Ia binding ligands and cAMP stimulate nuclear translocation of PKC in B lymphocytes

Involvement of second messengers in regulation of the low-density lipoprotein receptor gene

Transcription of the low-density lipoprotein receptor (LDL-R) gene in the human monocytic leukemic cell line THP-1 and in the human hepatocarcinoma cell line Hep-G2 is regulated by second messengers of the diacylglycerol-protein kinase C (DAG-PKC), inositol 1,4,5-triphosphate-Ca2+, and cyclic AMP pathways. Exogenous phospholipase C (which releases DAG and inositol 1,4,5-triphosphate), PKC activators (phorbol esters and DAG), Ca2+ ionophores, and a cyclic AMP analog all transiently induced accumulation of LDL-R mRNA. The effects of these three signal-transducing pathways were to a large extent additive. Furthermore, PKC stimulation effected an increase in LDL binding, which suggested that the increase in LDL-R mRNA resulted in an increase in functional cell surface receptor activity. These results suggest that uptake of cholesterol by these cells is under control of both intracellular cholesterol levels and external signals.

Ia-mediated signal transduction leads to proliferation of primed B lymphocytes

One of the most controversial questions in immunology is the molecular basis by which Th lymphocytes deliver activating signals to quiescent B lymphocytes during T cell-dependent immune responses. Recent studies suggest that T cell-dependent activation of quiescent B lymphocytes may involve signaling mediated by direct T helper cell-B cell contact. Since B cell membrane-associated MHC-encoded class II molecules (Ia) must be recognized by Th lymphocytes for generation of T cell-dependent humoral immune responses, they are obvious candidates for receptors of this signal. Here we report that stimulation of quiescent murine B cells with IL-4 and antibodies against the B cell antigen receptor for 12-16 h primes cells to proliferate in response to immobilized mIa binding ligands. In the presence of additional lymphokines, these B cells differentiate to secrete Ig of IgM and IgG classes. These results suggest that Ia molecules are receptors for direct, T helper cell-B cell contact mediated signaling that results in B cell proliferation.

B cell activation by synthetic lipopeptide analogues of bacterial lipoprotein bypassing phosphatidylinositol metabolism and proteinkinase C translocation

Synthetic analogues of bacterial lipoprotein induce proliferation of murine small resting B lymphocytes. We investigated the role of proteinkinase C (PKC) activation in lipopeptide-induced B cell stimulation. Using a standardized extraction procedure, there was no change in membrane bound and soluble PKC activity upon stimulation with lipopeptide. However, omitting Ca2+ chelators from the standard extraction medium resulted in a decrease of membrane bound PKC activity after stimulation. Lipopeptide failed to induce phosphoinositide degradation and the generation of the two second messengers cAMP and cGMP. To test whether guanosinetriphosphate-binding proteins are involved in lipopeptide-induced signal transfer we investigated the effect of LiCl, choleratoxin and pertussistoxin on B lymphocyte proliferation. LiCl and pertussistoxin did not inhibit cell activation, whereas choleratoxin reduced the proliferation rate at concentrations higher than 0.5 micrograms/ml. Similar results were observed when LPS was used as mitogen, whereas the anti-immunoglobulin-induced B cell activation was inhibited by all three compounds. Our results show, that B cell activation by bacterial lipopeptides bypasses phosphatidylinositol metabolism and PKC translocation.

Cyclic AMP can enhance mouse B cell activation by regulating progression into the late G1/S phase

This study demonstrates that several substances which raise intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels in different ways were able to enhance both RNA and DNA synthesis in mouse purified B cells co-stimulated by the protein kinase C activator phorbol 12-myristate 13-acetate and the Ca2+ ionophore ionomycin, while earlier activation events were not modified. These included early changes in cell size and chromatin decondensing demonstrated by light scatter properties at narrow and 90 degrees angles, increase in Ia expression and loss of surface IgD. We concluded that cAMP can up-regulate mouse B cell activation by controlling progression into the late G1 (G1B)/S phase, but not transition from G0 to early G1 (G1A). Furthermore, because cAMP could synergize with ionomycin but not with phorbol 12-myristate 13-acetate to induce RNA and DNA synthesis, we proposed that the cAMP effects in this model may be related to the protein kinase C pathway.

