Taccalonolides: A microtubule stabilizer poses a new puzzle with old pieces

Abstract

The structural diversity of microtubule targeting agents (MTAs) is breathtaking, from simple bicyclics, like combretastatins, to complex compounds, like epothilones and halicondrins. The taccalonolides, plant-derived steroids described by Risinger and Mooberry in the July 1st issue of Cell Cycle,1 fall somewhere in the middle of this structural complexity. Interest in MTAs as chemotherapeutics began with the use of vincristine to treat acute lymphoblastic leukemia in the 1950s and shows no sign of abating. All MTAs inhibit mitosis in rapidly growing cells, though clinical efficacy against patient tumors is likely due to non-mitotic activities.2 The success of MTAs as chemotherapeutics has fueled searches for new compounds, one result being the taccalonolides. Notably, unlike agents against other cellular targets, MTAs remain principally natural products or semi-synthetic derivatives. That diverse organisms produce MTAs as toxins underscores the crucial roles of MTs in many cellular functions, continuing to make MTs attractive targets. The variability of MTAs likely explains their diverse antitumor profiles and varying neurotoxicty. This variability may yet lead to agents with different antitumor activity and hopefully less neurotoxicity, and this possibility continues to encourage pursuit of MTAs as cancer therapeutics. In the July 1st issue, Risinger and Mooberry discussed several features of taccalonolides that distinguish them from paclitaxel and other MT-stabilizers: (1) a novel structure, (2) MT activity in cells not reproduced with purified tubulin or cell extracts, (3) cellular activity much less reversible than with other MTAs and (4) concentrations effective in interphase and mitotic cells that are very similar, unlike the differential seen with paclitaxel. We discuss these below. The MT activity of taccalonolides was demonstrated by mitotic arrest and accumulation of MT bundles in interphase cells, evidence that taccalonolides, like taxanes, epothilones, laulimalide and peloruside, stabilize MTs. Taccalonolides as steroids are quite different structurally from other MT stabilizers, but taccalonolides are not the only MT-stabilizing steroid. A synthetic derivative with paclitaxel-like activity was discovered years ago in a search of analogs of the natural estrogen metabolite 2-methoxyestradiol, itself a weak MT destabilizer. This steroid derivative demonstrated pronounced MT stabilization with purified tubulin but not with cells,3 the opposite effect of that observed with taccalonolides. Taccalonolides are unusual, since their cellular activity is not reproduced in vitro. Unlike paclitaxel and other MT stabilizers, taccalonolides do not induce MT assembly using purified tubulin or cell extracts. It is assumed that taccalonolides act by binding tubulin, but direct binding data demonstrating this are lacking, and, in fact, one study found an absence of binding and an inability to displace other MT stabilizers from tubulin.4 Other MTAs exhibit discordance between activity in cells and with purified proteins. Dolastatin 15 and the 2-methoxyestradiol derivative mentioned above are examples. Dolastatin 15 is a MT-destabilizing peptide that is very potent in cells (GI50 1000-fold lower than taccalonolides), but it inhibits polymerization of purified tubulin very poorly and does not inhibit binding of other MTAs to tubulin. Dolastatin 15 was ultimately shown to bind tubulin,5 but the discordance between its in vivo and in vitro activities remains notable compared with other MTAs and is worth remembering in the context of taccalonolide activities described by Risinger and Mooberry. The cellular effects of taccalonolides are less reversible on removal of drug than is observed with paclitaxel. This persistence suggests that the drug may be significantly retained once inside the cell compared with most MTAs. Again, MTA diversity provides another example. Dolastatin 10 binds tightly to tubulin and is highly retained in cells;6 these properties likely contribute to its very low GI50. In contrast, taccalonolides show GI50 values ~105-fold higher than dolastatin 10 and appear not to bind tightly to tubulin. It will be interesting to reevaluate this with a labeled taccalonolide. While durability of drug effect may be seen as desirable, it may be a mixed blessing for a chemotherapeutic, since irreversibility could increase neurotoxicity. Finally, the suggestion that taccalonolides may target interphase MTs more efficiently than other MTAs is intriguing. If this is a new wrinkle on MTA activity, it may manifest as greater activity against slow-growing cells and human tumors compared with other MTAs. These various activities make the taccalonolides compounds to watch as the puzzles described in this paper are solved.