Abstract
BRAF functions in the RAS-extracellular signal-regulated kinase (ERK) signaling cascade. Activation of this pathway is necessary to mediate the transforming potential of oncogenic BRAF, however, it may also cause a negative feedback that inhibits the epidermal growth factor receptor (EGFR). Mitogen-inducible gene-6 (MIG-6) is a potent inhibitor of the EGFR and has been demonstrated to function as a tumor suppressor. As MIG-6 can be induced via RAS-ERK signaling, we investigated its potential involvement in this negative regulatory loop. Focus formation assays were performed and demonstrated that MIG-6 significantly reduces malignant transformation induced by oncogenic BRAF. Although this genetic interaction was mirrored by a physical interaction between MIG-6 and BRAF, we did not observe a direct regulation of BRAF kinase activity by MIG-6. Interestingly, a selective chemical EGFR inhibitor suppressed transformation to a similar degree as MIG-6, whereas combining these approaches had no synergistic effect. By analyzing a range of BRAF mutated and wildtype cell line models, we could show that BRAF V600E causes a strong upregulation of MIG-6, which was mediated at the transcriptional level via the RAS-ERK pathway and resulted in downregulation of EGFR activation. This feedback loop is operational in tumors, as shown by the analysis of almost 400 patients with papillary thyroid cancer (PTC). Presence of BRAF V600E correlated with increased MIG-6 expression on the one hand, and with inactivation of the EGFR and of PI3K/AKT signaling on the other hand. Importantly, we also observed a more aggressive disease phenotype when BRAF V600E coexisted with low MIG-6 expression. Finally, analysis of methylation data was performed and revealed that higher methylation of MIG-6 correlated to its decreased expression. Taken together, we demonstrate that MIG-6 efficiently reduces cellular transformation driven by oncogenic BRAF by orchestrating a negative feedback circuit directed towards the EGFR.
Highlights
Somatic mutations within BRAF have been described in a broad range of human tumors, with melanoma, thyroid cancer and colorectal cancer affected most frequently [1,2,3]
We focused our attention on the role of Mitogen-inducible gene-6 (MIG-6) in epidermal growth factor receptor (EGFR) regulation, given that pharmacological inhibition of EGFR was equipotent and non-additive to MIG-6 in curbing cell transformation mediated by BRAF V600E
Our data obtained in papillary thyroid cancer (PTC) patients, showing correlation of BRAF V600E with increased expression of MIG-6, and with decreased activities of the EGFR and the PI3K/AKT pathways, further support this hypothesis and provide the basis for future studies addressing this issue
Summary
Somatic mutations within BRAF have been described in a broad range of human tumors, with melanoma, thyroid cancer and colorectal cancer affected most frequently [1,2,3]. The V600E mutation constitutes the most important alteration conferring high kinase activity and accounting for approximately 90% of BRAF mutations This mutation has been extensively studied and proved to be a bona fide oncogene as evidenced by in-vitro and in-vivo models [1, 3]. Prahallad and coworkers demonstrated that in colorectal cancer this resistance is caused by the ability of BRAF V600E to induce an inhibitory feedback circuit towards the EGFR [6]. They showed that pharmacologic inhibition of V600E resulted in reactivation of the EGFR, which supported continued proliferation and transformation. In recent studies of thyroid carcinoma and melanoma, SPRY2 and SOX10 have been identified as additional BRAF-EGFR feedback mediators, which suggests that feedback signaling of BRAF V600E to the EGFR might be more complex than initially thought and includes other, hitherto unknown proteins as well [7, 8]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