Type 3 protein kinase C localization to the nuclear envelope of phorbol ester-treated NIH 3T3 cells

We have examined the immunocytochemical localization of protein kinase C (PKC) in NIH 3T3 cells using mAbs that recognize Type 3 PKC. In control cells, the immunofluorescent staining was similar with mAbs directed to either the catalytic or the regulatory domain of PKC. Type 3 PKC localized in a diffuse cytoplasmic pattern, while the nuclei were apparently unstained. Cytoskeletal components also were Treatment of the cells with phorbol 12-myristate 13-acetate (PMA) resulted in a redistribution of PKC with a specific increase in nuclear PKC. Compared to control cells, the staining with the anticatalytic domain mAbs changed markedly, covering the entire cell surface. In contrast, the staining by the antiregulatory domain mAb did not cover the cell surface and the nuclei remained unstained; these results suggest that PKC activation leads to a conformational change of the regulatory domain such that the epitope recognized by the antiregulatory domain mAb is not readily accessible. We have demonstrated by three criteria that PMA treatment specifically increased PKC in the nucleus: (a) immunofluorescent staining in isolated nuclei increased; (b) Western blots showed that our mAbs detected only one protein, the 82-kD PKC, whose level increased in nuclear lysates from PMA-treated cells; and (c) PKC activity increased in nuclear lysates. In fractionation studies we demonstrated that PKC specifically localized to the nuclear envelope fraction. These results demonstrate that PMA activation leads to a rapid redistribution of Type 3 PKC to the nuclear envelope, and suggests that this isozyme may play a role in mediating PKC-induced changes in gene expression.

Altered I-A protein-mediated transmembrane signaling in B cells that express truncated I-Ak protein

Recent evidence suggests that the major histocompatibility complex class II molecules of B lymphocytes function as signal-transducing receptors during the generation of T lymphocyte-dependent humoral immune responses. By analogy with other receptors, we postulate that perturbation of the class II molecules is coupled to the generation of intracellular second messengers through interactions involving the transmembrane and/or cytoplasmic domains of the class II molecules. We report a series of experiments that assess which amino acids of the class II molecule I-Ak are required for coupling it to the signal-transduction pathway. We prepared a series of B-lymphocyte transfectants that express I-Ak molecules with COOH-terminal truncations of either the Ak alpha or Ak beta chain or both. The ability of each transfected class II molecule to transduce a signal after being bound by monoclonal antibody was found by monitoring the translocation of protein kinase C from the cytosol to the "nuclear compartment" of the transfected B lymphocyte. Results indicate that the Ak beta chain plays the dominant role in signal transduction and that the 6 cytoplasmic amino acids of Ak beta chain most proximal to the inner plasma membrane are of greatest importance in coupling I-Ak molecules to the molecules of the signaling cascade.

Biphasic regulation of macrophage attachment by activators of cyclic adenosine monophosphate-dependent kinase and protein kinase C

A method is described that enabled us to study the adhesiveness of J-774 murine macrophages. Cell attachment was stimulated by activators of kinase C (i.e., phorbol esters) as well as kinase A (cyclic adenosine monophosphate; cAMP). This novel effect of cAMP was observed when its levels were increased via receptor triggering (prostaglandin E1, beta-adrenergic agonists), activation of Ns (cholera toxin), or inhibition of phosphodiesterase (Ro 20-1724) or when the kinase was directly activated by Br8-cAMP. The simultaneous treatment with kinase A and kinase C activators at the time of attachment resulted in a partially additive response. On the other hand, preincubation of the cells in suspension with one of the activators rendered them refractory to subsequent stimulation at the onset of the adhesion assay, whatever agent was used. Such a refractoriness was also observed in cells preincubated with oleoyl-acetyl-glycerol (OAG). On the other hand, when added at the time of attachment, this near-physiological activator of kinase C evoked a biphasic response: the early stimulation of cell attachment was followed by an accelerated rate of "detachment." In conclusion, kinase C and kinase A play a role in the sequence of events leading to cell adhesion. The cross desensitization observed is distal and takes place at or beyond the kinase step.

