Regulation of the in vitro anamnestic antibody response by cyclic AMP. III. Cholera enterotoxin induces lymph node cells to release soluble factor(s) which enhance(s) antibody synthesis by antigen-treated lymph node cells.

Unprimed or KLH-primed rabbit lymph node cells were pulsed with cholera enterotoxin or KLH for 2 hr and washed. KLH-treated LNC were mixed with equal numbers of CT-treated LNC or boiled CT-treated LNC. Cocultivation of CT-treated LNC with KLH-treated cells resulted in at least a 100% increase in antibody synthesis compared to control cultures. Delaying cocultivation for 24 hr reduced enhancement to 25%. Thus it appears that an early event—before 24 hr—is involved in CT enhancement. Using ¹²⁵I-CT, it was shown that these effects were not due to CT carry-over. When KLH- and CT-pulsed LNC were cultured in chambers separated by polycarbonate membranes (0.2- to 0.4-μm pore size) antibody production was enhanced 50–80%. Supernates of CT-treated LNC also enhanced antibody production by KLH-treated LNC. These results suggest that CT triggers the release of soluble factor(s) which enhance(s) antibody synthesis by antigen-primed and antigen-challenged LNC.     

Regulatory substances produced by lymphocytes. V. Production of inhibitor of DNA synthesis (IDS) by proliferating T lymphocytes.

The conditions neccessary for production of inhibitor of DNA synthesis (IDS) by rat lymphocytes were investigated.In concanavalin A (Con A)-stimulated lymph node cell (LNC) cultures, IDS production was not detected in the culture supernatant during the first 24 hr, and it increased gradually after that to reach a maximum at 3 to 4 days.When the cells were pretreated with mitomycin C, IDS was not produced, suggesting that DNA synthesis of LNC or a LNC subpopulation is necessary for IDS production. In contrast, Con A-stimulated spleen cells priduced a high level of IDS within 24 hr, and its production fell off sharply thereafter. Con A-stimulated rat thymocytes also produced IDS reaching a maximum at 2 to 3 dyas. However, thymus cells from rats treated with hydrocortisone 48 hr previously did not produce IDS. This finding implies that cortisol-sensitive (cortical) thymocytes are capable of producing IDS and cortisol-resistant (medullary) thymocytes are not. IDS production by lymphoblasts was proportional to cell number and unaffected eith by cell density (1 to 10 x 106/ml) or by the concomitant presence of normal cells from spleen, lymph node, or thymus. Thus Con A-stimulated cells, after becoming blasts, appear to produce IDS automatically wihtout affecting or being affected by other cells. Both spleen and thymus cells from rats injected with a large dose of antigen (ovalbumin, 100 mg, i.p.) 24 hr in advance produced substantial amounts of IDS in culture within 24 hr in the absence of mitogen or additional antigen, but not the cells from rats injected with an immunizing dose (1 mg) of the same antigen. The cells producing IDS in the spleen were shown to be adherent to glass wool, and those in the thymus were partially so. IDS production by antigen-stimulated spleen cells was abrogated by injecting rats with bromodexyuridine (BUdR) at 0 and 12 hr after the ovalbumin. These findings suggest that a subpopulation ofadherent spleen cells (possibly resembling cortical thymocytes), which begins to proliferate within a few hours after a large dose of systemic antigen, produces IDS. This may account for increased nonspecific suppressor activity observed at the same time.     

The control of DNA synthesis in primary cultures of hepatocytes from adult and young rats: interactions of extracellular matrix components, epidermal growth factor, and the cell cycle

