Yeast ribosomal sensitivity and resistance to the Amaryllidaceae alkaloids

The Inhibition of RNA Viruses by Amaryllidaceae Alkaloids: Opportunities for the Development of Broad-Spectrum Anti-Coronavirus Drugs

The global COVID-19 pandemic has claimed the lives of millions and disrupted nearly every aspect of human society. Currently, vaccines remain the only widely available medical means to address the cause of the pandemic, the SARS-CoV-2 virus. Unfortunately, current scientific consensus deems the emergence of vaccine-resistant SARS-CoV-2 variants highly likely. In this context, the design and development of broad-spectrum, small-molecule based antiviral drugs has been described as a potentially effective, alternative medical strategy to address circulating and re-emerging CoVs. Small molecules are well-suited to target the least-rapidly evolving structures within CoVs such as highly conserved RNA replication enzymes, and this renders them less vulnerable to evolved drug resistance. Examination of the vast literature describing the inhibition of RNA viruses by Amaryllidaceae alkaloids suggests that future, broad-spectrum anti-CoV drugs may be derived from this family of natural products.

Insight to the antifungal properties of Amaryllidaceae constituents

BACKGROUND

Fungal pathogenesis continues to be a burden to healthcare structures in both developed and developing nations. The gradual and irreversible loss of efficacies of existing antifungal medicines as well as the emergence of drug-resistant strains have contributed largely to this scenario. There is therefore a pressing need for new drugs from diverse structural backgrounds with improved potencies and novel modes of action to fortify or replace contemporary antifungal schedules.

AIM

Alkaloids of the plant family Amaryllidaceae exhibit good growth inhibitory activities against several fungal pathogens. This review focuses on the mechanistic aspects of these antifungal activities. It achieves this by highlighting the molecular targets as well as structural features of Amaryllidaceae constituents which serve to enhance such action.

METHODS

During the information gathering stage extensive use was made of the three database platforms; Google Scholar, SciFinder and Scopus. In most instances articles were accessed directly from journals licensed to the University of KwaZulu-Natal. In the absence of such proprietary agreements the respective corresponding authors were approached directly for copies of papers.

RESULTS

Although several classes of molecules from the Amaryllidaceae have been probed for their antifungal effects, it is the key constituents lycorine and narciclasine which have together afforded the most profound mechanistic insights. These may be summarized as follows: (i) effects on the fungal cell wall and cell membrane; (ii) effects on morphology such as budding and hyphal growth; (iii) effects on fungal organelles such as ribosomes; (iv) effects on macromolecules such as DNA, RNA and proteins and; (v) identification of the active sites for these constituents.

CONCLUSION

The key feature in the antifungal effects of Amaryllidaceae alkaloids is the inhibition of protein synthesis. This involved the inhibition of peptide bond formation by binding to yeast ribosomes via the 60S subunit. Related effects involved the inhibition of both DNA and RNA synthesis. These adverse effects were reflected morphologically on both the fungal cell wall and cell membrane. Such observations should prove useful in the chemotherapeutic arena should efforts shift towards the development of a clinical candidate.

Antifungal constituents of the plant family Amaryllidaceae

Globalization, the modern lifestyle, immuno-suppressive agents, invasive surgical procedures, the loss of efficacies of existing drugs, and multidrug resistance are some of the factors used to explain the rise in fungal infections in recent years. Significant advances have been made in attempts to replace existing antifungal schedules, especially with synthetic targets. The identification of other platforms for drug discovery is now entrenched in research programs across the globe. Plants offer significant benefits owing to their numerical superiority, exceedingly broad chemical basis and appealing sustainability characteristics. Furthermore, plants have a long and rich historical association with traditional approaches towards fungal diseases. These have in numerous instances served as markers in the bioassay-guided identification of the active constituents. Although the plant family Amaryllidaceae is conventionally associated with cancer and motor-neuron disease chemotherapies, around 30 of its species have been examined for antifungal activities with microgram per millilitre inhibitory activities detected in several instances. This review focuses on the nearly 40 constituents from the family, mainly isoquinoline alkaloids, which have been screened against around 50 fungal pathogens. Encouragingly, microgram per millilitre growth inhibitory activities were applicable for several of the compounds with a minimum inhibitory concentration of 4 μg/ml seen to be the lowest.

