AMF-26, a novel inhibitor of the Golgi system, targeting ADP-ribosylation factor 1 (Arf1) with potential for cancer therapy

Novel effects of Brefeldin A (BFA) in signaling through the insulin receptor (IR) pathway and regulating FoxO1-mediated transcription

Brefeldin A (BFA) is a fungal metabolite best known for its ability to inhibit activation of ADP-ribosylation factor (Arf) and thereby inhibit secretory traffic. BFA also appears to regulate the trafficking of the GLUT4 glucose transporter by inducing its relocation from intracellular stores to the cell surface. Such redistribution of GLUT4 is normally regulated by insulin-mediated signaling. Hence, we tested whether BFA may intersect with the insulin pathway. We report that BFA causes the activation of the insulin receptor (IR), IRS-1, Akt-2, and AS160 components of the insulin pathway. The response is mediated through phosphoinositol-3-kinase (PI3K) and Akt kinase since the PI3K inhibitor wortmannin and the Akt inhibitors MK2206 and perifosine inhibit the BFA effect. BFA-mediated activation of the insulin pathway results in Akt-mediated phosphorylation of the insulin-responsive transcription factor FoxO1. This leads to nuclear exclusion of FoxO1 and a decrease in transcription of the insulin-responsive gene SIRT-1. Our findings suggest novel effects for BFA in signaling and transcription, and imply that BFA has multiple intracellular targets and can be used to regulate diverse cellular responses that include vesicular trafficking, signaling and transcription.

Pharmacological inhibitors of exocytosis and endocytosis: novel bullets for old targets

Pharmacological inhibitors of vesicle trafficking possess great promise as valuable analytical tools for the study of a variety of biological processes and as potential therapeutic agents to fight microbial infections and cancer. However, many commonly used trafficking inhibitors are characterized by poor selectivity that diminishes their use in solving basic problems of cell biology or drug development. Recent high-throughput chemical screens intensified the search for novel modulators of vesicle trafficking, and successfully identified a number of small molecules that inhibit exocytosis and endocytosis in different types of mammalian cells. This chapter provides a systematic overview of recently discovered inhibitors of vesicle trafficking. It describes cellular effects and mechanisms of action of novel inhibitors of exocytosis and endocytosis. Furthermore, it pays special attention to the selectivity and possible off-target effects of these inhibitors.

Fragment-based identification of a locus in the Sec7 domain of Arno for the design of protein-protein interaction inhibitors

By virtual screening using a fragment-based drug design (FBDD) approach, 33 fragments were selected within small pockets around interaction hot spots on the Sec7 surface of the nucleotide exchange factor Arno, and then their ability to interfere with the Arno-catalyzed nucleotide exchange on the G-protein Arf1 was evaluated. By use of SPR, NMR, and fluorescence assays, the direct binding of three of the identified fragments to Arno Sec7 domain was demonstrated and the promiscuous aggregate behavior evaluated. Then the binding mode of one fragment and of a more active analogue was solved by X-ray crystallography. This highlighted the role of stable and transient pockets at the Sec7 domain surface in the discovery and binding of interfering compounds. These results provide structural information on how small organic compounds can interfere with the Arf1-Arno Sec7 domain interaction and may guide the rational drug design of competitive inhibitors of Arno enzymatic activity.

Ridaifen-SB8, a novel tamoxifen derivative, induces apoptosis via reactive oxygen species-dependent signaling pathway

Tamoxifen is an anticancer agent widely used for treatment of estrogen receptor (ERα)-positive breast cancer. We previously developed a novel synthesis of tamoxifen and its derivatives, named Ridaifens (RIDs). Some of them, including RID-SB8, exhibited a stronger anticancer activity than tamoxifen in ERα-positive MCF-7 cells while having lost the affinity for ERα, suggesting an ERα-independent anticancer mode of action. In this study, we investigated the underlying mechanism by which RID-SB8 exerts anticancer activity. As expected, anticancer activity of RID-SB8 was not influenced upon knockdown of ERα expression in MCF-7 cells. RID-SB8 exerted similar anticancer effects on thirteen ERα-negative cancer cell lines including human gliosarcoma SF539 cells. In SF539 cells, RID-SB8 triggered loss of mitochondrial membrane potential (ΔΨ(m)) and progression of apoptosis accompanied by activation of caspases and translocation of apoptosis-inducing factor (AIF) to the nucleus. Furthermore, it induced reactive oxygen species (ROS), and a ROS scavenger, N-acetylcysteine (NAC), canceled loss of ΔΨ(m) and progression of apoptosis triggered by RID-SB8. Using fifteen human cancer cell lines, we demonstrated a significant correlation between RID-SB8 concentration required for ROS production and that required for cytotoxic effect across these cell lines, but such correlation was not observed for tamoxifen. Finally, the selective induction of ROS and cytotoxic effect on cancer cells by RID-SB8 were confirmed. From these results, we concluded that RID-SB8 exerts an anticancer effect via a mode of action distinct from tamoxifen, and that RID-SB8 could become a promising anticancer lead compound which selectively induces ROS formation and apoptosis in cancer cells.

