Rise and fall of cyclic AMP required for onset of lymphocyte DNA synthesis

A mini-review: phosphodiesterases in charge to balance intracellular cAMP during T-cell activation

T-cell activation is a pivotal process of the adaptive immune response with 3',5'-cyclic adenosine monophosphate (cAMP) as a key regulator of T-cell activation and function. It governs crucial control over T-cell differentiation and production of pro-inflammatory cytokines, such as IFN-γ. Intriguingly, levels of intracellular cAMP differ between regulatory (Treg) and conventional T-cells (Tcon). During cell-cell contact, cAMP is transferred via gap junctions between these T-cell subsets to mediate the immunosuppressive function of Treg. Moreover, the activation of T-cells via CD3 and CD28 co-stimulation leads to a transient upregulation of cAMP. Elevated intracellular cAMP levels are balanced precisely by phosphodiesterases (PDEs), a family of enzymes that hydrolyze cyclic nucleotides. Various PDEs play distinct roles in regulating cAMP and cyclic guanosine monophosphate (cGMP) in T-cells. Research on PDEs has gained growing interest due to their therapeutic potential to manipulate T-cell responses. So far, PDE4 is the best-described PDE in T-cells and the first PDE that is currently targeted in clinical practice to treat autoimmune diseases. But also, other PDE families harbor additional therapeutic potential. PDE2A is a dual-substrate phosphodiesterase which is selectively upregulated in Tcon upon activation. In this Mini-Review, we will highlight the impact of cAMP regulation on T-cell activation and function and summarize recent findings on different PDEs regulating intracellular cAMP levels in T-cells.

Treatment of Experimental Autoimmune Encephalomyelitis with an Inhibitor of Phosphodiesterase-8 (PDE8)

After decades of development, inhibitors targeting cyclic nucleotide phosphodiesterases (PDEs) expressed in leukocytes have entered clinical practice for the treatment of inflammatory disorders, with three PDE4 inhibitors being in clinical use as therapeutics for psoriasis, psoriatic arthritis, chronic obstructive pulmonary disease and atopic dermatitis. In contrast, the PDE8 family that is upregulated in pro-inflammatory T cells is a largely unexplored therapeutic target. We have previously demonstrated a role for the PDE8A-Raf-1 kinase complex in the regulation of myelin oligodendrocyte glycoprotein peptide 35–55 (MOG_35–55) activated CD4⁺ effector T cell adhesion and locomotion by a mechanism that differs from PDE4 activity. In this study, we explored the in vivo treatment of experimental autoimmune encephalomyelitis (EAE), a model for multiple sclerosis (MS) induced in mice immunized with MOG using the PDE8-selective inhibitor PF-04957325. For treatment in vivo, mice with EAE were either subcutaneously (s.c.) injected three times daily (10 mg/kg/dose), or were implanted subcutaneously with Alzet mini-osmotic pumps to deliver the PDE8 inhibitor (15.5 mg/kg/day). The mice were scored daily for clinical signs of paresis and paralysis which were characteristic of EAE. We observed the suppression of the clinical signs of EAE and a reduction of inflammatory lesion formation in the CNS by histopathological analysis through the determination of the numbers of mononuclear cells isolated from the spinal cord of mice with EAE. The PDE8 inhibitor treatment reduces the accumulation of both encephalitogenic Th1 and Th17 T cells in the CNS. Our study demonstrates the efficacy of targeting PDE8 as a treatment of autoimmune inflammation in vivo by reducing the inflammatory lesion load.

The Role of PDE8 in T Cell Recruitment and Function in Inflammation

Inhibitors targeting cyclic nucleotide phosphodiesterases (PDEs) expressed in leukocytes have entered clinical practice to treat inflammatory disorders, with three PDE4 inhibitors currently in clinical use as therapeutics for psoriasis, psoriatic arthritis, atopic dermatitis and chronic obstructive pulmonary disease. In contrast, the PDE8 family that is upregulated in pro-inflammatory T cells is a largely unexplored therapeutic target. It was shown that PDE8A plays a major role in controlling T cell and breast cancer cell motility, including adhesion to endothelial cells under physiological shear stress and chemotaxis. This is a unique function of PDE8 not shared by PDE4, another cAMP specific PDE, employed, as noted, as an anti-inflammatory therapeutic. Additionally, a regulatory role was shown for the PDE8A-rapidly accelerated fibrosarcoma (Raf)-1 kinase signaling complex in myelin antigen reactive CD4⁺ effector T cell adhesion and locomotion by a mechanism differing from that of PDE4. The PDE8A-Raf-1 kinase signaling complex affects T cell motility, at least in part, via regulating the LFA-1 integrin mediated adhesion to ICAM-1. The findings that PDE8A and its isoforms are expressed at higher levels in naive and myelin oligodendrocyte glycoprotein (MOG)_35–55 activated effector T (Teff) cells compared to regulatory T (Treg) cells and that PDE8 inhibition specifically affects MOG_35–55 activated Teff cell adhesion, indicates that PDE8A could represent a new beneficial target expressed in pathogenic Teff cells in CNS inflammation. The implications of this work for targeting PDE8 in inflammation will be discussed in this review.

A most versatile kinase: The catalytic subunit of PKA in T-cell biology

The cAMP-dependent protein kinase, more commonly referred to as protein kinase A (PKA), is one of the most-studied enzymes in biology. PKA is ubiquitously expressed in mammalian cells, can be activated in response to a plethora of biological stimuli, and phosphorylates more than 250 known substrates. Indeed, PKA is of central importance to a wide range of organismal processes, including energy homeostasis, memory formation and immunity. It serves as the primary effector of the second-messenger molecule 3',5'-cyclic adenosine monophosphate (cAMP), which is believed to have mostly inhibitory effects on the adaptive immune response. In particular, elevated levels of intracellular cAMP inhibit the activation of conventional T cells by limiting signal transduction through the T-cell receptor and altering gene expression, primarily in a PKA-dependent manner. Regulatory T cells have been shown to increase the cAMP levels in adjacent T cells by direct and indirect means, but the role of cAMP within regulatory T cells themselves remains incompletely understood. Paradoxically, cAMP has been implicated in promoting T-cell activation as well, adding another functional dimension beyond its established immunosuppressive effects. Furthermore, PKA can phosphorylate the NF-κB subunit p65, a transcription factor that is essential for T-cell activation, independently of cAMP. This phosphorylation of p65 drastically enhances NF-κB-dependent transcription and thus is likely to facilitate immune activation. How these immunosuppressive and immune-activating properties of PKA balance in vivo remains to be elucidated. This review provides a brief overview of PKA regulation, its ability to affect NF-κB activation, and its diverse functions in T-cell biology.

PDE8 controls CD4+ T cell motility through the PDE8A-Raf-1 kinase signaling complex

The levels of cAMP are regulated by phosphodiesterase enzymes (PDEs), which are targets for the treatment of inflammatory disorders. We have previously shown that PDE8 regulates T cell motility. Here, for the first time, we report that PDE8A exerts part of its control of T cell function through the V-raf-1 murine leukemia viral oncogene homolog 1 (Raf-1) kinase signaling pathway. To examine T cell motility under physiologic conditions, we analyzed T cell interactions with endothelial cells and ligands in flow assays. The highly PDE8-selective enzymatic inhibitor PF-04957325 suppresses adhesion of in vivo myelin oligodendrocyte glycoprotein (MOG) activated inflammatory CD4⁺ T effector (Teff) cells to brain endothelial cells under shear stress. Recently, PDE8A was shown to associate with Raf-1 creating a compartment of low cAMP levels around Raf-1 thereby protecting it from protein kinase A (PKA) mediated inhibitory phosphorylation. To test the function of this complex in Teff cells, we used a cell permeable peptide that selectively disrupts the PDE8A-Raf-1 interaction. The disruptor peptide inhibits the Teff-endothelial cell interaction more potently than the enzymatic inhibitor. Furthermore, the LFA-1/ICAM-1 interaction was identified as a target of disruptor peptide mediated reduction of adhesion, spreading and locomotion of Teff cells under flow. Mechanistically, we observed that disruption of the PDE8A-Raf-1 complex profoundly alters Raf-1 signaling in Teff cells. Collectively, our studies demonstrate that PDE8A inhibition by enzymatic inhibitors or PDE8A-Raf-1 kinase complex disruptors decreases Teff cell adhesion and migration under flow, and represents a novel approach to target T cells in inflammation.

