Enhancement of helicase activity and increase of eIF-4E phosphorylation in ornithine decarboxylase-overproducing cells

Myc, Oncogenic Protein Translation, and the Role of Polyamines

Deregulated protein synthesis is a common feature of cancer cells, with many oncogenic signaling pathways directly augmenting protein translation to support the biomass needs of proliferating tissues. MYC’s ability to drive oncogenesis is a consequence of its essential role as a governor linking cell cycle entry with the requisite increase in protein synthetic capacity, among other biomass needs. To date, direct pharmacologic inhibition of MYC has proven difficult, but targeting oncogenic signaling modules downstream of MYC, such as the protein synthetic machinery, may provide a viable therapeutic strategy. Polyamines are essential cations found in nearly all living organisms that have both direct and indirect roles in the control of protein synthesis. Polyamine metabolism is coordinately regulated by MYC to increase polyamines in proliferative tissues, and this is further augmented in the many cancer cells harboring hyperactivated MYC. In this review, we discuss MYC-driven regulation of polyamines and protein synthetic capacity as a key function of its oncogenic output, and how this dependency may be perturbed through direct pharmacologic targeting of components of the protein synthetic machinery, such as the polyamines themselves, the eukaryotic translation initiation factor 4F (eIF4F) complex, and the eukaryotic translation initiation factor 5A (eIF5A).

Endogenous polyamine function--the RNA perspective

Recent progress with techniques for monitoring RNA structure in cells such as ‘DMS-Seq’ and ‘Structure-Seq’ suggests that a new era of RNA structure-function exploration is on the horizon. This will also include systematic investigation of the factors required for the structural integrity of RNA. In this context, much evidence accumulated over 50 years suggests that polyamines play important roles as modulators of RNA structure. Here, we summarize and discuss recent literature relating to the roles of these small endogenous molecules in RNA function. We have included studies directed at understanding the binding interactions of polyamines with polynucleotides, tRNA, rRNA, mRNA and ribozymes using chemical, biochemical and spectroscopic tools. In brief, polyamines bind RNA in a sequence-selective fashion and induce changes in RNA structure in context-dependent manners. In some cases the functional consequences of these interactions have been observed in cells. Most notably, polyamine-mediated effects on RNA are frequently distinct from those of divalent cations (i.e. Mg²⁺) confirming their roles as independent molecular entities which help drive RNA-mediated processes.

Minor contribution of an internal ribosome entry site in the 5'-UTR of ornithine decarboxylase mRNA on its translation

The mechanism of synthesis of ornithine decarboxylase (ODC) at the level of translation was studied using cell culture and cell-free systems. Synthesis of firefly luciferase (Fluc) from the second open reading frame (ORF) in a bicistronic construct transfected into FM3A and HeLa cells was enhanced by the presence of the 5'-untranslated region (5'-UTR) of ODC mRNA between the two ORFs. However, cotransfection of the gene encoding 2A protease inhibited the synthesis of Fluc. Synthesis of Fluc from the second cistron in the bicistronic mRNA in a cell-free system was not affected significantly by the 5'-UTR of ODC mRNA. Synthesis of ODC from ODC mRNA in a cell-free system was inhibited by 2A protease and cap analogue (m7GpppG). Rapamycin inhibited ODC synthesis by 40-50% at both the G1/S boundary and the G2/M phase. These results indicate that an IRES in the 5'-UTR of ODC mRNA does not function effectively.

A limited screen for protein interactions reveals new roles for protein phosphatase 1 in cell cycle control and apoptosis

Protein phosphatase 1 (PP1) catalytic subunits typically combine with other proteins that modulate their activity, direct them to distinct substrates, or serve as substrates for PP1. More than 50 PP1-interacting proteins (PIPs) have been identified so far. Given there are approximately 10 000 phosphoproteins in mammals, many PIPs remain to be discovered. We have used arrays containing 100 carefully selected antibodies to identify novel PIPs that are important in cell proliferation and cell survival in murine fetal lung epithelial cells and human A549 lung cancer cells. The antibody arrays identified 31 potential novel PIPs and 11 of 17 well-known PIPs included as controls, suggesting a sensitivity of at least 65%. A majority of the interactions between PP1 and putative PIPs were isoform- or cell type-specific. We confirmed by co-immunoprecipitation that 9 of these proteins associate with PP1: APAF-1, Bax, E-cadherin, HSP-70, Id2, p19Skp1, p53, PCNA, and PTEN. We examined two of these interactions in greater detail in A549 cells. Exposure to nicotine enhanced association of PP1 with Bax (and Bad), but also induced inhibitory phosphorylation of PP1. In addition to p19Skp1, PP1alpha antibodies also coprecipitated cullin 1, suggesting that PP1alpha is associated with the SCF1 complex. This interaction was only detectable during the G1/S transition and S phase. Forced loss of PP1 function decreased the levels of p27Kip1, a well-known SCF1 substrate, suggesting that PP1 may rescue proteins from ubiquitin/proteasome-mediated destruction. Both of these novel interactions are consistent with PP1 facilitating cell cycle arrest and/or apoptosis.

