Selective targeting of death receptor 5 circumvents resistance of MG-63 osteosarcoma cells to TRAIL-induced apoptosis

In Silico Modeling of the Induction of Apoptosis by Cryptotanshinone in Osteosarcoma Cell Lines

Osteosarcoma (OS) is the most common primary malignant bone tumor of both children and pet canines. Its characteristic genomic instability and complexity coupled with the dearth of knowledge about its etiology has made improvement in the current treatment difficult. We use the existing literature about the biological pathways active in OS and combine it with the current research involving natural compounds to identify new targets and design more effective drug therapies. The key components of these pathways are modeled as a Boolean network with multiple inputs and multiple outputs. The combinatorial circuit is employed to theoretically predict the efficacies of various drugs in combination with Cryptotanshinone. We show that the action of the herbal drug, Cryptotanshinone on OS cell lines induces apoptosis by increasing sensitivity to TNF-related apoptosis-inducing ligand (TRAIL) through its multi-pronged action on STAT3, DRP1 and DR5. The Boolean framework is used to detect additional drug intervention points in the pathway that could amplify the action of Cryptotanshinone.

Bone morphogenetic protein and activin membrane-bound inhibitor suppress bone cancer progression in MG63 and SAOS cells via regulation of the TGF-β-induced EMT signaling pathway

Bone cancer is one of the most common tumor types that occurs in bones and their affiliated tissues. The prognosis remains poor due to the limited number of effective therapeutic targets. Downregulation of bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) has been observed in human cancer cells and BAMBI reconstitution can inhibit growth and metastasis of human cancer cells. In the present study, a potential mechanism mediated by BAMBI in osteosarcoma cells was investigated. The data demonstrated that BAMBI reconstitution suppressed the cell growth, migration and invasion of the osteosarcoma cell lines SAOS2 and MG63. Alterations to the epithelial-to-mesenchymal transition (EMT) marker expression were observed in BAMBI-treated osteosarcoma SAOS2 and MG63 cells. The apoptosis rate of SAOS2 and MG63 cells induced by cisplatin were increased in BAMBI-treated osteosarcoma SAOS2 and MG63 cells via downregulation of the anti-apoptosis genes P16, P21 and B-cell lymphoma 2. The potential mechanism investigated indicated that BAMBI administration downregulated the transforming growth factor-β (TGF-β) signaling pathway, whilst knockdown of BAMBI upregulated the TGF-β signaling pathway in SAOS2 and MG63 cells. Reconstitution of BAMBI in SAOS2 and MG63 cells resulted in a notable reduction of TGF-β-induced EMT, cell growth, migration and invasion in vitro. In conclusion, the results demonstrated that BAMBI reconstitution inhibited growth and invasiveness of osteosarcoma, as well as promoted the apoptotic sensibility, which indicated that the TGF-β-induced EMT signaling pathway may be regarded as a potential target for osteosarcoma therapy.

Combined application of Embelin and tumor necrosis factor-related apoptosis-inducing ligand inhibits proliferation and invasion in osteosarcoma cells via caspase-induced apoptosis

Embelin, as an inhibitor of the X-linked inhibitor of apoptosis protein (XIAP), may induce apoptosis in various types of cancer cells. The present study aimed to determine the effect of Embelin on the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis of osteosarcoma cells. Embelin and TRAIL were applied to U2OS and MG63 cells, respectively or in combination. MTT was initially used to detect the difference in survival rates between the group receiving combined application of 100 ng/ml TRAIL and 20 µmol/l Embelin and the individual application groups. Light microscopic quantification was used to detect the morphology of the osteosarcoma cells in each group. Determination of cell apoptosis was subsequently performed using flow cytometry. The invasive ability of the cells was detected by a Transwell assay, prior to relative protein expression being determined by western blot analysis. Based on all the test data, it was revealed that the survival rates and the invasive ability were significantly lower following the combined application of 100 ng/ml TRAIL and 20 µmol/l Embelin than following the individual application of either (P<0.01). Additionally, upregulating expression of caspases, as well as death receptor 5, and downregulating expression of XIAP and matrix metalloproteinase 9 (MMP-9), had more significant effects in the combined group compared with the individual group and the control group. All these results suggested that Embelin may enhance TRAIL-induced apoptosis and inhibit the invasion of human osteosarcoma cells.

