Inhibition of protein synthesis by didemnin B is not sufficient to induce apoptosis in human mammary carcinoma (MCF7) cells

Didemnin B (DB) is one member of a class of natural cyclic depsipeptides th at display potent cytotoxicity in vitro. The detailed mechanism of action o f DB is unknown, although it appears to involve the inhibition of protein b iosynthesis. Additional activities of DB have established DB as a rapid and potent inducer of apoptosis in HL-60 cells. Our aim was to determine if th e induction of apoptosis by DB is mediated through inhibition of protein sy nthesis in MCF-7 human breast: carcinoma cells. Apoptosis was observed only at greater than or equal to 100 nM DB, even though inhibition of protein s ynthesis occurred at much lower DB concentrations (IC50 = 12 nM). DB-induce d apoptosis was mediated by caspase activation, since cleavage of the caspa se substrate poly(ADP-ribose) polymerase was observed as early as 6 hr afte r DB exposure. Two additional protein synthesis inhibitors, cycloheximide ( CHX) and emetine (ET), failed to induce apoptosis at concentrations that co mpletely inhibited protein synthesis. Moreover, DB-induced apoptosis was en hanced only slightly by pre- and co-treatment with CHX and ET. Thus, inhibi tion of protein synthesis alone was not sufficient to induce apoptosis in t hese cells. As a measure of antiproliferative potential, DB (1-5 nM) inhibi ted the colony forming ability of MCF7 cells regardless of pretreatment wit h CHX. In conclusion, additional effects of DB, independent of protein synt hesis inhibition, are proposed to account for its ability to induce apoptos is and prevent cell proliferation. (C) 1999 Elsevier Science Ins.