Hepatocytes from adult and 4-week-old rats cultured on one of several extracellular matrix components were stimulated to replicate by epidermal growth factor (EGF). DNA synthesis was increased at 44-48 hr in adult hepatocytes and at 24, 48, and 72 hr in hepatocytes from young rats when EGF was added 2 hr after explantation. When EGF was added at 24 hr, maximal DNA synthesis of adult hepatocytes was observed at 48 hr, whereas that of 4-week-old hepatocytes was seen at 48 and 72 hr. Ten ng EGF per ml was the optimal concentration for maximal DNA synthesis in both adult and young cells. DNA synthesis decreased with increasing cell density, but this effect was less in hepatocytes from young than in those from adults. When hepatocytes were cultured on substrata consisting of individual extracellular matrix components, neither the time that adult cells needed to respond to EGF nor the time from stimulation by EGF to the peak of maximal DNA synthesis was altered in either adult or young cells. The optimal EGF concentration for maximal DNA synthesis and the cell density control of replication were also not altered by the substrata used. Substrata made from each of the extracellular matrix components studied enhanced DNA synthesis of adult and young hepatocytes stimulated by EGF in the following decreasing order: fibronectin, type IV collagen, type I collagen, and laminin. In both adult and young hepatocytes the enhancement of DNA synthesis was greatest when cultured on fibronectin. Thus the initiation and magnitude of DNA synthesis in primary cultures of rat hepatocytes were altered both by the age of the donor and the substratum on which the cells were explanted.     

Cyclic AMP and theophylline enhance DNA synthesis in L cells stimulated with anti-actin IgG and [(IgG)2protein A]2 complex by recruiting cells into S-phase

Surface binding of anti-actin IgG alone or in a M_r = 716 000 [(IgG)₂Protein A]₂ complex results in a stimulation of DNA synthesis and cell growth in L cells. Cyclic-AMP (0.01–1.0 mM) added to such cell cultures augmented DNA synthesis as measured by incorporation of [³H]thymidine into DNA. Theophylline (0.1–1.0 mM), a phosphodiesterase inhibitor which prevents enzymatic breakdown of cAMP, had similar effects, but cGMP (0.01–1.0 μM) reversed the effects of cAMP and theophylline upon DNA synthesis. Analysis of the cell cycle by flow cytometry revealed that antibody produced a shift (7%) of cells from the G₁-phase to the S-phase (DNA-synthetic) of the cell cycle at 72 hr of incubation. Addition of cAMP (0.5 mM) to cell cultures, however, produced significant shifts of antibody stimulated cells from G₁-phase to S-phase at all time points measured, i.e., 24 (12%),48 (22%),72 hr (23%). Thus, antibody recruited cells into S-phase of the cell cycle and cAMP greatly augmented the effect. These observations suggest that the mechanism of activation of L cell growth by antibody to surface antigens involves a recruitment of cells into the DNA-synthetic phase and that the effect may be mediated by cAMP.     

Butachlor is cytotoxic and clastogenic and induces apoptosis in mammalian cells

The ability of butachlor to induce cytotoxicity, clastogenicity and DNA damage was assessed using Chinese hamster ovary cells (CHO), Swiss mouse embryo fibroblasts (MEF) and human peripheral blood lymphocytes. A dose and time dependent loss of viability was evident upon treatment of CHO cells with butachlor. Cell killing to an extent of 50% was observed when cells were treated with 16.2 micrograms/ml of butachlor for 24 hr or with 11.5 micrograms/ml for 48 hr. The herbicide induced micronuclei significantly in cultured lymphocytes at 24 and 48 hr of treatment suggesting that it is clastogenic. To understand the mechanism of cell death caused by butachlor, its effect on DNA strand breaks was studied in MEF. A concomitant decrease in cell viability was observed with increase in DNA strand breaks. Agarose gel electrophoresis of DNA from herbicide treated CHO cells and cytochemical staining indicate the induction of apoptosis by butachlor.     

Pyruvate-Induced changes in hepatoma cell morphology and macromolecular synthesis