A mitochondrial power play in lymphoma

Deregulated energetics is a hallmark of malignancy, but metabolic heterogeneity among individual tumors is unknown. A study by Caro et al. in this issue of Cancer Cell demonstrates that a subset of lymphomas is defined by reliance on mitochondrial energy generation and is selectively killed when this pathway is impaired.

Glucagon-like peptide-1 receptor signalling selectively regulates murine lymphocyte proliferation and maintenance of peripheral regulatory T cells

AIMS/HYPOTHESIS

Glucagon-like peptide-1 receptor (GLP-1R) agonists improve glucose control in animals and humans with type 1 diabetes. However, there is little information on the role of the GLP-1R in the immune system. We studied the role of the GLP-1R in immune function in wild-type (WT) and nonobese diabetic (NOD) and Glp1r-/- mice.

METHODS

Glp1r mRNA expression was examined in sorted immune subpopulations by RT-PCR. The effects of GLP-1R activation were assessed on cAMP production and proliferation, migration and survival of primary immune cells from WT and NOD mice. The ability of primary cells from Glp1r-/- mice to proliferate, migrate or survive apoptosis was determined. Immunophenotyping studies were performed to assess the frequency of immune subpopulations in Glp1r-/- mice.

RESULTS

Ex vivo activation of the GLP-1R resulted in a modest but significant elevation of cAMP in primary thymocytes and splenocytes from both WT and NOD mice. GLP-1R activation did not increase proliferation of primary thymocytes, splenocytes or peripheral lymph node cells. In contrast, Glp1r-/- thymocytes exhibited a hypoproliferative response, whilst peripheral Glp1r-/- lymphocytes were hyperproliferative in response to mitogenic stimulation. Activation or loss of GLP-1R signalling did not modify apoptosis or chemotaxis in primary lymphocytes. Male Glp1r-/- mice exhibited a significantly lower percentage of peripheral regulatory T cells, although no differences were observed in the numbers of CD4+ and CD8+ T cells and B cells in the spleen and lymph nodes of Glp1r-/- mice.

CONCLUSIONS/INTERPRETATION

These studies establish that GLP-1R signalling may regulate lymphocyte proliferation and maintenance of peripheral regulatory T cells.

Inhibition of T-cell receptor signal transduction and viral expression by the linker for activation of T cells-interacting p12(I) protein of human T-cell leukemia/lymphoma virus type 1

The p12(I) protein of human T-cell leukemia/lymphoma virus type 1 (HTLV-1) is a small oncoprotein that increases calcium release following protein kinase C activation by phorbol myristate acetate, and importantly, this effect is linker for activation of T cells (LAT) independent. Here, we demonstrate that p12(I) inhibits the phosphorylation of LAT, Vav, and phospholipase C-gamma 1 and decreases NFAT (nuclear factor of activated T cells) activation upon engagement of the T-cell receptor (TCR) with anti-CD3 antibody. Furthermore, we demonstrate that p12(I) localizes to membrane lipid rafts and, upon engagement of the TCR, relocalizes to the interface between T cells and antigen-presenting cells, defined as the immunological synapse. A p12(I) knockout molecular clone of HTLV-1 expresses more virus upon antigen stimulation than the isogenic wild type, suggesting that, by decreasing T-cell responsiveness, p12(I) curtails viral expression. Thus, p12(I) has contrasting effects on TCR signaling: it down-regulates TCR in a LAT-dependent manner on one hand, and on the other, it increases calcium release in a LAT-independent manner. The negative regulation of T-cell activation by p12(I) may have evolved to minimize immune recognition of infected CD4(+) T cells, to impair the function of infected cytotoxic CD8(+) T cells, and to favor viral persistence in the infected host.

Cyclic AMP levels in several macroalgae and their relation to light quantity and quality

Total cAMP levels were measured in the macroalgae Dictyota dichotoma, Gelidium sesquipedale and Ulva rigida under different light conditions in order to study its regulation either by phytochrome or photosynthesis. Incubation in red or far-red light did not promote a phytochrome-like response; instead, it showed a synergistic effect upon cAMP accumulation. cAMP levels seemed to depend on the amount of energy applied. The correlation between photosynthetic oxygen evolution and cAMP variations at sub-saturating white light irradiance pointed to photosynthetic electron transport as involved in the regulation of cAMP accumulation at least in G. sesquipedale and U. rigida. Inhibitors of thylakoidal and mitochondrial electron transport chains reduced cAMP levels in 70 to 99%. We conclude that cAMP accumulation could be regulated by photosynthetic activity rather than phytochrome in the macroalgae studied.

Influence of antidepressants on intracellular levels of cyclic adenosine monophosphate in human peripheral blood mononuclear cells

This study examines the effects of paroxetine and imipramine on intracellular concentrations of cyclic adenosine monophosphate (cAMP) in human peripheral blood mononuclear cells. It was found that imipramine and paroxetine had no effect on basal cAMP-levels. Stimulation with lipopolysaccharides and phytohaemagglutinin increased intracellular cAMP concentrations. However, pre-incubation with imipramine or paroxetine, did not influence this increase. These data do not support the hypothesis that cAMP may be related to the in vitro anti-inflammatory effects of antidepressants.

Role of MRP4 and MRP5 in biology and chemotherapy

Nucleotide efflux (especially cyclic nucleotides) from a variety of mammalian tissues, bacteria, and lower eukaryotes has been studied for several decades. However, the molecular identity of these nucleotide efflux transporters remained elusive, despite extensive knowledge of their kinetic properties and inhibitor profiles. Identification of the subfamily of adenosine triphosphate (ATP) binding cassette transporters, multidrug resistance protein (MRP) subfamily, permitted rapid advances because some recently identified MRP family members transport modified nucleotide analogs (ie, chemotherapeutic agents). We first identified, MRP4, based on its ability to efflux antiretroviral compounds, such as azidothymidine monophosphate (AZT-MP) and 9-(2-phosphonyl methoxyethyl) adenine (PMEA), in drug-resistant and also in transfected cell lines. MRP5, a close structural homologue of MRP4 also transported PMEA. MRP4 and MRP5 confer resistance to cytotoxic thiopurine nucleotides, and we demonstrate MRP4 expression varies among acute lymphoblastic leukemias, suggesting this as a factor in response to chemotherapy with these agents. The ability of MRP4 and MRP5 to transport 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) suggests they may play a biological role in cellular signaling by these nucleotides. Finally, we propose that MRP4 may also play a role in hepatic bile acid homeostasis because loss of the main bile acid efflux transporter, sister of P-glycoprotein (SPGP) aka bile-salt export pump (BSEP), leads to a strong compensatory upregulation in MRP4 expression. Cumulatively, these studies reveal that the ATP-binding cassette (ABC) transporters MRP4 and MRP5 have a unique role in biology and in chemotherapeutic response.

