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

Constitutive expression of the E2F1 transcription factor in fibroblasts alters G0 and S phase transit following serum stimulation

The E2F1 transcription factor was constitutively expressed in NIH3T3 fibroblasts to determine its effect on the cell cycle. These E2F1 cell lines were not tightly synchronized in G0 phase of the cell cycle following serum starvation, as are normal fibroblasts. Instead, the cells are spread throughout G0 and G1 phase with a portion of the population initiating DNA synthesis. Upon serum stimulation, the remaining cells in G0/G1 begin to enter S phase immediately but with a reduced rate. Constitutive expression of E2F1 appears to primarily affect the G0 phase, since transit of proliferating E2F1 cell lines through G1 phase is the same as control cells. Consistent with a shortened G0 phase, the E2F1 cell lines have a significantly reduced cellular volume. Additionally, the first S phase after serum stimulation, but not subsequent S phases, is nearly doubled in the E2F1 cell lines compared with control cells. Cell lines expressing a deletion mutant of E2F1 (termed E2F1d87), known to significantly affect cell shape, have cell cycle and volume characteristics similar to the E2F1 expressing cells. However, all S phase durations are considerably lengthened and the cells demonstrate delayed growth after plating.

Effects of lymphocyte activation on transfer RNAs

The influences of mitogen activation on the functional capacity of rat splenic tRNAs were evaluated. The specific amino acid acceptor activity, pmol of a specific amino acid accepted per nmol of tRNA, of isolated splenic tRNAs from in vivo Concanavalin A (37 h)-treated rats were up to 8 times the specific amino acid acceptor activities of splenic tRNAs from control rats. Control splenic tRNAs were treated with purified liver tRNA nucleotidyltransferase in vitro to repair the 3'[CCA] terminus of tRNAs, and subsequently assayed in an aminoacylation reaction. The specific amino acid acceptor activities were slightly increased over those tRNAs not repaired with tRNA nucleotidyltransferase, indicating the presence of a low level of defective but repairable tRNAs in the control rat spleen. Furthermore, our results indicate that cyclosporin A (inhibitor of lymphocyte activation) blocks the Concanavalin A stimulation of tRNA charging ranging from 16 to 93%.

Intracellular regulation of the production and release of human erythroid-directed lymphokines

Erythroid burst-promoting activity (BPA) is released from B lymphocytes in soluble (sBPA) and membrane-bound (mBPA) forms. To study intracellular processes involved in production of these physically separable factors, we measured their time course release into serum-free medium from B cells that were pulse-exposed for 5-240 min to nonmitogenic base medium or inhibitors of energy-dependent metabolism (2,4-dinitrophenol, sodium azide, and 2-deoxy-D-glucose), transcription and translation (actinomycin D and cycloheximide), replicative DNA synthesis (cytosine arabinoside), or posttranslational processing (monensin). mBPA and sBPA were initially detectable after 1 and 2 h, respectively. Maximum cumulative levels of 8 +/- 0.6 and 9 +/- 1.0 U/ml, respectively, were reached after 8 h. In contrast, cumulative mBPA and sBPA levels in medium prepared from cells treated with metabolic inhibitors were reduced by up to 90%. Both surface exfoliation and mBPA expression by intact plasma membranes were diminished. Whereas pulse-exposure to cytosine arabinoside had no effect, treatment with actinomycin D or cycloheximide abolished BPA expression. Exposure to monensin reduced mBPA and sBPA levels to zero in a concentration-and time-dependent fashion. We conclude that production and release of BPA is an energy-dependent process, requiring mRNA synthesis and translation and posttranslational remodeling of the protein but not replicative DNA synthesis.

Effects of idebenone on mitogen-induced proliferation of human lymphocytes

In vitro effect of idebenone on human lymphocytes isolated from old and young donors was determined. The effects of drug were the same with the old and young donors cells. At concentrations of 2 microg/ml (6 microM) or less in the culture medium, idebenone showed no effect on phytohemagglutinin (PHA)-induced proliferation and protein synthesis, or on cell viability measured by Trypan Blue exclusion. Concentrations of 25 and 50 microg/ml showed dose-dependent suppression of the proliferation and protein synthesis which was associated with significant cytotoxicity. At concentrations of 8-10 microg/ml the compound appears to have just detectable effect on lymphocyte viability or ability to respond to PHA stimulation. It seems clear that such in vivo concentrations which would be associated with lymphopenia and immunologic suppression are not achieved with therapeutic doses of idebenone. The pattern of protein bands observed on fluorograms of sodium dodecyl sulfate-polyacrylamide gels of cells incubated with [(3)H]leucine and [(35)S]methionine was similar in control and idebenone-treated samples, consistent with a slight, nonspecific inhibitory effect on protein synthesis in cultures with higher doses of the compound. At these concentrations, idebenone induced a slight, but detectable, enhancement of the intracellular stress proteins, HSP70 and HSP90.

