Marine compound palytoxin induces apoptotic cell death via Mcl-1 and Bcl-2 down-regulation in human leukemia cells

Abstract

Marine ecosystems contribute to a huge repository of pharmacologically active compounds. Palytoxin, one of the most toxic marine compounds, is known to be involved in the transformation of Na+/K+-ATPase into a cation channel inducing massive intracellular Na+ influx. Anti-cancer activity of palytoxin is an emerging area of research and especially palytoxin-induced cancer cell death mechanisms remain to be elucidated. Here we show that palytoxin induced cell death of various leukemia cell lines at low picomolar concentrations. Importantly, palytoxin did not affect viability of peripheral blood mononuclear cells (PBMC) cells from healthy donors and did not create systemic toxicity in zebrafish, thus demonstrating excellent differential toxicity. Cell death was characterized by nuclear condensation as demonstrated by Hoechst staining as well as by caspase activation demonstrated by western blot and luminescent caspase assays. Palytoxin triggers cleavage of initiator pro-caspases-8 and -9 as well as executioner pro-caspases-3 and -7 after 6 hours of treatment in a dose-dependent manner. As caspase activation is sensitive to pan-caspase inhibitor zVAD, we conclude that palytoxin induces apoptotic cell death. From a molecular point of view, palytoxin down-regulates anti-apoptotic Bcl-2 family proteins Mcl-1 and Bcl-xL in a dose-dependent manner. MG-132, a proteasome inhibitor, was able to prevent proteolysis of Mcl-1 whereas the three major proteasomal enzymatic activities were up-regulated by palytoxin. Palytoxin-induced dephosphorylation of Bcl-2 further exacerbates the pro-apoptotic effect of Mcl-1 and Bcl-xL degradation. As okadaic acid could rescue cell death triggered by palytoxin, we hypothesize involvement of protein phosphatase (PP)2A in Bcl-2 phosphorylation and induction of apoptosis by palytoxin. Altogether, we provide here first evidence of the role of palytoxin as a very potent and promising cancer-specific cytotoxic agent acting at low picomolar concentrations.