Light regulation of cyclic-AMP levels in the red macroalga Porphyra leucosticta

Total cyclic-3'-5'-adenosine monophosphate (cAMP) levels were measured in the gametophyte of the red macroalga Porphyra leucosticta under different light conditions in order to study its regulation by phytochrome or photosynthesis. cAMP levels were relatively low when samples were incubated in darkness, or exposed to red or far-red light. Irradiation with red+far-red light induced a moderate increase in cAMP levels, while white light induced a pronounced increase in cAMP levels. When incubated under increasing white light irradiance, cAMP levels closely followed the increase in photosynthetic oxygen evolution rate, suggesting a direct relationship between photosynthesis and cAMP accumulation. cAMP levels were not dependent on cellular ATP concentration, as inhibitors of ATP synthesis did not significantly affect cAMP levels in light. We conclude that cAMP depends on photosynthetic activity regardless of ATP synthesis and concentration or phytochrome activity.

Inhibition of stimulated Jurkat cell adenosine 3',5'-cyclic monophosphate synthesis by the immunomodulatory compound HR325

HR325 (2-cyano-3-cyclopropyl-3-hydroxy-N-[3'-methyl-4'(trifluoromethyl)-phenyl]-propenamide) is an immunomodulatory compound through pyrimidine biosynthesis inhibition with antiproliferative properties which was derived from the isoxazol compound A77 1726 [2-cyano-3-cyclopropyl-3-hydroxy-enoic acid (4-trifluoromethylphenyl)-amide]. During studies of the effects on early signal transduction events of this type of compound, it was found that HR325 dose-dependently inhibited adenosine 3',5'-cyclic monophosphate (cAMP) synthesis by Jurkat cells stimulated with prostaglandin E(2), (PGE(2)), cholera toxin (CTX), or forskolin (FKN). The potency of inhibition by HR325 of FKN-stimulated cells (IC(50) 30.4 microM) was approximately 3-fold higher than that of the other agonists (11.6 and 11.7 microM) and was independent of time of preincubation for both PGE(2) and FKN. Interestingly, A77 1726, an analogue of HR325, displayed a markedly different profile of stimulus-dependent potencies. The inhibition of cAMP synthesis by HR325 when stimulated by both PGE(2) and FKN was unaffected by glucose supplementation, in contrast to HR325-inhibited ATP levels, which were restored under such conditions. Further studies revealed that HR325 reduced intracellular ATP levels by uncoupling oxidative phosphorylation, albeit with a 1000-fold lower potency than the antihelmintic drug niclosamide. In addition, glucose supplementation experiments showed that, in contrast to HR325, the niclosamide-mediated reduction of ATP levels was wholly responsible for its inhibition of PGE(2)- and FKN-stimulated cAMP synthesis.

Indomethacin-induced G1/S phase arrest of the plant cell cycle

In animal systems, indomethacin inhibits cAMP production via a prostaglandin-adenylyl cyclase pathway. To examine the possibility that a similar mechanism occurs in plants, the effect of indomethacin on the cell cycle of a tobacco bright yellow 2 (TBY-2) cell suspension was studied. Application of indomethacin during mitosis did not interfere with the M/G1 progression in synchronized BY-2 cells but it inhibited cAMP production at the beginning of the G1 phase and arrested the cell cycle progression at G1/S. These observations are discussed in relation to the putative involvement of cAMP biosynthesis in the cell cycle progression in TBY-2 cells.

Distinct roles of prostaglandin H synthases 1 and 2 in T-cell development

Prostaglandin G and H synthases, or cyclooxygenases (COXs), catalyze the formation of prostaglandins (PGs). Whereas COX-1 is diffusely expressed in lymphoid cells in embryonic day 15.5 thymus, COX-2 expression is sparse, apparently limited to stromal cells. By contrast, COX-2 is predominant in a subset of medullary stromal cells in three- to five-week-old mice. The isozymes also differ in their contributions to lymphocyte development. Thus, experiments with selective COX-1 inhibitors in thymic lobes from normal and recombinase-activating gene-1 knockout mice support a role for this isoform in the transition from CD4(-)CD8(-) double-negative (DN) to CD4(+)CD8(+) double-positive (DP). Concordant data were obtained in COX-1 knockouts. Pharmacological inhibition and genetic deletion of COX-2, by contrast, support its role during early thymocyte proliferation and differentiation and, later, during maturation of the CD4 helper T-cell lineage. PGE2, but not other PGs, can rescue the effects of inhibition of either isoform, although it acts through distinct EP receptor subtypes. COX-dependent PG generation may represent a mechanism of thymic stromal support for T-cell development.

Immunomodulatory effects of pharmacological elevation of cyclic AMP in T lymphocytes proceed via a protein kinase A independent mechanism

The role of the cAMP pathway as an immunomodulatory system has been an area of intensive research. Pharmacological elevation of the cAMP pathway inhibits T lymphocyte proliferation and production of Th1-type cytokines. The effects of cAMP are thought to be mediated via activation of the intracellular receptor, protein kinase A (PKA). We investigated the inhibitory effects of cAMP elevation on human lymphocyte proliferation and function by utilising a range of selective inhibitors of PKA. Elevation of cAMP activity by dbcAMP, Sp-cAMPS and forskolin induced significant decreases of Con A stimulated PBMC proliferation. Co-incubation with the selective PKA inhibitors HA1004, KT5720 and Rp-cAMPS showed these antiproliferative effects to persist, despite measurable PKA activity being inhibited to that of untreated cells or less. IL-2 production was also inhibited by dbcAMP in the presence of HA1004 and Rp-cAMPS. It has been demonstrated that the inhibitory effects of pharmacological elevations in cAMP on human T cell proliferation and IL-2 production do not require PKA activity. These observations indicate that control of lymphocyte proliferation and functional status by cAMP proceeds through PKA-independent events. Identification of the underlying mechanisms behind these effects would increase our understanding of the cAMP cascade and may provide a potentially novel target for immunomodulation.

Isolation and Analysis of a T Cell Clone Variant Exhibiting Constitutively Phosphorylated Ser133 cAMP Response Element-Binding Protein

In driving T cell proliferation, IL-2 stimulates a new program of gene expression that includes proliferating cell nuclear antigen (PCNA), a requisite processivity factor for DNA polymerase δ. PCNA transcription is regulated in part through tandem CRE sequences in the promoter and CRE binding proteins; IL-2 stimulates CREB phosphorylation in the resting cloned T lymphocyte, L2. After culturing L2 cells for greater than 91 days, we consistently isolate a stable variant that exhibits constitutive CREB phosphorylation. L2 and L2 variant cells were tested for IL-2 responsiveness and rapamycin sensitivity with respect to specific kinase activity, PCNA expression and proliferation. In L2 cells, IL-2 stimulated and rapamycin inhibited the following: cAMP-independent CREB kinase activity, PCNA expression and proliferation. In L2 variant cells, CREB kinase activity was constitutively high; IL-2 stimulated and rapamycin blocked PCNA expression and proliferation. These results indicate that IL-2 induces a rapamycin-sensitive, cAMP-independent CREB kinase activity in L2 cells. However, phosphorylation of CREB alone is not sufficient to drive PCNA expression and L2 cell proliferation in the absence of IL-2.

Expression and regulation of mRNA for distinct isoforms of cAMP-specific PDE-4 in mitogen-stimulated and leukemic human lymphocytes

We reported previously that the gene for PDE-1B1 is induced in isolated human peripheral blood lymphocytes (HPBL) following mitogenic stimulation (Jiang, X., Li, J., Paskind, M., and Epstein, P.M. [1996] Proc. Natl. Acad. Sci. USA 93, 11,236-11,241). Using reverse transcription-polymerase chain reaction (RT-PCR), we investigated possible changes in the expression of the four genes for cAMP-specific phosphodiesterase (PDE-4A-D) in HPBL under the same conditions. Isolated, quiescent HPBL express mRNA for PDE-4B as the principal transcript. Following mitogenic stimulation with phytohemagglutinin (PHA), mRNA for PDE-4A and PDE-4D are clearly induced. HPBL appear not to express PDE-4C under resting or stimulated conditions. The PHA induced increase in PDE-1B1, PDE-4A, and PDE-4D mRNA is mimicked by incubation of HPBL with dibutyryl cAMP (dBcAMP) and 1-methyl-3-isobutylxanthine (IBMX). The B-lymphoblastoid cell line, RPMI 8392, and the T-leukemic cell line, Molt 4, express PDE-4A mRNA as the most abundant transcript, but incubation with dBcAMP and IBMX induces an increase in the expression of mRNA for PDE-4B in both of these cell lines, and in PDE-4D3 in the RPMI 8392 cell line. These studies demonstrate that expression of mRNA for PDE-1B1 and some of the subtypes of PDE-4 are induced in HPBL following mitogenic stimulation, possibly secondarily to elevation of cAMP induced by the mitogen. As already indicated for PDE-1B1, some of these subtypes of PDE-4 might also provide additional therapeutic targets for treatment of immunoproliferative disorders and immune dysfunction.

