The autophagy inhibitor chloroquine overcomes the innate resistance of wild-type EGFR non-small-cell lung cancer cells to erlotinib.

INTRODUCTION: The epidermal growth factor receptor (EGFR) inhibitor erlotinib is much less effective in non-small-cell lung cancer (NSCLC) tumors with wild-type EGFR, than in tumors with activating EGFR mutations. Autophagy is a tightly regulated lysosomal self-digestion process, which may alternatively promote cell survival or type II cell death. This study assessed the role of autophagy in erlotinib-mediated cytotoxicity. METHODS: We used wild-type EGFR erlotinib-sensitive and erlotinib-resistant NSCLC cell lines to determine whether inhibiting autophagy by a therapeutic agent potentiated the antitumor activity of erlotinib in vitro and in vivo. RESULTS: Erlotinib at a clinically relevant concentration (2 µM) induced autophagy in NSCLC cells with wild-type EGFR, and the degree of induction was greater in cells that were resistant than sensitive, suggesting that autophagy is cytoprotective. This was confirmed by knockdown of the autophagy-related gene Atg-5, and by using the autophagy inhibitor chloroquine (CQ), both of which increased the cytotoxicity of erlotinib. The synergistic activity of CQ was not because of the potentiation of erlotinib's effects on autophagy, cell-cycle arrest, and inhibition of both EGFR or downstream signaling of EGFR. Rather, CQ markedly activated apoptosis in the cells. The ability of CQ to potentiate the antitumor activity of erlotinib was also seen in mice bearing NSCLC tumor xenografts. CONCLUSIONS: The ability to adapt to anti-EGFR therapy by triggering autophagy may be a key determinant for resistance to erlotinib in wild-type EGFR NSCLC. Inhibition of autophagy by CQ represents a novel strategy to broaden the spectrum of erlotinib efficacy in wild-type EGFR NSCLC tumors.

The phosphatase inhibitor menadione (vitamin K3) protects cells from EGFR inhibition by erlotinib and cetuximab.

PURPOSE: Skin toxicity is the main side effect of epidermal growth factor receptor (EGFR) inhibitors, often leading to dose reduction or discontinuation. We hypothesized that phosphatase inhibition in the skin keratinocytes may prevent receptor dephosphorylation caused by EGFR inhibitors and be used as a new potential strategy for the prevention or treatment of this side effect. EXPERIMENTAL DESIGN: Menadione (Vitamin K3) was used as the prototype compound to test our hypothesis. HaCat human skin keratinocyte cells and A431 human squamous carcinoma cells were used. EGFR inhibition was measured by Western blotting and immunofluorescence. Phosphatase inhibition and reactive oxygen species (ROS) generation were measured by standard ELISA and fluorescence assays. RESULTS: Menadione caused significant and reversible EGFR activation in a dose-dependent manner starting at nontoxic concentrations. EGFR activation by menadione was associated with reversible protein tyrosine phosphatase inhibition, which seemed to be mediated by ROS generation as exposure to antioxidants prevented both menadione-induced ROS generation and phosphatase inhibition. Short-term coincubation of cells with nontoxic concentrations of menadione and the EGFR inhibitors erlotinib or cetuximab prevented EGFR dephosphorylation. Seventy-two-hour coincubation of cells with the highest nontoxic concentration of menadione and erlotinib provided for a fourfold cell growth inhibitory protection in HaCat human keratinocyte cells. CONCLUSIONS: Menadione at nontoxic concentrations causes EGFR activation and prevents EGFR dephosphorylation by erlotinib and cetuximab. This effect seems to be mediated by ROS generation and secondary phosphatase inhibition. Mild oxidative stress in skin keratinocytes by topical menadione may protect the skin from the toxicity secondary to EGFR inhibitors without causing cytotoxicity.

PM02734 (elisidepsin) induces caspase-independent cell death associated with features of autophagy, inhibition of the Akt/mTOR signaling pathway, and activation of death-associated protein kinase.

