BRAF inhibition upregulates a variety of receptor tyrosine kinases and their downstream effector Gab2 in colorectal cancer cell lines
Herr R, Halbach S, Heizmann M, Busch H, Boerries M, Brummer T.
BRAF mutations occur in approximately 10% of colorectal cancer (CRC) and are associated with an aggressive and therapy-resistant phenotype. Inhibitors targeting BRAFV600E, the most common BRAF mutant, elicit only poor response rates in BRAF-mutant CRC as single agents. Mechanistically, this unresponsiveness was attributed to the loss of negative feedbacks on the epidermal growth factor receptor (EGFR) and initiated clinical trials that combine BRAF (and MEK) inhibitors, either singly or in combination, with the anti-EGFR antibodies. Although these combinations still elicited variable response rates, the trials demonstrated improved efficacy. Here, we show that BRAF inhibition leads to the up-regulation of a variety of receptor tyrosine kinases (RTKs) in CRC cell lines, including not only the EGFR, but also human epidermal growth factor receptor (HER) 2 and HER3. Importantly, combination of the BRAF inhibitors (BRAFi) vemurafenib (PLX4032), dabrafenib or encorafenib with inhibitors dually targeting the EGFR and HER2 (such as lapatinib, canertinib and afatinib) significantly reduced the metabolic activity and proliferation of CRC cells. This re-sensitization was also observed after genetic depletion of HER2 or HER3. Interestingly, BRAF inhibition also increased the expression of the GRB2-associated binders (Gab) 1 and Gab2, two important amplifiers of RTK signaling. Furthermore, Gab2 and Gab2-mediated Shp2 signaling were shown to be functionally important in BRAFi resistance. Therefore, this study highlights potential new escape mechanisms to combinatorial therapies and indicate that targeted therapy in BRAF mutant CRC might benefit from broad suppression of RTK signaling.
