Investigating the pro-apoptotic properties of portimine

Abstract

Portimine is a bioactive compound recently isolated from the marine dinoflagellate Vulcanodinium rugosum. It is a member of the fast acting class of toxins called cyclic imines, which are typically lethal to mammals at low doses. Portimine has been shown to have limited toxicity in mice, but it appears to be a very effective inducer of apoptosis in cultured cells. This study looked to understand more about the mechanism in which portimine mediates programmed cell death. Other cyclic imines are neurotoxic through binding of acetylcholine receptors. We hypothesized portimine also utilised a receptor to mediate cytotoxic effects, and therefore utilised the extracellular death receptor pathway. Jurkat T-lymphoma cells and mouse embryonic fibroblasts (MEFs) were exposed to portimine and the loss in cell viability was measured. Cell lines in which different members of apoptosis signalling pathways were knocked out or overexpressed were assessed to help determine the mechanism of apoptosis induction, and the effects of portimine was compared with other inducers of cell death in these cells. Portimine had an LC50 of 7.3 nM in wild-type Jurkat cells. It was unable to kill Jurkat cells overexpressing Bcl-2, even at very high concentrations. This contrasted dramatically with hydrogen peroxide, which could not kill Bcl-2 overexpressing Jurkat cells at low doses, but caused necrotic death at high concentrations. Therefore, portimine appears to kill via a selective apoptotic mechanism without causing significant cell damage. MEFs were also killed at low doses of portimine, with an LC50 of 5.2 nM. Interestingly, it took 48 h after portimine exposure to achieve optimal cell death. MEFs in which the apoptosis effectors Bax and Bak were knocked out were resistant to portimine, supporting an apoptosis-dependent mechanism. However, Bcl-2 was not protective as in Jurkat cells, and a Bid-/- cell line was also susceptible to portimine. Bid is important for the induction of death receptor-mediated apoptosis, suggesting our initial hypothesis is incorrect. There are potential applications for portimine or the novel pathway it uses, as a selective inducer of apoptosis in cells that display unregulated cell growth.