Abstract
ErbB3 is a RTK family member known to regulate proliferation and growth in the intestinal epithelium. Previous work by our lab determined that targeted Erbb3 deletion from the intestinal epithelium resulted in precocious Paneth cell development and an expanded population in mice. This was associated with increased expression of intestinal stem cell (ISC) markers. Conversely, ileal enteroids treated with the ErbB3 ligand neuregulin (NRG)-1β had significantly reduced expression of ISC markers, including Lgr5 and Bmi1. These data suggest that ErbB3 restricts secretory cell differentiation and stemness in the intestinal crypt. However, the underlying mechanisms are largely unknown. BMI1 is a proto-oncogene and member of the PRC complex. Bmi1+ intestinal epithelial cells represent a slow-cycling ISC population that can regenerate the epithelium following injury and loss of Lgr5+ ISCs. Recent evidence determined that Bmi1+ ISCs are enriched in enteroendocrine markers and can give rise to goblet cells, suggesting BMI1 may be critical for differentiation of secretory lineages. We hypothesized that ErbB3 restricts secretory cell differentiation through regulation of BMI1.
To define ErbB3-dependent regulation of Bmi1 and determine if ErbB3 restriction of Paneth cell development and differentiation is through suppression of Bmi1.
Mucosal scrapings were collected from Vil-Cre;Erbb3flox/flox mice and Erbb3flox/flox littermate controls for protein and gene expression analysis. Ileal enteroids and HT-29 colorectal adenocarcinoma cells were treated with vehicle DMSO, the PI3K inhibitor LY294002 (10 μM), the MEK inhibitor U0126 (5 μM), the BMI1 inhibitor PTC-209 (1 μM), and/or recombinant NRG-1β (10 ng/mL). Enteroids and cell monolayers were harvested and prepared for gene expression analysis. RT-qPCR for Bmi1 and Lyz1 was performed relative to Hprt as a loading control. Statistical analysis by one-way ANOVA was conducted using Prism 9.
Consistent with previous data, BMI1 protein and gene expression were significantly increased in small and large intestinal mucosal scrapings from Vil-Cre;Erbb3flox/flox mice versus littermate controls. To determine if Bmi1 expression is sensitive to PI3K/Akt or MAPK signaling downstream of ErbB3, HT-29 cells were treated with inhibitors followed by NRG-1β after 1 hour, then harvested 24 hours later. PI3K and MAPK inhibition significantly increased Bmi1 expression. NRG-1β treatment was able to overcome MAPK inhibition to reduce BMI1 expression. Similarly, in ileal enteroids, PI3K and MAPK blockade increased Bmi1 expression and NRG-1β treatment reversed this effect. To determine whether BMI1 regulates expression of the Paneth cell marker LYZ1, HT-29 cells were treated with the BMI1 inhibitor PTC-209. BMI1 inhibition decreased LYZ1 expression.
Our data demonstrate that ErbB3 regulates Bmi1 expression through PI3K/Akt and MAPK signaling in both human and mouse intestinal cells. Furthermore, BMI1 activity promotes LYZ1 expression. Together, these results support our hypothesis that ErbB3 regulates secretory cell differentiation through BMI1 in the intestinal epithelium.