Minnelide Inhibits Androgen Dependent, Castration Resistant Prostate Cancer Growth by Decreasing Expression of Androgen Receptor Full Length and Splice Variants.

BACKGROUND: With almost 30,000 deaths per year, prostate cancer is the second-leading cause of cancer-related death in men. Androgen Deprivation Therapy (ADT) has been the corner stone of prostate cancer treatment for decades. However, despite an initial response of prostate cancer to ADT, this eventually fails and the tumors recur, resulting in Castration Resistant Prostate Cancer (CRPC). Triptolide, a diterpene triepoxide, has been tested for its anti-tumor properties in a number of cancers for over a decade. Owing to its poor solubility in aqueous medium, its clinical application had been limited. To circumvent this problem, we have synthesized a water-soluble pro-drug of triptolide, Minnelide, that is currently being evaluated in a Phase 1 clinical trial against gastrointestinal tumors. In the current study, we assessed the therapeutic potential of Minnelide and its active compound triptolide against androgen dependent prostate cancer both in vitro as well as in vivo. METHODS: Cell viability was measured by a MTT based assay after treating prostate cancer cells with multiple doses of triptolide. Apoptotic cell death was measured using a caspase 3/7 activity. Androgen Receptor (AR) promoter-binding activity was evaluated by using luciferase reporter assay. For evaluating the effect in vivo, 22Rv1 cells were implanted subcutaneously in animals, following which, treatment was started with 0.21 mg/kg Minnelide. RESULTS: Our study showed that treatment with triptolide induced apoptotic cell death in CRPC cells. Triptolide treatment inhibited AR transcriptional activity and decreased the expression of AR and its splice variants both at the mRNA and the protein level. Our studies show that triptolide inhibits nuclear translocation of Sp1, resulting in its decreased transcriptional activity leading to downregulation of AR and its splice variants in prostate cancer cells. In vivo, Minnelide (0.21 mg/kg) regressed subcutaneous tumors derived from CRPC 22RV1 at our study endpoint. Our animal studies further confirmed that Minnelide was more efficacious than the standard of care therapies, Docetaxel and Enzalutamide. CONCLUSION: Our study indicates that Minnelide is very effective as a therapeutic option against CRPC at a dose that is currently tolerated by patients in the ongoing clinical trials. Prostate 77: 584-596, 2017. © 2017 Wiley Periodicals, Inc.

Renal Papillary Necrosis Caused by Protein C Deficiency Leading to Recurrent Hydronephrosis.

A patient with history of a solitary functioning kidney and protein C deficiency (PCD) presented with recurrent severe hydronephrosis causing acute kidney injury upon chronic kidney disease. Work-up with endoscopic evaluation revealed renal papillary necrosis (RPN) and sloughed renal papillae to be the true cause of the recurrent obstruction. Pathologic evaluation of the sloughed tissue confirmed the diagnosis of RPN. This is the first case reported in the literature illustrating the unique presentation of RPN in the setting of PCD.

A Novel Immunocompetent Mouse Model of Pancreatic Cancer with Robust Stroma: a Valuable Tool for Preclinical Evaluation of New Therapies.

A valid preclinical tumor model should recapitulate the tumor microenvironment. Immune and stromal components are absent in immunodeficient models of pancreatic cancer. While these components are present in genetically engineered models such as Kras(G12D); Trp53(R172H); Pdx-1Cre (KPC), immense variability in development of invasive disease makes them unsuitable for evaluation of novel therapies. We have generated a novel mouse model of pancreatic cancer by implanting tumor fragments from KPC mice into the pancreas of wild type mice. Three-millimeter tumor pieces from KPC mice were implanted into the pancreas of C57BL/6J mice. Four to eight weeks later, tumors were harvested, and stromal and immune components were evaluated. The efficacy of Minnelide, a novel compound which has been shown to be effective against pancreatic cancer in a number of preclinical murine models, was evaluated. In our model, consistent tumor growth and metastases were observed. Tumors demonstrated intense desmoplasia and leukocytic infiltration which was comparable to that in the genetically engineered KPC model and significantly more than that observed in KPC tumor-derived cell line implantation model. Minnelide treatment resulted in a significant decrease in the tumor weight and volume. This novel model demonstrates a consistent growth rate and tumor-associated mortality and recapitulates the tumor microenvironment. This convenient model is a valuable tool to evaluate novel therapies.

Triptolide abrogates growth of colon cancer and induces cell cycle arrest by inhibiting transcriptional activation of E2F.