Cells maintained in basal growth medium with 0.2–1.0% serum often require citric acid cycle intermediates for optimal viability. We have found that pyruvate added to minimal growth medium causes cellular flattening and formation of external processes accompanied by increaded DNA synthesis in cultured hepatoma cells (HTC cells). Cells were cultured in plalstic T-flasks (0.5, 1.0, or 2.0 × 10⁶ cells/flask) containing 5 ml medium (90% Eagle's Basal Medium (BME) and 10% Swim's S-77) with various concentrations of fetal calf serum (0.2,0.25, 0.5, 1.0, 2.0, 10%) and either pyruvate (50, 100, 250,500, 1,000μg/ml), or one of: dibutyryl cAMP (DBcAMP) or dibutyryl cGMP (DBcGMP) at 10^?3, 10^?4, or 10^?5 M. At 44–48 hr cultures were pulsed with tritiated thymidine, uridine, or lecucine. Cells became attached to the plastic surface within 24hr. Cells in medium with 0.25 to 2.0% serum had a rounded appearance. With added pyruvate, cellular flattening, process formation, and an increased adherence to the substratum was absorbed. By 48 hr, culture without pyruvate grew in rounded clusters; with pyruvate, cells formed extensive interconnecting processes that appeared loosely attached to the monolayer surface. At the cell densities tested, process formation was maximal with 250 to 500 μg/ml pyruvate. Cytochalasin B blocked flattening and process formation; EDTA (1 mg/ml) caused retraction of processes within 3 min, and a slow dissolution of these structures within cells was observed. DBcAMP or DBcGMP did not induce process formation. Flattening and process foormation in pyruvate-enriched cultures were accompanied by marked stimulation of DNA synthesis and smaller increases in RNA and protein synthesis. Cell number was not affected. These pyruvate-induced changes suggest that alterations in energy metabolism, or precursors that enhance viability and macromolecular synthesis in mammalian cell cultures, may exert marked effects on cellular morphology without corresponding changes in growth of neoplastic liver cells.     

Studies on T cell clonal expansion. II. The in vitro differentiation of pre-killer and memory T cells.

Spleen cells from C57BL/6 mice immunized with a DBA/2 mastocytoma (P815) were harvested at various stages of the immune response and cultured in vitro in the presence and absence of antigen. Killer T cell activity in immune spleens could not be demonstrated until 6 or 7 days after antigen, but spleen cells harvested as early as 3 or 4 days and cultured for 24 hr at 37 degrees C showed significant cytotoxicity. This increased activity was not augmented further by culturing with antigen. "Memory" T cells, whose in vitro differentiation into killer cells required the presence of antigen, could not be demonstrated until 9 or 10 days after alloantigenic stimulation. Once produced, however, these cells persisted for at least 6 months. Memory cells, like killer T cells bound avidly to homologous allogeneic monolayers. There were indications that the memory T cell pool was heterogeneous. On one hand, when cells harvested 10 days after stimulation were exposed to antigen in vitro, their lytic activity increased within 24 hr but showed no further increases when the culture period was extended. In contrast, 45-day-old immune cells showed increasing lytic activity throughout a 4-day exposure to antigen. Augmentation of lytic activity in both cell populations was independent of DNA synthesis through the first 24 hr of culture. Subsequent increases in the activity of 45-day cells was dependent upon cell proliferation. Both the antigen-independent augmentation of lytic activity which followed culturing of immune cells, and the antigen-induced differentiation of memory cells were reversibly inhibited by a series of drugs which raised lymphocyte cAMP levels.     

The in vitro life span and fate of murine B lymphocytes stimulated by endotoxin.

The in vitro life span of murine spleen lymphocytes stimulated by endotoxin (LPS) was determined. Lymphocytes synthesizing DNA spontaneously in culture and those stimulated to DNA synthesis early (24 hr) and later (48 hr) in culture by LPS had half-lives of approximately 24 hr. The continuing presence of LPS in culture did not prolong cell longevity nor did free LPS have to be present to allow successive rounds of DNA synthesis in committed cells. Once activated to DNA synthesis, blast cells and lymphoblast-like cells did not revert to small lymphocytes.     

Cytomegalovirus-induced membrane antigens in productively infected cells.

