Topoisomerase II (Top2) is a ubiquitous nuclear enzyme that relieves torsional stress in chromosomal DNA during various cellular processes. Agents that target Top2, involving etoposide, doxorubicin, and mitoxantrone, are among the most effective anticancer drugs used in the clinic. Mammalian cells possess two genetically distinct Top2 isoforms, both of which are the target of these agents. Top2␣ is essential for cell proliferation and is highly expressed in vigorously growing cells, whereas Top2␤ is nonessential for growth and has recently been implicated in treatment-associated secondary malignancies, highlighting the validity of a Top2␣-specific drug for future cancer treatment; however, no such agent has been hitherto reported. Here we show that NK314, a novel synthetic benzo[c]phenanthridine alkaloid, targets Top2␣ and not Top2␤ in vivo. Unlike other Top2 inhibitors, NK314 induces Top2-DNA complexes and double-strand breaks (DSBs) in an ␣ isoform-specific manner. Heterozygous disruption of the human TOP2␣ gene confers increased NK314 resistance, whereas TOP2␤ homozygous knock-out cells display increased NK314 sensitivity, indicating that the ␣ isoform is the cellular target. We further show that the absence of Top2␤ does not alleviate NK314 hypersensitivity of cells deficient in non-homologous end-joining, a critical pathway for repairing Top2-mediated DSBs. Our results indicate that NK314 acts as a Top2␣-specific poison in mammalian cells, with excellent potential as an efficacious and safe chemotherapeutic agent. We also suggest that a series of human knock-out cell lines are useful in assessing DNA damage and repair induced by potential topoisomerase-targeting agents. DNA topoisomerase II (Top2)2 is a ubiquitous nuclear enzyme that alters the topological structure of DNA and chromosomes through a transient DNA double-strand break (DSB) and subsequent religation of the DSB (1, 2). The enzyme has been implicated in many aspects of DNA metabolism, including DNA replication, repair, transcription, and chromosome condensation/segregation (1, 3). Top2 has been of considerable interest to human medicine, because it is an important target for cancer chemotherapy (4). Top2-targeting agents, involving etoposide, doxorubicin, and mitoxantrone, are among the most effective and widely used anticancer drugs in cancer chemotherapy (5, 6). These agents are referred to as "Top2 poisons," because they convert the essential enzyme into a highly cytotoxic DNA-damaging agent through the formation of "cleavage complex" (also called "cleavable complex"), in which a Top2-linked DNA strand-passing intermediate is stabilized, allowing the generation of a DSB (7,8).Mammalian cells possess two genetically distinct Top2 isoforms (9, 10). Despite their similar structural features (ϳ70% identity at the amino acid level) and biological properties, the two isoforms are differentially regulated and play different roles in living cells. Top2␣ is most abundantly expressed in rapidly growing tissues and its expression is cell cycle-regulated...

Overexpression of an anti-apoptotic protein cIAP1 caused by its genetic amplification was reported in certain cancers, such as hepatocellular carcinoma, esophageal squamous cell carcinoma, cervical cancer, and lung cancer, which confers resistance to chemotherapy and radiotherapy. Here we report cIAP1 to be selectively down-regulated by a class of small molecules ((؊)-N-[(2S,3R)-3-amino-2-hydroxy-4-phenyl-butyryl]-L-leucine methyl ester (ME-BS)), resulting in a sensitization of cancer cells to apoptosis. ME-BS directly interacts with the BIR3 domain of cIAP1, promotes auto-ubiquitylation dependent on its RING domain, and facilitates proteasomal degradation of cIAP1. Other IAPs such as XIAP and cIAP2 were not affected by ME-BS. These results suggest targeted destabilization of cIAP1 by small molecules as a novel method to treat cancers expressing cIAP1, which interferes with treatment. Manipulation of the intrinsic ubiquitinligase activity could be a novel strategy to develop small molecules for therapeutic purposes.IAPs (inhibitor of apoptosis proteins) are a family of antiapoptotic proteins containing one to three baculoviral IAP repeat (BIR) 2 domains (1-3), some of which are frequently overexpressed in malignant cells. Certain IAPs such as XIAP/ hILP/BIRC4, cIAP1/MIHB/hiap-2/BIRC2, cIAP2/MIHC/hiap-1/BIRC3, ML-IAP/Livin/BIRC7 and Apollon/BRUCE/BIRC6 directly interact with and regulate caspases (4 -9). The BIR domain plays an important role in the interaction with caspases (8, 10, 11). These IAPs also contain a domain involved in ubiquitin conjugation (RING finger domain or UBC domain), and facilitate proteasomal degradation of caspases and IAPs (8, 12, 13). cIAP1 (cellular inhibitor of apoptosis protein 1) is overexpressed in human cancers such as esophageal squamous cell carcinoma, hepatocellular carcinoma, cervical cancer, and lung cancer, because of its genetic amplification and is regarded as an oncogene (14 -17). cIAP1 overexpression in cervical cancers correlates with resistance to radiotherapy. In addition, a comparative study of the expression of IAP family proteins and the sensitivity to chemotherapeutic drugs in 60 cell lines revealed the level of cIAP1 significantly correlates with resistance against anti-cancer drugs such as carboplatin, cisplatin, etoposide, and cytosine arabinoside (18). This evidence suggests cIAP1 to be a promising target for cancer therapy.Bestatin, an inhibitor of aminopeptidase N, has an immunomodulatory activity and is approved in Japan to treat patients with adult acute nonlymphatic leukemia (19 -21). In a clinical trial with stage I squamous cell lung carcinoma patients, bestatin significantly prolonged survival of these patients (22). Bestatin induces apoptosis in human leukemia cells (23) and augments death ligand-induced apoptosis in human solid tumor cell lines (24). In this paper, we describe selective down-regulation of cIAP1 by esterified analogs of bestatin represented by bestatin-methyl ester (ME-BS) (see Fig. 1A), resulting in a sensitization of cancer cells to ...

