IGF-1 LR3 infusion for one week in fetal sheep reduces glucose-stimulated insulin secretion via an intrinsic islet defect
Background
Insulin and insulin-like growth factor-1 (IGF-1) are crucial hormones for normal fetal growth and development. In certain conditions, like pregnancies complicated by diabetes, fetal IGF-1 concentrations can be elevated, often leading to fetal overgrowth (macrosomia). While experimentally increasing IGF-1 levels boosts fetal weight, it paradoxically lowers fetal plasma insulin concentrations. This observation raises critical questions about the interplay within the insulin/IGF-1 axis and its impact on pancreatic β-cell function. Understanding how sustained high IGF-1 affects fetal insulin regulation is essential, as it may explain the long-term reduction in β-cell function observed in neonates born from such pregnancies. This study investigates the direct effects of an IGF-1 analog on insulin secretion machinery.
Study Design
The study used a late-gestation fetal sheep model. One group (n=8) received a continuous infusion of IGF-1 LR3, an IGF-1 analog with high affinity for the IGF-1 receptor but low affinity for IGF binding proteins. A control group (CON, n=9) received a vehicle infusion. The infusion lasted for one week. To measure in-vivo insulin response, fetal glucose-stimulated insulin secretion (GSIS) was assessed using a hyperglycemic clamp in 8 IGF-1 and 7 CON fetuses. Following the in-vivo studies, fetal pancreatic islets were isolated from 8 IGF-1 and 7 CON animals to measure insulin secretion in vitro via static incubations.
Results
The one-week IGF-1 LR3 infusion significantly altered fetal metabolic parameters. Fetuses receiving IGF-1 had lower plasma insulin concentrations (P=0.0135) and lower plasma glucose concentrations (P=0.0012) compared to the control group. During the hyperglycemic clamp designed to stimulate insulin release, the IGF-1 treated fetuses demonstrated a blunted response.
Insulin secretion during the GSIS study was significantly lower in the IGF-1 group compared to the control group (P=0.0453).
This impairment was not just a systemic effect. When pancreatic islets were isolated and tested in vitro, the defect persisted. Glucose-stimulated insulin secretion from islets of the IGF-1 fetuses remained significantly lower than that from control islets (P=0.0447). This finding points to an intrinsic defect within the pancreatic islets themselves, rather than a systemic regulatory issue. The paper's 'New & Noteworthy' section highlights that these changes occurred without differences in total pancreatic insulin content.
Key Findings
- IGF-1 LR3 infusion in fetal sheep lowered plasma insulin concentrations (P=0.0135).
- IGF-1 LR3 infusion also lowered plasma glucose concentrations (P=0.0012).
- Fetal glucose-stimulated insulin secretion (GSIS) was significantly lower in IGF-1 treated fetuses (P=0.0453).
- In vitro, isolated islets from IGF-1 fetuses also showed lower glucose-stimulated insulin secretion (P=0.0447).
- Changes in insulin secretion occurred without altering total pancreatic insulin content.
Why It Matters
This research provides a potential mechanistic explanation for the reduced β-cell function seen in overgrown newborns from pregnancies complicated by conditions like diabetes. The finding that elevated IGF-1 can directly induce an intrinsic insulin secretion defect in fetal pancreatic islets is a significant insight. For those exploring IGF-1 analogs for growth or other purposes, this study serves as a crucial caution: prolonged exposure, even for one week, may negatively impact pancreatic function and glucose homeostasis by blunting insulin response. This is not a transient systemic effect but a more persistent change at the cellular level within the islets. The study suggests that protocols involving IGF-1 or its long-acting analogs should carefully consider the potential for desensitizing or impairing the insulin secretory pathway, which could have long-term metabolic consequences. Further research is needed to determine if this effect is reversible and its relevance in postnatal or adult physiology.
