Bortezomib in combination with mitotic inhibitors as a novel strategy for CML: a model of the combined treatments-induced cell death.

<p>Our results show that bortezomib in combination with several mitotic inhibitors, known to suppress mitosis through different mechanisms, is able to downregulate total levels and/or phosphorylation of Bcr-Abl at the Tyr 177 site and to inactivate the Bcr-Abl downstream pathways, mediated by Lyn, CrkL or STAT3/STAT5. These effects are associated with caspase-mediated cell death. Tyr177 phosphorylation mediates Bcr-Abl downstream signaling by inducing the formation of a Lyn-Gab2-Bcr-Abl complex, and is required for Bcr-Abl-induced leukemia [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B26" target="_blank">26</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B27" target="_blank">27</a>]. This binding results in Lyn activation by phosphorylation [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B28" target="_blank">28</a>]. Lyn kinase further regulates survival and responsiveness of CML cells to inhibition of Bcr-Abl kinase [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B19" target="_blank">19</a>]. Interestingly, Lyn can also phosphorylate Tyr177 in Bcr-Abl [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B26" target="_blank">26</a>], resulting in a potential feedback mechanism. The Gab2-Bcr-Abl complex is mediated by Grb2 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B3" target="_blank">3</a>] and results in activation of downstream proliferative/survival pathways, such as Ras-ERK and PI3K-Akt. Gab proteins couple growth factor and cytokine receptors to downstream proteins, resulting in activation of the downstream pathways Ras-ERK, PI3K-Akt and JAK/STAT pathways [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B3" target="_blank">3</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B29" target="_blank">29</a>]. Bcr-Abl phosphorylates and activates STAT3 and STAT5 transcription factors inducing survival and proliferation. Constitutive activation of STAT3/STAT5 is critical for the maintenance of chronic myeloid leukemia [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B20" target="_blank">20</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B30" target="_blank">30</a>]. Bcr-Abl also binds the C-terminal Proline-rich region of the adaptor protein CrkL [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B30" target="_blank">30</a>]. Bcr-Abl phosphorylates CrkL, an event needed for Bcr-Abl-induced leukemia. CrkL can enhance cell migration and Bcr-Abl-mediated leukemogenesis [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B6" target="_blank">6</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B18" target="_blank">18</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B31" target="_blank">31</a>-<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077390#B34" target="_blank">34</a>].