Upregulation of cAMP-specific PDE-4 activity following ligation of the TCR complex on thymocytes is blocked by selective inhibitors of protein kinase C and tyrosyl kinases

We have previously shown that the major cAMP phosphodiesterase (PDE) isoforms present in murine thymocytes are the cGMP-stimulated PDE activity (PDE-2) and the cAMP-specific PDE activity (PDE-4), and that these isoforms are differentially regulated following ligation of the TCR (Michie, A.M., Lobban, M. D., Mueller, T., Harnett, M. M., and Houslay, M.D. [1996] Cell. Signalling 8, 97-110). We show here that the anti-CD3-stimulated elevation in PDE-4 activity in murine thymocytes is dependent on protein tyrosine kinase and protein kinase C (PKC)-mediated signals as the TCR-coupled increase in PDE-4 activity can be abrogated by both the tyrosine kinase inhibitor, genistein, and the PKC selective inhibitors chelerythrine and staurosporine. Moreover, the PKC-activating phorbol ester, phorbol-12-myristate, 13-acetate (PMA) caused an increase in PDE-4 activity, similar to that observed in cells challenged with anti-CD3 monoclonal antibodies and which was not additive with cochallenge using anti-CD3 antibodies. Both the PMA- and the anti-CD3 antibody-mediated increases in PDE-4 activity were blocked by treatment with either cycloheximide or actinomycin D. Despite the upregulation of PDE-4 activity consequent to TCR ligation, intracellular cAMP levels increased on challenge of thymocytes with anti-CD3 antibody, indicating that adenylate cyclase activity was also increased by TCR ligation. It is suggested that the anti-CD3-mediated increase in PDE-4 activity was owing to a rapid PKC-dependent induction of PDE-4 activity following crosslinking of the TCR complex. This identifies "crosstalk" occurring between the PKA and PKC signaling pathways initiated by ligation of the antigen receptor in murine thymocytes. That both adenylate cyclase and PDE-4 activities were increased may indicate the presence of compartmentalized cAMP responses present in these cells.

Co-stimulation of human peripheral blood mononuclear cells with IL-2 and anti-CD3 monoclonal antibodies induces phosphorylation of CREB

Phosphorylation of the cAMP-response element binding protein CREB within 1 h of CD2 but not CD3 cross-linking of human PBMC was recently demonstrated. The absence of P-CREB following CD3 cross-linking was unexpected, as other laboratories reported increased phosphorylation of CREB following CD3 cross-linking of the Jurkat lymphocyte cell line. Due to Jurkat T-cells being IL-2-independent, it was postulated that IL-2 might provide a necessary co-stimulus for phosphorylation of CREB in primary lymphocytes. Therefore, P-CREB was evaluated following co-stimulation of human PBMC through the IL-2 and CD2 or CD3 receptors. IL-2 did not further augment phosphorylation of CREB following CD2 cross-linking. However, while neither IL-2 nor CD3 cross-linking alone induced P-CREB, a 4.5-fold increase in phosphorylation of CREB within 1 h of IL-2/CD3 co-stimulation was observed. Phosphorylation was not associated with the induction of cAMP, and inhibition of PKA signaling had no effect on P-CREB. Consistent with signal transduction through p56lck or p59fyn, inhibition of PTK signaling reduced phosphorylation 50%. Interestingly, inhibiting PKC signaling with calphostin C further increased P-CREB levels 3-fold over that observed in IL-2/CD3 co-stimulated cells not pretreated with a PKC inhibitor. In contrast to previous studies performed in the absence of exogenous IL-2, no increase in binding of CREB to a 32P-labeled oligonucleotide probe was observed by electrophoretic mobility shift assay. These data suggest that the IL-2 and CD3 signaling pathways provide a necessary and co-operative stimulus promoting phosphorylation of CREB following receptor cross-linking.

Effect of indomethacin on cell cycle dependent cyclic AMP fluxes in tobacco BY-2 cells

The evolution of adenosine 3',5'-cyclic monophosphate (cAMP) levels was investigated in synchronised tobacco BY-2 cells by virtue of a method based on immunoaffinity purification and analysis on electrospray tandem mass spectrometry. A transient peak in cAMP content was observed during the S and G1 phases of the cell cycle. Application of the prostaglandin inhibiting drug indomethacin at early S phase resulted in the loss of the cAMP peak in S phase and inhibited mitotic division. This inhibition of cAMP accumulation suggests the presence of a prostaglandin-dependent adenylyl cyclase activity, analogous to animal cyclases. A potential role for cAMP during the plant cell cycle is postulated.

Therapeutic potential of phosphodiesterase type 4 inhibition in chronic autoimmune demyelinating disease

It was recently demonstrated that selective phosphodiesterase type 4 (PDE4) inhibition suppresses the clinical manifestations of acute experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), and inhibits the production of tumor necrosis factor-alpha (TNF-alpha), a pathogenetically central cytokine. Since the most common presentation of MS in humans is a relapsing-remitting course, we investigated the therapeutic potential of PDE4 inhibition in the relapsing-remitting EAE model of the SJL mouse. Administration of rolipram, the prototypic PDE4 inhibitor, reduced the clinical signs of EAE during both the initial episode of disease and subsequent relapses. In parallel, there was marked reduction of demyelination and also less inflammation throughout the central nervous system (CNS) of rolipram-treated animals. Gene expression of proinflammatory cytokines in the CNS was reduced in most of the rolipram-treated animals. Additional experiments demonstrated that PDE4 inhibition acted principally by inhibiting the secretion of Th1 cytokines, however, the encephalitogenic potential of myelin basic protein-specific T cells was not impaired. Our findings suggest that PDE4 inhibitors are a promising cytokine-directed therapy in chronic demyelinating disease.

Differential regulation of IFN-gamma, IL-10 and inducible nitric oxide synthase in human T cells by cyclic AMP-dependent signal transduction pathway

Expression of cytokines by T lymphocytes is a highly balanced process, involving stimulatory and inhibitory intracellular signalling pathways. In the present work, we attempted to clarify the role of cAMP on interferon-gamma (IFN-gamma), interleukin (IL)-10, IL-4 and IL-13 expression as well as on the inducible nitric oxide synthase (iNOS) expression. Treatment of phytohaemagglutinin (PHA)/phorbol 12-myristate 13-acetate (PMA)-activated Jurkat cells with either dibutyryl-cyclic adenosine monophosphate (cAMP) or pentoxifylline induced a strong inhibition of IFN-gamma mRNA expression as measured by reverse transcription (RT)-polymerase chain reaction (PCR), without affecting IL-10 expression. Both cholera toxin and prostaglandin E2 (PGE2) induced a strong inhibition of IFN-gamma mRNA expression, whereas IL-10 mRNA expression was significantly enhanced. This differential regulation of IFN-gamma and IL-10 expression was related to intracellular cAMP concentration. IL-13 and IL-4 mRNA expressions were not inhibited. We developed a new method based on immunofluorescence for intracellular cytokine detection followed by optical and computerized image processing, and our results showed that IFN-gamma protein was strongly inhibited when cells were treated with PGE2 or dibutyryl (db)-cAMP, whereas IL-10 protein was enhanced. This suggests that cAMP exerts its action at both the transcriptional and protein levels. iNOS mRNA expression was markedly elevated in the presence of PGE2. The generation of nitric oxide using sodium nitroprusside (SNP) induced a dramatic decrease of IFN-gamma, while IL-10 was enhanced; and conversely the inhibition of iNOS activity using 1-NG-monomethyl arginine (1-NMMA) induced a clear inhibition of IL-10 and IL-4, while IFN-gamma was enhanced. These results provide evidence that the protein kinase A (PKA) activation pathway plays a prominent role in the balance between the type 1 and type 2 cytokine profile in PHA/PMA-activated Jurkat cells. Data also suggest that iNOS expression is under the control of PKA activation, and that NO seems to be able to assume the polarization of activated T cells to the type 2 profile.

