br Results br Discussion Adaptive drug
Discussion Adaptive drug resistance is a major challenge to the clinical success of cancer therapies. Incomplete inhibition of oncogenic signaling allows survival of “drug-tolerant” tumor cells, which persist in that state for variable periods before acquiring additional genetic mutations associated with acquired drug-resistance and tumor relapse (Hata et al., 2016, Sharma et al., 2010). For example, complete suppression of ERK activity (over 85%) has been shown to be required for significant tumor response in BRAF mutant melanomas (Bollag et al., 2010). In this context, adaptive resistance is frequently associated with homeostatic mechanisms, such as negative feedback, which are mobilized upon target inhibition and lead to ERK signaling rebound in the presence of the drug. Activation of RTK signaling has been found to drive adaptive resistance in various ERK-dependent tumor contexts, including BRAF(V600E) melanoma, colorectal and thyroid cancers, and TNBC and RAS mutant tumors (Chandarlapaty, 2012, Corcoran et al., 2012, Duncan et al., 2012, Karoulia et al., 2016, Karoulia et al., 2017, Lito et al., 2012, Montero-Conde et al., 2013, Prahallad et al., 2012, Samatar and Poulikakos, 2014, Shaffer et al., 2017, Sun and Bernards, 2014). However, because adaptive resistance is mediated by various RTKs across various tumor types, or even in the same tumor, establishing effective approaches for combined targeting of ERK signaling and individual RTKs is challenging. Here, we characterized the strategy of targeting SHP2, a phosphatase that mediates RAS activation downstream of multiple RTKs to overcome adaptive resistance to ERK signaling inhibitors. SHP2 (PTPN11) has been found to be required for full activation of RAS/ERK pathway in several contexts (Dance et al., 2008); however, the mechanistic details of how SHP2 regulates RAS activity downstream of RTK signaling remain unclear. Catalytic (phosphatase) activity of SHP2 has been shown to be critical for RAS/ERK activation, and SHP2 has been reported to dephosphorylate a number of proteins, including platelet-derived growth factor R547 (PDGFR) (Klinghoffer and Kazlauskas, 1995), EGFR(Agazie and Hayman, 2003), and GAB (Montagner et al., 2005); however, the relevant SHP2 substrate has not been conclusively identified. On the other hand, SHP2 has been shown to act as a scaffold protein recruiting GRB2/SOS complex to the membrane and promoting RAS activation (Dance et al., 2008, Grossmann et al., 2010). The allosteric SHP2 inhibitor used in our study (SHP099) both inhibits the catalytic activity and stabilizes the inactive conformation of SHP2 (Chen et al., 2016), resulting in the disruption of SHP2 interaction with other adaptor proteins, such as GRB2 and GAB1, and the concomitant decrease of RAS activity. In this study, we used p(Y542)SHP2 as a surrogate marker for SHP2 activation. However, the regulatory role of the SHP2 C-terminal phosphorylation remains incompletely understood. It has been shown that the phosphorylation of tyrosine 542 and 580 at the C-terminal tail of SHP2 are the main recruitment events for GRB2/SOS and subsequent activation of downstream RAS/ERK signaling (Bennett et al., 1994, Vogel and Ullrich, 1996), whereas another report showed that mutation of those sites had no functional effect on SHP2 signaling (O’Reilly and Neel, 1998). Nonetheless, in our experiments using SHP099, the interaction of SHP2 with GRB2 and GAB1 and RAS/ERK activation always correlated with phosphorylation of SHP2 at Y542, suggesting that, at least in this context, p(Y542)SHP2 can serve as a marker of SHP2 activation downstream of RTK signaling. TNBC represents about 15% of breast tumors, and typically has a poorer outcome compared with other breast cancer subtypes because of an inherently more aggressive clinical behavior and the current lack of targeted therapeutic options (Bianchini et al., 2016). MEK inhibitors have shown preclinical activity in TNBC (Hoeflich et al., 2009), but feedback activation of upstream RTKs has been shown to limit their efficacy (Duncan et al., 2012). We show here that combined MEK and SHP2 inhibition had a profound inhibitory effect on both ERK signaling and cell growth in all TNBC lines tested, including RAS mutant and RTK-overexpressing TNBC tumor cells, suggesting that this combination provides a powerful therapeutic strategy for patients with this aggressive tumor type (Figure 7A).