Smac-mediated sensitization of human B-lymphoma cells to staurosporine- and lactacystin-triggered apoptosis is apoptosome-dependent

Peptidoglycan from Staphylococcus aureus has an anti-apoptotic effect in HaCaT keratinocytes mediated by the production of the cellular inhibitor of apoptosis protein-2

Colonization of epithelium by microorganisms leads to inflammatory responses. In some cases an anti-apoptotic response involving the cellular inhibitor of apoptosis protein-2 (cIAP-2) also occurs. Although strong expression of cIAP-2 has been observed in lesional skin from psoriatic patients and in HaCaT keratinocytes treated with peptidoglycan (PGN) from Staphylococcus aureus, anti-apoptotic responses induced in the skin by cIAP-2 have seldom been studied. In this study, the effect of PGN on TNF-α-induced apoptotic HaCaT keratinocytes was assessed. Morphological analysis, quantification of cells with DNA fragmentation and active caspase-3 detection was performed to assess apoptotic cell death. Greater LL-37 and cIAP-2 production was found in keratinocytes stimulated with PGN than in non-treated cells (P < 0.05). In comparison with cells treated with TNF-α only, a significant reduction in apoptotic cell death was observed when HaCaT were pretreated with PGN before inducing apoptosis with TNF-α (P < 0.05). In addition, an inhibitor of cIAP-2 activity (LCL161) stopped the PGN effect. These findings show that PGN from S. aureus has an anti-apoptotic effect in keratinocytes mediated by cIAP-2 production, suggesting that this anti-apoptotic activity could favor proliferation of keratinocytes in psoriasis.

Proapoptotic effects of the novel proteasome inhibitor b-AP15 on multiple myeloma cells and natural killer cells

The small molecule b-AP15 is a novel inhibitor of proteasome deubiquitination. Recent studies have shown that b-AP15 displays antitumor activity in several preclinical, solid tumor models. In this study, we show that b-AP15 triggers time- and dose-dependent apoptosis of the human multiple myeloma (MM) cell lines RPMI8226 and U266, as determined by phosphatidylserine exposure. Apoptosis was dependent on caspase activation and was partially dependent on cathepsin D. Furthermore, b-AP15 triggered processing of pro-caspase-3 and cleavage of poly (ADP-ribose) polymerase in MM cells. b-AP15 also induced caspase-independent apoptosis in primary human natural killer cells. We also demonstrate that b-AP15 induces activation of the mitochondrial apoptosis pathway in MM cells, with activation of the proapoptotic protein Bax and a pronounced loss of the mitochondrial transmembrane potential. The latter events, however, appeared largely independent of caspase activation. Our data suggest that proteasome deubiquitinase inhibitors may have potential for treatment of multiple myeloma patients.

Requirement of apoptotic protease-activating factor-1 for bortezomib-induced apoptosis but not for Fas-mediated apoptosis in human leukemic cells

Bortezomib is a highly selective inhibitor of the 26S proteasome and has been approved for clinical use in the treatment of relapsing and refractory multiple myeloma and mantle cell lymphoma. Clinical trials are also underway to assess the role of bortezomib in several other human malignancies, including leukemia. However, the mechanism(s) by which bortezomib acts remain to be fully understood. Here, we studied the molecular requirements of bortezomib-induced apoptosis using the human T-cell leukemic Jurkat cells stably transfected with or without shRNA against apoptotic protease-activating factor-1 (Apaf-1). The Apaf-1-deficient Jurkat T cells were resistant to bortezomib-induced apoptosis, as assessed by caspase-3 activity, poly(ADP-ribose) polymerase cleavage, phosphatidylserine externalization, and hypodiploid DNA content. In contrast, Apaf-1-deficient cells were sensitive to Fas-induced apoptosis. Bortezomib induced an upregulation of the pro-apoptotic protein Noxa, loss of mitochondrial transmembrane potential, and release of cytochrome c in cells expressing or not expressing Apaf-1. Transient silencing of Apaf-1 expression in RPMI 8402 T-cell leukemic cells also diminished bortezomib-induced apoptosis. Fas-associated death domain (FADD)-deficient Jurkat cells were resistant to Fas-mediated apoptosis yet remained sensitive to bortezomib. Our results show that bortezomib induces apoptosis by regulating pathways that are mechanistically different from those activated upon death receptor ligation. Furthermore, in silico analyses of public transcriptomics databases indicated elevated Apaf-1 expression in several hematologic malignancies, including acute lymphoblastic and myeloid leukemia. We also noted variable Apaf-1 expression in a panel of samples from patients with acute lymphoblastic leukemia. Our results suggest that the expression of Apaf-1 may be predictive of the response to proteasome inhibition.

cIAP2 as a therapeutic target in colorectal cancer and other malignancies

Colorectal cancer is one of the most common malignancies worldwide and 70% of tumors are resectable, but patients with metastatic diseases cannot be cured with current treatment modalities. Inhibition of the apoptotic pathway is one of the factors that may be responsible for carcinogenesis and drug resistance, and the inhibitor of apoptosis protein (IAP) family is thought to prevent apoptosis through inhibition of direct caspases and pro-caspases. Recently an increasing amount of evidence has been accumulated regarding cIAP2 and other IAP proteins of the antiapoptotic pathway and NF-kappaB signal transduction. IAPs are abnormally regulated and expressed in the majority of human malignancies at elevated levels. As a result, they have recently been reported to be therapeutic targets. The downregulation of cIAP2 efficiently enhances apoptosis through the activation of caspase 3/7 and 5-fluorouracil (5-FU) sensitivity in colorectal cancer cells exposed to 5-FU. This report reviews the evidence for cIAP2 and other IAP molecules as a therapeutic target for malignancies including colorectal cancer. So far, the information on colorectal cancer is limited; so this study includes other malignancies as well, in order to summarize the current knowledge of drug development targeting IAP molecules and provide an overview of the future course.