Celastrol increases osteosarcoma cell lysis by γδ T cells through up-regulation of death receptors

γδ T cells has been shown to exhibit profound antitumor effects in a broad range of tumor entities, including OS. However, resistance to γδ T cells is a serious problem in the management of OS. This study investigates the impact of celastrol on the expression of death receptors 4/5 (DR4/5) on OS cell lines (HOS, U2OS) and cancer cell lysis by γδ T cells. The results showed that celastrol increased transcription of DR4/5 in HOS and U2OS, leading to increased cell surface, and total DR4/5 protein expression. Celastrol sensitizes OS cell lines or autologous OS cells to healthy donors-derived or OS patient-derived γδ T cell cytotoxicity in vitro. The induction of DR4/5 molecules increased lysis of HOS and U2OS by γδ T cells which was abolished by addition of a blocking TRAIL antibody. Importantly, the cytotoxic activity of γδ T cells was unaltered by small-dose celastrol. Taken together, our data show that celastrol up-regulated DR4/5 on OS cells to be responsible for intercellular TRAIL/APO-2L crosslink that confers increased cancer cell lysis by γδ T cells. These results suggest the clinical evaluation of celastrol in OS, especially in combination with immunotherapy approaches employing adoptive γδ T cell transfer.

MicroRNA-124 suppresses growth and aggressiveness of osteosarcoma and inhibits TGF-β-mediated AKT/GSK-3β/SNAIL-1 signaling

Osteosarcoma is one of the most common malignant tumors in adolescent populations and the prognosis remains incompletely understand. Previous reports have demonstrated that microRNA‑124 (miR‑124) has inhibitory effects on various human malignancies and is associated with tumor progression. However, the clinical significance and potential mechanisms of miR‑124 in the progression of osteosarcoma is not clearly understood. In this study, the potential molecular mechanism of miR‑124 in osteosarcoma tumorigenesis, growth and aggressiveness was investigated. The growth, proliferation, apoptosis, migration and invasion of osteosarcoma cells were investigated following miR‑124 transfection were determined by colony formation assay, western blotting, immunofluorescence, migration/invasion assays and reverse transcription‑quantitative polymerase chain reaction. In vivo anti‑cancer effects of miR‑124 were analyzed by a tumor growth assay, immunohistochemistry and survival rate observations. The results demonstrated that miR‑124 transfection significantly decreased integrin expression in osteosarcoma cells, and further inhibited growth, proliferation, migration and invasion of osteosarcoma cells. Flow cytometry assays indicated that miR‑124 transfection attenuated apoptosis resistance of osteosarcoma to tunicamycin, potentially via the downregulation of P53 and Bcl‑2 apoptosis regulator expression. Mechanistic assays demonstrated that miR‑124 transfection suppressed TGF‑β expression in osteosarcoma. An animal study revealed that tumor growth was reduced in tumor cells transfected with miR‑124 compared with control cells, and the survival rate was prolonged in mice with miR‑124 transfected xenografts compared with control tumors. In conclusion, these results indicate that miR‑124 transection inhibits the growth and aggressive of osteosarcoma, potentially via suppression of TGF‑β‑mediated AKT/GSK‑3β/snail family transcriptional repressor 1 (SNAIL‑1) signaling, suggesting miR‑124 may be a potential anti‑cancer agent/target for osteosarcoma therapy.

Target therapy of TRIM-14 inhibits osteosarcoma aggressiveness through the nuclear factor-κB signaling pathway

