Distribution of lymphocyte messenger RNA during stimulation by phytohaemagglutinin

Changes in ribosomal binding activity of eIF3 correlate with increased translation rates during activation of T lymphocytes

The rate of protein synthesis in quiescent peripheral blood T lymphocytes increases dramatically following mitogenic activation. The stimulation of translation is due to an increase in the rate of initiation caused by the regulation of initiation factor activities. Here, we focus on eIF3, a large multiprotein complex that plays a central role in the formation of the 40 S initiation complex. Using sucrose density gradient centrifugation to analyze ribosome complexes, we find that most eIF3 is not bound to 40 S ribosomal subunits in unactivated T lymphocytes but becomes ribosome-bound following activation. Immunoblot analyses of sucrose gradient fractions for individual eIF3 subunits show that the small eIF3j subunit is unassociated with the eIF3 complex in quiescent T lymphocytes, but upon activation joins the other eIF3 subunits and binds 40 S ribosomal subunits. Because eIF3j has been shown to be required for eIF3 binding to 40 S ribosomes in vitro, the results suggest that mitogenic stimulation of T lymphocytes leads to an activation of eIF3j, thereby enabling eIF3 to bind to the larger ribosome-free eIF3 subunit complex, and then to the 40 S ribosomes. The association of eIF3j with the other eIF3 subunits appears to be inhibited by rapamycin, suggesting a mechanism that lies downstream from the mammalian target of rapamycin kinase. This association requires ionomycin together with a phorbol ester, which also suggests that calcium signaling is involved. We conclude that the complex formation of eIF3 and its association with the ribosomes might contribute to increased translation rates during T lymphocyte activation.

Genomic expression programs and the integration of the CD28 costimulatory signal in T cell activation

Optimal activation of T cells requires effective occupancy of both the antigen-specific T cell receptor and a second coreceptor such as CD28. We used cDNA microarrays to characterize the genomic expression program in human peripheral T cells responding to stimulation of these receptors. We found that CD28 agonists alone elicited few, but reproducible, changes in gene expression, whereas CD3 agonists elicited a multifaceted temporally choreographed gene expression program. The principal effect of simultaneous engagement of CD28 was to increase the amplitude of the CD3 transcriptional response. The induced genes whose expression was most enhanced by costimulation were significantly enriched for known targets of nuclear factor of activated T cells (NFAT) transcription factors. This enhancement was nearly abolished by blocking the nuclear translocation of NFATc by using the calcineurin inhibitor FK506. CD28 signaling promoted phosphorylation, and thus inactivation, of the NFAT nuclear export kinase glycogen synthase kinase-3 (GSK3), coincident with enhanced dephosphorylation of NFATc proteins. These results provide a detailed picture of the transcriptional program of T cell activation and suggest that enhancement of transcriptional activation by NFAT, through inhibition of its nuclear export, plays a key role in mediating the CD28 costimulatory signal.

Detection of early gene expression changes during activation of human primary lymphocytes by in vitro synthesis of proteins from polysome-associated mRNAs

The rapid increase in protein synthesis during the mitogenic stimulation of human peripheral blood lymphocyte is the result of global and specific translational control mechanisms. To study some of these mechanisms, we examined the in vitro translatability of mRNAs associated with the polyribosome fraction. Polyribosome fractions were isolated from lymphocytes after activation with ionomycin and the phorbol ester PMA. The associated PAmRNAs were translated in the presence of mRNA-depleted rabbit reticulocyte lysate and [(35)S]Met, and the protein products were analyzed by SDS--PAGE and autoradiography. There was little synthesis of protein from the PAmRNAs isolated from unactivated T cells, but the PAmRNAs isolated from activated T cells showed a rapid increase in translatability. Translation of the PAmRNAs was sensitive to edeine and m7GTP, suggesting their cap-dependent translation. With activation, the majority of proteins showed increasing in vitro translation, but two proteins, p72 and p33, were found to have increased synthesis within 30 min, which decreased in 1 h. Transcription inhibitors were used to ascertain if regulation of their expression was transcriptional or translational. To identify these proteins, we used biotinylated lysine during the in vitro translation reaction, and we extracted the biotinylated protein by using streptavidin magnetic beads. The protein product was analyzed by mass spectrometry. p33 was identified as a prohibitin-like protein (BAP37), but the identification of p72 was not found in the databases. The distinct up-regulation and down-regulation of their protein expression suggest their tightly controlled regulation during early T cell activation.