Adsorbed but not penetrated virus can be removed from the CMV-infected cell membrane by digestion with cystine-activated papain. Membrane antigens appear on 80-90% of the infected cells 14-20 hr after infection as a result of de novo protein synthesis. Antigen synthesis can be blocked with inhibitors of protein synthesis, but not with DNA inhibitors. In the early stage of infection, pooled human convalescent serum reacted well with the membrane antigen, whereas pooled antiserum of rabbits immunized with CMV virion suspension gave a positive reaction with a small proportion of the cells. After the 48th hr, both the human and the rabbit serum pool reacted with the membrane of the infected cells. Absorption with cell cultured for 24 hr after CMV infection reduced the neutralization titres of the antisera only slightly but the titre reduction was considerable when absorption was performed with cells cultured for more than 48 hr after infection. It is concluded that on the membrane of cells productively infected by CMV at least two membrane antigens are present, one coded for by the DNA of the parent virus and another which is the product of the DNA of the virus progeny. The two antigens can be differentiated serologically.     

Growth promotion of transfected hepatoma cells by liver fatty acid binding protein

Former studies have linked hepatocyte growth with liver fatty acid binding protein (L-FABP) of rat liver cytosol. In search for the roles of L-FABP in hepatocytes, we previously stably transfected rat L-FABP sense and antisense cDNAs into rat hepatoma HTC cells that do not contain L-FABP RNA or protein, thereby providing a zero-background, homologous cell model of L-FABP-expression suitable for controlled studies of its intracellular functions in hepatocyte-derived cells. The present study demonstrates the abilities of L-FABP to promote DNA synthesis and cell growth, preserve cell morphology, extend survival, and act cooperatively with unsaturated fatty acids in the transfected hepatoma cells in the absence of serum. Following removal of serum, the three control L-FABP-nonexpressing cell lines increased in cell lines increased in cell number for 24 hr and thereafter declined, whereas the three L-FABP-expressing cell lines exhibited a 39% higher rate of DNA synthesis per cell at 24 hr and grew in cell number for 48 hr. As a result, at 72 hr there were 2.5-fold (avg.) as many L-FABP-expressing cells than L-FABP-nonexpressing cells. In addition, the L-FABP-expressing cells retained their original polygonal morphology at 48 hr, when in contrast most of the control nonexpressing cells were spherical in shape with membrane blebs. In an effort to identify the agonists that collaborate with L-FABP in the growth promotion and preservation of cell morphology, various free fatty acids were examined at 48 hr for their ability to elminate the differences in behavior of the two cell types in the serum-free medium. The unsaturated fatty acids, oleic acid (18:1 ω9), linoleic acid (18:2ω6), α-linolenic acid (18: 3ω3), and arachidonic acid (20:4ω6), at 1 μM markedly elevated the level of DNA synthesis in the more depressed control L-FABP-nonexpressing cells and moderately raised it in the less depressed L-FABP-expressing cells. In accord, the control L-FABP-nonexpressing cells needed 10^?6–10^?5 M linoleic acid to achieve the extent of DNA synthesis attained by the expressing cells in the absence of added fatty acid. At 10 μM linoleic acid, their levels of DNA synthesis were equal. In contrast, five saturated fatty acids had no detectable effect on DNA synthesis. In addition, linoleic acid at 1 μM, but not the saturated fatty acid palmitic acid (16:0), prevented the above morphological alterations in the control L-FABP-nonexpressing cells observed in the absence of serum, thereby retaining their original polygonal morphology and that of the expressing cells. The findings are consistent with the concept that L-FABP improves the efficacy of the utilization of unsaturated fatty acid ligands of L-FABP in the formation, integrity, and fluidity of cell membranes that are involved in cell growth, morphology, and survival. © 1993 Wiley-Liss, Inc.     

Suppressing effects of purified eosinophils derived from guinea pigs sensitized with Ascaris antigen on lymphocyte-blastformation

Highly purified eosinophil (EOS) fractions were obtained from peritoneal exudate cells of guinea pigs immunized with with Ascaris lumbricoides suum antigen (Asc). These Eos suppressed the in vitro DNA synthesis of the lymph node cells (LNC) sensitized with Asc and then activated by this antigen or phytohemagglutinin (PHA). Although addition of Eos did not effect the viability of LNC in vitro, the blastformation of LNC was suppressed remarkably when 5-10 X 10(5) purified Eos were added to 10(6) LNC within 48 hr after the start of stimulation by Asc. The suppressive effects of Eos on the blastformation of LNC immunized with complete Freund's adjuvant with or without ovalbumin were observed when stimulated with purified protein derivates or ovalbumin. Such suppression were observed beyond the barrier of animal strain specificity; Eos from Hartley guinea pigs suppressed proliferation of LNC from either strain 13 or strain 2, and Eos from strain 13 suppressed that from strain 2. Such suppressing activity of Eos was reduced by heating them at 56 C for 1 hr or by sonication.     

