LGR4: Not Just for Wnt Anymore?

Leucine-rich repeat-containing G protein–coupled receptor 4 (LGR4) is best known for its role in regulating the ability of cells to respond to Wnt ligands. In this well-known role, LGR4 serves as a receptor for R-spondins and forms a complex with the ubiquitin E3 ligases ring finger protein 43 (RNF43) and zinc and ring finger 3 (ZNRF3). RNF43 and ZNRF3 ubiquitinate Frizzleds (FZD), which are a family of ten WNT receptors. This ubiquitination decreases FZD receptor levels on the cell surface, reducing Wnt ligands9 ability to activate signaling. While there were some previous indications of Wnt-independent functions of LGR4, this WNT-centric view has remained predominant. In this issue of Cancer Research, Yue and colleagues report that LGR4 also functions to regulate signaling through the EGF receptor. This work was stimulated by observing that while high levels of LGR4 expression in breast tumors correlated with poor patient outcomes, LGR4 levels did not correlate with a well-established Wnt-associated gene signature in these same patients. In contrast, high levels of Lgr4 expression strongly correlated with EGFR signaling. Reducing Lgr4 expression also inhibited signaling through the EGFR, potentially via regulation of the Casitas B-lineage lymphoma ubiquitin E3 ligase. Consistent with this model, LGR4 could be coimmunoprecipitated with a complex that contained EGFR and was capable of inhibiting EGFR ubiquitination. The implications of this work and how it challenges our understanding of the contributions of Wnt signaling and EGFR signaling in cancer are discussed as our several interesting future directions. See related article by Yue et al., p. 4441