Mitogen-induced preferential synthesis of proteins during the G0 to S phase transition in human lymphocytes

We have followed the induction of protein synthesis in mitogen-activated human peripheral blood mononuclear cells during the transition from quiescence, or G0, through the prereplicative phase and into first S phase. Doses of mitogens optimal for proliferative response preferentially enhance the synthesis of a subset of intracellular proteins during the approximately 24-h lag interval. The mitogenic lectin phytohemagglutinin (PHA) and OKT3, a mitogenic monoclonal antibody to the CD3 component of the T cell antigen receptor, preferentially enhance bands of the same molecular weight in one-dimensional SDS-PAGE. The proteins are low detergent soluble (0.1% Triton X-100) "cytoplasmic" cellular components and some have been identified as single spots on two-dimensional gels. Bands of 51 and 66 kDa are induced early in lag phase (4 h after stimulation) but are transiently synthesized, decreasing later in lag phase. The majority of the mitogen-induced proteins, 39, 51, 55, 60, 73, and 95 kDa are enhanced by mid lag phase (12 h after stimulation). With the exception of the 55-kDa band, five of these proteins are clearly enhanced in T cells purified after mitogen stimulation. The same five bands show sustained synthesis in actively cycling cells 42-48 h after stimulation and are major synthesized proteins, and corresponding bands are synthesized in a transformed T cell line, MOLT-4. Two of the proteins in this group that are most prominently synthesized during the lag interval have been previously identified as the heat shock proteins, HSP 90 (95-kDa band) and HSC 70 (73-kDa band). We speculate that this group of five proteins, including HSP 90 and HSC 70, may be coordinately expressed in actively replicating T cells and may have some common structural or functional role in sustaining the replicative state.

Mitogen activation induces the enhanced synthesis of two heat-shock proteins in human lymphocytes

We have used mitogenic lectin (PHA) and a monoclonal antibody (OKT3) to stimulate human peripheral blood (G0) lymphocytes, in the presence of monocytes, and have found two major preferentially synthesized proteins, 73 and 95 kD, which are induced by the mitogens. The elevated synthesis of both proteins begins approximately 4-6 h after mitogen addition (early to mid G0/G1) before entry into first S phase. Maximum synthesis of both proteins is reached by 12 h after mitogen addition when P95 synthesis represents approximately 4%, and P73 approximately 2%, of the total protein synthesis, compared with less than 0.5% for each protein in cells cultured without mitogen. Thus, the proteins appear to be major components of activated cells. We find that both P73 and P95 are induced by heat stress as well as mitogenic stimulation. The induction of the proteins is not affected by either deleting glucose from the culture media or, alternatively, by supplementing it. Using polyclonal antibodies prepared to each of the proteins isolated from mitogen activated cells and monoclonal antibodies that were raised to heat shock proteins, we are able to show that P95 is electrophoretically and immunologically identical to the HSP 90 induced by heat stress. P73 is one of the 70 kD HSPs, (termed HSC 70; Pelham, H. R. B. 1986. Cell. 46: 959-961), but is different from the most strongly heat inducible form of HSP 70 (72 kD). The distribution of both proteins in subcellular fractions of mitogen activated lymphocytes is similar to the reported localization of the respective HSP's in other cell types. The results suggest that HSP 90 and HSC 70 may have functional roles in stress response and growth processes of human lymphocytes.

Increased voltage-gated potassium conductance during interleukin 2-stimulated proliferation of a mouse helper T lymphocyte clone

Recent work has demonstrated the presence of voltage-gated potassium channels in human peripheral blood T lymphocytes (Matteson, R., and C. Deutsch, 1984, Nature (Lond.), 307:468-471; DeCoursey T. E., T. G. Chandy, S. Gupta, and M. D. Cahalan, 1984, Nature (Lond.), 307:465-468) and a murine cytolytic T-cell clone (Fukushima, Y., S. Hagiwara, and M. Henkart, 1984, J. Physiol., 351:645-656). Using the whole cell patch clamp, we have found a potassium conductance with similar properties in a murine noncytolytic T lymphocyte clone, L2. Under voltage clamp, a step from a holding potential of -70 mV to +50 mV produces an average outward current of 100-150 pA in "quiescent" L2 cells at the end of their weekly maintenance cycle. When these cells are stimulated with human recombinant interleukin 2 (rIL2, 100 U/ml), they grow in size and initiate DNA synthesis at approximately 24 h. Potassium conductance is increased as early as 8 h after stimulation with rIL2 and rises to a level 3-4 times that of excipient controls by 24 h. The level remains elevated through 72 h, but as the cells begin to leave the cell cycle at 72-96 h, the conductance decreases quickly to a value only slightly higher than the initial one. Quinine, a blocker of this conductance, markedly reduces the rate at which L2 cells traverse the cell cycle, while also reducing the rate of stimulated protein synthesis. The regulation of potassium conductance in L2 cells during rIL2-stimulated proliferation suggests that potassium channel function may play a role in support of the proliferative response.