Cytotoxic activity of tivantinib (ARQ 197) is not due solely to c-MET inhibition

The receptor tyrosine kinase c-MET is the high-affinity receptor for the hepatocyte growth factor (HGF). The HGF/c-MET axis is often dysregulated in tumors. c-MET activation can be caused by MET gene amplification, activating mutations, and auto- or paracrine mechanisms. Thus, c-MET inhibitors are under development as anticancer drugs. Tivantinib (ARQ 197) was reported as a small-molecule c-MET inhibitor and early clinical studies suggest antitumor activity. To assess whether the antitumor activity of tivantinib was due to inhibition of c-MET, we compared the activity of tivantinib with other c-MET inhibitors in both c-MET-addicted and nonaddicted cancer cells. As expected, other c-MET inhibitors, crizotinib and PHA-665752, suppressed the growth of c-MET-addicted cancers, but not the growth of cancers that are not addicted to c-MET. In contrast, tivantinib inhibited cell viability with similar potency in both c-MET-addicted and nonaddicted cells. These results suggest that tivantinib exhibits its antitumor activity in a manner independent of c-MET status. Tivantinib treatment induced a G(2)-M cell-cycle arrest in EBC1 cells similarly to vincristine treatment, whereas PHA-665752 or crizotinib treatment markedly induced G(0)-G(1) cell-cycle arrest. To identify the additional molecular target of tivantinib, we conducted COMPARE analysis, an in silico screening of a database of drug sensitivities across 39 cancer cell lines (JFCR39), and identified microtubule as a target of tivantinib. Tivantinib-treated cells showed typical microtubule disruption similar to vincristine and inhibited microtubule assembly in vitro. These results suggest that tivantinib inhibits microtubule polymerization in addition to inhibiting c-MET.

Target identification for small bioactive molecules: finding the needle in the haystack

Identification and confirmation of bioactive small-molecule targets is a crucial, often decisive step both in academic and pharmaceutical research. Through the development and availability of several new experimental techniques, target identification is, in principle, feasible, and the number of successful examples steadily grows. However, a generic methodology that can successfully be applied in the majority of the cases has not yet been established. Herein we summarize current methods for target identification of small molecules, primarily for a chemistry audience but also the biological community, for example, the chemist or biologist attempting to identify the target of a given bioactive compound. We describe the most frequently employed experimental approaches for target identification and provide several representative examples illustrating the state-of-the-art. Among the techniques currently available, protein affinity isolation using suitable small-molecule probes (pulldown) and subsequent mass spectrometric analysis of the isolated proteins appears to be most powerful and most frequently applied. To provide guidance for rapid entry into the field and based on our own experience we propose a typical workflow for target identification, which centers on the application of chemical proteomics as the key step to generate hypotheses for potential target proteins.

Total synthesis of AMF-26, an antitumor agent for inhibition of the Golgi system, targeting ADP-ribosylation factor 1

An effective method for the total synthesis of 1 (AMF-26), a potentially promising new anticancer drug that disrupts the Golgi system by inhibiting the ADP-ribosylation factor 1 (Arf1) activation, has been developed for the first time. The construction of the chiral linear precursor (a key to the synthesis) was achieved by the asymmetric aldol reaction followed by the computer-assisted predictive stereoselective intramolecular Diels-Alder reaction. The global antitumor activity of the totally synthetic 1 against a variety of human cancer cells was assessed using a panel of 39 human cancer cell lines (JFCR39), and it was shown that the synthetic 1 strongly inhibited the growth of several cancer cell lines at concentrations of less than 0.04 μM. Biological assays of novel derivatives, 26 and 31, which have different side-chains at the C-4 positions in the Δ¹,²-octalin backbone, disclosed the importance of the suitable structure of the side-chain containing conjugated multidouble bonds.

