IGF-I but not the IGF-I variant long R(3)IGF-I increases serum IGFBP-3 in adolescent monkeys

Previous work in rhesus monkeys has shown that both acute or chronic subcutaneous (s.c.) administration of insulin-like growth factor (IGF)-I elevates serum concentrations of IGF binding protein (IGFBP)-3. In order to determine whether an analog of IGF-I, which has a reduced affinity for the IGFBPs, has similar effects, a series of studies using adolescent female rhesus monkeys were conducted. In the first study, an s.c. injection of IGF-I (110 mg/kg;n = 6) significantly elevated serum IGFBP-3 concentrations through 7 h following treatment. In contrast, serum IGFBP-3 decreased throughout the day following an injection of Long R(3)IGF-I (110 mg/kg, s.c., n = 5). However, this decrease was not due to the analog treatment as serum IGFBP-3 also declined in a similar fashion in untreated females (n = 5) sampled on the same schedule. Serum GH levels were acutely suppressed by both IGFs but were not altered in untreated females. In the second study, serum IGFBP-3 were compared between untreated control females (n = 6) and females treated continuously by s.c. infusion with either Long R(3)IGF-I (120 mg/kg/day, s.c.;n = 5) or IGF-I (120 mg/kg/day, s.c.;n = 5) or IGF-I s.c.;n = 4). Serum IGFBP-3 was consistently elevated by IGF-I infusion, whereas levels in analog-treated monkeys were similar to those in control females. Although acute or chronic administration of Long R(3)IGF-I did not elevate serum IGFBP-3, chronic administration of the analog did not block the acute facilitating effects of IGF-I on serum IGFBP-3. The increase in serum IGFBP-3 following an acute injection of IGF-I (110 mg/kg, s.c.) was not significantly different between untreated females and females receiving a constant s.c. infusion of Long R(3)IGF-I. These data indicate either acutely or chronically administered IGF-I but not its analog Long R(3)IGF-I can elevate serum concentrations of IGFBP-3. Although the analog fails to increase serum IGFBP-3, it does not block the facilitating effects of IGF-I on concentrations of this IGFBP. Taken together, these data suggest that the increase in serum IGFBP-3 by exogenous IGF-I may not be a receptor mediated event but may be the result of IGF-I binding to IGFBP-3 and forming the binary and ternary complex, slowing IGFBP-3 degradation.