Osteosarcoma is the most common cause of cancer-associated mortality and the prognosis is yet to be fully elucidated due to the paucity of effective therapeutic targets that significantly influence the quality of life and mean survival rates of patients with osteosarcoma. Studies have showed that tripartite motif-containing (TRIM)-14 is a member of the TRIM protein family that has a vital role in tumor progression and metastasis and promotes angiogenesis, invasion and apoptotic resistance of bone cancer. In this study, a chimeric antibody targeting TRIM-14 (Chanti-TRIM) was constructed and the molecular mechanism of target therapy for TRIM-14 was investigated in osteosarcoma cells and xenograft mice. The growth, migration and invasion properties of U-2OS cells were analyzed following incubation with 10–160 mg/ml Chanti-TRIM. Apoptosis of U-2OS cells was detected after Chanti-TRIM treatment. Matrix metalloproteinase (MMP)-9-mediated nuclear factor-κB (NF-κB) signal pathway was analyzed in U-2OS cells treated with Chanti-TRIM. The inhibitory efficacy of Chanti-TRIM was studied in U-2OS-bearing xenograft mice. Our results demonstrated that neutralizing TRIM-14 expression markedly inhibited the growth, migration and invasion of osteosarcoma cells, in vitro and in vivo. We found that TRIM-14 depletion decreased cell viability and induced cells apoptosis in vitro. In addition, we identified Chanti-TRIM inhibited growth and promoted apoptosis induced by cisplatin through MMP-9-mediated NF-κB signal pathway. Furthermore, we observed that Chanti-TRIM treatment inhibited osteosarcoma growth in vivo. Histological analysis indicated that apoptotic bodies were increased and NF-κB nuclear translocation factors, including Ikkβ, p65 and IkBα, were decreased in tumors treated by Chanti-TRIM. In conclusion, these results showed that Chanti-TRIM markedly inhibited the progression of osteosarcoma, suggesting Chanti-TRIM may be a potential anti-cancer agent that functions via the activation of the NF-κB pathway for osteosarcoma.

Bmi‑1‑targeting suppresses osteosarcoma aggressiveness through the NF‑κB signaling pathway

Bone cancer is one of the most lethal malignancies and the specific causes of tumor initiation are not well understood. B‑cell‑specific Moloney murine leukemia virus integration site 1 protein (Bmi‑1) has been reported to be associated with the initiation and progression of osteosarcoma, and as a prognostic indicator in the clinic. In the current study, a full‑length antibody targeting Bmi‑1 (AbBmi‑1) was produced and the preclinical value of Bmi‑1‑targeted therapy was evaluated in bone carcinoma cells and tumor xenograft mice. The results indicated that the Bmi‑1 expression level was markedly upregulated in bone cancer cell lines, and inhibition of Bmi‑1 by AbBmi‑1 reduced the invasiveness and migration of osteosarcoma cells. Overexpression of Bmi‑1 promoted proliferation and angiogenesis, and increased apoptosis resistance induced by cisplatin via the nuclear factor‑κB (NF‑κB) signal pathway. In addition, AbBmi‑1 treatment inhibited the tumorigenicity of osteosarcoma cells in vivo. Furthermore, AbBmi‑1 blocked NF‑κB signaling and reduced MMP‑9 expression. Furthermore, Bmi‑1 promoted osteosarcoma tumor growth, whereas AbBmi‑1 significantly inhibited osteosarcoma tumor growth in vitro and in vivo. Notably, AbBmi‑1 decreased the percentages of Ki67‑positive cells and terminal deoxynucleotidyl transferase dUTP nick end labeling‑positive cells in tumors compared with Bmi‑1‑treated and PBS controls. Notably, MMP‑9 and NF‑κB expression were downregulated by treatment with AbBmi‑1 in MG‑63 osteosarcoma cells. In conclusion, the data provides evidence that AbBmi‑1 inhibited the progression of osteosarcoma, suggesting that AbBmi‑1 may be a novel anti‑cancer agent through the inhibition of Bmi‑1 via activating the NF‑κB pathway in osteosarcoma.

TNF-related apoptosis-inducing ligand (TRAIL) for bone sarcoma treatment: Pre-clinical and clinical data

Bone sarcomas are rare, highly malignant mesenchymal tumours that affect teenagers and young adults, as well as older patients. Despite intensive, multimodal therapy, patients with bone sarcomas have poor 5-year survival, close to 50%, with lack of improvement over recent decades. TNF-related apoptosis-inducing ligand (TRAIL), a member of the tumour necrosis factor (TNF) ligand superfamily (TNFLSF), has been found to induce apoptosis in cancer cells while sparing nontransformed cells, and may therefore offer a promising new approach to treatment. We cover the existing preclinical and clinical evidence about the use of TRAIL and other death receptor agonists in bone sarcoma treatment. In vitro studies indicate that TRAIL and other death receptor agonists are generally potent against bone sarcoma cell lines. Ewing's sarcoma cell lines present the highest sensitivity, whereas osteosarcoma and chondrosarcoma cell lines are considered less sensitive. In vivo studies also demonstrate satisfactory results, especially in Ewing's sarcoma xenograft models. However, the few clinical trials in the literature show only low or moderate efficacy of TRAIL in treating bone sarcoma. Potential strategies to overcome the in vivo resistance reported include co-administration with other drugs and the potential to deliver TRAIL on the surface of primed mesenchymal or immune cells and the use of targeted single chain antibodies such as scFv-scTRAIL.

