The Notch Inhibitors Isolated from Nerium indicum

PBI-05204, a supercritical CO2 extract of Nerium oleander, suppresses glioblastoma stem cells by inhibiting GRP78 and inducing programmed necroptotic cell death

Successful treatment of glioblastoma multiforme (GBM), an aggressive form of primary brain neoplasm, mandates the need to develop new therapeutic strategies. In this study, we investigated the potential of PBI-05204 in targeting GBM stem cells (GSCs) and the underlying mechanisms. Treatment with PBI-05204 significantly reduced both the number and size of tumor spheres derived from patient-derived GSCs (GBM9, GSC28 and TS543), and suppressed the tumorigenesis of GBM9 xenografts. Moreover, PBI-05204 treatment led to a significant decrease in the expression of CD44 and NANOG, crucial markers of progenitor stem cells, in GBM9 and GSC28 GSCs. This treatment also down-regulated GRP78 expression in both GSC types. Knocking down GRP78 expression through GRP78 siRNA transfection in GBM9 and GSC28 GSCs also resulted in reduced spheroid size and CD44 expression. Combining PBI-05204 with GRP78 siRNA further decreased spheroid numbers compared to GRP78 siRNA treatment alone. PBI-05204 treatment led to increased expression of pRIP1K and pRIP3K, along with enhanced binding of RIPK1/RIPK3 in GBM9 and GSC28 cells, resembling the effects observed in GRP78-silenced GSCs, suggesting that PBI-05204 induced necroptosis in these cells. Furthermore, oleandrin, a principle active cardiac glycoside component of PBI-05204, showed the ability to inhibit the self-renewal capacity in GSCs. These findings highlight the potential of PBI-05204 as a promising candidate for the development of novel therapies that target GBM stem cells.

Targeting Notch signaling pathways with natural bioactive compounds: a promising approach against cancer

Notch signaling pathway is activated abnormally in solid and hematological tumors, which perform essential functions in cell differentiation, survival, proliferation, and angiogenesis. The activation of Notch signaling and communication among Notch and other oncogenic pathways heighten malignancy aggressiveness. Thus, targeting Notch signaling offers opportunities for improved survival and reduced disease incidence. Already, most attention has been given to its role in the cancer cells. Recent research shows that natural bioactive compounds can change signaling molecules that are linked to or interact with the Notch pathways. This suggests that there may be a link between Notch activation and the growth of tumors. Here, we sum up the natural bioactive compounds that possess inhibitory effects on human cancers by impeding the Notch pathway and preventing Notch crosstalk with other oncogenic pathways, which provoke further study of these natural products to derive rational therapeutic regimens for the treatment of cancer and develop novel anticancer drugs. This review revealed Notch as a highly challenging but promising target in oncology.

Notch activator cyclopiazonic acid induces apoptosis in HL-60 cells through calcineurin activation

We screened a library of microbial extracts and compounds library using our constructed assay cells and found pulicatins F (1) and G (2), and cyclopiazonic acid (CPA) (3) as Notch activators. Pulicatin F (1) and (±)-pulicatin G were synthesized and their activities were evaluated. Notch activation of CPA (3) was investigated using Western blot and RT-PCR. CPA (3) increased protein level of HES1 and mRNA expression of HES1. Also, the expression of FMS-like tyrosine kinase 3 (FLT3), which was known to inhibit apoptosis, was also inhibited by CPA (3) addition. The Notch activation by CPA (3) and cytotoxicity against HL-60 were clearly canceled by addition of FK506, which is an inhibitor of calcineurin (CaN). In addition, it was revealed that CPA (3) induced apoptosis in HL-60 cells.