Despite significant progress in diagnostics and therapeutics, over 50 thousand patients die from colorectal cancer annually. Hence, there is urgent need for new lines of treatment. Triptolide, a natural compound isolated from the Chinese herb Tripterygium wilfordii, is effective against multiple cancers. We have synthesized a water soluble analog of triptolide, named Minnelide, which is currently in phase I trial against pancreatic cancer. The aims of the current study were to evaluate whether triptolide/Minnelide is effective against colorectal cancer and to elucidate the mechanism by which triptolide induces cell death in colorectal cancer. Efficacy of Minnelide was evaluated in subcutaneous xenograft and liver metastasis model of colorectal cancer. For mechanistic studies, colon cancer cell lines HCT116 and HT29 were treated with triptolide and the effect on viability, caspase activation, annexin positivity, lactate dehydrogenase release, and cell cycle progression was evaluated. Effect of triptolide on E2F transcriptional activity, mRNA levels of E2F-dependent genes, E2F1- retinoblastoma protein (Rb) binding, and proteins levels of regulator of G1-S transition was also measured. DNA binding of E2F1 was evaluated by chromatin immunoprecipitation assay. Triptolide decreased colon cancer cell viability in a dose- and time-dependent fashion. Minnelide markedly inhibited the growth of colon cancer in the xenograft and liver metastasis model of colon cancer and more than doubles the median survival of animals with liver metastases from colon cancer. Mechanistically, we demonstrate that at low concentrations triptolide induces apoptotic cell death but at higher concentrations it induces cell cycle arrest. Our data suggest that triptolide is able to induce G1 cell cycle arrest by inhibiting transcriptional activation of E2F1. Our data also show that triptolide downregulates E2F activity by potentially modulating events downstream of DNA binding. Therefore, we conclude that Triptolide and Minnelide are effective against colon cancer in multiple pre-clinical models.

Primary and liver metastasis-derived cell lines from KrasG12D; Trp53R172H; Pdx-1 Cre animals undergo apoptosis in response to triptolide.

OBJECTIVES: Pancreatic cancer has a 5-year survival rate of less than 5%, partly because of limited chemotherapeutic options, thereby highlighting the need for novel therapies. Triptolide, a diterpene triepoxide that was derived from a Chinese herb, has shown great promise in preclinical testing against pancreatic cancer using immunocompromised animals. RESULTS: In this study, we tested the ability of triptolide to induce cell death in cell lines derived from a primary tumor and adjacent liver metastases of immunocompetent animals (Kras, Trp53, Pdx-1 Cre [KPC]). Both cell lines were more aggressive in their ability to form tumors when compared with other pancreatic cancer cell lines and showed constitutive activation of the nuclear factor κ-light-chain-enhancer of activated B cells pathway. Triptolide induced apoptotic cell death in both cell lines, as evidenced by decreased cell viability as well as increased caspase 3/7 activity, annexin V positivity, and increased terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling positivity in tumors from KPC animals treated with Minnelide. In addition, triptolide decreased levels of HSP70, its transcription factor HSF1, as well as the antiapoptotic proteins Bcl-xL, Bcl-2, and Mcl-1, which are known to be up-regulated in pancreatic cancer. CONCLUSIONS: The ability of triptolide to cause cell death in cell lines derived from immunocompetent animals further validates its potential as a novel agent against pancreatic cancer.

CD133 initiates tumors, induces epithelial-mesenchymal transition and increases metastasis in pancreatic cancer.

CD133 has been implicated as a cancer stem cell (CSC) surface marker in several malignancies including pancreatic cancer. However, the functional role of this surface glycoprotein in the cancer stem cell remains elusive. In this study, we determined that CD133 overexpression induced "stemness" properties in MIA-PaCa2 cells along with increased tumorigenicity, tumor progression, and metastasis in vivo. Additionally, CD133 expression induced epithelial-mesenchymal transition (EMT) and increased in vitro invasion. EMT induction and increased invasiveness were mediated by NF-κB activation, as inhibition of NF-κB mitigated these effects. This study showed that CD133 expression contributes to pancreatic cancer "stemness," tumorigenicity, EMT induction, invasion, and metastasis.

Sorafenib and triptolide as combination therapy for hepatocellular carcinoma.

