BRAF^V600E mutation is one of the most therapeutic targets in thyroid cancers. However, its specific inhibitors have shown little clinical benefit because they can reactivate the MAPK/ERK and PI3K/AKT pathways by feedback upregulating the transcription of HER3. Peptidyl-prolyl cis/trans isomerase Pin1 has been proven to be closely associated with tumor progression. Here, we aimed to determine antitumor activity of Pin1 inhibitor API-1 in thyroid cancer and its effect on cellular response to BRAF inhibitors. The results showed that API-1 exhibited strong antitumor activity against thyroid cancer. Meanwhile, it improved the response of BRAF-mutant thyroid cancer cells to BRAF inhibitor PLX4032 and there was a synergistic effect between them. Specially, a combination therapy of API-1 and PLX4032 significantly inhibited cell proliferation, colony formation, and the growth of xenograft tumors as well as induced cell apoptosis in BRAF-mutant thyroid cancer cells compared with API-1 or PLX4032 monotherapy. Similar results were also observed in transgenic mice with Braf^V600E-driven thyroid cancer. Mechanistically, API-1 enhanced XPO5 ability to export pre-microRNA 20a (pre-miR-20a) from the nucleus to cytoplasm, thereby promoting the maturation of miR-20a-5p. Further studies showed that miR-20a-5p specifically targeted and down-regulated HER3, thereby blocking the reactivation of MAPK/ERK and PI3K/AKT signaling pathways caused by PLX4032. These results, taken together, demonstrate that Pin1 inhibitor API-1 significantly improves the sensitivity of BRAF-mutant thyroid cancer cells to PLX4032. Thus, this study not only determines the potential antitumor activity of Pin1 inhibitor API-1 in thyroid cancer but also offers an alternative therapeutic strategy for BRAF-mutant thyroid cancers by a combination of Pin1 inhibitor and BRAF kinase inhibitor.

BRAF ^V600E突变是甲状腺癌最具治疗作用的靶点之一。然而，其特异性抑制剂几乎没有显示出临床益处，因为它们可以通过反馈上调 HER3 的转录来重新激活 MAPK/ERK 和 PI3K/AKT 通路。肽基脯氨酰顺/反异构酶Pin1已被证明与肿瘤进展密切相关。在这里，我们的目的是确定 Pin1 抑制剂 API-1 在甲状腺癌中的抗肿瘤活性及其对细胞对 BRAF 抑制剂反应的影响。结果表明API-1对甲状腺癌表现出很强的抗肿瘤活性。同时，提高了BRAF突变型甲状腺癌细胞对BRAF抑制剂PLX4032的反应，两者之间存在协同作用。特别是，与API-1或PLX4032单一疗法相比，API-1和PLX4032的联合疗法显着抑制BRAF突变型甲状腺癌细胞的细胞增殖、集落形成和异种移植肿瘤的生长，并诱导细胞凋亡。在患有 Braf ^V600E驱动的甲状腺癌的转基因小鼠中也观察到了类似的结果。从机制上讲，API-1增强了XPO5将前microRNA 20a (pre-miR-20a)从细胞核输出到细胞质的能力，从而促进miR-20a-5p的成熟。进一步研究表明，miR-20a-5p特异性靶向并下调HER3，从而阻断PLX4032引起的MAPK/ERK和PI3K/AKT信号通路的重新激活。这些结果综合起来表明，Pin1 抑制剂 API-1 显着提高了 BRAF 突变型甲状腺癌细胞对 PLX4032 的敏感性。因此，本研究不仅确定了 Pin1 抑制剂 API-1 在甲状腺癌中的潜在抗肿瘤活性，而且通过 Pin1 抑制剂和 BRAF 激酶抑制剂的组合为 BRAF 突变型甲状腺癌提供了替代治疗策略。