PURPOSE: PM02734 (elisidepsin) is a synthetic marine-derived cyclic peptide of the kahalalide family currently in phase II clinical development. The mechanisms of cell death induced by PM02734 remain unknown. EXPERIMENTAL DESIGN: Human non-small-cell lung cancer (NSCLC) cell lines H322 and A549 were used to evaluate PM02734-induced cytotoxicity, apoptosis, and autophagy, as well as effects on cell death-related signaling pathways. RESULTS: PM02734 at clinically achievable concentrations (0.5-1 µmol/L) was cytotoxic to H322 and A549 cells but did not cause nuclear fragmentation, PARP cleavage, or caspase activation, suggesting that classical apoptosis is not its main mechanism of cell death. In contrast, PM02734-induced cell death was associated with several characteristics of autophagy, including an increase in acidic vesicular organelle content, levels of GFP-LC3-positive puncta, elevation of the levels of Atg-5/12 and LC3-II, and an associated compromise of the autophagic flux resulting in increased number of autophagosomes and/or autolysosomes. Cotreatment with 3-methyladenine (3-MA) and downregulation of Atg-5 gene expression by siRNA partially inhibited PM02734-induced cell death. PM02734 caused inhibition of Akt/mTOR signaling pathways and cotreatment with the Akt inhibitor wortmannin or with the mTOR inhibitor rapamycin led to a significant increase in PM02734-induced cell death. Furthermore, PM02734 caused the activation of death-associated protein kinase (DAPK) by dephosphorylation at Ser308, and downregulation of DAPK expression with siRNA caused also a partial but significant reduction of PM02734-induced cell death. In vivo, PM02734 significantly inhibited subcutaneous A549 tumor growth in nude mice (P < 0.05) in association with induction of autophagy. CONCLUSIONS: Our data indicate that PM02734 causes cell death by a complex mechanism that involves increased autophagosome content, due for the most part to impairment of autophagic flux, inhibition of the Akt/mTOR pathway, and activation of DAPK. This unique mechanism of action justifies the continued development of this agent for the treatment of NSCLC.

An A13 repeat within the 3'-untranslated region of epidermal growth factor receptor (EGFR) is frequently mutated in microsatellite instability colon cancers and is associated with increased EGFR expression.

Colorectal cancers (CRC) with microsatellite instability (MSI) have clinical, pathologic, genetic, and epigenetic features distinct from microsatellite-stable CRC. Examination of epidermal growth factor receptor (EGFR) mRNA and protein expression levels in a panel of colon cancer cell lines identified strong expression of EGFR in multiple cell lines with MSI. Although no relationship between EGFR overexpression and the length of a CA dinucleotide repeat in intron 1 was observed, a variant A13/A14 repeat sequence within the 3'-untranslated region (3'-UTR) of the EGFR gene was identified, which was mutated by either mononucleotide or dinucleotide adenosine deletions in 64% of MSI cell lines and 69% of MSI colon tumors. Using a Tet-Off system, we show that this mutation increases EGFR mRNA stability in colon cancer cells, providing a mechanistic basis for EGFR overexpression in MSI colon cancer cell lines. To determine whether this mutation is a driver or a bystander event in MSI colon cancer, we examined the effect of pharmacologic and molecular inhibition of EGFR in EGFR 3'-UTR mutant MSI cell lines. Cell lines with an EGFR 3'-UTR mutation and that were wild-type (WT) for downstream signaling mediators in the Ras/BRAF and PIK3CA/PTEN pathways were sensitive to EGFR inhibition, whereas those harboring mutations in these signaling mediators were not. Furthermore, in cell lines WT for downstream signaling mediators, those with EGFR 3'-UTR mutations were more sensitive to EGFR inhibition than EGFR 3'-UTR WT cells, suggesting that this mutation provides a growth advantage to this subset of MSI colon tumors.

Molecular pharmacodynamics of PM02734 (elisidepsin) as single agent and in combination with erlotinib; synergistic activity in human non-small cell lung cancer cell lines and xenograft models.

PM02734 (elisidepsin) is a novel marine-derived cyclic peptide belonging to the Kahalalide family of compounds currently under phase I development with early evidence of a positive therapeutic index. The cytotoxicity of PM02734 has been determined in a panel of human NSCLC (non-small cell lung cancer) cell lines. Western blot analysis showed a direct correlation between ErbB3 expression and cell sensitivity to PM02734. Furthermore, PM02734 was more effective in the induction of ErbB3 degradation and dephosphorylation than in that of ErbB2 and ErbB1 in human NSCLC cell lines. The combination of PM02734 and erlotinib was synergistic in all NSCLC cell lines tested, including erlotinib resistant cell lines, with combination indexes ranging between 0.59 and 0.81. The combination of PM02734 and erlotinib was more effective than either drug alone in mice inoculated intravenously (i.v.) with A549 cells. The combination of PM02734 and erlotinib was more effective in inhibiting AKT than either single agent alone in H322 cells. These results have provided a rational basis for an ongoing clinical trial to explore this combination in patients with advanced malignant solid tumours.

