Tumour suppressor RNF43 is a stem-cell E3 ligase that induces endocytosis of Wnt receptors

LGR5(+) stem cells reside at crypt bottoms, intermingled with Paneth cells that provide Wnt, Notch and epidermal growth factor signals(1). Here we find that the related RNF43 and ZNRF3 transmembrane E3 ubiquitin ligases are uniquely expressed in LGR5(+) stem cells. Simultaneous deletion of the two genes encoding these proteins in the intestinal epithelium of mice induces rapidly growing adenomas containing high numbers of Paneth and LGR5(+) stem cells. In vitro, growth of organoids derived from these adenomas is arrested when Wnt secretion is inhibited, indicating a dependence of the adenoma stem cells on Wnt produced by adenoma Paneth cells. In the HEK293T human cancer cell line, expression of RNF43 blocks Wnt responses and targets surface-expressed frizzled receptors to lysosomes. In the RNF43-mutant colorectal cancer cell line HCT116, reconstitution of RNF43 expression removes its response to exogenous Wnt. We conclude that RNF43 and ZNRF3 reduce Wnt signals by selectively ubiquitinating frizzled receptors, thereby targeting these Wnt receptors for degradation.

LGR5（+）干细胞位于隐窝底部，与提供Wnt、Notch和表皮生长因子信号的潘氏细胞混合在一起（1）。在此我们发现相关的RNF43和ZNRF3跨膜E3泛素连接酶在LGR5（+）干细胞中独特表达。在小鼠肠上皮中同时缺失编码这些蛋白质的两个基因会诱导快速生长的腺瘤，其中包含大量潘氏细胞和LGR5（+）干细胞。在体外，当抑制Wnt分泌时，源自这些腺瘤的类器官的生长会停止，这表明腺瘤干细胞依赖腺瘤潘氏细胞产生的Wnt。在HEK293T人类癌细胞系中，RNF43的表达阻断Wnt反应并将表面表达的卷曲受体靶向溶酶体。在RNF43突变的结直肠癌细胞系HCT116中，RNF43表达的重建消除了其对外源Wnt的反应。我们得出结论，RNF43和ZNRF3通过选择性地泛素化卷曲受体来减少Wnt信号，从而将这些Wnt受体靶向降解。