Fourmis : une chimiothèque de nouveaux anticancéreux

Ants: a chemical library of anticancer molecules

Équipe Venins et Activités Biologiques (VAcBio), EA 4357, PRES-Université de Toulouse, Institut Universitaire Jean-François Champollion, place de Verdun, 81000 Albi, France

Auteur correspondant : Angélique Vétillard, angelique.vetillard@univ-jfc.fr

Reçu : 7 Juin 2016

Résumé

La composition des venins est très compliquée à déterminer à cause de leur complexité et de leur diversité moléculaire, cependant l’étude transcriptomique des glandes à venin de la fourmi Tetramorium bicarinatum révèle une grande diversité fonctionnelle des peptides/protéines qui pourraient être produits par ces glandes. Ces molécules peptidiques correspondent pour partie à des toxines connues mais de nombreuses séquences ne sont répertoriées dans aucune base de données et pourraient correspondre à des peptides de venins originaux non décrits auparavant. Nos travaux étaient les premiers à rapporter le séquençage du transcriptome de la glande à venin chez une fourmi, d’abord par la technique de la SSH (Suppression Subtractive Hybridization), puis par une technique de séquençage profond permettant de progresser dans le degré d’informations que renferme le transcriptome (Illumina). Nos expériences montrent clairement que l’extrait de venin brut induit l’apoptose de certaines lignées de cellules tumorales résistantes à tous les traitements actuels et l’analyse en spectrométrie de masse nous a orientées vers une molécule dont la structure nous est encore inconnue. L’identification et la caractérisation du mécanisme d’action de la molécule responsable de cette cytotoxicité constituent donc un espoir de découvrir de nouvelles stratégies thérapeutiques dans la lutte contre cette pathologie encore aujourd’hui incurable. D’une manière générale, l’extraordinaire diversité taxonomique des fourmis laisse présager d’une grande diversité de leurs toxines. La probabilité de découvrir de nouvelles molécules d’intérêt pour l’homme n’en est que renforcée.

Abstract

Animal venoms are complex mixtures containing simple organic molecules, proteins, peptides, and other bioactive elements with extraordinary biological properties associated with their ability to act on a number of molecular receptors in the process of incapacitating their target organisms. In such a context, arthropod venoms are invaluable sources of bioactive substances, with therapeutic interest but the limited availability of some venom such as those from ants, has restricted the potential that these biomolecules could represent. We investigated for the first time transcriptomic expression from the ant species Tetramorium bicarinatum. Four hundred randomly selected clones from cDNA libraries were sequenced and a total of 374 expressed sequence tags (ESTs) were generated. Based on the results of BLAST searches, these sequences were clustered and assembled into 269 contigs. About 72% (269) of these matched BLASTx hits with an interesting diversity and unusual abundance of cellular transcripts (48%) related to gene and protein expression reflecting the specialization of this tissue. In addition, transcripts encoding transposases were relatively highly expressed (14%). It may be that transposable elements are present and that their presence accounts for some of the variation in venom toxins. About twenty per cent of the ESTs were categorized as putative toxins, the major part represented by allergens (48% of the total venom toxins) such as pilosulin 5, sol i 3 and Myp p I and II. Several contigs encoding enzymes, including zinc-metalloproteases (17%) that are likely involved in the processing and activation of venom proteins/peptides, were also identified from the library. In addition, a number of sequences (8%) had no significant similarity to any known sequence which indicates a potential source of for the discovery of new toxins. In order to provide a global insight on the transcripts expressed in the venom gland of the Brazilian ant species Tetramorium bicarinatum and to unveil the potential of their products, high-throughput expressed sequence tags were generated using Illumina paired-end sequencing technology. A total of 212 371 758 pairs of quality-filtered, 100-base-pair Illumina reads were obtained. The de novo assemblies yielded 36 042 contigs for which 27 873 have at least one predicted ORF among which 59.77% produce significant hits in the available databases. The investigation of the read mapping toxin class revealed and confirmed a high diversification with the major part consistent with the classical hymenopteran venom protein signature represented by venom allergen (33.3%) followed by a diverse toxin-expression profile including several distinct isoforms of phospholipase A₁ and A₂, venom serine protease, hyaluronidase, protease inhibitor and secapin. Moreover, our results revealed for the first time the presence of toxin-like peptides that have been previously identified from unrelated venomous animals such as waprin-like (snakes) and agatoxins (spiders and conus). These studies provide a first insight of the gene expression scenario of the venom gland of T. bicarinatum which might contribute to acquiring a more comprehensive view about the origin and functional diversity of venom proteins of this ant. Based on such results, we conducted cytotoxic tests from the crude venom of T.bicarinatum ant and reported toxic effect on tumoral cells lines from one of the fifth of the most frequently occurring cancers with a 3-year survival rate of only 30%. In such a context, new therapeutic strategies are essential and the discovery of new molecules in ant venom could be one possible avenue. Thus our project aims to characterize, from the crude venom of T.bicarinatum, the molecule(s) which have potential anti-cancerous toxicity as well as their mechanisms of action.