Inhibition of protein phosphatases activates P4 promoter of the human insulin-like growth factor II gene through the specific promoter element.

To understand the transcriptional regulation of the human insulin-like growth factor II (IGF-II) gene, we examined the effects of okadaic acid, a potent in vitro inhibitor of protein phosphatases, on the activation of human IGF-II gene expression. Treatment of A-549 human lung adenocarcinoma cells with okadaic acid increased expression of the IGF-II mRNAs. Since the 4.8-kb mRNA is transcribed under the control of human IGF-II P4 promoter, we examined the P4 promoter element responsible for the okadaic acid-mediated transcriptional activation. Transfection of IGF-II P4 promoter-chloramphenicol acetyltransferase constructs demonstrated that the effects of okadaic acid on the induction of IGF-II gene expression are mediated through multiple promoter elements, including an Egr-1 consensus element. We have also shown that okadaic acid induced the expression of the transcription factor Egr-1. Moreover, by using a GAL4-Egr-1 fusion protein, we have directly demonstrated that okadaic acid positively regulates Egr-1 transcriptional activity in vivo. These results indicate that protein phosphatases play an important role in the transcriptional regulation of the IGF-II.