Early biochemical changes in phytohemagglutinin-stimulated peripheral blood lymphocytes from normal and tumor-bearing rats

Purine metabolism and control of cell proliferation

Exposure of normal lymphocytes to phytohaemagglutinin or other lectin mitogens results in increased concentrations of 5-phosphoribosyl-1-pyrophosphate (PP-ribose-P) within minutes. Subsequently, synthesis of purine nucleotides by both the de novo and the salvage pathways is facilitated. This change is prevented by proliferation-inhibiting concentrations of exogenous adenosine. The capacity of lymphocytes to metabolize both adenine and adenosine is increased several-fold by incubation with phytohaemagglutinin but the specific activities of the respective first-step enzymes are not significantly altered. These results suggest that the relatively low quantity of PP-ribose-P available in normal lymphocytes is a major factor limiting the synthesis of purine nucleotides and may be important for the maintenance of the quiescent state. Increased availability of PP-ribose-P may also be associated with proliferative activation of fibroblast-like cells: chick embryo fibroblast cultures released from density-dependent inhibition of growth by insulin, trypsin or serum rapidly increase the rate of adenine incorporation into nucleotides. Chick embryo fibroblasts transformed by Rous sarcoma virus, but not cells infected with the respective non-transforming leukosis virus, show PP-ribose-P concentrations higher than those observed in normal cells.

Glutathione status during the mitogenic response of rat splenocytes. Effects of oxygen concentration: FO2 21% versus FO2 7%

Glutathione is known to be an important parameter for ConA proliferative response of murine splenocytes. We studied the glutathione status of ConA-stimulated rat splenocytes during the early and late phase of the mitogenic response under low (FO2 7%) and standard (FO2 21%) oxygen concentrations. We determined the intracellular total, oxidized and reduced glutathione levels after 6, 12, 24 and 48 h of culture with or without ConA and/or 2-ME, under FO2 7% and 21%. Our results showed that: The 2 GSSG/GSH + 2 GSSG ratio, which indicated the redox state of the cells, remained normal during the early period of culture (0-24 h), irrespective of culture conditions. After 48 h of culture, this ratio increased dramatically under FO2 21% and less under FO2 7%. The maintenance of the redox state seems to be an oxygen concentration-dependent phenomenon. ConA stimulation involved a glutathione consumption during the early stages of culture; under these conditions 2-ME increased the glutathione synthesis, which was higher under FO2 7% than under FO2 21%. On the other hand, the presence of 2-ME involved an increase of tritiated thymidine uptake in stimulated splenocytes, which was significantly higher under FO2 21% than under FO2 7%. Low oxygen tension (FO2 7%) can induce a higher increase of glutathione synthesis, whereas the respective ConA proliferative response is lower than that observed under standard O2 conditions.

Are cyclic nucleotides involved in the initiation of mitogenic activation of human lymphocytes?

In order to obtain more insight into the possible role of cyclic AMP or cyclic GMP in modulating the initial cellular processes following activation of lymphocytes, we measured the effects of the T-cell mitogen concanavalin A and other substances including hormones on the cyclic nucleotide levels in human peripheral blood lymphocytes. The enzyme activities of the corresponding nucleotide cyclases, adenylate cyclase and guanylate cyclase were measured in both isolated plasma membranes or the cytosol of resting or concanavalin A stimulated rabbit thymocytes. Concanavalin A in a mitogenic concentration of about 5-10 micrograms/ml caused small, but consistent increases in cAMP but no changes in cGMP levels during the first hour of activation. Concomitantly, the specific activity of plasma membrane-bound adenylate cyclase was always increased at least 1.5-fold 30 min after stimulation of rabbit thymocytes with concanavalin A, but no effect could be detected on the specific activities of plasma membrane-bound or soluble guanylate cyclase. At high, supraoptimal concentrations of concanavalin A (more than 20 micrograms/ml) cAMP levels dramatically increased in human lymphocytes within minutes, but cGMP levels again were unaffected. Forskolin and beta-adrenergic hormones elevated cAMP in human lymphocytes, whereas cGMP levels were increased by the addition of sodium nitroprusside or alpha-adrenergic hormones. Sodium nitroprusside, in concentrations which elevated cGMP in human lymphocytes, had no influence on the incorporation of [3H]uridine into RNA of resting or concanavalin A stimulated human lymphocytes. Addition of forskolin resulted in an increase of cAMP levels and a dose-dependent decrease of [3H]uridine incorporation into RNA of concanavalin A-stimulated lymphocytes with no effect on resting lymphocytes. The data suggest that cGMP does not play a role in the initial phase of mitogenic activation of lymphocytes, whereas cAMP may be involved in the blast transformation process as an inhibitory signal.

Kinetics of thymidine incorporation into detergent-soluble DNA of mouse lymphocytes

We reported recently that splenocytes from concanavalin A-stimulated mice rapidly incorporated [3H]thymidine into non-mitochondrial DNA that was detergent soluble and distributed in size classes between 200 and 5000 base pairs. In this report we show that small [3H]thymidine-labeled oligonucleotides (less than 100 base pairs long) appear by 15 min. Subsequently, [3H]thymidine in small oligonucleotides diminishes as incorporation into larger size classes of detergent-soluble DNA occurs in a pattern that is stable for at least 3 hr. Although incorporation of [3H]thymidine into the oligonucleotides is not sensitive to aphidicolin or hydroxyurea, the appearance of [3H]thymidine in larger species is blocked by both drugs. These results indicate that the enzymatic process involved in synthesis of oligonucleotides is somehow distinct from the process involved in synthesis of larger detergent-soluble size classes. Synthesis of the latter may be replication related.