Kinetics of IL-2 and interferon-gamma production, expression of IL-2 receptors, and cell proliferation in human mononuclear cells exposed to staphylococcal enterotoxin A

Staphylococcal Enterotoxin A (SEA) at picogram amounts induces high levels of interleukin 2 (IL-2) and interferon in human mononuclear cells. SEA is a stronger inducer of IL-2 than phytohemagglutinin, leukoagglutinin, and concanavalin A. The IL-2 induction is very rapid with maximal levels being reached after 18 to 24 hr. The IL-2 concentration decreases rapidly and almost no IL-2 activity can be detected in supernatants of cells cultured for 3 days or more. Maximal DNA synthesis is recorded 3 days after maximal IL-2 levels have been reached in the culture medium. The DNA synthesis shows a 24 hr delay as compared to the expression of the IL-2 receptor during the initiation phase. An increase in the level of IL-2 receptor expression is apparent as early as 12 hr after stimulation with SEA and maximal expression is reached 48 to 72 hr after stimulation. The percentage of cells expressing the IL-2 receptor is maximal at 96 hr after onset of culture but the surface concentration of the receptor is lower than at 72 hr. The decline in expression of the IL-2 receptor is accompanied by a decline in mean cell size and in DNA-synthesis. The concentration of the T-cell marker T11 increases in parallel with the growing expression of the IL-2 receptor. It remains increased over a longer period than the IL-2 receptor and is still significantly augmented after 10 days' exposure to SEA.     

Cell junction and cyclic AMP: II. Modulations of junctional membrane permeability,dependent on serum and cell density

Summary Junctional molecular transfer (as indexed by the number of cell interfaces transferring fluorescent-labelled molecules) and concentration of endogenous cAMP were determined in mammalian cells in culture at varying serum concentration and cell density. In several cell types, on stepping the serum concentration from 10% (the concentration to which the cells had been adapted) to zero, the junctional transfer rose (reversibly) within 48 hr, as the endogenous cAMP concentration rose. The junctional transfer was inversely related to serum concentration over a range, most steeply so the transfer of large and charged molecules. one cell type showed no junctional change in response to serum; it showed also no endogenous cAMP change. Junctional transfer varied inversely with cell density over the range of 0.7–7 (10⁴ cells/cm²) in 3T3 cells. In cultures seeded to various densities, or growing to various densities on their own, junctional transfer fell with rising density, and so did the endogenous cAMP concentration. Upon downstep from high density, junctional transfer rose over 24–48 hr. In B cells, junctional transfer was independent of cell density over the aforementioned range, and so was the endogenous cAMP concentration. These results, in conjunction with the effects of exogenous cAMP described in the preceding paper of this series, point to a cAMP-mediated junctional effect; a possible teleonomy for control of membrane junction is discussed.     

Organ culture of adult mouse intestine. II. Mitotic activity, DNA synthesis and cellular migration after 24 and 48 hours of culture.

DNA synthesis, mitotic activity and cell migration have been measured in organ culture of adult mouse jejunum during the first 48 hr. Explants cultured in DMEM-HEPES-NCTC-135 medium present a sharp decrease of mitotic activity in the first hours of culture, but mitoses are restored to 80% of the controls between 24 and 48 hr. DNA synthesis follows the same pattern. Cell migration continues during culture.     

