Wan Lixin, Chen Ming, Cao Juxiang, Dai Xiangpeng, Yin Qing, Zhang Jinfang, Song Su-Jung, Lu Ying, Liu Jing, Inuzuka Hiroyuki, Katon Jesse M, Berry Kelsey, Fung Jacqueline, Ng Christopher, Liu Pengda, Song Min Sup, Xue Lian, Bronson Roderick T, Kirschner Marc W, Cui Rutao, Pandolfi Pier Paolo, Wei Wenyi
Department of Pathology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts.
Department of Molecular Oncology, H. Lee Moffitt Cancer Center and Research Institute, Tampa, Florida.
Cancer Discov. 2017 Apr;7(4):424-441. doi: 10.1158/2159-8290.CD-16-0647. Epub 2017 Feb 7.
BRAF drives tumorigenesis by coordinating the activation of the RAS/RAF/MEK/ERK oncogenic signaling cascade. However, upstream pathways governing BRAF kinase activity and protein stability remain undefined. Here, we report that in primary cells with active APC, APC earmarks BRAF for ubiquitination-mediated proteolysis, whereas in cancer cells with APC-free FZR1, FZR1 suppresses BRAF through disrupting BRAF dimerization. Moreover, we identified FZR1 as a direct target of ERK and CYCLIN D1/CDK4 kinases. Phosphorylation of FZR1 inhibits APC, leading to elevation of a cohort of oncogenic APC substrates to facilitate melanomagenesis. Importantly, CDK4 and/or BRAF/MEK inhibitors restore APC E3 ligase activity, which might be critical for their clinical effects. Furthermore, depletion cooperates with AKT hyperactivation to transform primary melanocytes, whereas genetic ablation of synergizes with loss, leading to aberrant coactivation of BRAF/ERK and AKT signaling in mice. Our findings therefore reveal a reciprocal suppression mechanism between FZR1 and BRAF in controlling tumorigenesis. FZR1 inhibits BRAF oncogenic functions via both APC-dependent proteolysis and APC-independent disruption of BRAF dimers, whereas hyperactivated ERK and CDK4 reciprocally suppress APC E3 ligase activity. Aberrancies in this newly defined signaling network might account for BRAF hyperactivation in human cancers, suggesting that targeting CYCLIN D1/CDK4, alone or in combination with BRAF/MEK inhibition, can be an effective anti-melanoma therapy. .
BRAF通过协调RAS/RAF/MEK/ERK致癌信号级联反应的激活来驱动肿瘤发生。然而，调控BRAF激酶活性和蛋白质稳定性的上游通路仍不明确。在此，我们报道，在具有活性APC的原代细胞中，APC将BRAF标记为泛素化介导的蛋白水解的目标，而在无APC的癌细胞中，FZR1通过破坏BRAF二聚化来抑制BRAF。此外，我们确定FZR1是ERK和细胞周期蛋白D1/细胞周期蛋白依赖性激酶4（CDK4）激酶的直接靶点。FZR1的磷酸化抑制APC，导致一组致癌性APC底物水平升高，从而促进黑色素瘤发生。重要的是，CDK4和/或BRAF/MEK抑制剂可恢复APC E3连接酶活性，这可能对它们的临床效果至关重要。此外，FZR1的缺失与AKT过度激活协同作用，可使原代黑素细胞发生转化，而FZR1的基因敲除与APC缺失协同作用，会导致小鼠体内BRAF/ERK和AKT信号异常共激活。因此，我们的研究结果揭示了FZR1和BRAF在控制肿瘤发生过程中的相互抑制机制。FZR1通过APC依赖性蛋白水解和BRAF二聚体的APC非依赖性破坏来抑制BRAF的致癌功能，而过度激活的ERK和CDK4则相互抑制APC E3连接酶活性。这个新定义的信号网络中的异常可能是人类癌症中BRAF过度激活的原因，这表明单独或与BRAF/MEK抑制联合靶向细胞周期蛋白D1/CDK4可能是一种有效的抗黑色素瘤疗法。