A toxic shrub turned therapeutic: The dichotomy of Nerium oleander bioactivities

Nerium oleander L. is a medicinal plant, used for the treatment of cancers and hyperglycemia across the world, especially in Indian sub-continent, Turkey, Morocco, and China. Although clinical studies supporting its pharmacological effects remain critically underexplored, accidental and intentional consumption of any part of the plant causes fatal toxicity in animals and humans. While the polyphenolic fraction of oleander leaves has been attributed to its pre-clinical pharmacological activities, the presence of diverse cardiac glycosides (especially oleandrin) causes apoptosis to cancer cells in vitro and results in clinical signs of oleander poisoning. Thus, the dual pharmacological and toxicological role of oleander is a perplexing dichotomy in phytotherapy. The current investigative review, therefore, intended to analyze the intrinsic and extrinsic factors that likely contribute to this conundrum. Especially by focusing on gut microbial diversity, abundance, and metabolic functions, oleander-associated pharmacological and toxicological studies have been critically analyzed to define the dual effects of oleander. Electronic databases were extensively screened for relevant research articles (including pre-clinical and clinical) related to oleander bioactivities and toxicity. Taxonomic preference was given to the plant N. oleander L. and synonymous plants as per 'The World Flora Online' database (WCSP record #135196). Discussion on yellow oleander (Cascabela thevetia (L.) Lippold) has intentionally been avoided since it is a different plant. The review indicates that the gut microbiota likely plays a key role in differentially modulating the pharmacological and toxicological effects of oleander. Other factors identified influencing the oleander bioactivities include dose and mode of treatment, cardiac glycoside pharmacokinetics, host-endogenous glycosides, plant material processing and phytochemical extraction methods, plant genotypic variations, environmental effects on the phytochemical quality and quantity, gene expression variations, host dietary patterns and co-morbidity, etc. The arguments proposed are also relevant to other medicinal plants containing toxic cardiac glycosides.

Molecular Modes of Action of an Aqueous Nerium oleander Extract in Cancer Cells In Vitro and In Vivo

Cancer drug resistance remains a major obstacle in clinical oncology. As most anticancer drugs are of natural origin, we investigated the anticancer potential of a standardized cold-water leaf extract from Nerium oleander L., termed Breastin. The phytochemical characterization by nuclear magnetic resonance spectroscopy (NMR) and low- and high-resolution mass spectrometry revealed several monoglycosidic cardenolides as major constituents (adynerin, neritaloside, odoroside A, odoroside H, oleandrin, and vanderoside). Breastin inhibited the growth of 14 cell lines from hematopoietic tumors and 5 of 6 carcinomas. Remarkably, the cellular responsiveness of odoroside H and neritaloside was not correlated with all other classical drug resistance mechanisms, i.e., ATP-binding cassette transporters (ABCB1, ABCB5, ABCC1, ABCG2), oncogenes (EGFR, RAS), tumor suppressors (TP53, WT1), and others (GSTP1, HSP90, proliferation rate), in 59 tumor cell lines of the National Cancer Institute (NCI, USA), indicating that Breastin may indeed bypass drug resistance. COMPARE analyses with 153 anticancer agents in 74 tumor cell lines of the Oncotest panel revealed frequent correlations of Breastin with mitosis-inhibiting drugs. Using tubulin-GFP-transfected U2OS cells and confocal microscopy, it was found that the microtubule-disturbing effect of Breastin was comparable to that of the tubulin-depolymerizing drug paclitaxel. This result was verified by a tubulin polymerization assay in vitro and molecular docking in silico. Proteome profiling of 3171 proteins in the NCI panel revealed protein subsets whose expression significantly correlated with cellular responsiveness to odoroside H and neritaloside, indicating that protein expression profiles can be identified to predict the sensitivity or resistance of tumor cells to Breastin constituents. Breastin moderately inhibited breast cancer xenograft tumors in vivo. Remarkably, in contrast to what was observed with paclitaxel monotherapy, the combination of paclitaxel and Breastin prevented tumor relapse, indicating Breastin's potential for drug combination regimens.

