βII-tubulin and βIII-tubulin mediate sensitivity to peloruside A and laulimalide, but not paclitaxel or vinblastine, in human ovarian carcinoma cells

Increased abundance of βII- and βIII-tubulin isotypes in cancer cells confers resistance to vinca and taxoid site drugs; however, the role of these isotypes in the acquired resistance of cancer cells to non-vinca or non-taxoid site binding agents has not been described. Peloruside A (PLA) and laulimalide are the only known non-taxoid site microtubule-stabilizing agents. A human ovarian cancer cell line, 1A9-L4 (L4), previously selected in high concentrations of laulimalide, has both a single point mutation in bI-tubulin and overexpression of βII- and βIII-tubulin. The cells are highly resistant to PLA as well as laulimalide but show no cross-resistance to taxoid site drugs or drugs that bind to the vinca site on β-tubulin. To understand the functional significance of the βII- and βIII-tubulin changes in this resistant cell line, isotype-specific short interfering RNA was used to knock down the expression of the βII and βIII isotypes, and the cellular effects of PLA and laulimalide were examined before and after silencing. It was found that inhibition of βII- and βIII-tubulin partially sensitized L4 cells to PLA and laulimalide, as seen by increased potency of PLA and laulimalide for inducing growth inhibition, cellular tubulin polymerization, microtubule aberrations, and G ₂-M arrest in the resistant cells. The sensitivity to paclitaxel, vinblastine, ixabepilone, and cisplatin was unaffected by the inhibition of isotype expression. It was concluded that the increased βII- and βIII-tubulin contributed significantly to the resistance phenotype, along with the tubulin structural mutation, and that the altered isotype effect was binding site specific.