American Association for Cancer Research
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Figure S3 from Fibronectin Contributes to a BRAF Inhibitor–driven Invasive Phenotype in Thyroid Cancer through EGR1, Which Can Be Blocked by Inhibition of ERK1/2

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posted on 2023-09-01, 08:41 authored by Hannah M. Hicks, Nikita Pozdeyev, Sharon B. Sams, Umarani Pugazhenthi, Elise S. Bales, Marie-Claude Hofmann, Logan R. McKenna, Rebecca E. Schweppe

Figure S3. Sensitivity to BRAF inhibition does not correlate with fibronectin, α5β1 integrin, c-MET, or FAK/Src levels. BRAF inhibition increases fibronectin expression and secretion, which is blocked by ERK1/2 inhibition.


National Cancer Institute (NCI)

United States Department of Health and Human Services

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Mutations in BRAF are common in advanced papillary and anaplastic thyroid cancer (PTC and ATC). However, patients with BRAF-mutant PTC currently lack therapies targeting this pathway. Despite the approved combination of BRAF and MEK1/2 inhibition for patients with BRAF-mutant ATC, these patients often progress. Thus, we screened a panel of BRAF-mutant thyroid cancer cell lines to identify new therapeutic strategies. We showed that thyroid cancer cells resistant to BRAF inhibition (BRAFi) exhibit an increase in invasion and a proinvasive secretome in response to BRAFi. Using reverse-phase protein array (RPPA), we identified a nearly 2-fold increase in expression of the extracellular matrix protein, fibronectin, in response to BRAFi treatment, and a corresponding 1.8- to 3.0-fold increase in fibronectin secretion. Accordingly, the addition of exogenous fibronectin phenocopied the BRAFi-induced increase in invasion while depletion of fibronectin in resistant cells resulted in loss of increased invasion. We further showed that BRAFi-induced invasion can be blocked by inhibition of ERK1/2. In a BRAFi-resistant patient-derived xenograft model, we found that dual inhibition of BRAF and ERK1/2 slowed tumor growth and decreased circulating fibronectin. Using RNA sequencing, we identified EGR1 as a top downregulated gene in response to combined BRAF/ERK1/2 inhibition, and we further showed that EGR1 is necessary for a BRAFi-induced increase in invasion and for induction of fibronectin in response to BRAFi. Together, these data show that increased invasion represents a new mechanism of resistance to BRAF inhibition in thyroid cancer that can be targeted with an ERK1/2 inhibitor.

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