Prostaglandin E₂ rapidly stimulates insulin-like growth factor-I gene expression in primary rat osteoblast cultures: Evidence for transcriptional control

Osteoblast-enriched (Ob) cultures isolated from fetal rat bone synthesize insulin-like growth factor-I (IGF-I), which functions as a locally acting growth and differentiation factor in the skeleton. Consistent with prior studies demonstrating that IGF-I production is enhanced in bone by agents that induce cAMP, prostaglandin E₂ (PGE₂) stimulates both cAMP synthesis and IGF-I mRNA in Ob cells. However, little is known about how cAMP regulates IGF-I expression in this or any other cell system. In rat tissues, multiple mechanisms influence levels of IGF-I mRNA, including transcription from two promoters, differential RNA splicing and stability, and alternative RNA polyadenylation. To determine how cAMP influences IGF-I gene expression in Ob cultures, we examined the responses of these cells to treatment with PGE₂. PGE₂ rapidly enhanced the accumulation of both large and small IGF-I transcripts, with increases in IGF-I mRNA detected within 2 h of treatment and persisting for 24 h. Analysis of precursor RNA by a highly specific and sensitive ribonuclease protection assay demonstrated a rise in nascent IGF-I mRNA within 30 min of exposure to PGE₂, with a peak stimulation of 4-fold above control levels seen by 2 h and levels remaining elevated for up to 24 h. IGF-I transcripts in Ob cells were directed only by promoter 1, the more 5′ of the two rat IGF-I gene promoters. As additionally assessed using the RNA polymerase II inhibitor 5,6-dichloro-1-β-D-ribofuranosyl benzimidazole, PGE₁ treatment had little effect on IGF-I mRNA stability. In aggregrate, these studies show that in fetal rat Ob cultures, PGE₂ enhances IGF-I gene expression primarily through transcriptional mechanisms that an limited to a single IGF-I gene promoter. Ob cells, therefore, may be an excellent model for determining how cAMP regulates IGF-I gene transcription.