Palmdelphin facilitates R-spondin2 secretion to activate Wnt signaling and promote colorectal cancer stemness and tumorigenesis

The Wnt signaling pathway is a key driver of stemness and progression which contributes to mortality in colorectal cancer (CRC). R-spondins bind to LGR receptors to inhibit ubiquitin E3 ligases, thus protecting Frizzled from degradation and activating downstream Wnt signaling. Herein, we identify Palmdelphin (PALMD) as a functional marker of CRC stem cells, interspersed between intestinal and colonic crypt base epithelial cells, and predictive of aggressive CMS4 CRC and poor survival. Gene knockdown and overexpression studies revealed that PALMD initiates paracrine activation of Wnt/β-catenin signaling via interacting with and facilitating the secretion of R-spondin2 (RSPO2), resulting in enhanced stemness and tumor growth in vitro and in vivo. Physiologic or pharmacologic inhibition of the PALMD-RSPO2 axis using R-spondin2-specific antibody or 6-methyl-1,3,8-trihydroxyanthraquinone (emodin), respectively, attenuates PALMD-mediated Wnt reporter activation, self-renewal, and tumorigenesis in cell and patient-derived organoid models. Together, these findings identify PALMD as a previously unknown player in Wnt signaling in CRC, and underscore the pro-tumorigenic role of R-spondin2 in this context.