INTRODUCTION: Sorafenib is the only drug approved by the Food and Drug Administration for metastatic hepatocellular carcinoma (HCC). Triptolide, a diterpene triepoxide, exhibits antineoplastic properties in multiple tumor cell types. In this study, we examined the effects of these agents and their combination on HCC in vitro and in vivo models. METHODS: HuH-7 and PLC/PRF/5 cells were treated with triptolide (50 nM), sorafenib (1.25 or 2.5 µM), or a combination of both. Cell viability assay (CCK-8), caspase 3&7 activation, and nuclear factor κB assays were performed. For in vivo studies, 40 mice were implanted with subcutaneous HuH7 tumors and divided into four treatment groups (n = 10); saline control, sorafenib 10 mg/kg PO daily (S), Minnelide (a prodrug of triptolide) 0.21 mg/kg intraperitoneally7 daily (M), and combination of both (C). Tumor volumes were assessed weekly. RESULTS: The combination of triptolide and sorafenib was superior to either drug alone in inducing apoptosis and decreasing viability, whereas triptolide alone was sufficient to decrease nuclear factor κB activity. After 2 weeks of treatment, tumor growth inhibition rates were S = 59%, M = 84%, and C = 93%, whereas tumor volumes in control animals increased by 9-fold. When crossed over to combination treatment, control mice tumor growth volumes plateaued over the following 4 weeks. CONCLUSION: The combination of sorafenib and triptolide is superior to single drug treatment in increasing cell death and apoptosis in vitro. Combining sorafenib with Minnelide inhibited tumor growth with greater efficacy than single-agent treatments. Importantly, in vivo combination treatment allowed for using a lesser dose of sorafenib (10 mg/kg), which is less than 10% of currently prescribed dose for HCC patients. Therefore, combination treatment could have translational potential in the management of HCC.

Triptolide activates unfolded protein response leading to chronic ER stress in pancreatic cancer cells.

Pancreatic cancer is a devastating disease with a survival rate of <5%. Moreover, pancreatic cancer aggressiveness is closely related to high levels of prosurvival mediators, which can ultimately lead to rapid disease progression. One of the mechanisms that enables tumor cells to evade cellular stress and promote unhindered proliferation is the endoplasmic reticulum (ER) stress response. Disturbances in the normal functions of the ER lead to an evolutionarily conserved cell stress response, the unfolded protein response (UPR). The UPR initially compensates for damage, but it eventually triggers cell death if ER dysfunction is severe or prolonged. Triptolide, a diterpene triepoxide, has been shown to be an effective compound against pancreatic cancer. Our results show that triptolide induces the UPR by activating the PKR-like ER kinase-eukaryotic initiation factor 2α axis and the inositol-requiring enzyme 1α-X-box-binding protein 1 axis of the UPR and leads to chronic ER stress in pancreatic cancer. Our results further show that glucose-regulated protein 78 (GRP78), one of the major regulators of ER stress, is downregulated by triptolide, leading to cell death by apoptosis in MIA PaCa-2 cells and autophagy in S2-VP10 cells.

CD133+ tumor initiating cells in a syngenic murine model of pancreatic cancer respond to Minnelide.

PURPOSE: Pancreatic adenocarcinoma is the fourth leading cause for cancer-related mortality with a survival rate of less than 5%. Late diagnosis and lack of effective chemotherapeutic regimen contribute to these grim survival statistics. Relapse of any tumor is largely attributed to the presence of tumor-initiating cells (TIC) or cancer stem cells (CSC). These cells are considered as hurdles to cancer therapy as no known chemotherapeutic compound is reported to target them. Thus, there is an urgent need to develop a TIC-targeted therapy for pancreatic cancer. EXPERIMENTAL DESIGN: We isolated CD133(+) cells from a spontaneous pancreatic ductal adenocarcinoma mouse model and studied both surface expression, molecular markers of pancreatic TICs. We also studied tumor initiation properties by implanting low numbers of CD133(+) cells in immune competent mice. Effect of Minnelide, a drug currently under phase I clinical trial, was studied on the tumors derived from the CD133(+) cells. RESULTS: Our study showed for the first time that CD133(+) population demonstrated all the molecular markers for pancreatic TIC. These cells initiated tumors in immunocompetent mouse models and showed increased expression of prosurvival and proinvasive proteins compared to the CD133(-) non-TIC population. Our study further showed that Minnelide was very efficient in downregulating both CD133(-) and CD133(+) population in the tumors, resulting in a 60% decrease in tumor volume compared with the untreated ones. CONCLUSION: As Minnelide is currently under phase I clinical trial, its evaluation in reducing tumor burden by decreasing TIC as well as non-TIC population suggests its potential as an effective therapy.

