| The PI 3-kinase pathway plays a key role in the survival, growth and proliferation of mammalian cells, but despite its importance the gene expression program controlled by PI 3-kinase has not been well-characterized. To better understand how PI 3-kinase regulates gene expression, this study analyzed both transcriptional changes and mRNA degradation that occur downstream of PI 3-kinase signaling.;A low-level of continuous PI 3-kinase signaling is present in proliferating cells maintained in serum. To characterize the gene expression program controlled by steady-state PI 3-kinase levels, we used microarray analyses to identify genes that were differentially regulated upon inhibition of PI 3-kinase, many of which are involved in cell survival and proliferation. Computational and experimental analyses determined that FOXO and NF-kappaB transcription factors regulated approximately one-third of the genes that were either up- or down-regulated, respectively.;Steady-state mRNA levels can also be regulated by degradation, therefore the role of PI 3-kinase in regulating mRNA stability was investigated. Of the genes that were down-regulated upon inhibition of PI 3-kinase, ∼40% exhibited PI 3-kinase-regulated mRNA degradation. The 3' untranslated regions (UTRs) of these genes were enriched in AU-rich elements (AREs), and analyses of beta-globin/3' UTR fusion transcripts indicated that sequences within 3' UTRs were the primary determinants of rapid mRNA decay. siRNA experiments further showed that knockdown of either BRF1 or KSRP, both ARE binding proteins regulated by Akt, stabilized the mRNAs of a majority of the down-regulated genes.;In contrast to the continuous PI 3-kinase signaling in proliferating cells, PI 3-kinase is inactive in quiescent cells, thereby abolishing an inhibitory phosphorylation by Akt on the downstream target GSK-3. Genes that can be induced solely by inhibition of GSK-3 in the absence of growth factor stimulation were previously identified, and the current study has determined that GSK-3 regulates their expression in part by suppressing NF-kappaB through inhibition of IKB kinase.;This study identified novel programs of gene expression controlled by PI 3-kinase signaling in both proliferating and quiescent cells. PI 3-kinase was found to regulate expression at the transcriptional level by NF-kappaB, and at the level of mRNA stability by ARE-mediated mRNA degradation. |
