Abstract
The Madin Darby bovine kidney (MDBK) cell line was used to investigate the mechanisms underlying the cAMP regulation of insulin-like growth factor binding protein-3 (IGFBP-3) gene expression. Treatment of confluent monolayers either with forskolin or cAMP produced a 60- to 75-fold induction of IGFBP-3 mRNA and protein levels. This effect did not require new protein synthesis as inhibition of translation by cycloheximide actually caused a 2-fold increase in the cAMP induction. The rates of IGFBP-3 gene transcription, assessed by nuclear run-on assays, increased approximately 15-fold in cells exposed to cAMP. In addition, the half-life of the IGFBP-3 mRNA transcript was increased ∼3-fold in the presence of cAMP. Gel mobility shift and competition experiments revealed the specific binding of an approximately 42-kDa cytoplasmic protein factor to the 3′-untranslated region (3′-UTR) of the IGFBP-3 mRNA. A 21-nucleotide uridine-rich segment that contained no AUUUA motif was sufficient for the specific binding. The binding activity of this protein was reduced after cAMP treatment but was increased by phosphatase treatment. In conclusion, the cAMP induction of IGFBP-3 mRNA in MDBK cells occurred at both the transcriptional and posttranscriptional levels. The IGFBP-3 mRNA stabilization in MDBK cells probably involved the phosphorylation of a member of the family of U-rich region mRNA-binding proteins and is the first reported member whose RNA-binding activity is reduced by cAMP.
Intranuclear Localization of Insulin-Like Growth Factor Binding Protein-3 (IGFBP-3) During Cell Division in Human Keratinocytes