In vitro and in vivo anti-leukemic efficacy of cyclic AMP modulating agents against human leukemic B-cell precursors

We show that the adenylate cyclase activating diterpine, forskolin, the phosphodiesterase inhibitor, aminophylline, and the permeant cAMP analog dibutyryl cAMP inhibit the in vitro clonogenic growth of leukemic B-cell precursors. We also used a SCID mouse xenograft model of refractory human B-cell precursor leukemia to evaluate the anti-leukemic effect of aminophylline in vivo. Treatment with aminophylline (6 mg/kg bolus followed by 0.1-0.5 mg/kg/hour x 7 days) significantly prolonged the event-free survival of SCID mice (median survival of control mice, 39 days, N = 79; median survival of aminophylline-treated mice, 60 days, N = 10; P < 0.0001 by log-rank test) and it was more effective than treatment with vincristine (median survival = 51 days, N = 5) or L asparaginase (median survival = 44 days, N = 5). However, aminophylline was not as effective as methylprednisolone (median survival: 103 days, N = 5). These results indicate that cAMP modulating agents may be useful in treatment of refractory human B-cell precursor leukemia.

Human T-cell lymphotropic virus type 1 Tax mediates enhanced transcription in CD4+ T lymphocytes

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia/lymphoma and is associated with a variety of immunoregulatory disorders. HTLV-1 has been shown to bind to and infect a variety of hematopoietic and nonhematopoietic cells. However, both in vivo and in vitro, the provirus is mostly detected in and preferentially transforms CD4+ T cells. The molecular mechanism that determines the CD4+ T-cell tropism of HTLV-1 has not been determined. Using cocultures of purified CD4+ and CD8+ T cells with an HTLV-1 producing cell line, we measured viral transcription by using Northern (RNA) blot analysis, protein production by using a p24 antigen capture assay and flow cytometric analysis for viral envelope, and proviral integration by using DNA slot blot analysis. We further measured HTLV-1 long terminal repeat-directed transcription in purified CD4+ and CD8+ T cells by using transient transfection assays and in vitro transcription. We demonstrate a higher rate of viral transcription in primary CD4+ T cells than in CD8+ T cells. HTLV-1 protein production was 5- to 25-fold greater in CD4+ cocultures and mRNA levels were 5-fold greater in these cultures than in the CD8+ cocultures. Transient transfection and in vitro transcription indicated a modest increase in basal transcription in CD4+ T cells, whereas there was a 20-fold increase in reporter gene activity in CD4+ T cells cotransfected with tax. These data suggest that unique or activated transcription factors, particularly Tax-responsive factors in CD4+ T cells, recognize regulatory sequences within the HTLV-1 long terminal repeat, and this mediates the observed enhanced viral transcription and ultimately the cell tropism and leukemogenic potential of the virus.

The role of adrenoceptor-mediated signals in the modulation of lymphocyte function

Adrenoceptors are heterotrimeric glycoproteins that bind specific endogenous ligands, such as the sympathetic neurotransmitter norepinephrine and the neurohormone epinephrine. Ligand binding to an adrenoceptor expressed on the cell surface initiates a cascade of biochemical and molecular responses inside the cell that lead to a change in cellular activity. Initially, the stimulation of an adrenoceptor directly activates G proteins that stimulate enzymes to induce the production of second messengers. The cascade continues as the second messengers activate serine/threonine protein kinases, resulting in either an inhibition or enhancement of cellular activity. The resulting changes in cellular activity are mediated by changes in gene expression that are induced by the phosphorylation of specific transcription factors. Adrenoceptor subtypes are expressed by both T and B lymphocytes. The aim of this review is to summarize and discuss the results from the many studies that have examined the role of adrenoceptor-mediated intracellular signals in the modulation of lymphocyte function. Another aim of this review is to discuss how these studies have advanced our understanding of the mechanisms by which the sympathetic nervous system transmits information to both T and B lymphocytes to maintain immune homeostasis.

Promoter activity of the proliferating-cell nuclear antigen gene is associated with inducible CRE-binding proteins in interleukin 2-stimulated T lymphocytes

The proliferating-cell nuclear antigen (PCNA) gene encodes an auxiliary factor of DNA polymerase delta and functions in DNA replication during S phase. It is expressed at much higher levels in proliferating cells than in quiescent cells. We have studied the regulatory role of the 5'-flanking sequence of the murine PCNA gene in interleukin 2 (IL-2)-responsive cloned T cells (L2). Analysis of a set of deletion constructs in transient transfection assays measuring heterologous reporter gene (luciferase) activity demonstrated that the 182-bp 5'-flanking region provides full promoter activity in IL-2-stimulated L2 cells. While many elements contribute to PCNA promoter strength in IL-2-stimulated cells, the largest decrease in activity occurred with deletion of the tandem CRE (cyclic AMP response element) binding sites located at nucleotides -37 to -52. With a gel mobility shift assay, several IL-2-inducible DNA-protein complexes were detected, including CREB (CRE-binding) and ATF1 (activating transcription factor) proteins that are specific for the PCNA-CRE sequence. Methylation interference analysis confirmed specific binding of these proteins to the CRE sites. Mutation at the PCNA-CRE motif abolishes IL-2-inducible binding and reduces substantially PCNA promoter activity. These results indicate that IL-2-stimulated PCNA transcription may be partially mediated by these CRE-binding proteins.

Promoter activity of the proliferating-cell nuclear antigen gene is associated with inducible CRE-binding proteins in interleukin 2-stimulated T lymphocytes

The proliferating-cell nuclear antigen (PCNA) gene encodes an auxiliary factor of DNA polymerase delta and functions in DNA replication during S phase. It is expressed at much higher levels in proliferating cells than in quiescent cells. We have studied the regulatory role of the 5'-flanking sequence of the murine PCNA gene in interleukin 2 (IL-2)-responsive cloned T cells (L2). Analysis of a set of deletion constructs in transient transfection assays measuring heterologous reporter gene (luciferase) activity demonstrated that the 182-bp 5'-flanking region provides full promoter activity in IL-2-stimulated L2 cells. While many elements contribute to PCNA promoter strength in IL-2-stimulated cells, the largest decrease in activity occurred with deletion of the tandem CRE (cyclic AMP response element) binding sites located at nucleotides -37 to -52. With a gel mobility shift assay, several IL-2-inducible DNA-protein complexes were detected, including CREB (CRE-binding) and ATF1 (activating transcription factor) proteins that are specific for the PCNA-CRE sequence. Methylation interference analysis confirmed specific binding of these proteins to the CRE sites. Mutation at the PCNA-CRE motif abolishes IL-2-inducible binding and reduces substantially PCNA promoter activity. These results indicate that IL-2-stimulated PCNA transcription may be partially mediated by these CRE-binding proteins.