Identification of 1β,2α-epoxytagitinin C as a Notch inhibitor, oxidative stress mechanism and its anti-leukemia activity

Notch signaling plays crucial roles in cell differentiation and proliferation, but aberrant activation of this signaling results in tumorigenesis and cancer progression. Notch signaling is thus a promising drug target for oncotherapy, and the development of Notch signaling inhibitors is eagerly awaited. Notch inhibitory activity-guided fractionation of a Spilanthes acmella extract led to the identification of five sesquiterpene lactones: tagitinin A (1), 1β,2α-epoxytagitinin C (2), tagitinin C (3), orizabin (4), and 2α-hydroxytirotundin (5). 1β,2α-Epoxytagitinin C (2) exhibited Notch signaling inhibition, with an IC₅₀ of 25.6 μM, and was further evaluated for its activity against HPB-ALL, a Notch-activated leukemia cell line. Compound 2 showed potent cytotoxicity against HPB-ALL (IC₅₀ 1.7 μM) and arrested the cell cycle at the G2/M phase, but did not induce apoptotic cell death. Notch inhibitory mechanism analysis suggested that compound 2 transcriptionally suppresses Notch1 mRNA. In addition, we found that oxidative stress induction is critical for Notch signaling inhibition and the cytotoxicity of compound 2. This is the first mechanism of small molecule Notch inhibition. Our results demonstrate that 1β,2α-epoxytagitinin C (2) is a potential anti-leukemia agent and further investigation of this compound is warranted.

Targeting Notch to Maximize Chemotherapeutic Benefits: Rationale, Advanced Strategies, and Future Perspectives

Simple Summary

The Notch signaling pathway regulates cell proliferation, apoptosis, stem cell self-renewal, and differentiation in a context-dependent fashion both during embryonic development and in adult tissue homeostasis. Consistent with its pleiotropic physiological role, unproper activation of the signaling promotes or counteracts tumor pathogenesis and therapy response in distinct tissues. In the last twenty years, a wide number of studies have highlighted the anti-cancer potential of Notch-modulating agents as single treatment and in combination with the existent therapies. However, most of these strategies have failed in the clinical exploration due to dose-limiting toxicity and low efficacy, encouraging the development of novel agents and the design of more appropriate combinations between Notch signaling inhibitors and chemotherapeutic drugs with improved safety and effectiveness for distinct types of cancer.

Abstract

Notch signaling guides cell fate decisions by affecting proliferation, apoptosis, stem cell self-renewal, and differentiation depending on cell and tissue context. Given its multifaceted function during tissue development, both overactivation and loss of Notch signaling have been linked to tumorigenesis in ways that are either oncogenic or oncosuppressive, but always context-dependent. Notch signaling is critical for several mechanisms of chemoresistance including cancer stem cell maintenance, epithelial-mesenchymal transition, tumor-stroma interaction, and malignant neovascularization that makes its targeting an appealing strategy against tumor growth and recurrence. During the last decades, numerous Notch-interfering agents have been developed, and the abundant preclinical evidence has been transformed in orphan drug approval for few rare diseases. However, the majority of Notch-dependent malignancies remain untargeted, even if the application of Notch inhibitors alone or in combination with common chemotherapeutic drugs is being evaluated in clinical trials. The modest clinical success of current Notch-targeting strategies is mostly due to their limited efficacy and severe on-target toxicity in Notch-controlled healthy tissues. Here, we review the available preclinical and clinical evidence on combinatorial treatment between different Notch signaling inhibitors and existent chemotherapeutic drugs, providing a comprehensive picture of molecular mechanisms explaining the potential or lacking success of these combinations.

Screening study of cancer-related cellular signals from microbial natural products

To identify bioactive natural products from various natural resources, such as plants and microorganisms, we investigated programs to screen for compounds that affect several cancer-related cellular signaling pathways, such as BMI1, TRAIL, and Wnt. This review summarizes the results of our recent studies, particularly those involving natural products isolated from microbial resources, such as actinomycetes, obtained from soil samples collected primarily around Chiba, Japan.

