VCAM-1 Upregulation Contributes to Insensitivity of Vemurafenib in BRAF-Mutant Thyroid cancer

Vemurafenib, an inhibitor of mutant BRAF activity, is a promising anticancer agent for patients with BRAF-mutant metastatic melanoma. However, it is less effective in BRAF-mutant thyroid cancer, and the reason for this discrepancy is not yet fully elucidated. By RNA sequencing analysis, we identified vascular cell adhesion molecular-1 (VCAM-1) to be highly upregulated in both time- and dose-dependent manners during BRAF inhibition (BRAFi) in a BRAF-mutant papillary thyroid cancer cell line (BCPAP). Cell cytotoxicity and apoptosis assays showed that knockdown of the induced VCAM-1 in BCPAP cells augmented the antitumor effects of vemurafenib, with decreased IC50 values of 1.4 to 0.8 mu M. Meanwhile, overexpression of VCAM-1 in a BRAF-mutant anaplastic thyroid cancer cell line (FRO) reduced the sensitivity to vemurafenib, with increased IC50 values of 1.9 to 5.8 mu M. Further investigation showed that PI3K-Akt-mTOR pathway was activated during BRAFi. Co-treatment with Akt signaling inhibitor MK2206 decreased the induced expression of VCAM-1 during BRAFi. This combination further improved the efficacy of vemurafenib. Moreover, VCAM-1 promoted migration and invasion in thyroid cancer cells in vitro, which was also indicated in thyroid cancer patients. The present study is the first to demonstrate that VCAM-1 is upregulated in thyroid cancer cells treated with vemurafenib and contributes to vemurafenib resistance in BRAF-mutant thyroid cancer cells. Targeting the PI3K-Akt-mTOR pathway-mediated VCAM-1 response may be an alternative strategy to sensitize BRAF-mutant thyroid cancers to vemurafenib.