Triptolide sensitizes pancreatic cancer cells to TRAIL-induced activation of the death receptor pathway.

The tumor necrosis factor related apoptosis-inducing ligand (TRAIL) causes cancer cell death, but many cancers, including pancreatic cancer, are resistant to TRAIL therapy. A combination of TRAIL and the diterpene triepoxide, triptolide, is effective in inducing pancreatic cancer cell death. Triptolide increases levels of death receptor DR5 and decreases the pro-survival FLICE-like inhibitory protein (c-FLIP), which contribute to the activation of caspase-8. This combination further causes both lysosomal and mitochondrial membrane permeabilization, resulting in cell death. Our study provides a mechanism by which triptolide sensitizes TRAIL resistant cells, which may become a novel therapeutic strategy against pancreatic cancer.

Triptolide-induced cell death in pancreatic cancer is mediated by O-GlcNAc modification of transcription factor Sp1.

Pancreatic cancer, the fourth most prevalent cancer-related cause of death in the United States, is a disease with a dismal survival rate of 5% 5 years after diagnosis. One of the survival proteins responsible for its extraordinary ability to evade cell death is HSP70. A naturally derived compound, triptolide, and its water-soluble prodrug, Minnelide, down-regulate the expression of this protein in pancreatic cancer cells, thereby causing cell death. However, the mechanism of action of triptolide has not been elucidated. Our study shows that triptolide-induced down-regulation of HSP70 expression is associated with a decrease in glycosylation of the transcription factor Sp1. We further show that triptolide inhibits glycosylation of Sp1, inhibiting the hexosamine biosynthesis pathway, particularly the enzyme O-GlcNAc transferase. Inhibition of O-GlcNAc transferase prevents nuclear localization of Sp1 and affects its DNA binding activity. This in turn down-regulates prosurvival pathways like NF-κB, leading to inhibition of HSF1 and HSP70 and eventually to cell death. In this study, we evaluated the mechanism by which triptolide affects glycosylation of Sp1, which in turn affects downstream pathways controlling survival of pancreatic cancer cells.

A preclinical evaluation of Minnelide as a therapeutic agent against pancreatic cancer.

Pancreatic cancer is one of the most lethal human malignancies with an all-stage 5-year survival frequency of <5%, which highlights the urgent need for more effective therapeutic strategies. We have previously shown that triptolide, a diterpenoid, is effective against pancreatic cancer cells in vitro as well as in vivo. However, triptolide is poorly soluble in water, limiting its clinical use. We therefore synthesized a water-soluble analog of triptolide, named Minnelide. The efficacy of Minnelide was tested both in vitro and in multiple independent yet complementary in vivo models of pancreatic cancer: an orthotopic model of pancreatic cancer using human pancreatic cancer cell lines in athymic nude mice, a xenograft model where human pancreatic tumors were transplanted into severe combined immunodeficient mice, and a spontaneous pancreatic cancer mouse model (KRas(G12D); Trp53(R172H); Pdx-1Cre). In these multiple complementary models of pancreatic cancer, Minnelide was highly effective in reducing pancreatic tumor growth and spread, and improving survival. Together, our results suggest that Minnelide shows promise as a potent chemotherapeutic agent against pancreatic cancer, and support the evaluation of Minnelide in clinical trials against this deadly disease.

Prosurvival role of heat shock factor 1 in the pathogenesis of pancreatobiliary tumors.

Several mechanisms have evolved to ensure the survival of cells under adverse conditions. The heat shock response is one such evolutionarily conserved survival mechanism. Heat shock factor-1 (HSF1) is a transcriptional regulator of the heat shock response. By the very nature of its prosurvival function, HSF1 may contribute to the pathogenesis of cancer. The current study investigates the role of HSF1 in the pathogenesis of pancreatobiliary tumors. HSF1 was downregulated in pancreatic cancer (MIA PaCa-2 and S2-013) and cholangiocarcinoma (KMBC and KMCH) cell lines by HSF1-specific small interfering RNA (siRNA). Nonsilencing siRNA was used as control. The effect of HSF1 downregulation on viability and apoptosis parameters, i.e., annexin V, terminal deoxynucleotidyl transferase dUTP-mediated nick end labeling (TUNEL), and caspase-3, was measured. To evaluate the cancer-specific effects of HSF1, the effect of HSF1 downregulation on normal human pancreatic ductal cells was also evaluated. HSF1 is abundantly expressed in human pancreatobiliary cancer cell lines, as well as in pancreatic cancer tissue, as demonstrated by Western blot and immunohistochemistry, respectively. Inhibition of HSF1 expression by the HSF1 siRNA sequences leads to time-dependent death in pancreatic and cholangiocarcinoma cell lines. Downregulation of HSF1 expression induces annexin V and TUNEL positivity and caspase-3 activation, suggesting activation of a caspase-dependent apoptotic pathway. Although caspase-3 inhibition protects against cell death induced by HSF1 expression, it does not completely prevent it, suggesting a role for caspase-independent cell death. HSF1 plays a prosurvival role in the pathogenesis of pancreatobiliary tumors. Modulation of HSF1 activity could therefore emerge as a novel therapeutic strategy for cancer treatment.