Activation of ER stress and inhibition of EGFR N-glycosylation by tunicamycin enhances susceptibility of human non-small cell lung cancer cells to erlotinib.

PURPOSE: The epidermal growth factor receptor (EGFR), an N-glycosylated transmembrane protein, is the target of erlotinib, an orally bioavailable agent approved for treatment of patients with non-small cell lung cancer (NSCLC). In this study, we examined whether inhibition of EGFR N-glycosylation and stimulation of endoplasmic reticulum (ER) stress by tunicamycin enhances erlotinib-induced growth inhibition in NSCLC cell lines. METHODS: We examined the effects of tunicamycin and erlotinib on cytotoxicity of erlotinib-sensitive and resistant NSCLC cell lines, as well its effects on apoptotic pathways and on EGFR activation and subcellular localization. RESULTS: A minimally cytotoxic concentration of tunicamycin (1 microM) resulted in approximatey 2.6-2.9 fold and approximatey 6.8-13.5 fold increase in erlotinib-induced antiproliferative effects in sensitive (H322 and H358) and resistant cell lines (A549 and H1650), respectively. We found that tunicamycin generated an aglycosylated form of 130 kDa EGFR. Tunicamycin additionally affected EGFR activation and subcellular localization. Interestingly, the combination of tunicamycin and erlotinib caused more inhibitory effect on EGFR phosphorylation than that of erlotinib alone. Moreover, the combination induced apoptosis in H1650 cells through induction of CHOP expression, activation of caspase-12 and caspase-3, cleavage of PARP and bak, and down-regulation of anti-apoptotic proteins bcl-xL and survivin. CONCLUSIONS: Overall, our data demonstrate that tunicamycin significantly enhances the susceptibility of lung cancer cells to erlotinib, particularly sensitizing resistant cell lines to erlotinib, and that such sensitization may be associated with activation of the ER stress pathway and with inhibition of EGFR N-glycosylation.

RETRACTED: Tumor dependence on the EGFR signaling pathway expressed by the p-EGFR:p-AKT ratio predicts erlotinib sensitivity in human non-small cell lung cancer (NSCLC) cells expressing wild-type EGFR gene.

The Journal of Thoracic Oncology was alerted on August 16, 2021 that Figures 1 and 3 in this article are suspected of image manipulation. The editorial team promptly consulted with the authors and publisher. The authors stand by the validity of the conclusions and the scientific merit of the manuscript but acknowledge that some Figures might not have been properly prepared. Therefore, the editorial team asserts that the conclusions could be unreliable.

Erlotinib induces mitochondrial-mediated apoptosis in human H3255 non-small-cell lung cancer cells with epidermal growth factor receptorL858R mutation through mitochondrial oxidative phosphorylation-dependent activation of BAX and BAK.

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor erlotinib shows potent antitumor activity in some non-small-cell lung cancer (NSCLC) cell lines and is approved by the Food and Drug Administration as second and third line treatment for NSCLC. However, the molecular mechanisms by which erlotinib induces apoptosis remain to be elucidated. Here, we investigated the effect of erlotinib on apoptotic signal pathways in H3255 cells with the EGFR(L858R) mutation. Erlotinib induces apoptosis associated with the activation of caspases in a dose- and time-dependent manner. Erlotinib did not alter the expression of apoptotic receptors FAS and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), although it induced caspase-8 activation and BID cleavage. In addition, cell death caused by erlotinib was not prevented by coincubation with FAS and TRAIL antagonists, ZB-4 monoclonal antibody and TRAIL/Fc recombinant, suggesting that erlotinib-induced apoptosis is not associated with receptor-mediated pathways. Erlotinib induces loss of mitochondrial membrane potential and release of cytochrome c and second mitochondria-derived activator of caspases/direct IAP binding protein with low pI from mitochondria. Furthermore, erlotinib causes BAX translocation to mitochondria, BAX and BAK conformational changes, and oligomerization. Erlotinib did not induce reactive oxygen species generation, and cotreatment with antioxidants did not alter erlotinib-induced activation of BAX and BAK and apoptosis. However, cotreatment with inhibitors of mitochondrial oxidative phosphorylation significantly blocked erlotinib-induced activation of BAX and BAK and cell death. Benzyloxycarbiny-VAD-fluoromethyl ketone had no effect on erlotinib-induced BAX and BAK activation but effectively prevented apoptosis. Overexpression of BCL-2 caused a significant attenuation of erlotinib-induced cell death, but no effect on BAX and BAK activation. Down-regulation of BAX and BAK gene expression with small interfering RNA led to an effective reduction of erlotinib-induced apoptosis. Our data indicate that activation of BAX and BAK plays a critical role in the initiation of erlotinib-induced apoptotic cascades.