The role of brain-immune interactions in immunotoxicology

Certain xenobiotics (or the metabolites) can damage immunocompetence by directly interacting with one or more of the cells of the immune system and adversely affecting its function. It has also been proposed that xenobiotics may indirectly affect immune function by affecting other organ systems that will in turn affect immunocompetence. This review surveys evidence that supports the existence of a functional link between the brain and the immune system. In addition, we review data that support the concept that a xenobiotic-induced dysfunction in the neuroendocrine system may be associated with an immune dysfunction as well. Such chemicals do not necessarily interact directly with immunocompetent cells but would instead act to disrupt regulatory brain-immune interactions. This class of indirectly acting immunotoxic xenobiotics would not be detected in the typical in vitro screening assays.

Effect of PGE2 on thymocyte proliferation induced by Con A or IL-4 + PMA

Prostaglandin E2 (PGE2) is known to inhibit peripheral T-lymphocyte and thymocyte proliferation activated by antigens, mitogens or anti-CD3 antibodies. In this study, we have investigated, the effect of PGE2 on thymocyte proliferation induced by the combination of IL-4 plus PMA. PGE2 inhibits the proliferation of thymocytes activated by ConA, whatever the culture period; in contrast PGE2 shifts the kinetics of thymocyte proliferation after stimulation by IL-4 plus PMA, but does not sustain the proliferation beyond day 3. This effect depends upon cell density, IL-4 concentration and on the time that PGE2 is added to the culture. By use of the cAMP inducer, forskolin, or a cAMP analog, db-cAMP, we observed the same results, PGE2 increases the proliferation of CD8+ corticoresistant thymocytes (CRT) activated by IL-4 plus PMA, but inhibits that of CD4+ CRT. These results suggest that PGE2 can regulate thymocyte proliferation differently according to the activation pathway and the thymic subpopulations.

Effects of intracellular cyclic AMP and cyclic GMP levels on DNA synthesis of young-adult rat hepatocytes in primary culture

Possible roles of dibutyryladenosine 3',5'-cyclic monophosphate (cAMP) and dibutyryl-guanosine 3',5'-cyclic monophosphate (cGMP) in regulation of hepatocyte DNA synthesis were examined using primary cultures of young-adult rat hepatocytes maintained in arginine-free medium. Throughout the experimental period, nonparenchymal cells were hardly observed in the selective medium. When epidermal growth factor (EGF) was added to the cultures, a transient increase in the intracellular cAMP level preceded the elevation of hepatocyte DNA synthesis. EGF-stimulated hepatocyte DNA synthesis was remarkably enhanced by the elevation of the intracellular cAMP level induced by treatment with cAMP alone or a combination of cAMP and theophylline, an inhibitor of cyclic nucleotide phosphodiesterase. Furthermore, the early elevation of intracellular cAMP alone, which was induced by treatment with the combination of cAMP and theophylline, caused a remarkable increase in hepatocyte DNA synthesis. On the other hand, addition of EGF to the cultures caused a rapid decrease in the intracellular cGMP level followed by an increase in hepatocyte DNA synthesis. EGF-stimulated hepatocyte DNA synthesis was severely suppressed or completely inhibited by the elevation of the intracellular cGMP level induced by treatment with cGMP alone or a combination of cGMP and dipyridamole, a specific inhibitor of cGMP phosphodiesterase. These findings indicate that cAMP and cGMP act oppositely on the regulation of DNA synthesis of young-adult rat hepatocytes in primary culture: cAMP plays a positive role, whereas cGMP plays a negative role. Also it is strongly suggested that an early elevation of the intracellular cAMP level is essential for the onset of DNA synthesis in hepatocyte primary cultures.

Protein phosphorylation regulated by cyclic nucleotide-dependent protein kinases in cell extracts and in intact human lymphocytes

A specific 46,000/50,000 molecular weight protein substrate for both cAMP-dependent protein kinase (cAK) and cGMP-dependent protein kinase (cGK) extensively characterized and purified from human platelets was found to be present also in human T-lymphocytes, B-lymphocytes and other cells and tumour cell lines. This protein termed vasodilator-stimulated phosphoprotein (VASP) was present in cytosol and membranes of lymphocytes. Addition of exogenous purified cAK or cGK to lymphocyte cytosol or membranes converted 80-90% of VASP to its phosphoform. Endogenous VASP phosphorylation in both cytosol and membranes was stimulated by the addition of cAMP but not by cGMP. With intact lymphocytes, prostaglandin E1 (PGE1) and prostaglandin E2 (PGE2) induced an increase of cAMP and converted 70% of VASP to its phosphoform. In contrast, an increase of cGMP was not associated with VASP phosphorylation although cGK was detected in lymphocytes. These data support the hypothesis that VASP phosphorylation may be an important component of cAMP-mediated regulation of lymphocyte function.

Characterization of the 3',5'-cyclic adenosine monophosphate-mediated regulation of IL2 production by T cells and Jurkat cells

The effect of cyclic AMP-elevating agents on mitogen-stimulated IL2 production was examined. Prostaglandin E2 (PGE2) inhibited IL2 production by human peripheral blood T cells stimulated with PHA. In contrast, PGE2 did not inhibit PHA-stimulated IL2 production by the human leukemic T cell line. Jurkat, and often slightly enhanced IL2 production by those cells. Other cyclic adenosine monophosphate (cAMP) elevating agents (forskolin, isoproterenol, and the cAMP analogue, dibutyryl cAMP) also inhibited lectin-stimulated IL2 production by T cells, but could not inhibit IL2 production by Jurkat cells. Of the cAMP-elevating agents examined, only cholera toxin (CT) inhibited IL2 production by both Jurkat cells and peripheral blood T cells. Although phorbol myristate acetate (PMA) greatly enhanced PHA-stimulated IL2 production by Jurkat cells. CT remained markedly inhibitory. The combination of PMA and the calcium ionophore, ionomycin, also induced IL2 production by Jurkat cells, and this was similarly suppressed by CT, suggesting that a step after initial second messenger generation was inhibited. A prolonged increase in intracellular cAMP levels was induced by CT in both T cells and Jurkat cells, but the maximal level and the length of elevation achieved in T cells were much less than those observed in Jurkat cells. In contrast, PGE2 caused only a modest and transient increase in intracellular cAMP levels in Jurkat cells compared to that noted with T cells. PGE2 induced a more marked and sustained increase in cAMP levels in Jurkat cells treated with isobutylmethylxanthine (IBMX), a phosphodiesterase inhibitor. Moreover, in the presence of IBMX, PGE2 caused a marked inhibition of IL2 production by PHA-stimulated Jurkat cells. Differences in the capacity of PGE2 to induce cAMP could not be explained by disparities in the level of cAMP phosphodiesterase activity as this was comparable in Jurkat cells and in T cells. Thus, these observations indicate that IL2 production by both peripheral T cells and Jurkat cells can be modulated by cAMP-elevating agents. The data suggest that the diminished capacity of PGE2 to inhibit IL2 production by Jurkat cells reflects both a diminished capacity of PGE2 to induce increases in cAMP levels in these cells and an increase in the threshold of cAMP required to inhibit Jurkat cells.

The effect of lithium therapy on parameters thought to be involved in the development of autoimmune thyroid disease

This study has considered the effects of primary affective disorders and lithium therapy on a number of factors thought to be important in the development of autoimmune thyroid disease. These factors were examined in (a) controls with no history of any such disorders; (b) patients with primary affective disorders treated with drugs other than lithium and (c) patients with primary affective disorders treated with lithium alone. Eight of 40 patients who were receiving lithium therapy were found to be positive for thyroid microsomal and/or thyroglobulin antibodies, compared to only 3/40 patients who were receiving some other form of treatment for their depression. Peripheral blood mononuclear cells from patients receiving lithium were found to have significantly reduced numbers of suppressor/cytotoxic T cells (P less than 0.05). In addition, suppressor T cells from these patients showed a significantly reduced response to stimulation with concanavalin A (P less than 0.01). These effects were greatest in patients found to be antibody positive. Increased B cell activity, as measured by increased IgG and IgM release following mitogen stimulation, was seen in patients receiving lithium and in those patients receiving other forms of treatment for their depression. This would suggest that the increase is a feature of primary affective disorders and is not due specifically to lithium treatment. It would appear from this study that lithium therapy induces antibody formation in susceptible individuals and this may ultimately lead to the development of thyroid disease.