Role of Hsp-70 in triptolide-mediated cell death of neuroblastoma.

BACKGROUND: Our recent work demonstrated that treatment of neuroblastoma with triptolide causes apoptotic cell death in vitro and decreases tumor size in vivo. Triptolide therapy has been associated with reduced expression of Hsp-70, suggesting a mechanism of cell killing involving Hsp-70 inhibition. The principal objective of this study was to investigate the role of Hsp-70 in triptolide-mediated cell death in neuroblastoma. MATERIALS AND METHODS: Neuroblastoma cells were transfected with Hsp-70-specific siRNA. Viability, caspase activity, and phosphatidylserine externalization were subsequently measured. An orthotopic, syngeneic murine tumor model was developed, and randomized mice received daily injections of triptolide or vehicle. At 21 d, mice were sacrificed. Immunohistochemisty was used to characterize Hsp-70 levels in residual tumors, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) was performed to identify cells undergoing apoptosis. RESULTS: Targeted silencing of Hsp-70 with siRNA significantly decreased cellular viability, augmented caspase-3 activity, and resulted in increased annexin-V staining. These effects parallel those findings obtained following treatment with triptolide. Residual tumors from triptolide-treated mice showed minimal staining with Hsp-70 immunohistochemistry, while control tumors stained prominently. Tumors from treated mice demonstrated marked staining with the TUNEL assay, while control tumors showed no evidence of apoptosis. CONCLUSIONS: Use of siRNA to suppress Hsp-70 expression in neuroblastoma resulted in apoptotic cell death, similar to the effects of triptolide. Residual tumors from triptolide-treated mice expressed decreased levels of Hsp-70 and demonstrated significant apoptosis. These findings support the hypothesis that Hsp-70 inhibition plays a significant role in triptolide-mediated neuroblastoma cell death.

Triptolide induces cell death in pancreatic cancer cells by apoptotic and autophagic pathways.

BACKGROUND & AIMS: Pancreatic adenocarcinoma, among the most lethal human malignancies, is resistant to current chemotherapies. We previously showed that triptolide inhibits the growth of pancreatic cancer cells in vitro and prevents tumor growth in vivo. This study investigates the mechanism by which triptolide kills pancreatic cancer cells. METHODS: Cells were treated with triptolide and viability and caspase-3 activity were measured using colorimetric assays. Annexin V, propidium iodide, and acridine orange staining were measured by flow cytometry. Immunofluorescence was used to monitor the localization of cytochrome c and Light Chain 3 (LC3) proteins. Caspase-3, Atg5, and Beclin1 levels were down-regulated by exposing cells to their respective short interfering RNA. RESULTS: We show that triptolide induces apoptosis in MiaPaCa-2, Capan-1, and BxPC-3 cells and induces autophagy in S2-013, S2-VP10, and Hs766T cells. Triptolide-induced autophagy has a pro-death effect, requires autophagy-specific genes, atg5 or beclin1, and is associated with the inactivation of the Protein kinase B (Akt)/mammalian target of Rapamycin/p70S6K pathway and the up-regulation of the Extracellular Signal-Related Kinase (ERK)1/2 pathway. Inhibition of autophagy in S2-013 and S2-VP10 cells results in cell death via the apoptotic pathway whereas inhibition of both autophagy and apoptosis rescues cell death. CONCLUSIONS: This study shows that triptolide kills pancreatic cancer cells by 2 different pathways. It induces caspase-dependent apoptotic death in MiaPaCa-2, Capan-1, and BxPC-3, and induces caspase-independent autophagic death in metastatic cell lines S2-013, S2-VP10, and Hs766T, thereby making it an attractive chemotherapeutic agent against a broad spectrum of pancreatic cancers.

