Protein synthesis in resting and growth-stimulated human peripheral lymphocytes. Evidence for regulation by a non-messenger RNA

Rapid Quantification of Mitogen-induced Blastogenesis in T Lymphocytes for Identifying Immunomodulatory Drugs

Lymphocyte proliferation in response to antigenic or mitogenic stimulation is a readily quantifiable phenomenon useful for testing immunomodulatory (i.e., immunosuppressive or immunostimulatory) chemical compounds and biologics. One of the earliest steps during mitogenesis is cell enlargement or blastogenic transformation, whereupon the cell volume increases before division. It is usually detectable in the first several hours of T-lymphocyte stimulation. Here, we describe a rapid method to quantify blastogenesis in T lymphocytes isolated from mouse spleens and human peripheral blood mononuclear cells (PBMCs) using an automated cell counter. Various commonly used proliferation assays for the most part are laborious and only reflect the overall population effect rather than individual cellular effects within a population. In contrast, the presented automated cell counter assay provides rapid, direct, and precise measurements of cell diameters that can be used for assessing the effectiveness of various mitogens and immunomodulatory drugs in vitro.

The Levels of a Universally Conserved tRNA Modification Regulate Cell Growth

N(6)-Threonylcarbamoyl-adenosine (t(6)A) is a universal modification occurring at position 37 in nearly all tRNAs that decode A-starting codons, including the eukaryotic initiator tRNA (tRNAi (Met)). Yeast lacking central components of the t(6)A synthesis machinery, such as Tcs3p (Kae1p) or Tcs5p (Bud32p), show slow-growth phenotypes. In the present work, we show that loss of the Drosophila tcs3 homolog also leads to a severe reduction in size and demonstrate, for the first time in a non-microbe, that Tcs3 is required for t(6)A synthesis. In Drosophila and in mammals, tRNAi (Met) is a limiting factor for cell and animal growth. We report that the t(6)A-modified form of tRNAi (Met) is the actual limiting factor. We show that changing the proportion of t(6)A-modified tRNAi (Met), by expression of an un-modifiable tRNAi (Met) or changing the levels of Tcs3, regulate target of rapamycin (TOR) kinase activity and influences cell and animal growth in vivo. These findings reveal an unprecedented relationship between the translation machinery and TOR, where translation efficiency, limited by the availability of t(6)A-modified tRNA, determines growth potential in eukaryotic cells.

Interchromatin granules during phytohemagglutinin stimulation of human lymphocytes. A cytochemical study

We studied the formation of interchromatin granules (IGs) in phytohemagglutinin (PHA)-stimulated lymphocytes. The bismuth staining method was used for the visualization of IGs, and we also applied high-resolution autoradiography after incubating cells in the presence of 3H-leucine during different stages of lymphocyte activation. The disaggregation of chromatin and the enlargement of interchromatinic areas in stimulated lymphocytes were found to be accompanied by an increase in the number of IGs, and it was shown that IGs were formed during all of the investigated stages of lymphocyte stimulation.

Transfer ribonucleic acid populations in concanavalin-A-stimulated bovine lymphocytes

Transfer RNA isolated from lymphocytes stimulated by concanavalin A and that from resting cells were compared with respect to amino-acid acceptance, integrity of the CCA-terminus, extent of modification and isoacceptor distribution. Following growth stimulation the overall amino-acid acceptance of the tRNA is elevated, in particular the relative acceptor activities for threonine and arginine are increased. The reduced acceptor activity of the tRNA from the quiescent cells is not due to a preferential degradation of the CCA-end, since it persists even in the presence of ATP(CTP):tRNA nucleotidyltransferase. We therefore conclude that this reduced activity is caused by structural differences of the tRNAs. The content of modified nucleotides in newly synthesized tRNA from lymphocytes cultured in the presence and absence of concanavalin A was determined. tRNA from resting cells was found to be undermodified with respect to pseudouridine and dihydrouridine. Upon monitoring the tRNA isoacceptor distribution by affinity chromatography on immobilized elongation factor Tu and subsequent two-dimensional gel electrophoresis, a preferential synthesis of particular lysine- and threonine-accepting tRNAs was observed upon mitogenic stimulation. Evidently, a specific tRNA population is needed by the proliferating cells. These results are discussed in view of the hypothesis that the commitment of lymphocytes to proliferation is at least in part under translational control.

Lymphocyte blastogenesis. Post-transcriptional controls of protein synthesis

To better understand the regulation of macromolecular synthesis in the response of lymphocytes to a mitogen, we have used two-dimensional electrophoresis to search for specificity in the early increase seen in protein synthesis in human lymphocytes treated with phytohemagglutinin and examined the role of new RNA synthesis in this response. Our results confirm a major increase in overall protein synthesis after 4 h of phytohemagglutinin treatment. A further disproportionate increase in the synthetic rates of certain polypeptides was observed using two-dimensional polyacrylamide gel electrophoresis. While actinomycin-D reduced protein synthesis to a level below that of untreated cells, phytohemagglutinin nonetheless enhanced total protein synthesis even in the presence of actinomycin. Some, but not all, of the disproportionate increases in synthesis seen for certain polypeptides are blocked by actinomycin. These results imply the existence of multiple mechanisms controlling protein synthesis early in the course of lymphocyte stimulation. At least some of these do not require new RNA synthesis and thus operate at a post-transcriptional level.