Transcriptional activation of the human cytotoxic serine protease gene CSP-B in T lymphocytes

The cytotoxic serine protease B (CSP-B) gene is activated during cytotoxic T-lymphocyte maturation. In this report, we demonstrate that the PEER T-cell line (bearing gamma/delta T-cell receptors) accumulates CSP-B mRNA following exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA) and N6-2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (bt2cAMP) because of transcriptional activation of the CSP-B gene. TPA and bt2cAMP act synergistically to induce CSP-B expression, since neither agent alone causes activation of CSP-B transcription or mRNA accumulation. Chromatin upstream from the CSP-B gene is resistant to DNase I digestion in untreated PEER cells, but becomes sensitive following TPA-bt2cAMP treatment. Upon activation of PEER cells, a DNase I-hypersensitive site forms upstream from the CSP-B gene within a region that is highly conserved in the mouse. Transient transfection of CSP-B promoter constructs identified two regulatory regions in the CSP-B 5'-flanking sequence, located at positions -609 to -202 and positions -202 to -80. The region from -615 to -63 is sufficient to activate a heterologous promoter in activated PEER cells, but activation is orientation specific, suggesting that this region behaves as an upstream promoter element rather than a classical enhancer. Consensus AP-1, AP-2, and cAMP response elements are found upstream from the CSP-B gene (as are several T-cell-specific consensus elements), but the roles of these elements in CSP-B gene activation have yet to be determined.

Transcriptional activation of the human cytotoxic serine protease gene CSP-B in T lymphocytes

The cytotoxic serine protease B (CSP-B) gene is activated during cytotoxic T-lymphocyte maturation. In this report, we demonstrate that the PEER T-cell line (bearing gamma/delta T-cell receptors) accumulates CSP-B mRNA following exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA) and N6-2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (bt2cAMP) because of transcriptional activation of the CSP-B gene. TPA and bt2cAMP act synergistically to induce CSP-B expression, since neither agent alone causes activation of CSP-B transcription or mRNA accumulation. Chromatin upstream from the CSP-B gene is resistant to DNase I digestion in untreated PEER cells, but becomes sensitive following TPA-bt2cAMP treatment. Upon activation of PEER cells, a DNase I-hypersensitive site forms upstream from the CSP-B gene within a region that is highly conserved in the mouse. Transient transfection of CSP-B promoter constructs identified two regulatory regions in the CSP-B 5'-flanking sequence, located at positions -609 to -202 and positions -202 to -80. The region from -615 to -63 is sufficient to activate a heterologous promoter in activated PEER cells, but activation is orientation specific, suggesting that this region behaves as an upstream promoter element rather than a classical enhancer. Consensus AP-1, AP-2, and cAMP response elements are found upstream from the CSP-B gene (as are several T-cell-specific consensus elements), but the roles of these elements in CSP-B gene activation have yet to be determined.

Beta-endorphin stimulates rat T lymphocyte proliferation

beta-Endorphin 1-31 and several structurally related peptides were tested for their ability to alter mitogen-induced T cell proliferation. Rat beta-endorphin 1-31 and human beta-endorphin 1-27 increased phytohemagglutinin (PHA)-induced [3H]thymidine incorporation into rat lymph node cells. However, when PHA-induced proliferation was suppressed by the inclusion of prostaglandin E1 (PGE1), human beta-endorphin 1-31 and a number of structurally similar peptides, including some peptides that did not alter mitogen-induced proliferation, significantly reduced the PGE1 inhibition of PHA-stimulated T cell proliferation. Although the N-terminus of beta-endorphin was necessary for potency, inclusion of the opioid antagonist naloxone together with beta-endorphin 1-31 did not alter the blockage of PGE1 inhibition of PHA-induced proliferation caused by beta-endorphin. The inhibition of mitogen-stimulated proliferation by either cholera toxin or forskolin, two additional compounds that like PGE1 also elevate cyclic AMP levels, was not blocked by beta-endorphin. Verapamil suppression of proliferation was not modified by beta-endorphin, indicating that the beta-endorphin stimulatory effect was probably not due to Ca2+ influx through verapamil-sensitive Ca2+ channels. These data suggest that beta-endorphin, acting through a nonopioid beta-endorphin receptor, may modulate immunocompetence by stimulating T cell proliferation and by counteracting the inhibitory effects of PGE1.

Dual effects of glucagon and cyclic AMP on DNA synthesis in cultured rat hepatocytes: stimulatory regulation in early G1 and inhibition shortly before the S phase entry

Although several lines of evidence implicate cyclic AMP in the humoral control of liver growth, its precise role is still not clear. To explore further the role of cyclic AMP in hepatocyte proliferation, we have examined the effects of glucagon and other cyclic AMP-elevating agents on the DNA synthesis in primary cultures of adult rat hepatocytes, with particular focus on the temporal aspects. The cells were cultured in a serum-free, defined medium and treated with epidermal growth factor (EGF), insulin, and dexamethasone. Exposure of the hepatocytes to low concentrations (10 pM-1 nM) of glucagon in the early stages of culturing (usually within 6 h from plating) enhanced the initial rate of S phase entry without affecting the lag time from the plating to the onset of DNA synthesis, whereas higher concentrations inhibited it. In contrast, glucagon addition at later stages (24-45 h after plating) produced only the inhibition. Thus, if glucagon was added at a time when there was a continuous EGF/insulin-induced recruitment of cells to S phase, the rate of G1-S transition was markedly decreased within 1-3 h. This inhibitory effect occurred at low glucagon concentrations (ID50 less than 1 nM) and was mimicked by cholera toxin, forskolin, isobutyl methylxanthine, and 8-bromo cyclic AMP. The results indicate that cyclic AMP has dual effects on hepatocyte proliferation with a stimulatory modulation early in the prereplicative period (G0 or early G1), and a marked inhibition exerted immediately before the transition from G1 to S phase.

A lymphocyte blastogenesis inhibitory factor (LBIF) reversibly arrests a human melanoma cell line, A375, at G1 and G2 phases of cell cycle

A lymphocyte blastogenesis inhibitory factor, LBIF, has been found in the culture supernatant of a human macrophage-like cell line, U937. The factor has been purified by fast protein liquid chromatography. Partial amino acid sequencing analysis showed that LBIF was a novel immunoregulatory factor. Recent study has demonstrated that LBIF possesses a remarkable tumor growth inhibitory activity. In this study, the cell growth inhibitory activity of LBIF was characterized on the proliferation of a human melanoma cell line A375 in vitro. LBIF strongly inhibits the proliferation of A375 cells. The inhibitory activity was cytostatic and reversible by Day 5 although the lethal effect became apparent at Day 7. Cell cycle analysis by flow cytometry showed that LBIF arrested A375 cells at both G1 and G2/M phases. Mitotic index analysis indicated that A375 cells were arrested in G1 and G2 phases. LBIF function was not attributed to the elevation of intracytoplasmic cyclic-AMP levels. Thus, these results suggest that LBIF plays an important role in controlling cell cycle and there is a similarity between the mechanisms of G1 and G2 arrests in eukaryotic cell proliferation. LBIF-induced reversible cell-cycle arrest of A375 cells can be a useful system to analyze the signal transduction for cell proliferation and cell-cycle arrest.