Kinetics of interaction between ADP-ribosylation factor-1 (Arf1) and the Sec7 domain of Arno guanine nucleotide exchange factor, modulation by allosteric factors, and the uncompetitive inhibitor brefeldin A

The GDP/GTP nucleotide exchange of Arf1 is catalyzed by nucleotide exchange factors (GEF), such as Arno, which act through their catalytic Sec7 domain. This exchange is a complex mechanism that undergoes conformational changes and intermediate complex species involving several allosteric partners such as nucleotides, Mg(2+), and Sec7 domains. Using a surface plasmon resonance approach, we characterized the kinetic binding parameters for various intermediate complexes. We first confirmed that both GDP and GTP counteract equivalently to the free-nucleotide binary Arf1-Arno complex stability and revealed that Mg(2+) potentiates by a factor of 2 the allosteric effect of GDP. Then we explored the uncompetitive inhibitory mechanism of brefeldin A (BFA) that conducts to an abortive pentameric Arf1-Mg(2+)-GDP-BFA-Sec7 complex. With BFA, the association rate of the abortive complex is drastically reduced by a factor of 42, and by contrast, the 15-fold decrease of the dissociation rate concurs to stabilize the pentameric complex. These specific kinetic signatures have allowed distinguishing the level and nature as well as the fate in real time of formed complexes according to experimental conditions. Thus, we showed that in the presence of GDP, the BFA-resistant Sec7 domain of Arno can also associate to form a pentameric complex, which suggests that the uncompetitive inhibition by BFA and the nucleotide allosteric effect combine to stabilize such abortive complex.

Small inhibitors of ADP-ribosylation factor activation and function in mammalian cells

Small GTP-binding proteins of the ADP-ribosylation factor (Arf) family control various cell functional responses including protein transport and recycling between different cellular compartments, phagocytosis, proliferation, cytoskeletal remodelling, and migration. The activity of Arfs is tightly regulated. GTPase-activating proteins (GAPs) inactivate Arfs by stimulating GTP hydrolysis, and guanine nucleotide exchange factors (GEFs) stimulate the conversion of inactive GDP-bound Arf to the active GTP-bound conformation. There is increasing evidence that Arf small GTPases contribute to cancer growth and invasion. Increased expression of Arf6 and of Arf-GEPs, or deregulation Arf-GAP functions have been correlated with enhanced invasive capacity of tumor cells and metastasis. The spatiotemporal specificity of Arf activation is dictated by their GEFs that integrate various signals in stimulated cells. Brefeldin A (BFA), which inactivates a subset of Arf-GEFs, has been very useful for assessing the function of Golgi-localized Arfs. However, specific inhibitors to investigate the individual function of BFA-sensitive and insensitive Arf-GEFs are lacking. In recent years, specific screens have been developed, and new inhibitors with improved selectivity and potency to study cell functional responses regulated by BFA-sensitive and BFA-insensitive Arf pathways have been identified. These inhibitors have been instrumental for our understanding of the spatiotemporal activation of Arf proteins in cells and demonstrate the feasibility of developing small molecules interfering with Arf activation to prevent tumor invasion and metastasis.

Implications of the Golgi apparatus in prostate cancer

The classical view of the Golgi apparatus is of a small membranous organelle involved in protein transport and secretion. Recent descriptions of the molecular network connecting the Golgi to other organelles demonstrate the essential roles of the Golgi in cellular activities as a stress sensor, apoptosis trigger, lipid/protein modifier, mitotic checkpoint, and a mediator of malignant transformation. Thus, the Golgi function should have a fundamental impact on cancer cell survival. Prostate cancer is initially responsive to androgenic hormones; however, it almost invariably progresses to a castration-refractory or hormone-insensitive state. Nevertheless, androgen signaling remains active at this stage and is important as a therapeutic target. Certain Golgi-associated molecules have recently been demonstrated to be regulated by androgen action, and the Golgi is emerging as a new therapeutic target in prostate cancer. The key Golgi-associated molecules essential for prostate cancer development and the potential therapeutic options targeting the Golgi apparatus are discussed.