Haplotype-specific differences in signaling by transfected class II molecules to a Ly-1+ B-cell clone

CH12.LX B cells are responsive to antigen-dependent differentiative signals transmitted through their surface Ek molecules. Although CH12.LX cells express surface Ak molecules with normal protein sequence, the Ak molecules do not deliver differentiative signal to these B cells. To determine whether introduction of a new A molecule into CH12.LX cells would correct this deficiency, CH12.LX cells were transfected with the genes encoding new Aalpha and/or Abeta molecules. It was found that transfected cells responded to antigen-specific signals delivered via the Ealphak Ebetak, Aalphab Abetab, or Aalphak Abetab molecule. However, the B cells did not respond to signals generated by the molecule Aalpha kAbetak, Aalphab Abetak, Aalphak Abetad, or Aalpha kAbetau. Comparison of these sequences suggests that two Abeta residues, His-47 and Trp-197, are important to the transmission of differentiative signals to B cells.

Transmembrane signaling via both CD3 and CD2 human T cell surface molecules involves protein kinase-C translocation

The activation of T lymphocytes by appropriate pairs of anti-CD2 monoclonal antibodies has been shown to involve phospholipase-C and phosphoinositide hydrolysis. In this paper we show that the stimulation of the human cloned leukemic T cell line Jurkat by anti-CD2 as well as anti-CD3 monoclonal antibodies induces translocation from cytosol to cell membrane of protein kinase-C (PKC), which is dependent on the formation of 1,2-diacylglycerol from inositol 4,5-diphosphate. PKC translocation is rapid and transient: the kinetics of enzyme redistribution are similar for CD2 and CD3. These results further stress that CD2 and CD3 T cell activation pathways use similar signal transducing mechanisms.

Membrane IgM cross-linking is not coupled to protein kinase C translocation in WEHI-231 B lymphoma cells

The early molecular events involved in the process of signal transduction via membrane immunoglobulins (mIg) include phosphatidyl inositol metabolism, intracellular Ca2+ mobilization, and protein kinase C (PKC) activation. Anti-mIg antibodies exert either stimulating or inhibitory effects depending on the activation state and/or the differentiation stage of B cells. WEHI-231 is a murine B lymphoma that becomes inactivated upon anti-mIg treatment. This lymphoma has an immature B cell phenotype and is considered as a model for tolerance induction in B lymphocytes. In this study, we have investigated the relationship between mIg triggering, Ca2+ elevation, PKC translocation, and growth inhibition in WEHI-231 cells. Monoclonal antibodies to mu and kappa chains of the mIgM receptor promoted a rapid increase in intracytoplasmic Ca2+ and were potent inhibitors of cell growth. Ca2+ elevation and PKC translocation have been previously shown to be associated in B lymphocytes. To study the subcellular distribution of PKC in WEHI-231 cells, we used enzymatic assays and immunodetection methods. Although phorbol 12-myristate 13-acetate induced a rapid and almost complete redistribution of cytosolic PKC to the membrane fraction, anti-mIg treatment failed to modify the compartmentalization of PKC. These findings extend recent observations suggesting that B cell triggering through mIg receptors may involve additional pathways independent from PKC activation. PKC activation in normal B cells is also believed to provide a regulatory signal which limits the magnitude of the early signals produced by anti-mIg. Such a regulatory control is unlikely in WEHI-231 cells, due to the dissociation between Ca2+ mobilization and PKC translocation. Our findings therefore suggest that the sensitivity of immature B cells such as WEHI-231 to the inhibitory effects of anti-mIg antibodies may result in part from alterations of the phosphoinositide signal transduction pathway.

