KRAS-MEK1核转位-YAP新通路的确证及其在肠癌直接靶向突变KRAS分子治疗中的意义

KRAS mutations occur in approximately 40% of colorectal cancers, but no effective pharmacological inhibitors of KRAS oncoproteins have reached the clinic. Thus, it would be very meaningful to illuminate the mechanisms of resistance to mutant KRAS-targeted inhibitors in colorectal cancer. The cytoplasmic kinase MEK1, a downstream effector of RAS pathway, could also shuttle rapidly into and out of the nucleus. In the preliminary work, we unexpectedly found that mutant KRAS knock down could induce MEK1 nuclear translocation, and MEK1 nuclear translocation could inhibit the degradation of YAP, a core component of Hippo pathway, which could promote the progression of colorectal cancer. Hereby, we raised a presumption: MEK1 nuclear translocation could stabilize YAP, and result in the resistance to mutant KRAS-targeted therapy in colorectal cancer. The project plans to focus on the verification of mutant KRAS-MEK1 nuclear translocation-YAP axis and its mechanisms, and identify whether MEK inhibitor trametinib, which was found to inhibit MEK1 nuclear translocation in our study, could sensitize colorectal cancer cells to mutant KRAS-targeted therapy. This study will offer new perspectives of the relationship between mutant KRAS/MEK1 nuclear translocation/YAP, and provide a novel combined therapeutic strategy for the KRAS mutantcolorectal cancer.

约40%的肠癌存在KRAS突变，而直接靶向突变KRAS分子的治疗研究鲜有进展。最近研究显示即使完全敲除突变KRAS，肿瘤仍可继续存活，因此阐明肠癌中靶向突变KRAS的抵抗机制意义重大。RAS通路中的胞浆激酶MEK1可短暂进出胞核，课题组前期研究意外发现：干扰突变KRAS能促进MEK1入核；而MEK1入核可明显抑制Hippo通路的核心蛋白YAP的降解，后者则可促进肠癌细胞的存活。据此我们提出假设：MEK1核转位促进YAP稳定可引起肠癌对靶向突变KRAS的抵抗。针对该假设，本项目拟通过体内外实验对突变KRAS调控MEK1核转位/YAP稳定的具体机制，及MEK抑制剂曲美替尼（我们发现其可以抑制MEK1核转位）能否增强肠癌对直接靶向突变KRAS治疗的敏感性等问题进行系统深入的研究。该项目将确证突变KRAS/MEK1核转位/YAP这一新通路，并为靶向KRAS突变型肠癌提供新的联合治疗策略。

丝裂原活化蛋白激酶激酶（mitogen-activated protein kinase kinase, MEK）是直接激活细胞外调节蛋白激酶（extracellular regulated protein kinase, ERK）的一种蛋白激酶。由于ERK通路上游EGFR/RAS/RAF的突变，MEK在很多肿瘤中都呈异常活化状态。虽然靶向MEK的小分子抑制剂已成功应用于临床黑色素瘤的治疗，但MEK蛋白的功能并未被研究透彻。MEK由于其氨基端带有核输出信号（nuclear export signal , NES）而主要定位于胞浆，但实际大部分MEK分子都是在细胞核和细胞浆中来回穿梭，而目前关于MEK的这种动态易位的调节机制和功能仍不清楚。. YAP蛋白是一个转录共激活因子，调控着器官大小、组织再生、干细胞更新和肿瘤形成。β-TrCP蛋白是E3泛素化连接酶中的底物识别组分，可以使YAP发生泛素化和降解。在胰腺癌、结直肠癌和肺癌等人类肿瘤中，体细胞KRAS突变是最常见的致癌突变。但KRAS靶向治疗一直面临着药效不足以及耐药产生这两大难题。研究发现，在KRAS依赖性肿瘤中抑制KRAS活性后，将代偿性激活YAP活性进而促进肿瘤重新进展，但其内在的分子机制仍然不清楚。.本研究发现：MEK核转位通过促进β-TrCP定位于胞核，抑制YAP的泛素化进而促进YAP稳定；曲美替尼通过抑制MEK核转位下调YAP蛋白的水平；IQGAP1是一个新的MEK胞浆锚定蛋白，在KRAS突变型细胞中限制MEK入核；突变型KRAS通过IQGAP1将MEK限制在胞浆进而导致YAP下调；抑制突变型KRAS协同MEK抑制剂曲美替尼可以明显抑制KRAS突变型细胞生长。. 综上所述，我们发现MEK蛋白一种新功能，即MEK核易位可以通过调节β-TrCP亚细胞分布调节YAP蛋白稳定性；确定了一条非经典的突变型KRAS通路，即突变型KRAS可以通过IQGAP1限制MEK核转位下调YAP水平。最后，我们发现了曲美替尼可以抑制MEK核转位，并且提供了一种治疗KRAS突变型肿瘤的新思路，即联合抑制KRAS活性和MEK核转位。