MRNA Degradation Plays a Significant Role in the Program of Gene Expression Regulated by Phosphatidylinositol 3-kinase Signaling

Control of gene expression by the phosphatidylinositol (PI) 3-kinase/Akt pathway plays an important role in mammalian cell proliferation and survival, and numerous transcription factors and genes regulated by PI 3-kinase signaling have been identified. Because steady-state levels of mRNA are regulated by degradation as well as transcription, we have investigated the importance of mRNA degradation in controlling gene expression downstream of PI 3-kinase. We previously performed global expression analyses that identified a set of approximately 50 genes that were downregulated following inhibition of PI 3-kinase in proliferating T98G cells. By blocking transcription with actinomycin D, we found that almost 40% of these genes were regulated via effects of PI 3-kinase on mRNA stability. Analyses of β-globin-3' untranslated region (UTR) fusion transcripts indicated that sequences within 3' UTRs were the primary determinants of rapid mRNA decay. Small interfering RNA (siRNA) experiments further showed that knockdown of BRF1 or KSRP, both ARE binding proteins (ARE-BPs) regulated by Akt, stabilized the mRNAs of a majority of the downregulated genes but that knockdown of ARE-BPs that are not regulated by PI 3-kinase did not affect degradation of these mRNAs. These results show that PI 3-kinase regulation of mRNA stability, predominantly mediated by BRF1, plays a major role in regulating gene expression.