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 beta II- and beta III-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 beta I-tubulin and overexpression of beta II- and beta III-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 beta-tubulin. To understand the functional significance of the beta II- and beta III-tubulin changes in this resistant cell line, isotype-specific short interfering RNA was used to knock down the expression of the beta II and beta III isotypes, and the cellular effects of PLA and laulimalide were examined before and after silencing. It was found that inhibition of beta II- and beta III-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 G2-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 beta II- and beta III-tubulin contributed significantly to the resistance phenotype, along with the tubulin structural mutation, and that the altered isotype effect was binding site specific. Mol Cancer Ther; 11(2); 393-404. (C) 2011 A ACR.