Abstract 20: Raf Kinase Inhibitors Can Induce Raf Dimerization, Downstream Signaling, and Cell Growth

Abstract Genetic alterations in the Ras/Raf/MEK/ERK pathway are among the most common in human cancers. Up to 70% of melanomas harbor B-Raf mutations, and roughly 90% of pancreatic tumors have K-Ras mutations. To address these Raf pathway-driven cancers, small molecule Raf kinase inhibitors have been developed and are currently under clinical investigation. In B-RafV600E cells, Raf compounds inhibit signaling through MEK and ERK, resulting in the expected anti-proliferative effects. Paradoxically, in wild-type Raf cells and in mutant Ras cells, these compounds induce downstream signaling and can induce cell growth in some settings in vitro. While the induction of downstream signaling has previously been attributed to published Raf pathway feedback loops, this has not been proven directly. In fact, we show here that induction of pMEK and pERK can occur within minutes of Raf compound treatment, even before reported feedback phosphorylation events are seen on B-Raf and C-Raf. Interestingly, the induction of signaling and cell growth both occur in a biphasic pattern, with low compound concentrations (0.01-0.1 uM) causing maximal induction, and higher compound concentrations (1-10 uM) causing less profound induction. Such a biphasic pattern is also observed in biochemical assays with purified wild-type B-Raf or C-Raf. The biphasic pattern is suggestive of a mechanism involving the interaction of two signaling subunits. In addition, recent literature data (Rajakulendran, Nature, 461:542-6) has demonstrated that Raf dimerization can upregulate pMEK, not through trans-phosphorylation of Raf molecules but presumably by conformational activation of the kinase. Consistent with that model, we show that Raf compound treatment induces B/C-Raf dimer formation in cells. In addition, knockdown of A-, B- or C-Raf with siRNA does not abrogate the Raf compound induction of pMEK and pERK, suggesting that induction might be mediated by Raf homo- as well as hetero-dimerization. Notably, knockdown of K-Ras in K-RasMUT cells also does not abolish the induction, implying that this effect is not mediated by Ras. Taken together, these data suggest a model in which compound binding to one Raf molecule induces dimerization and conformational activation of a partner Raf molecule in the dimer. These observations can explain why wild-type Raf and mutant Ras tumors are insensitive to selective Raf kinase inhibitors and might also have important implications for toxicity, since induction of strong mitogenic signaling could lead to hyperproliferation of normal tissues. Understanding the Raf compound induction mechanism may lead not only to the design of improved inhibitors, but also to methods for overcoming the induction seen with current development compounds. Toward that end, we show that combining a MEK inhibitor with a Raf compound causes inhibition of both pERK and cell growth and may therefore have significant advantages in the clinic. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 20.