Repression of Intestinal Stem Cell Function and Tumorigenesis through Direct Phosphorylation of β-Catenin and Yap by PKCζ

Abstract

SUMMARY Intestinal epithelial homeostasis requires continuous renewal supported by stem cells located in the base of the crypt. Disruption of this balance results in failure to regenerate and initiates tumorigenesis. The β-catenin and Yap pathways in Lgr5+ stem cells have been shown to be central to this process. However, the precise mechanisms by which these signaling molecules are regulated in the stem cell population are not totally understood. PKCζ has been previously demonstrated to be a negative regulator of intestinal tumorigenesis. We show here that PKCζ suppresses intestinal stem cell function by promoting the downregulation of β-catenin and Yap through direct phosphorylation. PKCζ deficiency results in increased stem cell activity in organoid cultures and in vivo, accounting for the increased tumorigenic and regenerative activity response of Lgr5+-specific PKCζ-deficient mice. This demonstrates that PKCζ is central to the control of stem cells in intestinal cancer and homeostasis.

Protein kinase C: perfectly balanced

“…Additionally, a frequently observed mutation in PKCι is neomorphic: mutation of R471C in PKCι changes its substrate specificity (Linch et al 2013). As noted above, low levels of PKCζ correlate with poor patient outcome in colon cancer, and functional studies in intestinal cells reveal that loss of PKCζ promotes metabolic reprogramming by two mechanisms – regulating the activity of a key metabolic enzyme, 3-phosphoglycerate dehydrogenase, and regulating the nuclear translocation of the transcription factors YAP and β-catenin (Ma et al 2013; Llado et al 2015). PKCι has also been proposed to have a tumor-suppressive function in the intestine: this isozyme is lost in the intestinal epithelium of patients with Crohn’s disease, a pathology associated with high risk of cancer, and mice lacking PKCι in their intestinal epithelium have increased inflammation and tumorigenesis (Nakanishi Y et al 2016).…”

YAP and TAZ: a nexus for Hippo signaling and beyond

“…A negative regulator of both β-catenin and YAP was also recently identified. Protein kinase C zeta (PKCζ) was identified to phosphorylate and thereby inhibit both β-catenin and YAP, highlighting the atypical PKCs as mediators of intestinal homeostasis and regeneration by modulating both YAP and β-catenin activity [91]. A general picture therefore emerges whereby nuclear YAP/TAZ potentiates Wnt signaling whereas cytoplasmic YAP/TAZ, due to an activated Hippo pathway, inhibits Wnt signaling [15,16,92].…”

Glycogen Synthase Kinase 3β Is Positively Regulated by Protein Kinase Cζ-Mediated Phosphorylation Induced by Wnt Agonists

“…In this respect, the atypical isoforms, PKC and PKC, have been demonstrated to be critical components of cell survival signal transduction pathways, frequently suppressing apoptosis by activation of prosurvival NF-B and MAPK signaling pathways (21,23,24), but it has also been reported that the loss of PKC in mice results in enhanced intestinal tumorigenesis and in increased stem cell activity (25,26). These atypical isoforms differ from other PKCs in that their catalytic activity is not dependent upon diacylglycerol, calcium, or phosphatidylserine.…”