Bispecific antibody suppresses osteosarcoma aggressiveness through regulation of NF-κB signaling pathway

Osteosarcoma is one of the most lethal malignancies, and the prognosis remains dismal due to the paucity of effective therapeutic targets. Bmi-1 and TRIM-14 are associated with the initiation and progression of osteosarcoma, which could promote angiogenesis, invasion, and apoptotic resistance in bone cancer tissue. In this study, we constructed a bispecific antibody of BsAbBmi/TRIM targeting Bmi-1 and TRIM-14 and investigated the therapeutic value in bone carcinoma cells and xenograft mice. Our results showed that Bmi-1 and TRIM-14 expression levels were markedly upregulated correlated with nuclear factor-κB nuclear translocation in bone cancer cells and clinical carcinoma tissues. Results have demonstrated that overexpression of Bmi-1 and TRIM-14 promoted growth, proliferation, aggressiveness, and apoptosis resistance of osteosarcoma cells. BsAbBmi/TRIM administration significantly inhibited nuclear factor-κB expression derived by matrix metalloproteinase-9 promoter. BsAbBmi/TRIM administration inhibited growth of osteosarcoma cells and downregulated Bmi-1 and TRIM-14 expression levels. Data also demonstrated that migration and invasion of osteosarcoma cells were also inhibited by BsAbBmi/TRIM. In addition, results illustrated that BsAbBmi/TRIM inhibited tumor growth and tumorigenicity by blockaded sensor expression in nuclear factor-κB signal pathway. Furthermore, in vivo study showed that BsAbBmi/TRIM treatment markedly inhibited the tumorigenicity and growth of osteosarcoma cells compared to either AbBmi-1 or AbTRIM-14 treatment. Notably, survival of xenograft mice was prolonged by BsAbBmi/TRIM treatment compared to either AbBmi-1 or AbTRIM-14 treatment. In conclusion, these results provided new evidence that BsAbBmi/TRIM inhibited the progression of osteosarcoma, which suggest that BsAbBmi/TRIM may be a novel anti-cancer agent for osteosarcoma therapy.

Triptolide induces the cell apoptosis of osteosarcoma cells through the TRAIL pathway

Research on triptolide, a diterpenoid epoxide found in the Thunder God Vine Tripterygium wilfordii, has increased our knowledge of the pharmacology, pharmacokinetics, toxicology and clinical application of this agent. In the present study, we aimed to identify the effects of triptolide on the apoptosis of osteosarcoma cells and to evaluate the anti-proliferative action of this agent. MG-63 cells were treated either with various doses of triptolide (50, 100 or 200 nM) or DMSO for 6, 12 and 24 h. Treatment with triptolide effectively suppressed the cell viability and induced the apoptosis of osteosarcoma MG-63 cells as detected by MTT assay and flow cytometry, respectively. In addition, by using caspase-3, caspase-8 and caspase-9 activity assays and western blot analysis, the anticancer effects of triptolide against osteosarcoma growth were found to involve activation of the DR-5/p53/Bax/caspase-9/ caspase-3 signaling pathway and the DR-5/FADD/caspase-8/lysosomal/cathepsin B/caspase-3 signaling pathway in the MG-63 cells. An important factor in the anticancer effects of triptolide against osteosarcoma was TRAIL-DR-5. The data suggest that triptolide may be a potential novel chemotherapeutic agent for osteosarcoma and acts through the TRAIL-DR-5 signaling pathway.