Transcriptional and translational control of ornithine decarboxylase during Ras transformation

ODC (ornithine decarboxylase) activity is induced following ras activation. However, the Ras effector pathways responsible are unknown. These experiments used NIH-3T3 cells expressing partial-loss-of-function Ras mutants to activate selectively pathways downstream of Ras and examined the contribution of each pathway to ODC induction. Overexpression of Ras12V, a constitutively active mutant, resulted in ODC activities up to 20-fold higher than controls. Stable transfections of Ras partial-loss-of-function mutants and constitutively active forms of MEK (MAPK kinase) and Akt indicated that activation of more than one Ras effector pathway is necessary for the complete induction of ODC activity. The increase in ODC activity in Ras12V-transformed cells is not owing to a substantial change in ODC protein half-life, which increased by <2-fold. Northern-blot analysis and reporter assays suggested that the mechanism of ODC induction involves both a modest increase in the transcription of ODC mRNA and a much more considerable increase in the translation of mRNA into protein. ODC transcription was controlled through a pathway dependent on Raf/MEK/ERK (where ERK stands for extracellular-signal-regulated kinase) activation, whereas activation of the phosphoinositide 3-kinase and the Raf/MEK/ERK pathways were necessary for translational regulation of ODC. The increase in ODC synthesis was accompanied by changes in phosphorylation of eukaryotic initiation factor 4E and its binding protein 4E-BP1. Results show that the phosphoinositide 3-kinase pathway regulates phosphorylation of both proteins, whereas the Raf/MEK/ERK pathway affects only the eukaryotic initiation factor 4E phosphorylation.

Increase in cap- and IRES-dependent protein synthesis by overproduction of translation initiation factor eIF4G

The role of eIF4G during the initiation of protein synthesis was studied using mouse mammary carcinoma FM3A cells and FM4G cells that overproduce an N-terminally truncated form of eIF4G, which lacks the binding site of poly(A)-binding protein. An increase in eIF4G was correlated with an increase in protein synthesis and RNA helicase activity. Translation of mRNAshaving both short and long 5'-untranslated regions (5'-UTR) increased significantly in FM4G cells compared to that in FM3A cells. Both full-length and N-terminally truncated eIF4G transfectants of NIH3T3 cells formed colonies in soft agar and increased the saturation density of cell growth, indicating that both eIF4Gs function similarly. We also found that an internal ribosome entry site (IRES) exists in the 5'-UTR of ornithinedecarboxylase mRNA and that IRES-dependent protein synthesis increased in FM4G cells. Our results indicate that an increase in eIF4G contributes to the formation of active eIF4F similarly to that caused by an increase in eIF4E, as well as to a stimulation of IRES-dependent protein synthesis.

Translational regulation of ornithine decarboxylase and other enzymes of the polyamine pathway

It has long been known that polyamines play an essential role in the proliferation of mammalian cells, and the polyamine biosynthetic pathway may provide an important target for the development of agents that inhibit carcinogenesis and tumor growth. The rate-limiting enzymes of the polyamine pathway, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC), are highly regulated in the cell, and much of this regulation occurs at the level of translation. Although the 5' leader sequences of ODC and AdoMetDC are both highly structured and contain small internal open reading frames (ORFs), the regulation of their translation appears to be quite different. The translational regulation of ODC is more dependent on secondary structure, and therefore responds to the intracellular availability of active eIF-4E, the cap-binding subunit of the eIF-4F complex, which mediates translation initiations. Cell-specific translation of AdoMetDC appears to be regulated exclusively through the internal ORF, which causes ribosome stalling that is independent of eIF-4E levels and decreases the efficiency with which the downstream ORF encoding AdoMetDC protein is translated. The translation of both ODC and AdoMetDC is negatively regulated by intracellular changes in the polyamines spermidine and spermine. Thus, when polyamine levels are low, the synthesis of both ODC and AdoMetDC is increased, and an increase in polyamine content causes a corresponding decrease in protein synthesis. However, an increase in active eIF-4E may allow for the synthesis of ODC even in the presence of polyamine levels that repress ODC translation in cells with lower levels of the initiation factor. In contrast, the amino acid sequence that is encoded by the upstream ORF is critical for polyamine regulation of AdoMetDC synthesis and polyamines may affect synthesis by interaction with the putative peptide, MAGDIS.

Translational control: the cancer connection

There is now a growing body of evidence which suggests links between the regulation of protein synthesis and the disruption of cell behaviour that typifies cancer. This directed issue of the International Journal of Biochemistry and Cell Biology presents several review articles of relevance to this field. The topics covered include the significance of the regulation and overexpression of polypeptide chain initiation factors for cell transformation and malignancy, the role of mRNA structure in the control of synthesis of key growth regulatory proteins, the actions of the eIF2 alpha-specific protein kinase PKR in the control cell growth and apoptosis, and the involvement of the elongation factor eEF1 in oncogenesis. The purpose of this article is to give an overview of the field and to indicate where we may expect developments to occur in the next few years.

Differential expression of vascular endothelial growth factor mRNA vs protein isoform expression in human breast cancer and relationship to eIF-4E

Angiogenesis is the formation of new blood vessels from the existing vasculature. Vascular endothelial growth factor (VEGF) is an endothelium-specific angiogenic factor strongly implicated in pathological angiogenesis. In this study, the mRNA and protein expression of the four alternatively spliced VEGF isoforms (121, 165, 189 and 206 amino acids) were examined in normal and malignant breast tissues. Three VEGF transcripts were detected in both (121>165>189), whereas only VEGF165 protein was detected. The tumours expressed more VEGF mRNA (P = 0.02) and protein (P < 0.0001), with eight-fold more VEGF protein generated per mRNA unit (P = 0.009). To examine this further, the expression of eIF-4E, a translation initiation factor, was examined. Increased eIF-4E mRNA levels were detected in the tumours (P < 0.0001) that correlated with VEGF mRNA (P = 0.0002), implying co-regulation of these genes. VEGF mRNA expression was elevated in tumours expressing the epidermal growth factor receptor (P < 0.01), but there was no difference according to oestrogen receptor status (P = 0.9), node status (P = 0.09) or between differing histologies (P = 0.4). These data suggest that elevated VEGF protein expression, by both enhanced transcription and translation, is a potential means by which tumour angiogenesis is induced in breast carcinomas. VEGF expression is also significantly associated with factors correlating with a poor outcome, implying a role in progression of this disease.