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 exam ined the regulation of IGFBP-2 expression by IGF-I and insulin in 293 cells, a cell line derived from human embryonic kidney. The predominan t 34 kDa IGFBP in media conditioned by unstimulated 293 cells was iden tified as IGFBP-2 by immunoprecipitation. IGFBP-2 levels were increase d 6 to 7-fold following incubation with IGF-I, IGF-II or insulin for 4 8 h. A corresponding increase in IGFBP-2 mRNA was not observed, sugges ting that regulation occurred at the translational or post-translation al level. The stimulation of IGFBP-2 by IGF-I and insulin was reversib ly abolished by incubation with protein synthesis inhibitors such as c ycloheximide. Biosynthetic labeling of quiescent 293 cells using [S-35 ]cysteine indicated that incubation with insulin or IGF-I for 24h incr eased the synthesis of total cell proteins (predominantly intracellula r) and IGFBP-2 (predominantly secreted) to a similar extent (2- to 4-f old). These results suggest that the increase in IGFBP-2 secreted by 2 93 cells after incubation with IGF-I or insulin largely results from a general stimulation of protein synthesis.