Signal transduction pathways that contribute to increased protein synthesis during T-cell activation

Protein synthesis rates were maximally stimulated in human lymphocytes by ionomycin and the phorbol ester PMA (I+P), which promotes proliferation, whereas PMA alone, which does not promote proliferation, stimulated protein synthesis to a lesser degree. Three translation-associated activities, eIF4E phosphorylation, eIF2B activity and 4E-BP1 phosphorylation also increased with stimulation by I+P and PMA, but only 4E-BP1 phosphorylation was differentially stimulated by these conditions. Correspondingly, signaling pathways activated in T cells were probed for their connection to these activities. Immunosuppressants FK506 and rapamycin partially blocked the protein synthesis rate increases by I+P stimulation. FK506 had less of an inhibitory effect with PMA stimulation suggesting that its mechanism mostly affected ionomycin-activated signals. I+P and PMA equally stimulated phosphorylation of ERK1/2, but I+P more strongly stimulated Akt, and p70(S6K) phosphorylation. An inhibitor that blocks ERK1/2 phosphorylation only slightly reduced protein synthesis rates stimulated by I+P or PMA, but greatly reduced eIF4E phosphorylation and eIF2B activity. In contrast, inhibitors of the PI-3 kinase and mTOR pathways strongly blocked early protein synthesis rate stimulated by I+P and PMA and also blocked 4E-BP1 phosphorylation and release of eIF4E suggesting that these pathways regulate protein synthesis activities, which are important for proliferation in T cells.

Increased phosphorylation of eukaryotic initiation factor 4 alpha during early activation of T lymphocytes correlates with increased initiation factor 4F complex formation

Mature porcine peripheral blood mononuclear cells (PPBMCs) exist in a resting state both in vivo and when maintained in culture, with low translation rates consistent with their non-proliferative state. When cultured in the presence of the appropriate mitogen, there is a 2-4-fold increase in the rate of protein synthesis per ribosome within 4 h of stimulation [Kay, J. E., Ahern, T. and Atkins, M. (1971) Biochim. Biophys. Acta 247, 322-334]. Studies on extracts prepared from unstimulated cells have suggested lesions in initiation factor activity, primarily affecting the binding of mRNA to ribosomes [Ahern, T., Sampson, J. and Kay, J. E. (1974) Nature 248, 519-521]. In these studies, we have demonstrated that activation of quiescent PPBMCs with the phorbol ester phorbol 12-myristate 13-acetate or concanavalin A leads to a rapid 2-4-fold increase in the rate of protein synthesis within 1 h or 4 h, respectively, which is insensitive to the transcriptional inhibitor, 5,6-dichlorobenzimidazole riboside. Relative to control cells, both phorbol ester and concanavalin A induce a 2-4-fold increase in labelling of the eukaryotic initiation factor eIF-4 alpha with phosphate in vivo, which primarily reflects a small net increase in phosphorylation rather than phosphate turnover on eIF-4 alpha. Similarly, with the human leukaemic T cell line JURKAT, stimulation of the T cell receptor with the monoclonal antibody, OKT-3, or treatment with phorbol ester induces a 2-3-fold increase in eIF-4 alpha phosphorylation within 30 min. Analysis of phosphorylation by two-dimensional gel electrophoresis and measurement of kinase activity towards synthetic peptides, indicate that this increased labelling also reflects increased eIF-4 alpha kinase activity rather than phosphate turnover on eIF-4 alpha. Of central importance is the finding that, concomitant with increased rates of protein synthesis following stimulation of PPBMCs with either phorbol ester or concanavalin A, there is a significant increase in the level of eIF-4 alpha recovered in high-molecular-mass complexes. These data suggest that, in quiescent PPBMCs, eIF-4F may be limiting and that the association of eIF-4 alpha and eIF-4 gamma into high-molecular-mass complexes is regulated by phosphorylation and may play a pivotal role in translational control.

Regulation of eukaryotic translation initiation factor expression during T-cell activation

Primary T-cells are metabolically quiescent, with little DNA, RNA or protein synthesis. Upon mitogenic stimulation the rate of protein synthesis increases 10-fold. We have studied the role of eIF-2 and eIF-4 alpha (eIF-4E) expression in the mechanism of translational activation. During this period, the levels of eIF-2 alpha and eIF-4 alpha mRNA increase some 50-fold. Similar to the increase in ribosomes and mRNA, the number of eIF-2 alpha, eIF-2 beta, and eIF-4 alpha molecules per cell also increase 2-3-fold. This suggests that in addition to an increase in the pool size of translational components, an additional mechanism exists which results in an increased efficiency of factor utilization. We have looked at initiation factor phosphorylation. We find that eIF-2 alpha does not undergo significant changes in its phosphorylation state nor is there a change in the efficiency of eIF-2 utilization. However, there is a rapid increase in the phosphorylation state of eIF-4 alpha which correlates with the rapid increase in translational activity. It thus appears there are 2 distinct components responsible for the translational activation of quiescent T-cells during mitogenic stimulation. The first is the phosphorylation of eIF-4 alpha, with a concomitant increase in the efficiency of eIF-4 alpha utilization. The second is an increase in the pool sizes of eIF-2 and eIF-4 alpha.

