ABSTRACT Mycalolides are marine‐derived trisoxazole macrolides that exhibit potent actin‐depolymerizing activity and inhibit the proliferation, migration, and invasion of various cancer cells. The C25–C35 side‐chain moiety of mycalolides is important for their actin‐binding properties; however, the biological function of the C1–C24 trisoxazole macrolactone moiety remains largely unexplored. In this study, using a photoaffinity biotin probe, we identified tubulin as a new, actin‐independent target of mycalolide C (MyC). Ultracentrifugation, fluorescence‐based tubulin polymerization assays, and transmission electron microscopy of negatively stained microtubules (MTs) demonstrated that MyC and trisoxazole macrolactones exert potent synergistic effects on paclitaxel‐induced MT assembly and stabilization. These compounds enhanced the antiproliferative activity of paclitaxel by 4.2‐ to 6.7‐fold in HCT‐116 human colon cancer cells but showed little synergy in 3T3‐L1 murine fibroblast. Molecular modeling studies suggest that MyC stabilizes the protein–protein interactions between α/β‐tubulin heterodimers. Given that the C1–C24 trisoxazole macrolactone moiety and its analogues exhibit little cytotoxicity, our findings may contribute to the development of new lead compounds for MT inhibitors with reduced side effects.
Ebihara et al. (Tue,) studied this question.