TRAIL and triptolide: an effective combination that induces apoptosis in pancreatic cancer cells.

INTRODUCTION: An emerging therapy in oncology is the induction of apoptotic cell death through anti-death receptor therapy. However, pancreatic cancer is resistant to apoptosis including anti-death receptor therapy. We have previously described how triptolide decreases resistance to apoptosis in pancreatic cancer cells in vitro and in vivo. We hypothesized that triptolide decreases tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) resistance in pancreatic cancer cells. The aim of this study was to evaluate the effects that combined therapy with TRAIL and triptolide have on different parameters of apoptosis. METHODS: Four different pancreatic cancer cell lines were exposed to triptolide, TRAIL, or a combination of both drugs. We assessed the effects that combined therapy with TRAIL and triptolide has on cell viability, apoptosis, caspase-3 and caspase-9 activities, and poly(ADP)-ribose polymerase cleavage. RESULTS: Pancreatic cancer cells were resistant to TRAIL therapy; however, combined therapy with triptolide and TRAIL significantly decreased the cell viability in all the cell lines and increased apoptotic cell death as a result of caspase-3 and caspase-9 activation. CONCLUSIONS: Pancreatic cancer is highly resistant to anti-death receptor therapy, but combined therapy with TRAIL and triptolide is an effective therapy that induces apoptotic cell death in pancreatic cancer cells.

Triptolide therapy for neuroblastoma decreases cell viability in vitro and inhibits tumor growth in vivo.

BACKGROUND: Heat shock protein (Hsp)-70 is overexpressed in several human malignancies, and its inhibition has been shown to kill cancer cells. Our objectives were to assess the effectiveness of triptolide, an Hsp-70 inhibitor, in treating neuroblastoma in vitro and in vivo, and to measure the associated effects on Hsp-70 levels and apoptosis markers. METHODS: After exposing N2a and SKNSH cell lines to triptolide, cell viability was assessed. Caspase-3 and -9 activities were measured and annexin staining performed to determine if cell death occurred via apoptosis. Hsp-70 protein and mRNA levels were determined using Western blot and real-time polymerase chain reaction. In an orthotopic tumor model, mice received daily triptolide injections and were humanely killed at study completion with tumor measurement. RESULTS: Triptolide treatment resulted in dose- and time-dependent N2a cell death and dose-dependent SKNSH killing. Triptolide exposure was associated with dose-dependent increases in caspase activity and annexin staining. Triptolide decreased Hsp-70 protein and mRNA levels in a dose-dependent fashion. Mice receiving triptolide therapy had significantly smaller tumors than controls. CONCLUSION: Triptolide therapy decreased neuroblastoma cell viability in vitro and inhibited tumor growth in vivo. Our studies suggest that triptolide killed cells via apoptosis and in association with inhibition of Hsp-70 expression. Triptolide may provide a novel therapy for neuroblastoma.

Heat shock protein 70 inhibits apoptosis in cancer cells through simultaneous and independent mechanisms.

BACKGROUND & AIMS: Heat shock proteins (HSPs) are highly conserved and serve a multitude of functions that mediate cell survival. HSP70, the only inducible form of the 70-kilodalton subfamily of HSPs, is overexpressed in pancreatic cancer cells and has been shown to inhibit caspase-dependent apoptosis. We aimed to elucidate the mechanism by which HSP70 inhibits apoptosis in cancer cells. METHODS: HSP70 expression was down-regulated in cultured pancreatic cancer cells by exposure to quercetin, triptolide, or short interfering RNAs. Intracellular Ca2+, cytosolic cathepsin B activity, caspase-3 activity, cell viability, and lysosome integrity were measured using colorimetric assays. Immunofluorescence assays were used to localize cathepsin B and Lamp2. BAPTA-AM was used to chelate intracellular Ca2+. RESULTS: Inhibition of HSP70 increased intracellular Ca2+ levels in pancreatic and colon cancer cell lines and led to loss of lysosome integrity in pancreatic cancer cells. The release of intracellular Ca2+ and lysosomal enzymes activated caspase-dependent apoptosis independently and simultaneously. CONCLUSIONS: HSP70 inhibits apoptosis in cancer cells by 2 mechanisms: attenuation of cytosolic calcium and stabilization of lysosomes. HSP70-mediated cell survival might occur in other types of cancer cells.