Chemical mediation as a structuring element in marine gastropod predator-prey interactions

Some diterpenoid compounds protect the sacoglossansElysiasp. andCyerce nigricansfrom their carnivorous predator the dorid nudibranch,Gymnodorissp., unlike chemically unprotected gastropods that are consumed by this voracious nudibranch (photo Philippe Bourseiller).

Inhibition of topoisomerase II by the marine alkaloid ascididemin and induction of apoptosis in leukemia cells

Ascididemin (ASC) is a pentacyclic DNA-intercalating agent isolated from the Mediterranean ascidian Cystodytes dellechiajei. This marine alkaloid exhibits marked cytotoxic activities against a range of tumor cells, but its mechanism of action remains poorly understood. We investigated the effects of ASC on DNA cleavage by human topoisomerases I and II. Relaxation assays using supercoiled DNA showed that ASC stimulated double-stranded cleavage of DNA by topoisomerase II, but exerted only a very weak effect on topoisomerase I. ASC is a conventional topoisomerase II poison that significantly promoted DNA cleavage, essentially at sites having a C on the 3' side of the cleaved bond (-1 position), as observed with etoposide. The stimulation of DNA cleavage by topoisomerase I in the presence of ASC was considerably weaker than that observed with camptothecin. Cytotoxicity measurements showed that ASC was even less toxic to P388 leukemia cells than to P388CPT5 cells resistant to camptothecin. In addition, the marine alkaloid was found to be equally toxic to HL-60 leukemia cells sensitive or resistant to mitoxantrone. It is therefore unlikely that topoisomerases are the main cellular targets for ASC. This alkaloid was found to strongly induce apoptosis in HL-60 and P388 leukemia cells. Cell cycle analysis showed that ASC treatment was associated with a loss of cells in the G1 phase accompanied with a large increase in the sub-G1 region. Cleavage experiments with poly(ADP-ribose) polymerase (PARP) revealed that caspase-3 was a mediator of the apoptotic pathway induced by ASC. The DNA of ASC-treated cells was severely fragmented. Collectively, these findings indicate that ASC is a potent inducer of apoptosis in leukemia cells.

Multiwavelength videomicrofluorometric study of cytotoxic effects of a marine peptide, didemnin B, on normal and MDR resistant CCRF-CEM cell lines

The development of multidrug resistance (MDR) in heterogeneous cell sensitive and resistant populations to a variety of clinically important cytotoxic drugs poses a major obstacle to cancer chemotherapy. Didemnin B, a marine cyclic depsipeptide, displays interesting biological properties: antiviral activity, inhibition of DNA, RNA and protein synthesis, initiation of apoptosis and ability to block the cell cycle. As very little is known about its mode of action, we studied the effect of increasing doses of Didemnin B on sensitive and resistant human leukemic lymphoblast cell lines. The fluorescence of living cells simultaneously stained with Hoechst 33,342, Rhodamine 123 and Nile Red, were analyzed in a multiparametric approach involving multiwavelength microfluorometry. High concentrations of Didemnin B induced, in the sensitive cell line, a very early decrease in the energetic state of the mitochondria that occurs before a significant decrease of nuclear DNA content, observed simultaneously on sensitive and resistant cells, that could be related to an apoptosis process. Furthermore low Didemnin doses (50 nM) affected CEM-WT and CEM VLB differently, while higher doses (200 nM-250 nM and over) affected the two cell lines in the same way. This indicated that, at these doses, the membranar Pgp has no effect on the mode of action of Didemnin, suggesting that Didemnin does not need to be internalized to be active.

Multiwavelength videomicrofluorometric study of cytotoxic properties of a marine peptide, didemnin B, using adriamycin as reference compound

Didemnin B (DB), a marine natural product, has very encouraging biological activity in vitro (Antineoplastic, immunosuppressive, antiviral). To learn more about its intracellular effects and targets, videomicrofluorometry on single living cells and a protocol of multiple labeling: Hoechst 342 for nuclear DNA, Rhodamine 123 for mitochondria and Nile Red for plasma membrane, have been used. DB behaves differently from Adriamycin, inducing at its IC50 dose of (20 nM) an accumulation of the CEM-WT lymphoblasts in the S phase of the cell cycle while we observed a 50% decrease of the mitochondrial labeling by R123, showing a decrease of the mitochondrial energetic state. Cytostatic dose of DB (250 nM) confirms these observations. However the treatment with a dose reported as apoptotic (1000 nM) induces a much faster effect (corresponding to that of 72 hours at the IC50 dose), 24 hours incubation induced a drastic decrease of nuclear DNA content as well as of the mitochondria energetic state. The evolution of NAD(P)H cellular content exhibited an increase that seems to indicate that the decrease of mitochondrial energetic state was dependent on inhibition of the mitochondrial activity due to an effect of DB at the mitochondrial level, either direct or mediated. Furthermore, the decrease of mitochondrial labeling appears as a very early event in the mechanisms leading to apoptosis.

Binding to DNA and cytotoxic evaluation of ascididemin, the major alkaloid from the Mediterranean ascidian Cystodytes dellechiajei

The isolation of ascididemin from the Mediterranean ascidian Cystodytes dellechiajei is described. This alkaloid consists of a planar pentacyclic chromophore which was investigated for its DNA-binding and cytotoxic properties. Spectroscopic measurements provided evidence that the drug intercalates into DNA. DNase I footprinting assays indicated that the binding of ascididemin to GC-rich sequences is favoured over binding to AT-rich and mixed sequences. Chemical probes were used to detect ligand-induced structural changes in DNA. The alkaloid induces a hyper-reactivity of the DNA towards potassium permanganate, but not towards diethylpyrocarbonate, just as is the case with ethidium bromide; it has little effect on the catalytic activities of topoisomerases I and II. Ascididemin exhibits marked cytotoxicity towards human leukaemic cells in vitro and appears to be practically equally toxic for drug-sensitive and multidrug-resistant cell lines. The results suggest that DNA, but not topoisomerases, may represent the critical cellular target at which this marine alkaloid exhibits its potent cytotoxic properties in vitro.