Issn Print: 0960-8931
Publication Date: 2006/09/01
A novel BH3 mimetic reveals a MAPK-dependent mechanism of melanoma cell survival controlled by p53 and reactive oxygen species
M. Verhaegen; J. Bauer; G. Wang; K. Wolter; C. Brenner; Z. Nikolovska-Coleska; T. Carey; S. Wang; M. Soengas
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Excerpt
The RAS/BRAF/MEK/ERK mitogen activated protein kinase (MAPK) pathway is emerging as a key modulator of melanoma initiation and progression. However, a variety of clinical studies indicate that inhibiting the MAPK pathway is insufficient per se to effectively kill melanoma cells. Here we report on a genetic and pharmacological approach to identify survival factors responsible for the resistance of melanoma cells to MEK/ERK antagonists. In addition, we describe a new tumor-cell selective means to bypass this resistance in vitro and in vivo. By generating a panel of isogenic cell lines with specific defects in the apoptotic machinery, we found that the ability of melanoma cells to survive in the absence of functional MEK relies on an ERK-independent expression of the anti-apoptotic factor Mcl-1 (and to a lesser extent, Bcl-xL and Bcl-2). Using computer-based modeling, we developed a novel Bcl-2 homology domain 3 (BH3) mimetic. This compound, named TW-37, is the first rationally designed small molecule with a high affinity for Mcl-1, Bcl-xL and Bcl-2. Importantly, TW-37 was able to promote a cell-selective killing of aggressive metastatic melanoma cells in the presence of MEK inhibitors. This synergy between TW-37 and MEK inhibition was also translated into an improved anti-tumor activity in mouse xenograft models. Mechanistic analyses of the mode of action of TW-37 revealed an unanticipated interplay between Bcl-2 family members and the MAPK pathway. BH3 mimetics are expected to engage the activation of the pro-apoptotic proteins BAX or BAK by relieving their blockage by anti-apoptotic Bcl-2 members. Therefore, an appealing feature of BH3 mimetics is their potential to bypass the requirement for upstream initiators of the mitochondrial pathway, for example the tumor suppressor p53. In contrast to this notion, we found that the cytotoxic effect of TW-37 relied on a feed-forward loop involving p53. Interestingly, the driving force behind the induction of p53 by TW-37 relied on a tumor-cell selective dysregulation of reactive oxygen species (ROS). However, in the presence of a functional MAPK pathway, p53 and ROS were maintained at low levels in TW-37-treated melanoma cells, which remained viable. MEK inhibitors relieved the protective effect against ROS and p53, and subsequently promoted cell death. Of note, normal melanocytes did not induce ROS, p53 or cell death in response to TW-37 and/or MEK inhibitors. In summary, we have designed and validated a new strategy based on a BH3 mimetic to engage pro-apoptotic activities of endogenous p53 and overcome the resistance of melanoma cells to MEK inhibitors. Moreover, from a mechanistic point of view, TW-37 unveiled an unexpected role of the MAPK pathway in the control ROS production that is dispensable for normal melanocytes. Our results suggest that this newly identified ROS/p53 feedback loop is a point of vulnerability of melanoma cells that can be exploited for rational drug design.
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