Expression of intracellular biochemical defects of lymphocytes in aging: proposal of a general aging mechanism which is not cell-specific

There is a decline in immune capacity with age which is expressed on the organismic level by association with numerous immune-related diseases, on the cellular level by impaired mitogenesis, on the biochemical level by impaired metabolic pathways, and on the molecular level by decreased protein synthesis and degradation. Defects in various cofactors such as calcium and several nucleotides also occur and may be related to the impaired enzyme function during mitogenesis in the aged. The central cause for decreased mitogenesis in the aged could be a decrease in protein synthesis which appears to cause impaired enzyme induction. This impaired enzyme induction accounts in part for the decreased glycolytic flux and DNA synthesis in these cells. Decreased protein synthesis also has been associated with a decreased synthesis of lymphokines which help these cells to proliferate. Numerous other intracellular age-related defects of lymphocytes also occur which may collectively play important interdependent roles in the impaired lymphocyte function of the aged. A potential general underlying mechanism of cellular senescence is proposed based on a genetic "slowing-cycle" effect of transcription, translation, and enzyme induction with immunosenescence presented as an example of an expression of these basic defects.

Early synthesis of specific cytoplasm proteins is correlated with the rate of exit of lymphocytes from the resting state

We investigated the initiation of synthesis of proteins in human lymphocytes exposed to the mitogen phytohemagglutinin (PHA) for 6 h. Radiolabeled proteins in three subcellular fractions, cytoplasmic, nuclear salt wash, and nuclear, were separated on polyacrylamide gels. Compared with cells incubated for the same time in the absence of PHA only two cytoplasmic proteins of Mr 51 and 60 kd showed increased synthesis in a dose-dependent fashion. Synthesis of the 60-kd protein shows the strongest correlation with rate of entry into the first S phase and with rate of cellular aggregation. Thus, the 60-kd protein appears to be a major early response-associated protein for entry of lymphocytes into the first S phase after PHA stimulation.

Poly(A)-containing messenger ribonucleoprotein complexes from sea urchin eggs and embryos: polypeptides associated with native and UV-crosslinked mRNPs

Fertilization of sea urchin eggs results in the rapid recruitment of stored messages into polyribosomes. Whether translational control in sea urchin eggs is mediated by macromolecules associated with the stored messages remains unknown, since preparations of messenger ribonucleoprotein complexes (mRNPs) were active in protein synthesis in a rabbit reticulocyte lysate. To facilitate the study of mRNPs, chromatography on oligo(dT)-cellulose was used to purify poly(A)-containing mRNPs from eggs and embryos of the sea urchin Strongylocentrotus purpuratus. Nonpolyribosomal mRNPs purified from eggs had a similar sedimentation in sucrose to unpurified mRNPs, a peak buoyant density in metrizamide of 1.22 g/cm3, and peak buoyant densities in Cs2SO4 in 1.42 g/cm3 after fixation with glutaraldehyde and 1.46 g/cm3 without fixation. Nonpolyribosomal mRNPs from eggs and zygotes contained 5-10 major proteins on sodium dodecylsulfate (SDS) polyacrylamide gels, and numerous minor bands. UV-irradiation of living eggs of the sea urchin Arbacia punctulata produced cross-linked mRNPs which contained a similar pattern of polypeptides to noncross-linked mRNPs. The polypeptides associated with embryonic polyribosomal mRNPs were also qualitatively similar to those present in nonpolyribosomal mRNPs, although stoichiometric differences may exist.

Lectin-induced changes among polyadenylated and non-polyadenylated mRNA in lymphocytes. mRNAs for actin, tubulin and calmodulin respond differently

The differentiation of T-cell-enriched concanavalin-A-stimulated bovine lymphocytes was studied in vitro. mRNA was isolated from resting and stimulated cells. The amount of polyadenylated RNA increases from 3.6 X 10(-9) to 12 X 10(-9) micrograms/cell during 40 h concanavalin A stimulation. The estimated maximum length of the 3'-poly(A) tract in this RNA is reduced from 240 to 220 residues in stimulated cells. During translation in the rabbit reticulocyte lysate in vitro mRNA from stimulated cells consistently incorporates about 1.6-3 times more radioactivity/micrograms RNA into proteins than mRNA from resting cells. Three translation products have been identified on two-dimensional gels as actin, tubulin and calmodulin. Large quantitative shifts are seen between proteins translated from mRNAs isolated from resting cells and stimulated cells respectively. Actin and calmodulin are already major products from resting cell mRNA. Actin, however, increases about fivefold after stimulation while calmodulin does not change. Tubulin appears in substantial amounts only among stimulated cell mRNA products. Tubulin and calmodulin, on the other hand, remain mainly in the polyadenylated RNA fraction after stimulation while 50% of the actin together with a group of about six other major products is found among proteins translated from non-polyadenylated RNA. We conclude that in lectin-stimulated lymphocytes, besides a general increase in the amount of mRNA, alterations in post-transcriptional processing reactions are active in determining the fate of individual mRNAs.

Stability of polyadenylated mRNA in pig lymphocytes

The stability of cytoplasmic poly(A)-containing mRNA in unstimulated pig lymphocytes has been determined by [3H]poly(U) hybridization after actinomycin D treatment. This approach has shown that the majority of the mRNA is much more unstable than has been suggested by earlier pulse-chase experiments, and has a half-life of between 1 and 2 h.