Yi He Ling

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 (Δψm), and release of cytochrome c from mitochondria. With human H460 lung cancer cells, bortezomib exposure at 0.1 μm 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 Δψm, 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 Δψm, and cytochrome c release. Tiron, an antioxidant agent, blocked the bortezomib-induced ROS production, Δψm 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 Δψm, 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, Δψm 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 Δψm increase. Thus ROS generation plays a critical role in the initiation of the bortezomib-induced apoptotic cascade by mediation of the disruption of Δψm and the release of cytochrome c from mitochondria.

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 G1-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.

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 IC50 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.

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 p27KIP1, and retinoblastoma hypophosphorylation. To further understand the role of p27KIP1 in G1/S arrest and cell growth inhibition by erlotinib, we determined its effect on the expression of p27KIP1 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 p27KIP1 half-life by inhibiting p27KIP1 phosphorylation at Thr187 and by down-regulating Skp2 expression. Furthermore, immunofluorescence staining and cell fractionation showed that erlotinib treatment led to p27KIP1 translocation to the nucleus. Knockdown of p27KIP1 expression with p27KIP1 small interfering RNA significantly abrogated erlotinib-induced G1 phase arrest and cell growth inhibition, suggesting that induction of p27KIP1 is required for G1 arrest and cell growth inhibition by erlotinib. It is noteworthy that we found that G1 arrest and p27KIP1 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 p27KIP1 expression and by suppressing cell-cycle events involved in the G1/S transition.

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. Clin Cancer Res; 17(16); 5353–66. ©2011 AACR.

Bortezomib: Understanding the Mechanism of Action

Commentary on: nn[Yi-He Ling, Leonard Liebes, Bruce Ng, Michael Buckley, Peter J. Elliott, Julian Adams, Jian-Dong Jiang, Franco M. Muggia, and Roman Perez-Soler. PS-341, a Novel Proteasome Inhibitor, Induces Bcl-2 Phosphorylation and Cleavage in Association with G2–M Phase Arrest and Apoptosis .

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. Clin Cancer Res; 17(21); 6766–77. ©2011 AACR.

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

Introduction: This study was undertaken to identify molecular determinants of tumor dependency on the epidermal growth factor receptor (EGFR) signaling pathway for predicting clinical benefit from erlotinib monotherapy in non-small cell lung cancer (NSCLC) patients with tumors expressing wild-type EGFR gene. Methods: The effect of erlotinib on the total and phosphorylated protein expression of EGFR and key downstream signaling molecules was determined by immunoblots in a panel of NSCLC cells expressing wild-type EGFR gene. The parameters that correlate with cell sensitivity and resistance to erlotinib was analyzed. Results: Individual assessment of total or phosphorylated protein expression of EGFR or a downstream signaling molecule does not correlate with sensitivity to erlotinib in these NSCLC tumors. Resistance of NSCLC cells to erlotinib is associated with failed inhibition of at least one phosphorylated downstream signaling molecule. The dependency of NSCLC cells on the activated EGFR axis was measured by the ratio of p-EGFR to a phosphorylated downstream protein. A high ratio should indicate that activation of a downstream signaling molecule primarily results from the activation of upstream EGFR; and a low ratio should indicate that activation of a downstream signaling molecule primarily results from the activation of a upstream receptors other than EGFR. The p-EGFR:p-AKT ratio was 10-fold higher in erlotinib-sensitive cells than erlotinib-resistant cells (p = 0.03). It was the best predictor of erlotinib sensitivity among all parameters analyzed in this panel of NSCLC cell lines. Conclusions: The p-EGFR:p-AKT ratio deserves further investigation as a predictive parameter for clinical response to erlotinib in NSCLC tumors expressing wild-type EGFR gene.