Microtubule-targeting agents augment the toxicity of DNA-damaging agents by disrupting intracellular trafficking of DNA repair proteins

The paradigm that microtubule-targeting agents (MTAs) cause cell death via mitotic arrest applies to rapidly dividing cells but cannot explain MTA activity in slowly growing human cancers. Many preferred cancer regimens combine a MTA with a DNA-damaging agent (DDA). We hypothesized that MTAs synergize with DDAs by interfering with trafficking of DNA repair proteins on interphase microtubules. We investigated nine proteins involved in DNA repair: ATM, ATR, DNA-PK, Rad50, Mre11, p95/NBS1, p53, 53BP1, and p63. The proteins were sequestered in the cytoplasm by vincristine and paclitaxel but not by an aurora kinase inhibitor, colocalized with tubulin by confocal microscopy and coimmunoprecipitated with the microtubule motor dynein. Furthermore, adding MTAs to radiation, doxorubicin, or etoposide led to more sustained γ-H2AX levels. We conclude DNA damage-repair proteins traffic on microtubules and addition of MTAs sequesters them in the cytoplasm, explaining why MTA/DDA combinations are common anticancer regimens.First developed as anticancer agents in the 1950s, microtubule targeting agents (MTAs) are used in the treatment of a wide variety of malignancies and until now have been thought to kill cells by arresting them in mitosis (1, 2). Although this explanation applies to rapidly dividing cells in preclinical models, it cannot explain the activity of these agents in tumors in humans because these cells divide much more slowly. For the latter situtation, a different paradigm must explain the activity of MTAs, and we have proposed that interfering with microtubule (MT) trafficking in interphase cells is the principal mechanism of MTA action (3–5). In breast, ovarian, lung, and head and neck cancers, as well as in most lymphomas, combination regimens that include a MTA and a DNA-damaging agent (DDA) are preferred (Table S1). Although the frequency with which these combinations are used might be fortuitous, it is likely there is a mechanistic basis for this outcome. We hypothesized that by hampering the trafficking of essential DNA repair proteins, MTAs synergize with DDAs, augmenting their toxicity. To explore this theory further we studied the effects of combining a MTA and a DDA in a number of cell models and examined the distribution and biology of nine different proteins involved in DNA repair. We have confirmed the hypothesis and report our findings.