Insulin-like growth factor-I and insulin stimulate the synthesis of IGF-binding protein-2 in a human embryonic kidney cell line.

Insulin and insulin-like growth factor (IGF)-I are thought to be major metabolic regulators of IGF-binding protein-2 (IGFBP-2). We have examined the regulation of IGFBP-2 expression by IGF-I and insulin in 293 cells, a cell line derived from human embryonic kidney. The predominant 34 kDa IGFBP in media conditioned by unstimulated 293 cells was identified as IGFBP-2 by immunoprecipitation. IGFBP-2 levels were increased 6 to 7-fold following incubation with IGF-I, IGF-II or insulin for 48 h. A corresponding increase in IGFBP-2 mRNA was not observed, suggesting that regulation occurred at the translational or post-translational level. The stimulation of IGFBP-2 by IGF-I and insulin was reversibly abolished by incubation with protein synthesis inhibitors such as cycloheximide. Biosynthetic labeling of quiescent 293 cells using [35S]cysteine indicated that incubation with insulin or IGF-I for 24 h increased the synthesis of total cell proteins (predominantly intracellular) and IGFBP-2 (predominantly secreted) to a similar extent (2- to 4-fold). These results suggest that the increase in IGFBP-2 secreted by 293 cells after incubation with IGF-I or insulin largely results from a general stimulation of protein synthesis.