The role of cyclic AMP in modulating cytotoxic T lymphocytes. II. Sequential changes during culture in responsiveness of cytotoxic lymphocytes to cyclic AMP-active agents

Agents that increase cyclic AMP (cAMP) levels inhibited the activity of cytotoxic T lymphocytes (CTL) obtained from spleens of mice immunized with allogeneic cells. Cultured CTL, however, were desensitized to cAMP-active agents, in that the capacity of these agents to inhibit the activity of cultured CTL was markedly reduced. The capacity to inhibit CTL activity was reduced more rapidly for some agents than for others; the percent inhibition by histamine and PGE2 was reduced after 4 hr and was reduced more than 20% after 24 hr, whereas the percent inhibition by dibutyryl cAMP, theophylline, and cholera enterotoxin was reduced less than 6% after 24 hr, and was reduced significantly only after 48 hr. Culture in the presence of antigen accelerated desensitization to the latter three agents. CTL populations were also tested for their capacity to increase cAMP levels in response to agonists. The capacity of histamine to increase cAMP levels of the CTL was lost within 4 hr (i.e., as rapidly as its capacity to inhibit CTL activity) and was never restored, whereas the capacity of PGE2 to increase cAMP levels persisted throughout culture. These results suggest that culture induces multiple alterations in cAMP metabolism of CTL. These alterations, which result in dissociation of CTL activity from cAMP-mediated regulatory steps, may include loss of histamine receptors and/or histamine receptor-adenylate cyclase coupling, and also loss of one or more biochemical reactions that link elevated cAMP levels to inhibition of lysis.     

Dissociation of differentiation and proliferation in the primary induction of cytolytic T lymphocytes by alloantigens

The requirement for DNA synthesis in the induction of cytolytic T lymphocytes (CTL) by alloantigens has been investigated. C57BL/6 splenic T cells purified by passage on nylon wool columns were stimulated in vitro in mixed leukocyte culture (MLC) and assayed for cytotoxicity against 51Cr-labeled target cells. With this system, CTL activity was detectable after 24 hr of MLC and reached high levels after 48 hr. Addition of cytosine arabinoside (ARA-C) or hydroxyurea to such cultures at concentrations that were sufficient to inhibit DNA synthesis by greater than 98% did not reduce CTL activity measured after 24 hr; however, the increase in activity that occurred between 24 and 48 hr in control cultures was strongly reduced (or abolished) by these drugs. Velocity sedimentation analysis of MLC cells activated for 48 hr in the presence of ARA-C further revealed that CTL precursor lymphocytes had enlarged into medium- to large-sized CTL under these conditions. These studies provide direct evidence that the primary induction of CTL by alloantigens can be dissociated into a differentiation step, which occurs within 24 hr in the absence of DNA synthesis and is accompanied by blast transformation, and a subsequent proliferation.     

Lymphoid cell subpopulations. II. Characterization of cell populations responsible for syngery in the mixed lymphocyte interaction.

Mixtures of isogeneic lymph node cells (LNC) and thymocytes (TC) exhibit far greater responsiveness in the murine MLI, as measured by proliferation and development of cytotoxic effector cells, than either cell type cultured alone. Pretreatment of either lnc or TC with mitomycin-C or ultraviolet irradiation completely abolished their synergistic interaction. Administration of cortisone acetate to cell donors 20 hr before sacrifice reduced the capacity of LNC and enhanced the capacity of TC to synergize. The LNC and TC populations participating in synergy, were found to be thymus dependent. LNC were shown to be responsible for the bulk of proliferative and effector activity observed in synergizing cultures, whereas TC appeared to amplify the activation of LNC. These findings provide the basis for a three cell model of MLI responsiveness.     

In vitro chondrogenesis and cell viability

Anterior somites cultured with (NSA) or without (SA) notochord, and posterior somites cultured with (NSP) or without notochord (SP) were compared with respect to changes in their DNA content, their potential to synthesize the active sulfate principle phosphoadenosine phosphosulfate (PAPS), and their ability to accumulate ³⁵S-sulfate.Chondrogenesis was observed in the NSA, NSP, and SP explants, but was rarely noted in the SA explants. A decrease in DNA content during the initial 48 hr of culture was common to all explants. After this initial decrease, DNA content increased most in those explants forming cartilage. The synthesis of PAPS by cell-free extracts of each type of somite explant also decreased during the initial period of culture. Only extracts of those explants undergoing chondrogenesis showed increases in PAPS synthesis with continued culture. Each type of somite explant accumulated ³⁵S-sulfate into chondroitin sulfate during the first hours of culture. The non-chondrogenic SA explants accumulated little ³⁵S-sulfate during the period of culture. At varying times after 24 hr the chondrifying explants (NSA, SP, and NSP) initiated an increased rate of accumulation of ³⁵S-sulfate.Cartilage nodules, increases in DNA content, PAPS synthesis and ³⁵S-sulfate accumulation occurred within the same 24 hr period, during the 2nd day in NSP explants, the 3rd day in NSA explants, and between the 3rd and 4th day for SP explants. A hypothesis of in vitro somite chondrogenesis based on differential cell viability is presented.     

