Insulin-like growth factor binding protein-4 inhibits epithelial growth and proliferation in the rodent intestine

Abstract

Insulin-like growth factor binding protein-4 (IGFBP-4) is a binding protein that modulates the action of IGF-1, a growth factor whose presence is required for the intestinotrophic effects of glucagon-like peptide-2 (GLP-2). GLP-2 is a gut hormone that utilizes both IGF-1 and epidermal growth factor (EGF) as intermediary factors to promote intestinal growth. Thus, to elucidate the mechanism through which IGFBP-4 regulates IGF-1 activity in the intestine, proliferation assays were conducted using rat intestinal epithelial (IEC-6) cells. IGF-1 and EGF synergistically enhanced proliferation, an effect that was dose-dependently decreased by IGFBP-4 (p<0.05-0.001) in an IGF-1 receptor(R)- and MEK1/2- but not PI3K-dependent manner (p>0.05 for IGFBP-4 effects with IGF-1R- and MEK1/2-inhibitors). Intestinal organoids derived from IGFBP-4 knockout mice demonstrated significantly greater Ki67 expression and an enhanced surface area increase in response to IGF-1 treatment, compared to organoids from control mice (p<0.05-0.01). GLP-2 is also known to increase the mucosal expression of IGFBP-4 mRNA. To investigate whether this occurs through the actions of its intermediaries, IGF-1 and EGF, inducible intestinal epithelial-IGF-1R knockout and control mice were treated for 10d with and without the pan-ErB inhibitor, CI-1033. However, no differences in mucosal IGFBP-4 mRNA expression were found for any of the treatment groups (p>0.05). Consistently, IEC-6 cells treated with IGF-1 and/or EGF displayed no alteration in IGFBP-4 mRNA or in cellular and secreted IGFBP-4 protein (p>0.05). Overall, this study establishes that endogenous IGFBP-4 plays an important role in inhibiting IGF-1-induced intestinal epithelial proliferation, and that mucosal IGFBP-4 expression is independent of IGF-1 and EGF.