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

Insulin-like growth factor-binding protein-4 (IGFBP-4) is a binding protein that modulates the action of insulin-like growth factor-1 (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 uses both IGF-1 and epidermal growth factor (EGF) as intermediary factors to promote intestinal growth. Therefore, to elucidate the mechanism through which IGFBP-4 regulates IGF-1 activity in the intestine, proliferation assays were conducted using rat intestinal epithelial cells (IEC-6). 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 a phosphatidylinositol 3-kinase-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 Ki-67 expression and an enhanced surface area increase in response to IGF-1 treatment, compared with 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 10 days with and without the pan-ErbB 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. NEW & NOTEWORTHY This study demonstrates, for the first time, the inhibitory role of locally expressed insulin-like growth factor-binding protein-4 (IGFBP-4) on the intestinal proliferative actions of IGF-1 and supports the notion of the synergistic roles of IGF-1 and EGF in promoting intestinal epithelial growth. In turn, intestinal IGFBP-4 expression was not found to be regulated by IGF-1 and/or EGF.