Oncogenic KRAS signalling promotes the Wnt/β-catenin pathway through LRP6 in colorectal cancer.
Oncogenic KRAS signalling promotes the Wnt/β-catenin pathway through LRP6 in colorectal cancer.
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DOI：
10.1038/onc.2014.416
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发表时间：
2015-09-17
期刊：
影响因子：
8
通讯作者：
Rivard N
中科院分区：
文献类型：
--
作者：
Lemieux E;Cagnol S;Beaudry K;Carrier J;Rivard N
Wnt/β-连环蛋白信号通路的异常调节是结直肠癌（CRC）的主要原因之一。 APC 的功能丧失突变常见于 CRC，导致经典 Wnt 信号传导的不当激活。相反，高达 60% 的 CRC 中检测到 KRAS 和 BRAF 基因的功能获得性突变。尽管 KRAS/丝裂原激活蛋白激酶 (MAPK) 和经典 Wnt/β-连环蛋白通路对于肠道肿瘤发生至关重要，但在 CRC 发育过程中整合这两个重要信号通路的机制尚不清楚。本文结果表明，致癌形式的 KRAS、BRAF 或 MEK1 对正常肠上皮细胞 (IEC) 的转化与 β-catenin/TCF4 和 c-MYC 启动子转录活性以及 c-Myc、Axin2 和 Lef1 mRNA 水平的显着增加相关。值得注意的是，T 细胞因子 4 (ΔNTCF4) 的显性失活突变体的表达严重减弱了致癌性 MEK1 诱导的 IEC 转化，并显着降低了免疫功能低下小鼠的致瘤和转移潜力。有趣的是，Frizzled 辅助受体 LRP6 在转化的 IEC 和人 CRC 细胞系中以 MEK 依赖性方式磷酸化。 LRP6突变体的表达显着降低了β-连环蛋白/TCF4转录活性，其中每个特定脯氨酸脯氨酸丝氨酸/苏氨酸脯氨酸基序中的丝氨酸/苏氨酸残基突变为丙氨酸(LRP6-5A)。因此，人 CRC 细胞中的 MEK 抑制显着降低了 β-catenin/TCF4 转录活性以及 c-MYC mRNA 和蛋白质水平，而不影响 β-catenin 表达或稳定性。最后，与健康的邻近正常组织相比，LRP6 磷酸化在人类结直肠肿瘤（包括腺瘤）中也增加。我们的数据表明，KRAS/BRAF/MEK 信号传导的致癌激活会刺激经典的 Wnt/β-catenin 通路，进而促进肠道肿瘤的生长和侵袭。此外，ERK1/2 引起的 LRP6 磷酸化可能在肿瘤发生过程中提供 KRAS/MAPK 和 Wnt/β-catenin 信号传导之间的独特汇聚点。
Aberrant regulation of the Wnt/β-catenin signaling pathway is one of the major causes of colorectal cancer (CRC). Loss-of-function mutations in APC are commonly found in CRC, leading to inappropriate activation of canonical Wnt signaling. Conversely, gain-of-function mutations in KRAS and BRAF genes are detected in up to 60% of CRCs. Whereas KRAS/mitogen-activated protein kinase (MAPK) and canonical Wnt/β-catenin pathways are critical for intestinal tumorigenesis, mechanisms integrating these two important signaling pathways during CRC development are unknown. Results herein demonstrate that transformation of normal intestinal epithelial cells (IECs) by oncogenic forms of KRAS, BRAF or MEK1 was associated with a marked increase in β-catenin/TCF4 and c-MYC promoter transcriptional activities and mRNA levels of c-Myc, Axin2 and Lef1. Notably, expression of a dominant-negative mutant of T-Cell Factor 4 (ΔNTCF4) severely attenuated IEC transformation induced by oncogenic MEK1 and markedly reduced their tumorigenic and metastatic potential in immunocompromised mice. Interestingly, the Frizzled co-receptor LRP6 was phosphorylated in a MEK-dependent manner in transformed IECs and in human CRC cell lines. Expression of LRP6 mutant in which serine/threonine residues in each particular ProlineProlineProlineSerine/ThreonineProline motif were mutated to alanines (LRP6-5A) significantly reduced β-catenin/TCF4 transcriptional activity. Accordingly, MEK inhibition in human CRC cells significantly diminished β-catenin/TCF4 transcriptional activity and c-MYC mRNA and protein levels without affecting β-catenin expression or stability. Lastly, LRP6 phosphorylation was also increased in human colorectal tumors, including adenomas, in comparison with healthy adjacent normal tissues. Our data indicate that oncogenic activation of KRAS/BRAF/MEK signaling stimulates the canonical Wnt/β-catenin pathway, which in turn promotes intestinal tumor growth and invasion. Moreover, LRP6 phosphorylation by ERK1/2 may provide a unique point of convergence between KRAS/MAPK and Wnt/β-catenin signalings during oncogenesis.