Jonquailine, a new pretazettine-type alkaloid isolated from Narcissus jonquilla quail, with activity against drug-resistant cancer

A new alkaloid, belonging to the pretazettine group of Amaryllidaceae alkaloids, was isolated from dried bulbs of Narcissus jonquilla quail and named jonquailine. Its structure, including the absolute configuration, was elucidated using various NMR, ECD and ESI MS techniques. Initial biological evaluation revealed significant antiproliferative effects against glioblastoma, melanoma, uterine sarcoma and non-small-cell lung cancer cells displaying various forms of drug resistance, including resistance to apoptosis and multi-drug resistance. Jonquailine was also found to synergize with paclitaxel in its antiproliferative action against drug-resistant lung cancer cells. The results obtained compared with literature data also showed that the hydroxylation at C-8 is an important feature for the anticancer activity but this seems unaffected by the stereochemistry or the acetalization of the lactol.

Mechanistic Insights to the Cytotoxicity of Amaryllidaceae Alkaloids

With over 500 individual compounds, the Amaryllidaceae alkaloids represent a large and structurally diverse group of phytochemicals. Coupled to this structural diversity is the significant array of biological properties manifested by many of its members, of which their relevance in motor neuron disease and cancer chemotherapy has attracted considerable attention. To this extent, galanthamine has evolved into a successful commercial drug for Alzheimer's disease since its approval by the FDA in 2001. Concurrently, there have been several positive indicators for the emergence of an anticancer drug from the Amaryllidaceae due to the potency of several of its representatives as cell line specific antiproliferative agents. In this regard, the phenanthridones such as pancratistatin and narciclasine have offered most promise since their advancement into clinical trials, following which there has been renewed interest in the cytotoxic properties of these alkaloids. Given this background, this review seeks to highlight the various mechanisms which have been invoked to corroborate the cytotoxic effects of Amaryllidaceae alkaloids.

Lycorine reduces mortality of human enterovirus 71-infected mice by inhibiting virus replication

Human enterovirus 71 (EV71) infection causes hand, foot and mouth disease in children under 6 years old and this infection occasionally induces severe neurological complications. No vaccines or drugs are clinical available to control EV71 epidemics. In present study, we show that treatment with lycorine reduced the viral cytopathic effect (CPE) on rhabdomyosarcoma (RD) cells by inhibiting virus replication. Analysis of this inhibitory effect of lycorine on viral proteins synthesis suggests that lycorine blocks the elongation of the viral polyprotein during translation. Lycorine treatment of mice challenged with a lethal dose of EV71 resulted in reduction of mortality, clinical scores and pathological changes in the muscles of mice, which were achieved through inhibition of viral replication. When mice were infected with a moderate dose of EV71, lycorine treatment was able to protect them from paralysis. Lycorine may be a potential drug candidate for the clinical treatment of EV71-infected patients.

Chemical and biological aspects of Narcissus alkaloids

This chapter discusses the chemical and biological aspects of Narcissus alkaloids. Numerous alkaloids have been isolated from Narcissus speciesasaresult of the continuing search for novel alkaloids with pharmacological activity in the Amaryllidaceae family. The alkaloids isolated from this genus, classified in relation to the different skeleton types. The different Narcissus wild species and intersectional hybrids, grouped into subgenera and sections, with their corresponding alkaloids, arranged according to their ring system are listed. The biosynthetic pathways of Narcissus alkaloids includes: (1) enzymatic preparation of the precursors, (2) primary cyclization mechanisms, (3) enzymatic preparation of intermediates, (4) secondary cyclization, diversification, and restructuring. The chapter discusses proton nuclear magnetic resonance (1H NMR), carbon nuclear magnetic resonance (13C NMR), and mass spectrometry (MS) for Narcissus alkaloids. A list of the different Narcissus alkaloids, their spectroscopic properties, and literature with the most recent spectroscopic data is given. Several Narcissus extracts shows the following activities: antiviral, prophage induction, antibacterial, antifungal, antimalarial, insecticidal, cytotoxic, antitumor, antimitotic, antiplatelet, hypotensive, emetic, acetylcholine esterase inhibitory, antifertility, antinociceptive, chronotropic, pheromone, plant growth inhibitor, and allelopathic.