Characterization and subcellular localization of nucleotide cyclases in calf thymus lymphocytes

The role of cyclic nucleotides in the regulation of lymphocyte growth and differentiation remains controversial, as an adequate characterization of the key enzymes, adenylate cyclase and guanylate cyclase, in the plasma membrane of lymphocytes is still lacking. In this study, calf thymus lymphocytes were disrupted by nitrogen cavitation and various cellular fractions were isolated by differential centrifugation and subsequent sucrose density ultracentrifugation. As revealed by the chemical composition and the activities of some marker enzymes, the plasma membrane fraction proved to be highly purified. Nucleotide cyclases were present in the plasma membranes in high specific activities, basal activities of adenylate cyclase being 13.7 pmol/mg protein per min and 34.0 pmol/mg protein per min for the guanylate cyclase, respectively. Adenylate cyclase could be stimulated by various effectors added directly to the enzyme assay, including NaF, GTP, 5'-guanylyl imidodiphosphate, Mn2+ and molybdate. Addition of beta-adrenergic agonists only showed small stimulating effects on the enzyme activity in isolated plasma membranes. Basal activity of adenylate cyclase as well as activities stimulated by NaF or 5'-guanylyl imidodiphosphate exhibited regular Michaelis-Menten kinetics. Activation by both agents only marginally affected the Km values, but largely increased Vmax. The activity of the plasma membrane-bound guanylate cyclase was about 10-fold enhanced by the nonionic detergent Triton X-100 and high concentrations of lysophosphatidylcholine, but was slightly decreased upon addition of the alpha-cholinergic agonist carbachol. Basal guanylate cyclase indicated to be an allosteric enzyme, as analyzed by the Hill equation with an apparent Hill coefficient close to 2. In contrast, Triton X-100 solubilized enzyme showed regular substrate kinetics with increasing Vmax but unaffected Km values. Thus the lymphocyte plasma membrane contains both adenylate cyclase and guanylate cyclase at high specific activities, with properties characteristic for hormonally stimulated enzymes.

Cellular and molecular aspects of immune system aging

We begin with a brief discussion of the importance and advantages of immune studies to the problem of aging. This is followed by a short over-view of immune system aging at the systemic level. The major portion of the article is a review of observation, both at the cellular and molecular level, of changes in aging immune cells, with sections on intercellular communication, membrane phenomena, cyclic nucleotides, and molecular genetic changes.

Diabetes and cancer: a postulated relationship

Diabetes leads to periodic fluctuations in levels of nutrients and cyclic nucleotides in plasma and various tissues. Variations in cyclic nucleotide levels, periods of nutrient elevation and surges of insulin combine to stimulate growth. Growth stimulation may be associated with the promotional phase of carcinogenesis. Diabetes should therefore predispose the individual to carcinogenesis and limited epidemiological data available suggest that it does.

Do plasma glycoproteins induce lymphocyte hyporesponsiveness and insulin resistance?

Depression of lymphocyte transformation and an increase in insulin resistance are common to pregnancy, oral contraceptive use, widespread malignancy, infection, and tissue destruction. We suggest that these abnormalities are caused by a rise in the plasma-glycoprotein level which is also common to these clinical states. There is evidence that glycoproteins can inhibit cell division, lymphocyte transformation, and the action of hormones on target cells. Because of the increase in plasma glycoprotein the cells in many organs and their hormone receptors may have a thicker coating of glycoproteins which blunts their response to variuos stimuli.

Cyclic nucleotide levels in resting and mitogen-stimulated spleen cell suspensions from young and old mice

The levels of cyclic adenosine 3', 5'-monophosphate (cAMP) in suspensions of unstimulated spleen cells from tumor-free 30-month old (C57BL/10Sn X C3H/HeDiSn)F1 hybrid mice averaged only 14% of that of 6-month old mice. By contrast, the level of cyclic guanosine 3', 5'-monophosphate (cGMP) in spleen cell suspensions from old mice was about 270% that of young mice. The cAMP/cGMP ratio for the unstimulated (resting) state showed a decline by 30 months to about 5% of its 6-month value. Cyclic nucleotide levels were also measured in cell suspensions from old and young mice at intervals over a two hour period following in vitro stimulation with the plant mitogens phytohemagglutinin, concanavalin-A and pokeweed mitogen. Quantitative and in some instances qualitative differences in responses were noted. These results might conceivably reflect either age-related changes in the splenic lymphoid cell subpopulations or intrinsic cellular alterations or both. It is unlikely that changes of this degree could be wholly explained by population shifts. An imbalance in cyclic nucleotide levels in both resting and stimulated lymphoid cells in older animals might contribute to the immune dysfunction known to occur with normal aging.

Phosphorylation of lymphocyte nuclear acidic proteins: regulation by cyclic nucleotides

Guanosine 3',5'-monophosphate (cyclic GMP) and cholinergic agents stimulate incorporation of phosphate into specific nuclear acidic proteins of horse peripheral blood lymphocytes. Agents that raise intracellular adenosine 3',5'-monophosphate (cyclic AMP) inhibit nuclear acidic protein phosphorylation. The opposing effects of cyclic GMP and cyclic AMP upon nuclear protein phosphorylation parallel the effects of the cyclic nucleotides upon induction of lymphocyte proliferation.