Reversible defect in cAMP metabolism in lymphocytes in malaria infection

Peripheral blood lymphocytes were observed to have a defect in adenosine 3',5'-monophosphate (cAMP) metabolism during acute malaria infection which reversed once parasites were eliminated from the host circulation. The defect was characterized by decreased intracellular cAMP levels in lymphocytes and by hyporesponsiveness to adenosine or forskolin stimulation of cAMP production. These biochemical changes appeared to correlate functionally with a reduction in the proliferative response of lymphocytes to concanavalin A. A defect in the second messenger role of cAMP in immune effector cells may underlie immunosuppression in malaria infection.

Changes in T and B lymphocyte proliferative responses in adjuvant-arthritic rats: antagonism by indomethacin

The effects of a potent non-steroidal antiinflammatory drug, indomethacin, on the inflammatory response and lymphocyte proliferation were investigated in adjuvant-arthritic rats. As shown by others, adjuvant produced a time-dependent swelling of the contralateral paw which was maximal within 14 days after administration. Upon the intraperitoneal injection of either 1 or 2 mg/kg of indomethacin, twice daily for 3 days, the swelling of the contralateral paw was completely reduced. Selective changes in proliferative responses of splenic T and B lymphocytes to mitogens were found to accompany these effects. Adjuvant arthritic rats were found to have increased T cell proliferation when stimulated with phytohemagglutinin, whereas proliferation of B lymphocytes in the presence of lipopolysaccharide was completely suppressed. Indomethacin treatment at either the 1 or 2 mg/kg dose was found to depress the proliferative responses of T cells by 60 and 95%, respectively. In contrast, the hyporesponsiveness of B lymphocytes in arthritic animals was partially reversed with 2 mg/kg indomethacin. These results suggest that indomethacin may be exerting an antiinflammatory effect through a selective alteration of T and B lymphocyte activities in lymphoid tissue.

Elevation of cyclic adenosine monophosphate levels independently down regulates IL-1, IL-2, and IL-2 receptor (CD25) syntheses

In view of the central involvement of interleukin-1 (IL-1) in T-cell functions and the negative effects exerted by cyclic adenosine monophosphate (cAMP) on T-cell responses, we wondered whether these inhibitions rely on defects in IL-1 generation. We investigated the effect of a known cAMP elevating agent, cholera toxin (CT), on the generation of IL-1 from peripheral blood adherent cells as well as the role of IL-1 whenever IL-2 synthesis and IL-2 receptor (CD25 antigen) expression are inhibited. While augmenting intracellular cAMP concentration, CT inhibits from 20 to 40% the generation of IL-1 activity from E. coli lipopolysaccharide (LPS)-stimulated adherent cells. Theophylline (TH), a cAMP degradation blocking agent, induces the same decrease in IL-1 activity. The B chain of CT, devoid of cAMP activating potency, is not inhibitory. In systems where CT and TH dramatically inhibit the generation of IL-2 activity (80%), addition of exogenous IL-1 does not restore the ability of T-cells to produce or release IL-2. Moreover, CT- and dibutyryl (db)cAMP-induced inhibition of CD25 antigen expression is not overcome by exogenous IL-1, IL-2, nor by both interleukins. It is concluded that inhibition of IL-1 and IL-2 production are independent and that inhibition of CD25 antigen expression is independent of IL-1 and IL-2 modulation. Cholera toxin and cAMP influences on interleukin synthesis are discussed.

Co-ordinated changes in the cyclic AMP signalling system and the phosphorylation of two nuclear proteins of Mr 130,000 and 110,000 during proliferative stimulation of the rat parotid gland by isoprenaline. Possible identity of the two proteins with pp135 and nucleolin

Parotid glands were stimulated to growth by repeated injection of the beta-agonist isoprenaline into rats. Incubation of intact parotid-gland lobules with [32P]Pi and subsequent analysis of nuclear proteins revealed in the stimulated glands an increased 32P incorporation into two acid-soluble non-histone proteins with apparent Mr values of 110,000 and 130,000 (p110 and p130). After a single injection of isoprenaline, leading to a biphasic increase in DNA synthesis (maximum at 24 h), the same two proteins showed a transiently increased 32P incorporation at 17 h after injection. At this time point at the onset of DNA synthesis the total activity of soluble cyclic AMP-dependent protein kinase decreased. No change in p110/p130 phosphorylation was observed at 0.3 h after stimulation, a time of maximal stimulation of secretion. Administration of the beta-antagonist propranolol 8 h after the injection of isoprenaline suppressed the increase in DNA synthesis, the preceding changes in the concentration of cyclic AMP and in the activity of cyclic AMP-dependent protein kinase, as well as the increased phosphorylation of p110 and p130. Cross-reactivity of p110 and p130 with specific antisera against two nucleolar phosphoproteins of similar molecular mass (nucleolin and pp135), as well as their localization in a nucleolar cell fraction, indicated a possible identity of p110 and p130 with these two proteins. Our results suggest that nucleolin and pp135 are nuclear target proteins of cyclic AMP in the cyclic AMP-influenced regulation of the transition of cells from the G1 to the S phase.

Differential activation of the 21-base-pair enhancer element of human T-cell leukemia virus type I by its own trans-activator and cyclic AMP

A transcriptional trans-acting factor p40tax of human T-cell leukemia virus type I (HTLV-I) functions as an inducer for expression of HTLV-I provirus via activation of the enhancer in the long terminal repeat of HTLV-I. In addition to p40tax and a tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, we report here that forskolin, an activator of adenyl cyclase, also induces function of the HTLV-I enhancer. Experiments with mutants of the HTLV-I enhancer revealed that TPA-induced activation was not mediated by solely a 21-base-pair (bp) sequence that is repeated three times in the enhancer, whereas the 21-bp enhancer element can act as a sufficient cis-acting sequence for activation by both p40tax and forskolin. In addition, we found that nuclear factor(s) like the cyclic AMP-responsive element (CRE) binding factor could bind to the HTLV-I 21-bp enhancer element. However, a difference was found in sequences required for activation by p40tax and forskolin. A CRE related sequence present in the 21-bp enhancer element was enough for forskolin-induced activation. On the other hand, p40tax required a much longer sequence that is overlapping but not identical to the CRE related sequence, suggesting that the forskolin-induced cyclic AMP pathway may be partly involved in, but not sufficient for p40tax-mediating trans-activation of the HTLV-I enhancer.

Inhibitory effect of papaverine on HIV replication in vitro

The ability of papaverine to inhibit human immunodeficiency virus (HIV) replication in H9 cell line and in peripheral blood mononuclear cell (PBMC) culture was examined. HIV-infected H9 cells were exposed to different concentrations of papaverine for 20 days. Reverse transcriptase (RT) activity and the presence of p24 in the supernatant were determined to assess the level of viral replication in treated and control cultures. The most effective concentration of papaverine in the culture medium was 10 micrograms/ml, a dose that did not significantly affect cell proliferation. At this drug concentration the treatment resulted in no RT activity or p24 expression in the supernatant and no virus antigen detection at the cellular level as demonstrated by Western blot (WB) analysis. The activity of the drug occurred in a short period of time (60 hours) as shown by radioimmunoprecipitation (RIP) assay and affected the synthesis of the env precursor protein gp160. The drug was also effective in inhibiting HIV replication in PBMC cultures and influenced specific viral markers, namely, RT and p24. Evidence of the efficacy of papaverine treatment was enforced by the finding in the treated PBMC cultures, compared with the untreated ones, of a reduced percentage of cells forming syncitia and of the inhibition of the virus-induced decrease in the number of cells. When an equal number of virus-infected H9 cells exposed or unexposed to papaverine was analyzed for HIV-specific proteins, a marked decrease in the expression of the viral proteins was observed in the treated cultures. At the same time, one cellular protein of molecular weight 69,000 was not inhibited by papaverine. This may indicate that, at least for one protein, synthesis may not be affected by the drug. Our data suggest that papaverine merits attention as a possible nontoxic candidate for the treatment of HIV infection.