Cell survival under stress is enhanced by a mitochondrial ATP-binding cassette transporter that regulates hemoproteins

AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO Overexpression of \#946;III-tubulin is associated with resistance to tubulin-binding agents (TBAs) in ovarian, breast and non-small cell lung cancer (NSCLC) tumors. We have recently shown that silencing \#946;III-tubulin expression hypersensitized NSCLC cells to paclitaxel, vincristine and DNA damaging agents (1). To determine whether \#946;III-tubulin mediates its effect on drug sensitivity by differentially regulating microtubule behavior, the effects of \#946;III-tubulin silencing on microtubule dynamics were analyzed in H460 NSCLC cells stably expressing GFP-\#946;I-tubulin. Interphase cells were examined at three concentrations of paclitaxel and vincristine; (i) the concentration that inhibited cell proliferation, (ii) the concentration that induced 5-10% mitotic arrest and (iii) the concentration that induced 30-40% mitotic arrest. In the absence of either drug, no significant changes in any parameters of microtubule dynamic instability were detected between \#946;III-tubulin knockdowns and control siRNA cells. At 1.6 nM paclitaxel (IC50 from drug-treated clonogenic assays), the overall dynamicity was significantly suppressed in \#946;III-tubulin knockdowns (-20.9%) compared to the controls (+0.3%). Similar results were obtained with vincristine treatment, suggesting that knockdown of \#946;III-tubulin induces hypersensitivity by enhancing stabilization of microtubule dynamics. Paradoxically, \#946;III-tubulin knockdowns had no further stabilizing effect on dynamicity at concentrations that induced 30-40% mitotic arrest. Mitotic index experiments revealed that \#946;III-tubulin knockdowns had reduced mitotic arrest in response to TBA treatment but nevertheless showed increased apoptosis compared to control siRNA cells. These results suggest that \#946;III-tubulin knockdown enhances the effectiveness of TBAs via two mechanisms: suppression of microtubule dynamics at low concentrations and a mitosis-independent mechanism of cell death at higher drug concentrations. 1. Gan PP, Pasquier E, Kavallaris M. (2007) Class III beta-tubulin mediates sensitivity to chemotherapeutic drugs in non-small cell lung cancer. Cancer Res . 67:9356-63 Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5560.