| An important feature in most cases of non-small cell lung cancer (NSCLC) is the deregulation of the classical MAPK (mitogen-activated protein kinase) pathway. The MAPK pathway consists of enzymes that under normal conditions activate one another in a tightly regulated fashion in order to promote cell proliferation, differentiation and growth. Activated receptors (e.g., EGFR) activate the small protein Ras, which in turn activates Raf. Activated Raf phosphorylates and activates MEK, which in turn phosphorylates and activates ERK (EGFR/Ras/Raf/MEK/ERK cascade). Activated ERK phosphorylates multiple substrates both in the cell cytoplasm and in the nucleus that promote a number of growth-related functions. Mutations in components of the pathway result in constitutively activated molecules and deregulated activation of the cascade. In NSCLC, activating mutations have been found in EGFR, Ras and the Raf isoform BRAF. There is accumulating evidence that in certain contexts, tumors depend on one or more pathways that are activated by mutated enzymes. The addiction of certain tumors to a specific mutation provides opportunity for therapeutic intervention with specific inhibitors (targeted therapy). This concept has been validated in cell culture models, in mice and in patients with other cancers, such as CML (chronic myelogenous leukemia) and GIST (gastrointestinal stromal tumors).
NSCLC has not only proven highly lethal, it has also been extremely refractory to broadly applied chemotherapy. Targeted therapy of the MAPK pathway in NSCLC is highly attractive because it may offer effective therapy with less toxicity. Recently, EGFR inhibitors conferred improved survival to patients with advanced NSCLC when combined with chemotherapy. Our lab recently demonstrated that tumors harboring BRAF mutations depend on MEK/ERK signaling and are sensitive to inhibition of the downstream target, MEK. In collaboration with Dr. G. Bollag (Plexikkon Inc) we are now developing and evaluating the highly potent and selective Raf kinase inhibitor PLX4032. PLX4032 is currently in a Phase I clinical trial involving patients with metastatic melanoma in Memorial Sloan-Kettering Cancer Center and at three additional Cancer Centers in the U.S. Our preliminary data suggest that, even though Raf and MEK are considered to lie within a linear signaling pathway, the effects of Raf inhibition differ substantially from those of MEK inhibition. The most profound difference is that, unlike MEK inhibition which effectively downregulates ERK in all cell lines, Raf inhibitors selectively inhibit ERK only in cells carrying mutant BRAF. We hypothesize that such differences in the consequences of MEK inhibition and Raf inhibition are due to feedback regulation of the MAPK pathway at the level of Raf. Moreover, we propose that Raf inhibition can be an effective therapeutic strategy for the specific subset of NSCLC patients whose tumors depend on Raf activity.
Our proposal is outlined in the following specific aims:
Aim 1. To compare the effect of the Raf kinase inhibitor PLX4032 to MEK inhibition in NSCLC in lung cancer cells and in lung tumors.
Aim 2. To biochemically characterize the effects of Raf inhibition. Accumulating evidence suggests that the interplay between the Raf isoforms is a critical point of MAPK regulation. We will determine the effects of Raf inhibition on the heterodimerization between BRAF and c-Raf and on the formation of the signaling complex with Ras and MEK.
Our project has two parallel aims that bridge basic and translational research: We will validate the use of Raf kinase inhibitor PLX4032 for targeted therapeutic intervention in NSCLC. In addition, we will use this specific inhibitor as a tool to interrogate the regulation of the MAPK pathway. Our findings will help elucidate mechanisms of regulation of Raf activity, which shall ultimately have important clinical implications.
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