Genetic and biochemical characterization of mutants of CHO cells resistant to the protein synthesis inhibitor trichodermin

Mutants resistant to the protein synthesis inhibitor trichodermin have been selected in Chinese hamster ovary (CHO) cells. The mutants vary in their stability from those which rapidly lose their resistance to others which are relatively stable after prolonged growth in nonselective medium. Protein synthesis in extracts from the latter class of mutants (Trir) is resistant to the inhibitory action of trichodermin as compared to similar extracts from wild-type cells. After dissociation into subunits, the ability of the 60S ribosomal subunits from Trir cells to function in a protein-synthesizing system is greatly diminished. This subunit also shows reduced binding of [acetyl-14C]TRICHODERMIN. The lesion in Trir mutants therefore seems to have affected this ribosomal subunit. Trir X Tris hybrids are sensitive to trichodermin indicating that the Trir mutation behaves recessively to Tris in hybrids. The Emtr and Trir markers segregate independently from hybrid cells showing that the Trir mutation is probably not linked to the Emtr locus, which as we have shown earlier affects the 40S ribosomal subunit.

Binding of [3H]narciclasine to eukaryotic ribosomes. A study on a structure-activity relationship

[3H]Narciclasine is a specific inhibitor of peptide bond formation on eukaryotic ribosomes and binds to 60-S ribosomal subunits. Binding of [3H]-narciclasine to yeast ribosomes is inhibited by many other inhibitors of peptide bond formation including anisomycin, several sequiterpene antibiotics (trichodermin, trichothecin, fusarenon X and verrucarin A) several Cephalotaxus alkaloids (harringtonine, homoharringtonine and isoharringtonine), several Amaryllidaceae alkaloids (pretazettine, haemanthamine, lycorine, pseudolycorine and dihydrolycorine) and the narciclasine derivatives trans-dihydronarciclasine, trans-dihydronarciclasine acetonide and isonarciclasine. Binding is also inhibited, although to a very small extent, by methylnarciclasine and cisdihydronarciclasine. In contrast, no inhibition of [3H]narciclasine binding was observed in the presence of certain other inhibitors of peptide bond formation including blasticidin S, gougerotin, sparsomycin and puromycin.

Inhibition of translation in eukaryotic systems by harringtonine

The Cephalotaxus alkaloids harringtonine, homoharringtonine and isoharringtonine inhibit protein synthesis in eukaryotic cells. The alkaloids do not inhibit, in model systems, any of the steps of the initiation process but block poly(U)-directed polyphenylalanine synthesis as well as peptide bond formation in the fragment reaction assay, the sparsomycin-induced binding of (C)U-A-C-C-A-[3H]Leu-Ac, and the enzymic and the non-enzymic binding of Phe-tRNA to ribosomes. These results suggest that the Cephalotaxus alkaloids inhibit the elongation phase of translation by preventing substrate binding to the acceptor site on the 60-S ribosome subunit and therefore block aminoacyl-tRNA binding and peptide bond formation. However, the Cephalotaxus alkaloids do not inhibit polypeptide synthesis and peptidyl[3H]puromycin formation in polysomes. Furthermore, these alkaloids strongly inhibit [14C]trichlodermin binding to free ribosomes but hardly affect the interaction of the antibiotic with yeast polysomot interact with polysomes and therefore only inhibit cycles of elongation. This explains the polysome run off that has been observed by some workers in the presence of harringtonine.