Serum and growth factors stimulate ribosomal RNA processing in Syrian hamster embryo cells: divergence of this signalling pathway from immediate-early gene expression

RIOK2 phosphorylation by RSK promotes synthesis of the human small ribosomal subunit

Ribosome biogenesis lies at the nexus of various signaling pathways coordinating protein synthesis with cell growth and proliferation. This process is regulated by well-described transcriptional mechanisms, but a growing body of evidence indicates that other levels of regulation exist. Here we show that the Ras/mitogen-activated protein kinase (MAPK) pathway stimulates post-transcriptional stages of human ribosome synthesis. We identify RIOK2, a pre-40S particle assembly factor, as a new target of the MAPK-activated kinase RSK. RIOK2 phosphorylation by RSK stimulates cytoplasmic maturation of late pre-40S particles, which is required for optimal protein synthesis and cell proliferation. RIOK2 phosphorylation facilitates its release from pre-40S particles and its nuclear re-import, prior to completion of small ribosomal subunits. Our results bring a detailed mechanistic link between the Ras/MAPK pathway and the maturation of human pre-40S particles, which opens a hitherto poorly explored area of ribosome biogenesis.

Cell signaling in protein synthesis ribosome biogenesis and translation initiation and elongation

Protein synthesis is a highly energy-consuming process that must be tightly regulated. Signal transduction cascades respond to extracellular and intracellular cues to phosphorylate proteins involved in ribosomal biogenesis and translation initiation and elongation. These phosphorylation events regulate the timing and rate of translation of both specific and total mRNAs. Alterations in this regulation can result in dysfunction and disease. While many signaling pathways intersect to control protein synthesis, the mTOR and MAPK pathways appear to be key players. This chapter briefly reviews the mTOR and MAPK pathways and then focuses on individual phosphorylation events that directly control ribosome biogenesis and translation.

Heat shock inhibits pre-rRNA processing at the primary site in vitro and alters the activity of some rRNA binding proteins

The effect of heat shock on pre-rRNA processing at the primary site within external transcribed spacer region 1 (ETS1) was studied in S-100 extract derived from mouse lymphosarcoma cells. In vivo labeling with [32P]orthophosphate showed that the synthesis of the rRNA precursor and its processing to 28S and 18S rRNAs were inhibited significantly due to heat shock. The processing activity was reduced by 50% at 1 h and was completely blocked following 2-h exposure of cells at 42 degrees C. Mixing S-100 extracts from the control and heat-treated cells did not affect the processing activity in the control extract, which proves the absence of a nuclease or other inhibitor(s) of processing in the extract from the heat-shocked cells. Heat shock did not affect interaction between pre-rRNA and U3 snoRNA, a prerequisite for the processing at the primary site, but significantly altered RNA-protein interaction. Three polypeptides of 200, 110, and 52 kDa that specifically cross-link to pre-rRNA spanning the primary processing site were inactivated after heat shock. Hyperthermia did not alter 3' end processing of SV40L pre-mRNA.

Problems related to the interpretation of autoradiographic data on gene expression using common constitutive transcripts as controls

The 28S rRNA, a ribosomal RNA, and the ACTB and GAPD mRNAs, coding respectively for beta-actin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), are frequently presented as controls of modulated gene expression. These transcripts were quantified by replicate slot-blot autoradiography and image analysis in mammary epithelial cells and fibroblasts from breast tissues. Each cell-type group comprised strains with different pathological backgrounds, growth rates, antigenic phenotypes and culture histories. The effects of a differentiating agent (cholera toxin) and/or a tumor promoter (12-O-tetradecanoyl-phorbol-13-acetate) were also examined. Despite the impression that visual examination of autoradiographs might create, image analysis suggests that 28S rRNA, ACTB and GAPD are substantially and independently influenced by the above biological factors and by the drugs. Therefore, these transcripts represent specifically regulated cellular activities and may not be taken as alternative indicators of the overall transcription rate or of the amount of material being examined. Instead, such nonspecific variation may be accurately measured and removed from quantitative data using a principal component function. A methodology that allows comparison of expression (or amplification) patterns between genes, between experiments or, even, between laboratories is presented with an example of quantification of transcripts related to cell-growth, differentiation, signaling and cancer.