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

In vivo and in vitro studies show that the stem-cell E3 ubiquitin ligases RNF43 and ZNRF3 act as tumour suppressors in colorectal cancer models, and are involved in the negative regulation of the cancer-associated Wnt signalling pathway through limiting the cell-surface expression of Wnt receptors. RNF43 protein in Wnt-linked colon cancer Wnt signalling is critical for the function of intestinal stem cells; it also drives colorectal tumorigenesis. Bon-Kyoung Koo et al . find that two targets of Wnt signalling, the E3 ligases RNF43 and ZNFR3, are also important negative-feedback regulators of Wnt signalling. They act by limiting the cell-surface expression of Wnt receptors. Deletion of both genes in the mouse intestine leads to expansion of LGR5 + intestinal stem cells and the development of adenomas. Furthermore, in human colon cancer cells, the expression of RNF43 reduces Wnt signalling. Mutated RNF43 has been found in human colorectal cancers, indicating that Wnt-pathway inhibitors that act at the level of Wnt secretion or Wnt-receptor activation may have therapeutic potential. 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.