HORMONAL-REGULATION OF ASTROGLIAL INSULIN-LIKE GROWTH-FACTOR (IGF)-BINDING PROTEIN GENE-EXPRESSION BY IGFS AND INSULIN

Primary astroglial cells produce insulin-like growth factor (IGF)-binding proteins (IGFBPs), which modulate the biological activity of IGFs on the developing astroglia. Alterations in the synthesis of astroglial IGFBPs by hormones and growth factors may, therefore, influence the paracrine regulatory actions of IGFs. The objective of this study was to examine the regulation of astroglial IGFBP biosynthesis by IGF-I, IGF-II, and insulin. Primary astroglial cells were cultured from newborn rat cerebral cortices. Conditioned media from astroglial cultures without (control) or with hormonal treatment (10, 100, and 200 ng/ml IGF-I or IGF-II and 0.1, 1.0, and 10 mug/ml insulin) were collected and subjected to Western ligand blot analysis. Total RNAs were extracted from the same cultures and subjected to Northern blot analysis. Two IGFBP species of 34K (IGFBP-2) and 40K (IGFBP-3) were identified. After 24 h of treatment, IGF-I, IGF-II, and high concentrations of insulin increased IGFBP-2 and IGFBP-3 levels. At 24 h, IGFBP-2 stable mRNA levels showed an increase corresponding to the protein levels; however, IGFBP-3 stable mRNA levels did not. Time-course studies demonstrated that IGF-I rapidly induced the mRNA levels of both IGFBP-2 and IGFBP-3 within 7.5-12 h. IGFBP-2 mRNA levels showed a biphasic response to IGF-I, a rapid increase at 7.5 h, followed by a decline at 12-18 h, and then another increase at 24 h. In contrast, IGFBP-3 mRNA levels showed a response that peaked at 7.5-12 h and decreased to control levels at 24 h. The stability of IGFBP-2 mRNA at 24 h of IGF-I treatment was not significantly altered from that under control conditions, indicating that the changes in IGFBP-2 mRNA levels were not due to alterations in the stability of the mRNA. In situ hybridization studies demonstrated an increase in the abundance per cell of IGFBP-2 and IGFBP-3 mRNAs in either one or both types of astroglial cells after IGF-I or IGF-II treatment. We conclude that, 1) IGFs differentially modulate the production of astroglial IGFBPs; 2) the increase in IGFBP mRNA levels by IGFs is rapid and biphasic; 3) the effects of IGFs on IGFBP stable mRNAs may occur at the level of gene transcription; and 4) the effect of IGFs on IGFBP gene expression may be cell type specific. The changes in IGFBP biosynthesis may be a mechanism by which the growth regulatory actions of IGFs and insulin on developing neural tissue are modulated.