Osteoblasts display different responsiveness to TRAIL-induced apoptosis during their differentiation process

Apoptosis can occur throughout the life span of osteoblasts (OBs), beginning from the early stages of differentiation and continuing throughout all stages of their working life. Here, we investigated the effects of tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) on normal human OBs showing for the first time that the expression of TRAIL receptors is modulated during OB differentiation. In particular, the TRAIL receptor ratio was in favor of the deaths because of the low expression of DcR2 in undifferentiated OBs, differently it was shifted toward the decoys in differentiated ones. Undifferentiated OBs treated with TRAIL showed reduced cell viability, whereas differentiated OBs displayed TRAIL resistance. The OB sensitiveness to TRAIL was due to the up-regulation of DR5 and the down-regulation of DcR2. The main death receptor involved in TRAIL-reduced OB viability was DR5 as demonstrated by the rescue of cell viability observed in the presence of anti-DR5 neutralizing antibody. Besides the ratio of TRAIL receptors, the sensitivity of undifferentiated OBs to TRAIL-cytotoxic effect was also associated with low mRNA levels of intracellular anti-apoptotic proteins, such as cFLIP, the activation of caspase-8 and -3, as well as the DNA fragmentation. This study suggests that apoptotic effect exerted by TRAIL/TRAIL-receptor system on normal human OB is strictly dependent upon cell differentiation status.

Oxaliplatin sensitizes OS cells to TRAIL-induced apoptosis via down-regulation of Mcl1

PURPOSE

To investigate the killing effect on OS cells of a combination of oxaliplatin and TRAIL and related molecular mechanisms.

METHODS

TRAIL and oxaliplatin were applied to OS732 cells singly or jointly and survival inhibition rates were measured by MTT assay, changes of cellular shape being assessed with inverted phase contrast and fluorescence microscopy. Apoptotic rates were analyzed by flow cytometry (FCM) and immunocytochemistry was used to examine Mcl1 expression of OS732 cells.

RESULTS

The survival inhibition rate of combined application of 100 μg/ml TRAIL and 1 μg/ml oxaliplatin on OS-732 cells was significantly higher than that of either agent singly (p<0.01). Changes of cellular shape and apoptotic rates also indicated apoptosis-inducing effects of combined application to be much stronger than those of individual application. Oxaliplatin had the effect of down-regulating Mcl1 expression and sensitizing OS cells to TRAIL-induced apoptosis.

CONCLUSION

A combination of TRAIL and oxaliplatin exerts strong killing effects on OS-732 cells which might be related to down-regulation of Mcl1 expression.

TRAIL receptor signaling and therapeutic option in bone tumors: the trap of the bone microenvironment

Tumor Necrosis Factor-Related Apoptosis Inducing Ligand (TRAIL/TNFSF10) has been reported to specifically induce malignant cell death being relatively nontoxic to normal cells. Since its identification 15 years ago, the antitumor activity and therapeutic value of TRAIL have been extensively studied. Five receptors quickly emerged, two of them being able to induce programmed cell death in tumor cells. This review takes a comprehensive look at this ligand and its receptors, and its potential role in primary bone tumors (osteosarcoma and Ewing's sarcoma) therapy. The main limit of clinical use of TRAIL being the innate or acquired resistance mechanisms, different possibilities to sensitize resistant cells are discussed in this review, together with the impact of bone microenvironment in the regulation of TRAIL activity.

Bisphosphonate enhances TRAIL sensitivity to human osteosarcoma cells via death receptor 5 upregulation

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), a member of the TNF superfamily of cytokines, is one of the most promising candidates for cancer therapeutics. However, many osteosarcomas are resistant to TRAIL. Bisphosphonates are very effective in the treatment of bone problems associated with malignancies; the antitumor effects are due to the inhibition of protein prenylation that is essential for cell function and survival. The purpose of this study was to determine the effects of bisphosphonates on TRAIL-resistant MG 63 human osteosarcoma cells. The cells showed no response to TRAIL alone; however, pre-treatment with bisphosphonates significantly increased TRAIL-mediated apoptosis and cellular activation of caspase-3. Bisphosphonates significantly induced mRNA and protein expression of the TRAIL receptor, DR5. Bisphosphonates induced protein unprenylation in MG 63 cells; in addition, co-treatment with TRAIL also significantly increased protein unprenylation. Blocking of protein unprenylation using geranylgeraniol attenuated the cellular responses, including cell apoptosis and protein unprenylation induced by bisphosphonates and TRAIL. This is the first study to demonstrate that bisphosphonates markedly enhanced TRAIL-induced apoptosis in human osteosarcoma cells. These findings suggest that bisphosphonates may be a new and effective anticancer treatment with TRAIL proteins for TRAIL-resistant cancer cells.