Evaluation of Naturally Occurring HIF-1 Inhibitors for Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a rare and severe progressive disorder characterized by high pulmonary artery pressure. Chronic hypoxia causes a metabolic disorder and the Warburg effect in pulmonary arterial smooth muscle cells (PASMCs). Pyruvate dehydrogenase kinase 1 (PDK1) is a key enzyme in Warburg effect increased by hypoxia-inducible factor (HIF-1). We constructed a cell-based luciferase assay system for HIF-1 inhibitors. Using this system, six HIF-1 inhibitors were identified. Among these inhibitors, the effect of tagitinin C (1) on PASMC was investigated. Tagitinin C (1) clearly decreased the amount of HIF-1β and the HIF-1 target PDK1. This result indicates that HIF-1 inhibitors effectively decrease PDK1 activity, which is a cause of the metabolic disorder and Warburg effect observed in PASMCs. Identifying naturally occurring HIF-1 inhibitors could provide novel insights into the development of PAH medications.

Isolation of nocobactin NAs as Notch signal inhibitors from Nocardia farcinica, a possibility of invasive evolution

Notch signaling inhibitors with the potential of immune suppressor production by pathogenic bacteria for easy host infection were searched from extracts of Nocardia sp. Nocobactin NA-a (compound 1) and nocobactin NA-b (compound 2), which have been suggested as pathogenesis factors, were isolated from N. farcinica IFM 11523 isolated from the sputum of a Japanese patient with chronic bronchitis. Compounds 1 and 2 showed Notch inhibitory activities with IC₅₀ values of 12.4 and 17.6 μM, respectively. Compound 1 and 2 decreased of Notch1 protein, Notch intracellular domain, and hairy and enhancer of split 1, which is a Notch signaling target protein. In addition, compounds 1 and 2 showed cytotoxicity against mouse macrophage-like cell line RAW264.7 with IC₅₀ values of 18.9 and 21.1 μM, respectively. These results suggested that the Notch inhibitors production by pathogenic bacteria may increase pathogen infectivity.

Targeting Notch in oncology: the path forward

Notch signalling is involved in many aspects of cancer biology, including angiogenesis, tumour immunity and the maintenance of cancer stem-like cells. In addition, Notch can function as an oncogene and a tumour suppressor in different cancers and in different cell populations within the same tumour. Despite promising preclinical results and early-phase clinical trials, the goal of developing safe, effective, tumour-selective Notch-targeting agents for clinical use remains elusive. However, our continually improving understanding of Notch signalling in specific cancers, individual cancer cases and different cell populations, as well as crosstalk between pathways, is aiding the discovery and development of novel investigational Notch-targeted therapeutics.

ASR490, a Small Molecule, Overrides Aberrant Expression of Notch1 in Colorectal Cancer

Notch1 activation triggers significant oncogenic signaling that manifests as enhanced metastatic potential and tumorigenesis in colorectal cancer. Novel small-molecule inhibitors, mainly plant-derived analogs, have low toxicity profiles and higher bioavailability. In this study, we have developed a small molecule, ASR490, by modifying structure of naturally occurring compound Withaferin A. ASR490 showed a growth-inhibitory potential by downregulating Notch1 signaling in HCT116 and SW620 cell lines. Docking studies and thermal shift assays confirmed that ASR490 binds to Notch1, whereas no changes in Notch2 and Notch3 expression were seen in colorectal cancer cells. Notch1 governs epithelial-to-mesenchymal transition signaling and is responsible for metastasis, which was abolished by ASR490 treatment. To further confirm the therapeutic potential of ASR490, we stably overexpressed Notch1 in HCT-116 cells and determined its inhibitory potential in transfected colorectal cancer (Notch1/HCT116) cells. ASR490 effectively prevented cell growth in both the vector (P = 0.005) and Notch1 (P = 0.05) transfectants. The downregulation of Notch1 signaling was evident, which corresponded with downregulation of mesenchymal markers, including N-cadherin and β-catenin and induction of E-cadherin in HCT-116 transfectants. Intraperitoneal administration of a 1% MTD dose of ASR490 (5 mg/kg) effectively suppressed the tumor growth in control (pCMV/HCT116) and Notch1/HCT116 in xenotransplanted mice. In addition, downregulation of Notch1 and survival signaling in ASR-treated tumors confirmed the in vitro results. In conclusion, ASR490 appears to be a potent agent that can inhibit Notch1 signaling in colorectal cancer.