PIK3CA mutation/PTEN expression status predicts response of colon cancer cells to the epidermal growth factor receptor inhibitor cetuximab.

Cetuximab is a monoclonal antibody that targets the human epidermal growth factor receptor (EGFR). Although approved for use in EGFR-overexpressing advanced colorectal cancer, recent studies have shown a lack of association between EGFR overexpression and cetuximab response, requiring the identification of novel biomarkers predictive of response to this agent. To do so, 22 colon cancer cell lines were screened for cetuximab response in vitro and sensitive and resistant lines were identified. In sensitive cell lines, cetuximab induced a G(0)-G(1) arrest without inducing apoptosis. Notably, cetuximab-sensitive but not cetuximab-resistant cell lines were preferentially responsive to EGF-stimulated growth. Whereas neither EGFR protein/mRNA expression nor gene copy number correlated with cetuximab response, examination of the mutation status of signaling components downstream of EGFR showed that cell lines with activating PIK3CA mutations or loss of PTEN expression (PTEN null) were more resistant to cetuximab than PIK3CA wild type (WT)/PTEN-expressing cell lines (14 +/- 5.0% versus 38.5 +/- 6.4% growth inhibition, mean +/- SE; P = 0.008). Consistently, PIK3CA mutant isogenic HCT116 cells showed increased resistance to cetuximab compared with PIK3CA WT controls. Furthermore, cell lines that were PIK3CA mutant/PTEN null and Ras/BRAF mutant were highly resistant to cetuximab compared with those without dual mutations/PTEN loss (10.8 +/- 4.3% versus 38.8 +/- 5.9% growth inhibition, respectively; P = 0.002), indicating that constitutive and simultaneous activation of the Ras and PIK3CA pathways confers maximal resistance to this agent. A priori screening of colon tumors for PTEN expression status and PIK3CA and Ras/BRAF mutation status could help stratify patients likely to benefit from this therapy.

Schedule-dependent interaction between the proteosome inhibitor bortezomib and the EGFR-TK inhibitor erlotinib in human non-small cell lung cancer cell lines.

INTRODUCTION: Both erlotinib (E) and bortezomib (B) have shown single-agent activity in patients with non-small cell lung cancer. We studied the combination of these two novel biologic agents in vitro using a panel of non-small cell lung cancer cell lines. METHODS: The growth inhibitory effect of E and B were determined by MTT assay on seven non-small cell lung cancer cell lines. The data obtained from the growth inhibition assay in response to the combination of E and B were subject to isobologram analysis. The effects of each individual drug as well as combination in different sequences on cell cycle and apoptosis were determined by flow cytometry. RESULTS: Two of seven cell lines are sensitive to E. However, B has narrower range of cytotoxicity. The combination is neither synergistic nor additive in two of four cell lines tested. In H358 bronchoalveolar cell lines, the combination is more active than either agent alone. E causes G1 cell cycle arrest and B causes G2/M cell cycle arrest. In sequential studies, 24-hour previous exposure to E causes G1 arrest and abrogates the cytotoxic effect of B. This is observed in both E-sensitive as well as E-resistant cells and is accompanied by an increase in cell survival and a decrease in apoptosis. CONCLUSIONS: The combination of E and B is neither additive nor synergistic in two of four cell lines tested. In H358 bronchoalveolar cell, the combination is more active than either agent alone. The schedule-dependent antagonistic effect of E pre-exposure was observed. E pre-exposure causes G1 cell cycle arrest and abrogates the activity of B. Further in vivo studies of this combination are warranted.

Schedule-dependent cytotoxic synergism of pemetrexed and erlotinib in human non-small cell lung cancer cells.