Therapeutic peptides for cancer therapy. Part II - cell cycle inhibitory peptides and apoptosis-inducing peptides

BACKGROUND

Therapeutic peptides have great potential as anticancer agents owing to their ease of rational design and target specificity. However, their utility in vivo is limited by low stability and poor tumor penetration.

OBJECTIVE

The authors review the development of peptide inhibitors with potential for cancer therapy. Peptides that arrest the cell cycle by mimicking CDK inhibitors or induce apoptosis directly are discussed.

METHODS

The authors searched Medline for articles concerning the development of therapeutic peptides and their delivery.

RESULTS/CONCLUSION

Inhibition of cancer cell proliferation directly using peptides that arrest the cell cycle or induce apoptosis is a promising strategy. Peptides can be designed that interact very specifically with cyclins and/or cyclin-dependent kinases and with members of apoptotic cascades. Use of these peptides is not limited by their design, as a rational approach to peptide design is much less challenging than the design of small molecule inhibitors of specific protein-protein interactions. However, the limitations of peptide therapy lie in the poor pharmacokinetic properties of these large, often charged molecules. Therefore, overcoming the drug delivery hurdles could open the door for effective peptide therapy, thus making an entirely new class of molecules useful as anticancer drugs.

Proteasome inhibition induces apoptosis in primary human natural killer cells and suppresses NKp46-mediated cytotoxicity

BACKGROUND

Bortezomib is a selective and potent inhibitor of the proteasome and has prominent effects in vitro and in vivo against tumors. Very recently, cytotoxic effects of bortezomib on immune-competent cells such as T cells and dendritic cells were also revealed. The aim of the study was to investigate the effects of this agent on natural killer cell survival and function.

DESIGN AND METHODS

We investigated cytotoxic properties of bortezomib on natural killer cell apoptosis and function. Primary resting natural killer cells were purified from peripheral blood mononuclear cells of healthy donors by negative selection. The apoptotic cells were quantified by dual labeling of recombinant annexin V and propidium iodide. Mitochondrial membrane potential and expression of natural killer cell activating receptors were also quantified by flow cytometry. Natural killer cell cytotoxicity against murine and human tumor cells was tested by chromium 51 release assay.

RESULTS

Our results demonstrate that bortezomib induces apoptosis in resting natural killer cells in a dose- and time-dependent manner. Glutathione, a reactive oxygen species scavenger, prevented the loss of mitochondrial membrane potential and conferred protection against bortezomib-induced apoptosis in resting natural killer cells, indicating a role for oxidative stress. Additionally, bortezomib significantly decreased expression of the natural killer activating receptor NKp46 in non-apoptotic resting natural killer cells in a dose-dependent manner, and as a result the redirected cytotoxicity mediated via NKp46 activation was diminished. Bay 11-7082, a pharmacological inhibitor of NF-kappaB activation, also reduced NKp46 expression and suppressed redirected cytotoxicity.

CONCLUSIONS

Bortezomib induces apoptosis in primary resting natural killer cells in a dose- and time-dependent manner, and reduces NKp46 receptor expression as well as natural killer cell cytotoxicity mediated by the NKp46 activation pathway, suggesting that bortezomib may disrupt natural killer cell-mediated immunity through at least two different mechanisms: induction of natural killer cell apoptosis, and suppression of NKp46 receptor-mediated cytotoxicity.

Natural compounds with proteasome inhibitory activity for cancer prevention and treatment

The proteasome is a multicatalytic protease complex that degrades most endogenous proteins including misfolded or damaged proteins to ensure normal cellular function. The ubiquitin-proteasome degradation pathway plays an essential role in multiple cellular processes, including cell cycle progression, proliferation, apoptosis and angiogenesis. It has been shown that human cancer cells are more sensitive to proteasome inhibition than normal cells, indicating that a proteasome inhibitor could be used as a novel anticancer drug. Indeed, this idea has been supported by the encouraging results of the clinical trials using the proteasome inhibitor Bortezomib (Velcade, PS-341), a drug approved by the US Food and Drug Administration (FDA). Several natural compounds, including the microbial metabolite lactacystin, green tea polyphenols, and traditional medicinal triterpenes, have been shown to be potent proteasome inhibitors. These findings suggest the potential use of natural proteasome inhibitors as not only chemopreventive and chemotherapeutic agents, but also tumor sensitizers to conventional radiotherapy and chemotherapy. In this review, we will summarize the structures and biological activities of the proteasome and several natural compounds with proteasome inhibitory activity, and will discuss the potential use of these compounds for the prevention and treatment of human cancers.

Big wheel keeps on turning: apoptosome regulation and its role in chemoresistance

Apoptosis, a form of programmed cell death, enables organisms to maintain tissue homeostasis through deletion of extraneous cells and also serves as a means to eliminate potentially harmful cells. Numerous stress signals have been shown to engage the intrinsic pathway of apoptosis, with the release from mitochondria of proapoptotic factors such as cytochrome c and the subsequent formation of a cytosolic complex between apoptotic protease-activating factor-1 (Apaf-1) and procaspase-9, known as the apoptosome. Recent studies have led to the identification of an array of factors that control the formation and activation of the apoptosome under physiological conditions. Moreover, deregulation of apoptosome function has been documented in various forms of human cancer, and may play a role in both carcinogenesis and chemoresistance. We discuss how the apoptosome is regulated in normal and disease states, and how targeting of apoptosome-dependent, post-mitochondrial stages of apoptosis may serve as a rational approach to cancer treatment.