Regulation of the in vitro anamnestic antibody response by cyclic AMP: I. Antigen-induced uptake of nucleotides and nucleosides by lymphocytes

Addition of N⁶, O²′-dibutyryl cAMP (DbcAMP) to keyhole limpet hemocyanin (KLH)-primed rabbit lymph node cells for 1 hr, followed by its removal and the addition of KLH, had no effect on the subsequent antibody response, whereas addition of KLH for 1 hr followed by DbcAMP resulted in a 100% enhancement of antibody synthesis. Addition of cholera enterotoxin (CT), which rapidly and irreversibly binds to lymphocytes and activates adenylate cyclase, either before or after the addition of antigen, elevated the antibody response by 100%. These results suggested that some antigen-induced event(s) may be required for DbcAMP to exert its enhancing effects on the antibody response. The effect of KLH on the uptake of DbcAMP by KLH-primed lymph node cells was investigated. One and one hundred micrograms of KLH, which induce optimal and supraoptimal antibody synthesis, respectively, promoted maximal uptake of DbcAMP. This induced uptake was first detectable about 12 hr after addition of KLH, and it peaked during 24–48 hr of culture. DbcAMP uptake induced by a brief exposure of KLH (0–1 hr) was equivalent to that observed with long-term KLH addition (0–24 hr). KLH-induced DbcAMP uptake required KLH-reactive lymphocytes and represented active transport. Antibody to rabbit T lymphocytes inhibited this antigen-induced uptake. The mitogens concanavalin A (Con A) (T cells) and goat anti-rabbit Fab' (anti-Fab') (B cells) also stimulated DbcAMP uptake, as did human serum albumin (HSA) and myoglobulin (Mb) when added to homologously primed cells, indicating the generality of the phenomenon. [³H]DbcAMP entered the cells as di- or monobutyryl cAMP with about 40% metabolized to 5′AMP. This uptake could be competitively inhibited by other adenine or guanine nucleotides and nucleosides.     

Treatment of myeloid leukemic cells with the phosphatase inhibitor okadaic acid induces cell cycle arrest at either G1/S or G2/M depending on dose.

The phosphatase inhibitor okadaic acid was found to induce cell cycle arrest of human myeloid leukemic cell lines HL-60 and U937 in a concentration- and time-dependent manner. Exposure to low concentrations of okadaic acid (2-8nM) for 24-48 hr caused greater than 70% of cells to arrest at G2/M, with up to 40% of the cells arrested in early mitosis. Cell viability decreased rapidly after 48 hr of treatment, and morphological and DNA structure analysis indicated that this was primarily due to the induction of apoptosis. The cells arrested in mitosis by 8 nM okadaic acid could be highly enriched by density gradient centrifugation and underwent apoptosis when further cultured either with or without okadaic acid, indicating that the effects of okadaic acid were irreversible. In contrast to the effects of low concentrations of okadaic acid, high concentrations (500 nM), inhibited proliferation in less than 3 hr. Remarkably, the majority of cells also entered a mitosis-like state characterized by dissolution of the nuclear membrane and condensation and partial separation of chromosomes. However, these cells had a diploid content of DNA, indicating that the cell cycle arrest occurred at G1/S with premature chromosome condensation (PCC), rather than at G2/M. If cells were first blocked at G1/S with hydroxyurea and then treated with okadaic acid, greater than 90% developed PCC in less than 3 hr without replicating their DNA. Caffeine was not able to induce PCC in these cells, either with or without prior inhibition of DNA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)