PURPOSE: This study was undertaken to select the optimal combination schedule of erlotinib and pemetrexed for the treatment of relapsed non-small cell lung cancer (NSCLC) using a panel of human NSCLC lines. EXPERIMENTAL DESIGN: Human NSCLC cell lines, with variable expression of the known molecular determinants of erlotinib sensitivity, were exposed to pemetrexed and erlotinib using different schedules. Antitumor effect was measured by growth inhibition by cell count and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, cell cycle distribution and apoptosis by flow cytometry, and expression of cell cycle mediators by immunoblots. The cytotoxic interaction between pemetrexed and erlotinib (i.e., synergistic, additive, or antagonistic) was determined by median effect analysis. RESULTS: When cells were exposed to concurrent pemetrexed and erlotinib or sequential pemetrexed followed by erlotinib, cytotoxic synergism was observed in both erlotinib-sensitive and erlotinib-resistant human NSCLC cell lines. This was independent of the mutation status of epidermal growth factor receptor or K-Ras genes. Synergism was associated with a combination of cell cycle effects from both agents. In contrast, exposure of cells to erlotinib followed by pemetrexed was mostly antagonistic in erlotinib-sensitive cells and additive at best in erlotinib-resistant cells. Antagonism was associated with erlotinib-induced G(1)-phase blockade of erlotinib-sensitive cells, which protects cells from pemetrexed cytotoxicity. Pemetrexed induced an epidermal growth factor receptor-mediated activation of the phosphatidylinositol 3-kinase/AKT pathway, which was inhibited by erlotinib and a specific phosphatidylinositol 3-kinase inhibitor, LY294002. CONCLUSIONS: The combination of pemetrexed and erlotinib is synergistic in NSCLC in vitro if exposure to erlotinib before pemetrexed is avoided, particularly in tumors that are sensitive to erlotinib. Based on these findings, a randomized phase II study comparing the progression-free survival between an intermittent combination of erlotinib and pemetrexed (experimental arm) and pemetrexed alone (control arm) in patients with relapsing NSCLC has been initiated.

Erlotinib, an effective epidermal growth factor receptor tyrosine kinase inhibitor, induces p27KIP1 up-regulation and nuclear translocation in association with cell growth inhibition and G1/S phase arrest in human non-small-cell lung cancer cell lines.

Erlotinib, a small-molecule epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor, has been shown to have potent antitumor effects against human non-small-cell lung cancer (NSCLC) cell growth; however, the mechanism of such an effect is not elucidated. Here, we demonstrate that erlotinib-induced cell growth inhibition in EGFR high-expressing human H322 NSCLC cells was accompanied by G1/S phase arrest, which was largely caused by a decrease in expression of G1/S-related cyclins, suppression of activities of cyclin-dependent kinase (CDK) 2 and CDK4, induction of CDK inhibitor p27(KIP1), and retinoblastoma hypophosphorylation. To further understand the role of p27(KIP1) in G1/S arrest and cell growth inhibition by erlotinib, we determined its effect on the expression of p27(KIP1) at transcriptional and posttranscriptional levels. Studies using real-time reverse transcription-polymerase chain reaction analysis and p27 promoter-driven luciferase reporter showed that erlotinib treatment resulted in the promotion of p27 gene transcription. In addition, erlotinib treatment led to an increase in p27(KIP1) half-life by inhibiting p27(KIP1) phosphorylation at Thr187 and by down-regulating Skp2 expression. Furthermore, immunofluorescence staining and cell fractionation showed that erlotinib treatment led to p27(KIP1) translocation to the nucleus. Knockdown of p27(KIP1) expression with p27(KIP1) small interfering RNA significantly abrogated erlotinib-induced G1 phase arrest and cell growth inhibition, suggesting that induction of p27(KIP1) is required for G1 arrest and cell growth inhibition by erlotinib. It is noteworthy that we found that G1 arrest and p27(KIP1) up-regulation by erlotinib occurred in the tested sensitive cell lines but to a lesser extent in the resistant cell lines. Taken together, these results suggest that erlotinib inhibits human NSCLC cell growth predominantly by inducing p27(KIP1) expression and by suppressing cell-cycle events involved in the G1/S transition.

Synthesis and structure-activity relationships of carbazole sulfonamides as a novel class of antimitotic agents against solid tumors.

