Suppression of WNT signaling by RNF43 in intestinal stem cells inhibits tumor formation.
Major finding: Suppression of WNT signaling by RNF43 in intestinal stem cells inhibits tumor formation.
Mechanism: RNF43 and ZNRF3 promote the ubiquitination and lysosomal degradation of Frizzled WNT receptors.
Impact: Blocking WNT receptor activation may be effective in tumors with RNF43 mutations.
Intestinal stem cells are characterized by the expression of leucine-rich repeat–containing G protein-coupled receptor 5 (LGR5), and self-renewal of these cells is regulated in part via WNT ligands secreted by adjacent terminally differentiated Paneth cells. The balance of positive and negative WNT regulators is crucial for stem-cell homeostasis, and disruption of this signaling is associated with adenoma formation. Koo and colleagues investigated additional mechanisms regulating WNT activity and identified Rnf43 and Znrf3 via expression profiling as genes enriched in LGR5-positive stem cells. Simultaneous deletion of these genes, which encode for 2 related RING-type E3 ubiquitin ligases, in intestinal epithelium resulted in the rapid formation of hyperproliferative adenomas. These tumors displayed elevated WNT signaling, as evidenced by high expression of β-catenin and induction of WNT target genes, suggesting that RNF43 and ZNRF3 downregulate the WNT pathway. In addition, stem cells deficient in both E3 ligases exhibited hypersensitivity to paracrine WNT stimulation; the growth of double-mutant organoids was independent of the LGR5 ligand R-spondin1 but was blocked by inhibition of WNT secretion from Paneth cells. Conversely, RNF43 expression resulted in growth suppression of organoid cultures and colon carcinoma cells expressing mutant RNF43 and abrogated the response to exogenous WNT signals. This inhibitory effect was mediated via ubiquitination of Frizzled receptors by RNF43, which promoted receptor internalization and lysosomal degradation, thereby reducing WNT activity. These results are supported by other recent studies that showed a similar role for ZNRF3 in negatively modulating WNT signaling and by the presence of RNF43 mutations in multiple cancers. Together, these data implicate RNF43 as a tumor suppressor and suggest that inhibitors of WNT secretion or receptor activation may be therapeutically useful in tumors with RNF43 mutations.