Division of Endocrinology, Metabolism, and Diabetes, Department of Medicine, University of Colorado School of Medicine, Aurora, Colorado.
Department of Biomedical Informatics, University of Colorado School of Medicine, Aurora, Colorado.
Mol Cancer Res. 2023 Sep 1;21(9):867-880. doi: 10.1158/1541-7786.MCR-22-1031.
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.
BRAF 突变在晚期甲状腺乳头状癌和间变性甲状腺癌（PTC 和 ATC）中很常见。然而，目前 BRAF 突变型 PTC 患者缺乏针对该途径的治疗方法。尽管 BRAF 和 MEK1/2 抑制联合批准用于 BRAF 突变型 ATC 患者，但这些患者常常进展。因此，我们筛选了一组 BRAF 突变型甲状腺癌细胞系，以确定新的治疗策略。我们表明，对 BRAF 抑制（BRAFi）有抗药性的甲状腺癌细胞在受到 BRAFi 治疗时表现出侵袭性增加和促侵袭性的分泌组。使用反相蛋白质阵列（RPPA），我们发现，在对 BRAFi 治疗的反应中，细胞外基质蛋白纤维连接蛋白的表达增加了近 2 倍，而纤维连接蛋白的分泌增加了 1.8 至 3.0 倍。因此，外源性纤维连接蛋白的添加模拟了 BRAFi 诱导的侵袭性增加，而在耐药细胞中耗尽纤维连接蛋白则导致侵袭性增加的丧失。我们进一步表明，BRAFi 诱导的侵袭可以通过抑制 ERK1/2 来阻断。在 BRAFi 耐药的患者衍生异种移植模型中，我们发现 BRAF 和 ERK1/2 的双重抑制减缓了肿瘤生长并降低了循环纤维连接蛋白。通过 RNA 测序，我们发现 EGR1 是对联合 BRAF/ERK1/2 抑制反应中下调最多的基因，我们进一步表明，EGR1 是 BRAFi 诱导的侵袭性增加和对 BRAFi 反应中纤维连接蛋白诱导所必需的。
这些数据表明，侵袭性增加代表了甲状腺癌对 BRAF 抑制的一种新的耐药机制，可以通过 ERK1/2 抑制剂靶向。