Two series of carbazole sulfonamides related to Combretastatin A4 (1) were synthesized and evaluated for antiproliferative activity. Thirteen of the 26 new sulfonamides exhibited IC(50) values of <1 muM against CEM leukemia cells. Five compounds were evaluated against a panel of eight human tumor cell lines. 9-Ethyl-N-(3,4,5-trimethoxyphenyl)-carbazole-3-sulfonamide (11a) showed significant antitumor activity in two human xenograft models (MCF-7 and Bel-7402). Preliminary studies with 11a showed that the mode of action involves arrest of M-phase cell cycle and induction of apoptosis by increasing expression of p53 and promoting bcl-2 phosphorylation. Unexpectedly, 11a only weakly inhibits tubulin polymerization, which suggests that the mode of action of 11a differs from 1 and involves an unidentified target(s). Also, the SAR information gleaned from ring A-substituted analogues varies significantly from that of 1. Carbazole sulfonamides are a novel promising class of antimitotic agents with clinical development potential.

Enhanced sensitivity to the HER1/epidermal growth factor receptor tyrosine kinase inhibitor erlotinib hydrochloride in chemotherapy-resistant tumor cell lines.

PURPOSE: Erlotinib (Tarceva, OSI-774) is a potent and specific inhibitor of the HER1/epidermal growth factor receptor (EGFR) tyrosine kinase. In phase II clinical studies, oral erlotinib monotherapy has shown antitumor activity in patients with advanced non-small cell lung cancer, head and neck cancer, and ovarian cancer after the failure of standard chemotherapy. We hypothesized that some tumors treated with multiple cytotoxic therapies may become more dependent on the HER1/EGFR signaling pathways for survival. EXPERIMENTAL DESIGN: The growth-inhibitory effect of erlotinib was tested on 10 pairs of chemosensitive, parental, and chemoresistant tumor cell lines. RESULTS: Enhanced sensitivity to erlotinib was observed in the doxorubicin-resistant human breast cancer cell line MCF-7, paclitaxel-resistant human ovarian carcinoma cell line A2780, and cisplatin-resistant human cervical carcinoma cell line ME180. The IC(50) values of erlotinib in the resistant cell lines were 2- to 20-fold lower than those in the corresponding parental cell lines. This enhanced sensitivity to erlotinib correlated with higher HER1/EGFR and phospho-HER1/EGFR expression when compared with the corresponding parental cell lines. Acquired resistance to cytotoxic agents was not associated with cross-resistance to erlotinib. AE-ME180/CDDP-resistant xenografts showed greater sensitivity to erlotinib than parental ME180 xenografts did. CONCLUSIONS: Our findings suggest that acquired resistance to cytotoxic therapy in some tumors is associated with enhanced sensitivity to HER1/EGFR inhibitors, which correlates with increased HER1/EGFR expression. These data may explain some of the observed clinical activity of HER1/EGFR inhibitors in patients previously treated with multiple therapies. HER1/EGFR tyrosine kinase inhibitors may be more effective as second- or third-line treatment for certain patients with tumors that were previously treated with multiple chemotherapy regimens.

Exogenous expression of H-cadherin in CHO cells regulates contact inhibition of cell growth by inducing p21 expression.

The impact of the cadherins in human cancers is becoming better understood. However, few studies have directly tested the hypothesis that H-cadherin, a tailless cadherin, is actually a tumor suppressor, and no published studies have addressed the question of how H-cadherin suppresses cellular transformation. We report here the influence that exogenous expression of H-cadherin imposes on growth, morphology, clonogenicity and tumorigenicity of Chinese hamster ovarian (CHO) cells. H-cadherin expression in CHO cells resulted in tighter adhesion of multicellular aggregates and reduced cell proliferation. In addition to enhancement of cell-cell adhesion, exogenous H-cadherin expression also inhibited cell proliferation and the ability to form colonies in soft agar. Furthermore, expression of H-cadherin in CHO cells led to complete suppression of subcutaneous tumor growth in nude mice. Seeding the H-cadherin expressing CHO cells on culture plates coated with recombinant H-cadherin amino-terminal fragments resulted in inhibition of cell proliferation that was accompanied by increased expression of the cdk inhibitor p21. These results support the role of H-cadherin as a tumor suppressor participating in contact inhibition of cell growth, possibly by inducing p21 expression.