Isolation and evaluation of cardenolides from Lansium domesticum as Notch inhibitors

Since Notch signaling plays important roles in cell proliferation and differentiation, aberrant activation of this signaling contributes to cancer progression. In neural stem cells, Notch signaling inhibits differentiation by activating HES1 expression. Therefore, Notch signaling inhibitors may be candidates for new anticancer drugs or have applications in neural regenerative medicine. In this study, six naturally occurring Notch inhibitors were isolated from the methanol (MeOH) extract of Lansium domesticum using our novel cell-based assay. Hongherin (2), a cardiac glycoside, demonstrated potent Notch inhibitory activity with an IC₅₀ of 0.62 μM and was found to be cytotoxic in HPB-ALL human T cell acute lymphoblastic leukemia cells. Hongherin (2) also induced the differentiation of C17.2 neural stem cells to neurons, causing a 65% increase in differentiation compared to the control. Mechanistically, hongherin (2) reduced the amount of Notch1 (full length) and mastermind-like protein (MAML). This indicates that hongherin (2) inhibits Notch signaling through a dual mechanism involving the reduction of both Notch1 and MAML protein levels.

The Botanical Glycoside Oleandrin Inhibits Human T-cell Leukemia Virus Type-1 Infectivity and Env-Dependent Virological Synapse Formation

At present, there are no antiretroviral drugs that inhibit incorporation of the envelope glycoprotein into newly-synthesized virus particles. The botanical glycoside, oleandrin, derived from extracts of Nerium oleander, has previously been shown to reduce the levels of the gp120 envelope glycoprotein on human immunodeficiency virus type-1 (HIV-1) particles and inhibit HIV-1 infectivity in vitro. We therefore tested whether oleandrin or an extract from N. oleander could also inhibit the infectivity of the human T-cell leukemia virus type-1 (HTLV-1): A related enveloped retrovirus and emerging tropical infectious agent. The treatment of HTLV-1+ lymphoma T-cells with either oleandrin or a N. oleander extract did not significantly inhibit viral replication or the release of p19^Gag-containing particles into the culture supernatants. However, the collected virus particles from treated cells exhibited reduced infectivity on primary human peripheral blood mononuclear cells (huPBMCs). Unlike HIV-1, extracellular HTLV-1 particles are poorly infectious and viral transmission typically occurs via direct intercellular interactions across a virological synapse. We therefore investigated whether oleandrin or a N. oleander extract could inhibit virus transmission from a GFP-expressing HTLV-1+ lymphoma T-cell-line to huPBMCs in co-culture assays. These results demonstrated that both oleandrin and the crude phytoextract inhibited the formation of virological synapses and the transmission of HTLV-1 in vitro. Importantly, these findings suggest oleandrin may have broad antiviral activity against enveloped viruses by reducing the incorporation of the envelope glycoprotein into mature particles, a stage of the infection cycle not targeted by modern HAART.

Potential Anticancer Agents Characterized from Selected Tropical Plants

Higher plants are well known for their value in affording clinically useful anticancer agents, with such compounds acting against cancer cells by a range of mechanisms of action. There remains a strong interest in the discovery and development of plant secondary metabolites as additional cancer chemotherapeutic lead compounds. In the present review, progress on the discovery of plant-derived compounds of the biflavonoid, lignan, sesquiterpene, steroid, and xanthone structural types is presented. Several potential anticancer leads of these types have been characterized from tropical plants collected in three countries as part of our ongoing collaborative multi-institutional project. Preliminary structure-activity relationships and work on in vivo testing and cellular mechanisms of action are also discussed. In addition, the relevant work reported by other groups on the same compound classes is included herein.