Voltage-gated potassium channels are required for human T lymphocyte activation

The calcium channel blockers, verapamil and diltiazem, inhibit phytohemagglutinin (PHA)-induced mitogenesis at concentrations that block the T lymphocyte K channel currents. K channel blockers also inhibit the allogeneic mixed lymphocyte response in a dose-dependent manner with the same potency sequence as for block of K currents. K channel blockers inhibit PHA-stimulated mitogenesis only if added during the first 20-30 h after PHA addition, but not later, indicating a requirement for functional K channels during this period. We investigated the effect of K channel blockers on various aspects of protein synthesis for two reasons: first, protein synthesis appears to be necessary for the events leading to DNA synthesis, and second, the increase in the protein synthetic rate commences during the first 24-48 h after PHA addition. PHA-induced total protein synthesis was reduced to the level in unstimulated T lymphocytes by K channel blockers in a dose-dependent manner with the same potency sequence as for the block of K currents and inhibition of [3H]thymidine incorporation. Two-dimensional gel electrophoresis demonstrated that although the synthesis of the majority of proteins was reduced by K channel blockers to the level in unstimulated T cells, some proteins continued to be synthesized at an enhanced rate compared with resting cells. Two proteins, S and T, detected by two-dimensional gel electrophoresis in unstimulated T lymphocytes, appeared to be reduced in intensity in gels of PHA-treated T lymphocytes, in contrast to the increased synthesis of the remaining proteins. 4-Aminopyridine (4-AP), at concentrations that inhibit protein synthesis, prevented the apparent PHA-induced reduction of proteins S and T. These proteins may play a role in maintaining the T lymphocyte in a resting state and may be related to the translation inhibitory factors reported to be present at a higher specific activity in quiescent T lymphocytes than in PHA-activated T cells. The expression of the IL-2 receptor (Tac) during T lymphocyte activation was not altered by K channel blockers, whereas the production of interleukin 2 (IL-2) was reduced to the level in unstimulated T lymphocytes. Exogenous IL-2 partially relieved the inhibition of mitogenesis by low, but not by high, concentrations of 4-AP. These experiments clarify the role of K channels in T lymphocyte activation and suggest that functional K channels are required either for protein synthesis or for events leading to protein synthesis.

Chromatin structure and nucleic acid synthesis in human lymphocyte activation by phytohemagglutinin

Alterations in chromatin structure were explored as a function of time following phytohemagglutinin (PHA) activation of human lymphocytes. Nuclear refringency (related to chromatin status), ultrastructural changes, and modifications of nucleic acid synthesis were compared. Chromatin dispersion was observed within 20 min of stimulation, with a concomitant increase in [3H]uridine incorporation; both were inhibited by actinomycin D. Recondensation of chromatin had occurred at 60 min when RNA synthesis was still increasing. The early chromatin dispersion thus was a reversible step and one of the most striking cellular events during the first phase of PHA activation. Late chromatin dispersion of blastic cells during the second phase of PHA response (after 24 h) was related to DNA synthesis in mitotic cells. This step was also inhibited by actinomycin D. These results suggest a relationship between the morphological changes in chromatin structure and the induction of RNA and DNA syntheses. The regulation of transcriptional and replicational states of the nucleus is discussed in terms of modifications of chromatin structure, considering PHA-induced calcium and cyclic nucleotide metabolism and interleukin II receptor expression.

Cd2+ responses of cultured human blood cells

Cd2+ cytotoxicity, uptake, and partitioning, and Cd2+-induced metallothioneine synthesis were studied in cultured peripheral human blood cells. Mononuclear cells were found to resist relatively high levels of Cd2+. Few cells were killed below 50 microM Cd2+. Above this value, survival decreased exponentially with dose. The mean LD50 for mononuclear cells cultured in Cd2+ for 40 hr was 100 microM. Polymorphonuclear cells (granulocytes) were found to be more resistant, with a significantly higher threshold and LD50, and a more complex dose response. Most of the Cd2+ incorporated by blood cells was taken up by nucleated cells. Despite their greater resistance, polymorphonuclear cells incorporated more Cd2+ at higher doses (50 to 150 microM) than did mononuclear cells. No Cd2+ was bound to metallothioneine in polymorphonuclear cells following exposure to Cd2+ for even extended periods of time (18 hr) at high doses of 109Cd2+ (25 microM). Instead Cd2+ appeared in a Sephadex G-75 peak of approximately 60,000 Da, as well as in the void peak. No significant amount of preexisting metallothioneine (MT) or metallothioneine mRNA was found in the mononuclear cells. However, MT synthesis was induced rapidly following exposure to Cd2+. [109Cd2+]MT appeared within 1 hr following exposure to 50 microM 109Cd2+, and MT synthesis rates measured from [35S]cysteine incorporation were found to be maximal within 4 hr.