EGFR blockade prevents glioma escape from BRAFV600E targeted therapy

Abstract

Mutational activation of BRAF (BRAFV600E) occurs in pediatric glioma and drives aberrant MAPK signaling independently of upstream cues. Targeted monotherapy against BRAFV600E displays efficacy in pre-clinical models of glioma, however xenograft tumors adapt rapidly and escape from the growth-inhibitory effects of BRAF-targeted therapy. Here, we show that intrinsic resistance to a BRAFV600E specific inhibitor stems, in part, from feedback activation of EGFR and downstream signaling pathways. BRAFV600E inhibition suppresses MAPK signaling, which in turn downregulates the EGFR phosphatase PTPN9, resulting in sustained EGFR phosphorylation and enhanced EGFR activity. We demonstrated that overexpression of PTPN9 reduces EGFR phosphorylation and cooperates with BRAFV600E inhibitor PLX4720 to reduce MAPK and Akt signaling, resulting in decreased glioma cell viability. Moreover, pharmacologic inhibition of EGFR combined with inhibition of BRAFV600E to reduce growth of glioma cell lines and orthotopic glioma xenograft by decreasing tumor cell proliferation while increasing apoptosis, with resultant significant extension of animal subject survival. Our data support clinical evaluation of BRAFV600E and EGFR targeted therapy in treating BRAFV600E glioma.

Concurrent MEK targeted therapy prevents MAPK pathway reactivation during BRAFV600E targeted inhibition in a novel syngeneic murine glioma model

“…Nonetheless, our results shown that 2341 grafts adapt to Tr monotherapy, whereas combination treatment delayed acquired resistance (Figure 6A). Our data are consistent with those obtained by studying human BRAF V600E mutant glioma cell lines, which point to re-activation of MAPK pathway and increased AKT and EGFR signaling activity as a mechanisms of adaptation and resistance to BRAF V600E inhibition (Supplementary Figure S3) [21, 46]. These similarities suggest that our mouse model has clinical relevance.…”

Autophagy inhibition overcomes multiple mechanisms of resistance to BRAF inhibition in brain tumors

“…This includes mutations in PTEN as shown above, as well as RAS mutations and activation of receptor tyrosine kinase signaling (Luke and Hodi, 2012). Feedback activation of EGFR has specifically been suggested as an escape pathway in BRAF V600E CNS tumor cells (Yao et al, 2015). The above data suggest that autophagy inhibition is able to overcome BRAFi resistance in different tumor types, and that distinct mechanisms of resistance can be similarly targeted by this approach.…”

“…Feedback activation of EGFR, represents another mechanistically-distinct resistance mechanism that can provide an escape pathway in BRAF V600E CNS tumor cells (Yao et al, 2015). Therefore, we also developed cell lines with EGFR overexpression (EGFRoe) and evaluated their response to autophagy inhibition.…”

“…The initial excitement for BRAF inhibitors (BRAFi) in other tumors was tempered because the majority of patients who initially respond to RAF inhibition quickly develop resistance to therapy (Hartsough et al, 2014; Sun et al, 2014). This is a significant issue in brain tumors as well (Levy et al, 2014; Yao et al, 2015). …”

“…EGFR is another potential secondary target in melanoma, brain, and colorectal cancer. Although encouraging preclinical results have been obtained in these tumors (Yao et al, 2015; Corcoran et al, 2012; Girotti et al, 2013), combined BRAF/EGFR inhibition similarly leads to incomplete and short-term responses in people (Ahronian et al, 2015; Pietrantonio et al, 2016). …”