Abstract 5481: Differential inhibition of microtubule-dependent transport and signaling pathways important for cancer cell growth and survival
Authors: Rohena Cristina C. , Mooberry Susan L. 
Published: May 2015

Microtubule targeting agents are some of the most valuable drugs used in the treatment of cancer. Recent studies demonstrate that the effects of these agents on interphase cells are likely central to their anticancer actions. The ability of microtubule targeting agents to interrupt multiple microtubule-dependent oncogene-driven signaling cascades might be one mechanism by which they initiate cancer cell death. The effects of diverse microtubule targeting agents on two microtubule-dependent processes essential for cancer cell survival were evaluated; TGF-β-mediated SMAD2/3 transport into the nucleus and the internalization and endocytosis of the EGFR. The ability of diverse microtubule targeting agents including paclitaxel, taccalonolide AJ, laulimalide and vinblastine that bind within different sites on tubulin/microtubules to interrupt TGF-β-mediated signaling pathways that promote cell growth, invasion and metastasis was evaluated. Using a variety of microscopy techniques, we show that microtubule stabilizers inhibit this pathway by preventing SMAD2/3 nuclear accumulation and subsequent activation of target genes. On the other hand, vinblastine caused an apparent increase in nuclear SMAD2/3 as compared to vehicle-treated cells. These effects, however, were not seen in cell lines driven by TGF-β pathways. Our results suggest that in cell lines with amplified TGF-β signaling, the microtubule regulatory component is lost.

The effects of the microtubule targeting agents on EGFR internalization and endocytosis were also evaluated in A549 NSCLC cells and BT549 triple negative breast cancer cells; both of which overexpress EGFR. Taccalonolide AJ initiated distinct changes in the localization of the EGFR, impaired its internalization and endosomal sorting, leading to faster internalization and enhanced sorting to late endosomes. The data suggest that taccalonolide AJ causes premature degradation of EGFR and thus inhibited downstream signaling. Vinblastine on the other hand, did not impair the internalization of the EGFR or its degradation. Surprisingly the microtubule targeting agents had different effects on AKT phosphorylation, suggesting that they modulate EGFR dependent pathways in distinct ways. Identification of the specific signaling pathways and downstream targets interrupted by the different microtubule targeting agents might identify new rational drug combinations. This has the potential to identify patient populations that might respond better to certain microtubule targeting agents and begins to shed some light into the complex and subtly different mechanisms of action of these drugs.

Citation Format: Cristina C. Rohena, Susan L. Mooberry. Differential inhibition of microtubule-dependent transport and signaling pathways important for cancer cell growth and survival. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5481. doi:10.1158/1538-7445.AM2014-5481