The ErbB3 receptor tyrosine kinase regulates expression of Notch target genes and intestinal stem cell markers.

Abstract

BackgroundStem cell‐driven regeneration of the intestinal epithelium is important for recovery of damage from insults including inflammation, chemotherapy, and radiation. This process is controlled through tight regulation of intestinal stem cells (ISCs). Our lab has previously reported that intestine‐specific knockout of the receptor tyrosine kinase ErbB3 leads to precocious development of Paneth cells and increased Paneth cell numbers in mice the mouse intestinal epithelium. Since Paneth cells can play a supportive role for ISCs, we tested the hypothesis that altered ErbB3 signaling results in changes in the ISC compartment.MethodsWe crossed ErbB3flox/flox mice with mice expressing villin promoter‐driven Cre to generate an intestinal epithelial‐specific ErbB3 knockout mouse model (ErbB3‐IEKO). Ileal samples were collected from these mice, fixed, and embedded in paraffin for immunostaining. Tissue samples from the ileum were also processed for RNA and protein for qPCR and immunoblots. Samples were probed for Notch target genes and Notch ligands due to the importance of Notch signaling in maintaining the intestinal stem cell compartment. Enteroids were generated from ileum of C57Bl/6 mice and treated with the ErbB3 ligand Nrg‐1β. To activate and inactivate Notch experimentally, enteroids and HT29 cells were treated with valproic acid (VPA) and the γ‐secretase inhibitor DAPT respectively.ResultsqPCR of ErbB3‐IEKO ileum showed increased expression of Lgr5 (rapid‐cycling ISCs) and Bmi1 (facultative ISCs or secretory progenitors capable of regenerating ISCs). Ileal sections also showed increased numbers of Lgr5+ cells by in situ hybridization, and western blots showed increased protein levels of Bmi1 in ErbB3‐IEKO ileum. qPCR of ErbB3‐IEKO ileum showed increased expression of Notch target genes Hey1, Hey2, and Heyl, as well as expression of Notch ligands Dll1, Jag1, and Jag2. Enteroids treated with Nrg‐1β had decreased expression of Hey1, Lgr5, and Bmi1. Enteroids and HT29 cells treated with VPA to activate Notch had decreased expression of ErbB3 RNA; this was abrogated by DAPT treatment.ConclusionErbB3 negatively regulates expression of Lgr5 and Bmi1, markers for fast‐cycling and quiescent ISCs, respectively, indicating a possible role for ErbB3 in the ISC niche. Furthermore, ErbB3 and Notch appear to negatively regulate one another, suggesting a possible mechanism for ErbB3’s effects on ISCs and intestinal cell differentiation.Support or Funding InformationThis work was supported by NIH award R01DK095004.