Enhanced production and proteolytic degradation of insulin-like growth factor binding protein-2 in proliferating rat astrocytes.

Insulin-like growth factors (IGFs) protect neurons, are important for oligodendrocyte survival and myelin production, and stimulate the proliferation of astrocytes. The effects of IGFs are regulated by a family of IGF binding proteins (IGFBPs). Astrocytes express predominantly IGFBP-2. In the present study, primary neonatal rat astrocytes were cultivated in a chemically defined medium to initiate a differentiated cell status. After stimulation with fetal calf serum, astrocytes became hypertrophic and increased proliferation. Western blot analysis of cell lysate of proliferating astrocytes displayed an increased expression of IGFBP-2. This finding was supported by immunocytochemical images. Semiquantitative polymerase chain reaction analysis demonstrated equal mRNA levels in both differentiated and proliferating astrocytes, suggesting that the increase in IGFBP-2 production in proliferating astrocytes was exerted at the translational level. Concentrated medium of proliferating cells, however, displayed lower levels of IGFBP-2 than differentiated cells. When recombinant IGFBP-2 was incubated with culture media, we found degradation in the medium of proliferating cells, but not in medium of differentiated cells. This degradation could be inhibited with protease inhibitors, indicating that lower levels of IGFBP-2 in the medium of proliferating astrocytes are due to the presence of proteases. Our results suggest that, in proliferating astrocytes, IGFBP-2 may help target IGFs to IGF-1 receptors, and IGFBP-2 proteases may play a role in enhancing the availability of IGFs.