TIMP-1 interaction with αvβ3 integrin confers resistance to human osteosarcoma cell line MG-63 against TNF-α-induced apoptosis

Tumor necrosis factor-α (TNF-α) is a pleiotropic cytokine affecting diverse cellular responses. TNF-α is cytotoxic in many systems, but it can also act as an anti-apoptotic signal to promote cell survival pathways activated through integrins and extracellular matrix components. This is particularly evident in cancer cells. To unravel the basis of resistance to TNF-α-induced apoptosis, human osteosarcoma MG-63 cell line was used. Our data showed that resistance to apoptosis was accompanied by high levels of TIMP-1 expression in part mediated by NF-κB activation, whereas under apoptotic conditions, in the presence of cycloheximide (CHX), TIMP-1 and αvβ3 integrin protein levels were significantly reduced. Silencing TIMP-1 using siRNA led to increased apoptosis following treatment with TNF-α, whereas exogenously-added recombinant TIMP-1 reduced the extent of apoptosis. Immunoprecipitation and confocal microscopy experiments demonstrated that TIMP-1 interacted with αvβ3 integrins. The biological role of this interaction was revealed by the use of echistatin, an antagonist of αvβ3 integrin. In the presence of echistatin, decreased protection against apoptosis by recombinant TIMP-1 was observed.

Tumor Necrosis Factor-related apoptosis-inducing ligand (TRAIL) Receptor-1 and Receptor-2 agonists for cancer therapy

IMPORTANCE OF THE FIELD

Agents that activate the TNF-related apoptosis-inducing ligand death receptors, TRAIL-R1 and TRAIL-R2, have attracted substantial attention and investment as potential anti-cancer therapies. Preclinical studies of TRAIL-R agonists indicate that they may be efficacious in a wide range of tumor types, especially when combined with chemotherapeutic agents.

AREAS COVERED IN THIS REVIEW

The rationale for clinical development of TRAIL-R agonists is described, including the basis for combining these agents with other agents that modulate the 'checks and balances' of the apoptotic pathways. Accruing data that highlight differences between TRAIL-R1 and TRAIL-R2 that could affect the clinical significance of their specific agonists are described. The clinical experience to date with each of the agonists is summarized.

WHAT THE READER WILL GAIN

The reader will gain an understanding of the rationale for the clinical development of TRAIL-R agonists, as well as the current status of clinical trials of these interesting new agents.

TAKE HOME MESSAGE

Ongoing clinical trials will provide important information regarding the future development of TRAIL-R agonists.

Targets for cancer therapy in childhood sarcomas

Development of chemotherapeutic treatment modalities resulted in a dramatic increase in the survival of children with many types of cancer. Still, in case of some pediatric cancer entities including rhabdomyosarcoma, osteosarcoma and Ewing's sarcoma, survival of patients remains dismal and novel treatment approaches are urgently needed. Therefore, based on the concept of targeted therapy, numerous potential targets for the treatment of these cancers have been evaluated pre-clinically or in some cases even clinically during the last decade. This review gives an overview over many different potential therapeutic targets for treatment of these childhood sarcomas, including receptor tyrosine kinases, intracellular signaling molecules, cell cycle and apoptosis regulators, proteasome, hsp90, histone deacetylases, angiogenesis regulators and sarcoma specific fusion proteins. The large number of potential therapeutic targets suggests that improved comparability of pre-clinical models might be necessary to prioritize the most effective ones for future clinical trials.

Sanguinarine induces apoptosis in A549 human lung cancer cells primarily via cellular glutathione depletion

Sanguinarine is a plant-derived benzophenanthridine alkaloid and has been shown to possess anti-tumor activities against various cancer cells. In this study, we investigated whether sanguinarine induces apoptosis in A549 human lung cancer cells. Treatment of A549 cells with sanguinarine induced apoptosis in a dose- and time-dependent manner. Treatment with sanguinarine led to activation of caspases and MAPKs as well as increased MKP-1 expression. Importantly, pretreatment with z-VAD-fmk, a pan caspase inhibitor suppressed the sanguinarine-induced apoptosis in A549 cells. Moreover, pretreatment with NAC, a sulfhydryl group-containing reducing agent strongly suppressed the apoptotic response and caspase activation to sanguinarine. However, the sanguinarine-mediated cytotoxicity in A549 cells was not protected by pharmacological inhibition of MAPKs or MKP-1 siRNA-mediated knockdown of MKP-1. These results collectively suggest that sanguinarine induces apoptosis in A549 cells through cellular glutathione depletion and the subsequent caspase activation.