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Pironetin is a potent inhibitor of tubulin assembly and arrests cell cycle progression in M phase. Analyses of its structure-activity relationships suggested that pironetin covalently binds tubulin. To determine the binding site of pironetin, we synthesized biotinylated pironetin, which inhibited tubulin assembly both in vitro and in situ. The biotinylated pironetin selectively and covalently bound with tubulin. Partial digestion of biotinylated pironetin-treated tubulin by several proteases revealed that the binding site is the C-terminal portion of alpha-tubulin. By systematic alanine scanning, the pironetin binding site was determined to be Lys352 of alpha-tubulin. Lys352 is located at the entrance of a small pocket of alpha-tubulin, and this pocket faces the beta-tubulin of the next dimer. This is the first compound that covalently binds to the alpha subunit of tubulin and Lys352 of alpha-tubulin and inhibits the interaction of tubulin heterodimers.

Crystallographic studies have elucidated the binding mechanism of forskolin and P-site inhibitors to adenylyl cyclase. Accordingly, computer-assisted drug design has enabled us to identify isoform-selective regulators of adenylyl cyclase. After examining more than 200 newly synthesized derivatives of forskolin, we found that the modification at the positions of C6 and C7, in general, enhances isoform selectivity. The 6-(3-dimethylaminopropionyl) modification led to an enhanced selectivity for type V, whereas 6-[N-(2-isothiocyanatoethyl) aminocarbonyl] and 6-(4-acrylbutyryl) modification led to an enhanced selectivity for type II. In contrast, 2-deoxyadenosine 3-monophosphate, a classical and 3-phosphate-substituted P-site inhibitor, demonstrated a 27-fold selectivity for inhibiting type V relative to type II, whereas 9-(tetrahydro-2-furyl) adenine, a ribose-substituted P-site ligand, showed a markedly increased, 130-fold selectivity for inhibiting type V. Consequently, on the basis of the pharmacophore analysis of 9-(tetrahydro-2-furyl) adenine and adenylyl cyclase, a novel nonnucleoside inhibitor, 2-amino-7-(2-furanyl)-7,8-dihydro-5(6H)-quinazolinone (NKY80), was identified after virtual screening of more than 850,000 compounds. NKY80 demonstrated a 210-fold selectivity for inhibiting type V relative to type II. More importantly, the combination of a type III-selective forskolin derivative and 9-(tetrahydro-2-furyl) adenine or NKY80 demonstrated a further enhanced selectivity for type III stimulation over other isoforms. Our data suggest the feasibility of adenylyl cyclase isoform-targeted regulation of cyclic AMP signaling by pharmacological reagents, either alone or in combination.The G protein 1 -sensitive, membrane-bound form of adenylyl cyclase consists of a large family; nine isoforms have been isolated and extensively studied (1-3). These isoforms are characterized by distinct biochemical properties and tissue distribution. For example, calcium-inhibitable type V is expressed in the heart as a major isoform (4); protein kinase C-sensitive type II is expressed in lungs (5); calmodulin-sensitive type I and type VIII are expressed exclusively in neuronal tissues (6); type III is expressed abundantly in olfactory tissues (7, 8) but also in other tissues including lungs (9), atria (10), and adipose tissue (11); type IV and VII are widely expressed (12, 13). Therefore, it is now accepted that the content and mixture of adenylyl cyclase isoforms provide a biochemical signature of tissue cyclic AMP generation. Forskolin, like digitalis, is a natural plant extract that has been used in traditional medicine (14). Forskolin directly activates adenylyl cyclase to increase the concentration of intracellular cyclic AMP. This mechanism for activation is now explained as follows. Forskolin binds to the catalytic core at the opposite end of the same ventral cleft that contains the active site and activates the enzyme by gluing together the two cytoplasmic domains in the core (C 1 and C 2 ) using a combination of hydropho...

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We previously reported that the aminopeptidase inhibitor bestatin induced apoptosis in several human leukemia cell lines. The present study was performed to examine whether bestatin can also induce apoptosis in solid tumor cell lines. Bestatin alone exhibited neither direct growth inhibition nor induction of apoptosis in the tumor cell lines examined. However, it significantly augmented the growth-inhibitory effect and induction of apoptosis by agonistic anti-Fas antibody (CH11). The augmentation by bestatin was also observed with other death ligands including tumor necrosis factor-␣ (TNF-␣) in EBC-1 cells, a cell line sensitive to these death ligands. However, the HeLa S3 cell line, which is insensitive to TNF-␣, showed no growth inhibition even by combination treatment. Bestatin methyl ester, a more cell-permeable derivative of bestatin with similar inhibitory activity to cytosolic neutral aminopeptidase, potentiated cell growth inhibition of CH11 more efficiently than bestatin. Other cytosolic neutral aminopeptidase inhibitors such as actinonin and puromycin also augmented cell growth suppression by CH11, while an enantiomer of bestatin lacking aminopeptidase inhibitory action did not increase the growth-inhibitory effects of CH11. The combination of 10 g/ml of bestatin with CH11 promoted processing of capase 3 to the active form p17 and efflux of mitochondrial cytochrome c into the cytosol more quickly and more intensely than CH11 alone. Inhibition of aminopeptidase was not involved in dATP-and cytochrome c-dependent caspase 3-activation in a cell-free system. Bestatin significantly augmented activation of caspase 8, which is upstream of cytochrome c efflux in the apoptosis cascade. These results suggested that intracellular neutral aminopeptidase might play an important role in Fas-or TNF-␣-induced solid tumor cell apoptosis.

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