Reactive oxygen species generation and mitochondrial dysfunction in the apoptotic response to Bortezomib, a novel proteasome inhibitor, in human H460 non-small cell lung cancer cells.

Bortezomib, a proteasome inhibitor, shows substantial anti-tumor activity in a variety of tumor cell lines, is in phase I, II, and III clinical trials and has recently been approved for the treatment of patients with multiple myeloma. The sequence of events leading to apoptosis following proteasome inhibition by bortezomib is unclear. Bortezomib effects on components of the mitochondrial apoptotic pathway were examined: generation of reactive oxygen species (ROS), alteration in the mitochondrial membrane potential (Delta psi m), and release of cytochrome c from mitochondria. With human H460 lung cancer cells, bortezomib exposure at 0.1 microM showed induction of apoptotic cell death starting at 24 h, with increasing effects after 48-72 h of treatment. After 3-6 h, an elevation in ROS generation, an increase in Delta psi m, and the release of cytochrome c into the cytosol, were observed in a time-dependent manner. Co-incubation with rotenone and antimycin A, inhibitors of mitochondrial electron transport chain complexes I and III, or with cyclosporine A, an inhibitor of mitochondrial permeability transition pore, resulted in inhibition of bortezomib-induced ROS generation, increase in Delta psi m, and cytochrome c release. Tiron, an antioxidant agent, blocked the bortezomib-induced ROS production, Delta psi m increase, and cytochrome c release. Tiron treatment also protected against the bortezomib-induced PARP protein cleavage and cell death. Benzyloxycarbonyl-VAD-fluoromethyl ketone, an inhibitor of pan-caspase, did not alter the bortezomib-induced ROS generation and increase in Delta psi m, although it prevented bortezomib-induced poly(ADP-ribose) polymerase cleavage and apoptotic death. In PC-3 prostate carcinoma cells (with overexpression of Bcl-2), a reduction of bortezomib-induced ROS generation, Delta psi m increase was correlated with cellular resistance to bortezomib and the attenuation of drug-induced apoptosis. The transient transfection of wild type p53 in p53 null H358 cells caused stimulation of the bortezomib-induced apoptosis but failed to enhance ROS generation and Delta psi m increase. Thus ROS generation plays a critical role in the initiation of the bortezomib-induced apoptotic cascade by mediation of the disruption of Delta psi m and the release of cytochrome c from mitochondria.

Mechanisms of proteasome inhibitor PS-341-induced G(2)-M-phase arrest and apoptosis in human non-small cell lung cancer cell lines.

PURPOSE: PS-341 is a novel dipeptide boronic acid proteasome inhibitor with in vitro and in vivo antitumor activity that induces mechanisms of apoptosis by unknown mechanisms. EXPERIMENTAL DESIGN: Human non-small cell lung cancer cell lines were used to investigate effects PS-341 on cell proliferation, cell cycle progression, and the induction of apoptosis. RESULTS: PS-341 was 38-360-fold more cytotoxic against H460 cells when compared with the proteasome inhibitors MG-132 and PSI. Differential PS-341 cytotoxic effects were found with respect to P53 function: H322 cells (p53 mutant) were 6-fold less sensitive as compared with H460 cells (p53 wild type); and H358 cells (p53 null) were 1.6-fold more sensitive as compared with H460 cells (p53 wild type). A concentration- and time-dependent cell cycle blockade at G(2)-M phase was seen for H460 cells without any direct effects on microtubule polymerization or depolymerization. PS-341 exposure in H460 cells led to stabilization of p53, induction of p21(cip/waf-1) and MDM2 expression, an increase in cyclin B and cyclin A, and the activation of cyclin B and cyclin A kinases. MDM2 induction was found only in H460 cells, whereas in H322 and H358 cells, G(2)-M-phase arrest, p21(cip/waf-1) induction, and an increase in cyclin B1 were found. The commitment of G(2)-M-phase cells to apoptosis was verified by the activation of caspase-3 and cleavage of poly(ADP-ribose) polymerase in drug-free medium. CONCLUSIONS: Our data suggest that the PS-341-induced G(2)-M-phase arrest may be associated with the inhibition of degradation of cell cycle regulators and that the up-regulation of p21(cip/waf-1) expression may be via p53-dependent and/or -independent pathways. The resulting disturbance of cell cycle progression leads either to growth inhibition or to the initiation of apoptotic pathways.