases in cell size and granularity. XK469/SH80-treated cultures accumulated tetraploid (4N) DNA-containing cells within 24 h of treatment, an 8N population within 3 days, and a 16N population within 5 days. Increases in ploidy correlated with the appearance of multinucleated cells. Under no circumstances did cells exhibit evidence of furrow formation. Both drugs suppressed cytokinesis in additional mammalian cell lines. Cytotoxic concentrations of XK469 elevated DEVDase activities (a measure of procaspase-3/7 activation) and enhanced cellular staining by a fluorescent analog of the pan caspase inhibitor valine-alanine-aspartic acid-fluoromethyl ketone within 48 to 96 h of treatment. Within 48 h of treatment, cytostatic and cytotoxic concentrations of XK469 elevated p21 contents, reduced Bcl-2 and Bcl-XL contents, and induced autophagy, as monitored by the accumulation of phosphatidylethanolamine-modified cleavage product of microtubule-associated protein light chain 3 (LC3-II). Cultures treated with > or =10 microM XK469 or SH80 for 5 days could not be induced to divide upon removal of drugs. Such cultures maintained high LC3-II contents, exhibited reduced cyclin E and D1 contents, and extensively expressed senescence-associated beta-galactosidase within 14 to 17 days of cessation of drug treatment. Hence, XK469 and SH80 inhibit cytokinesis, promote polyploidy, and induce senescence in Melan-a cells.

Suppression of autophagy enhances the cytotoxicity of the DNA-damaging aromatic amine p-anilinoaniline

p-Anilinoaniline (pAA) is an aromatic amine that is widely used in hair dying applications. It is also a metabolite of metanil yellow, an azo dye that is commonly used as a food coloring agent. Concentrations of pAA between 10 and 25 microM were cytostatic to cultures of the normal human mammary epithelia cell line MCF10A. Concentrations >or=50 microM were cytotoxic. Cytostatic concentrations induced transient G(1) and S cell cycle phase arrests; whereas cytotoxic concentrations induced protracted arrests. Cytotoxic concentrations of pAA caused DNA damage, as monitored by the alkaline single-cell gel electrophoresis (Comet) assay, and morphological changes consistent with cells undergoing apoptosis and/or autophagy. Enzymatic and western blot analyses, and binding analyses of fluorescent labeled VAD-FMK, suggested that caspase family members were activated by pAA. Western blot analyses documented the conversion of LC3-I to LC3-II, a post-translational modification involved in the development of the autophagosome. Suppression of autophagosome formation, via knockdown of ATG7 with shRNA, prevented pAA-induced vacuolization, enhanced the activation of pro-caspase-3, and increased susceptibility of ATG7-deficient cells to the cytostatic and cytotoxic activities of markedly lower concentrations of pAA. Cells stably transfected with a nonsense shRNA behaved like parental MCF10A cells. Collectively, these data suggest that MCF10A cultures undergo autophagy as a pro-survival response to concentrations of pAA sufficient to induce DNA damage.

Apoptosis and autophagy after mitochondrial or endoplasmic reticulum photodamage

Photodynamic therapy (PDT) can cause lethal photodamage by both direct and indirect mechanisms. Direct modes of cell death relate to nonspecific necrosis and the initiation of signaling pathways that elicit apoptosis, autophagy or both. In this report, effects of low-dose and high-dose PDT are explored, comparing sensitizers that localize in the endoplasmic reticulum (the porphycene termed CPO) or mitochondria (mesochlorin). To explore the role of autophagy, two cell lines were examined--the murine L1210 leukemia and an Atg7 knockdown derivative of L1210. The Atg7 gene is central to the process of autophagy. High-dose PDT with either sensitizer resulted in a substantial loss of the Bcl-2 protein. As Bcl-2 regulates both apoptosis and autophagy, loss of this protein can lead to initiation of either or both processes. Low-dose PDT with either sensitizer resulted in the initiation of apoptosis in the L1210/Atg7- cell line and a 20% loss of viability. In contrast, the same PDT dose led to the rapid appearance of autophagic cells in the L1210 line, less apoptosis and only a 5% loss of viability. These results are consistent with autophagy serving as a pro-survival response via the recycling of damaged organelles. At a higher PDT dose more apoptosis was again seen in the L1210/Atg7- line, but both cell lines exhibited comparable cytotoxicity in colony formation assays. We conclude that autophagy offers protection from the phototoxic effects of low-dose PDT, but can serve as an alternate death mode when the PDT dose is increased.

Apoptotic and autophagic responses to Bcl-2 inhibition and photodamage

Among the cellular responses to photodamage initiated by photodynamic therapy (PDT) are autophagy and apoptosis. While autophagy is a reversible process that can be both a survival and a death pathway, apoptosis is irreversible, leading only to cell death. In this study, we followed the fate of mouse leukemia L1210 cells after photodamage to the endoplasmic reticulum (ER) using a porphycene photosensitizer, where Bcl-2 was among the PDT targets. In wild-type cells, we observed a rapid wave of autophagy, presumed to represent the recycling of some damaged organelles, followed by apoptosis. Using shRNA technology, we created a Bax knockdown line (L1210/Bax(-)). In the latter cell line, we found a marked decrease in apoptosis after photodamage or pharmacologic inactivation of Bcl-2 function, but this did not affect PDT efficacy. Loss of viability was associated with a highly-vacuolated morphology consistent with autophagic cell death. Previous studies indicated pro-survival attributes of autophagy after low-dose PDT, suggesting that autophagy may be responsible for the 'shoulder' on the dose-response curve. It appears that attempts at extensive recycling of damaged organelles are associated with cell death, and that this phenomenon is amplified when apoptosis is suppressed.