TRANSCRIPTIONAL REGULATION OF INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-5 BY PROSTAGLANDIN E(2) IN OSTEOBLAST CELLS

Insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) is an aut ocrine and paracrine factor that modulates the effects of IGFs. We exa mined the mechanisms that regulate IGFBP-5 synthesis by PGE(2) in oste oblast-enriched cells from fetal rat calvaria (Oh cells). PGE(2) at 1 mu M for 2-8 h increased IGFBP-5 heterogeneous nuclear RNA levels and did not change the half-life of IGFBP-5 messenger RNA in Ob cells, sug gesting that PGE(2) stimulates IGFBP-5 transcription. To analyze the e lements responsible for this effect, regions of the mouse IGFBP-5 prom oter from -2695 to +120 bp were ligated into pGL-2-basic and transient ly transfected into Ob cells. PGE(2) caused a time- and dose-dependent increase in IGFBP-5 promoter activity. Further analysis revealed two potential PGE(2)-responsive regions in the -2695 to -1470 and the -989 to -332 fragments. The effect of PGE(2) on IGFBP-5 messenger RNA and heterogeneous nuclear RNA levels was mimicked by forskolin and inhibit ed by the PKA inhibitor H-89, suggesting that part of the PGE(2) effec t was mediated through a cAMP-dependent pathway. H-89 also blocked bas al and PGE(2)-stimulated IGFBP-5 promoter activities. We conclude that PGE(2) regulates IGFBP-5 synthesis in Ob cells by transcriptional mec hanisms. PKA-dependent pathways account for part of the effect of PGE( 2) on IGFBP-5 expression. Deletion analysis of the IGFBP-5 promoter su ggests the presence of two PGE(2)-responsive regions.