Intracellular events associated with inhibition of B cell activation by monoclonal antibodies to HLA class II antigens

We have investigated several aspects of the inhibitory effects of monoclonal antibodies (mAb) directed against MHC class II antigens in B cell activation/proliferation, using a panel of mAb specifically reactive with antigens encoded by HLA class II loci (DP, DQ, DR). All mAb except the anti-DP mAb inhibited significantly anti-mu plus B cell growth factor-induced DNA synthesis. Only one mAb, however, which was reactive with gene products of all three class II loci (DP, DQ, DR) inhibited anti-mu-induced DNA synthesis as well as c-myc mRNA expression. In addition, the same mAb inhibited the early events induced by anti-mu stimulation alone, including phosphatidylinositol turnover and elevation of [Ca2+]i. In contrast to previous findings in the murine system, none of the anti-MHC class II mAb used in this study increased the cAMP levels.

Lack of correlation between translocation and biological effects mediated by protein kinase C: an appraisal

Protein kinase C is involved in the mechanism of action of hormones, growth factors, mitogens and tumour promoters. The correlation between the extent of the biological effects mediated by protein kinase C and its fractional activation shows cell type specific patterns of behaviour. The discrepancy between enzyme activity and biological effects elicited by protein kinase C is particularly relevant to lymphoblastic cells. In B cells, the full expression of some biological responses mediated by protein kinase C may be achieved by activation of less than 5% of the enzyme activity.

Interleukin 1 and cyclic AMP induce kappa immunoglobulin light-chain expression via activation of an NF-kappa B-like DNA-binding protein

Interleukin 1 (IL-1) induces the synthesis of kappa immunoglobulin light chains and the expression of surface immunoglobulin in the murine pre-B-cell line 70Z/3 (J. G. Giri, P. W. Kincade, and S. B. Mizel, J. Immunol. 132:223-228, 1984). In the present study, we found that these effects of IL-1 are mimicked by cyclic AMP (cAMP) analogs and cAMP-elevating drugs. The induction of kappa immunoglobulin light-chain gene expression by IL-1 was associated with an increase in intracellular cAMP levels. Incubation of 70Z/3 cells with IL-1 or cAMP resulted in the activation of the kappa immunoglobulin enhancer, as detected by the induction of chloramphenicol acetyltransferase (CAT) in cells transfected with a kappa enhancer-CAT expression plasmid. In contrast, CAT plasmids lacking a kappa immunoglobulin enhancer were inactive in the presence of IL-1 or cAMP. Furthermore, IL-1 and cAMP analogs and inducers were found to induce the activation of a NF-kappa B-like DNA-binding protein that exhibited specificity for the kappa immunoglobulin enhancer. These results suggest that cAMP may play an important role as a second messenger for IL-1 in the induction of kappa immunoglobulin light-chain synthesis in pre-B cells via the activation of a DNA-binding protein that is similar or identical to NF-kappa B.

Adenosine 3',5'-cyclic monophosphate modulates the mitogenic responses of murine B lymphocytes

Adenosine 3',5'-cyclic monophosphate (cAMP) acts to inhibit a number of lymphocyte activities. The extent of this inhibition was tested by evaluating the effects of two cAMP-raising agents on B cell S phase entry induced by several different mitogenic regimens. It was found that both dibutyryl cAMP (dbcAMP) and isobutylmethylxanthine (IBMX) enhanced S phase entry induced by some regimens but inhibited S phase entry induced by others. The observed enhancing activity stands in contrast to the general notion of cAMP as being a "negative regulator," and it confirms that the observed inhibiting activity does not simply reflect cytotoxicity. Mitogenic regimens that appear to mimic each other, such as F(ab')2 fragments of goat anti-mouse immunoglobulin and the combination of a calcium ionophore and a phorbol ester, were distinguished by their responses to the addition of the two cAMP-raising agents. B cell responses were enhanced or inhibited even when dbcAMP was added 18-24 hr after the establishment of cultures. Cyclic AMP may regulate in a complex fashion S phase entry in cells of the immune system.

A role for MHC class II antigens in B-cell activation

Class II antigens of the major histocompatibility complex (MHC) have a well-defined role in restricting cellular interactions and presenting processed antigen to T cells. In addition, a fundamental role for Class II antigens in cellular activation has been suggested, following studies demonstrating that Class II antigen binding alters the proliferation of various cell types. This is further supported by biochemical evidence of signal transduction by second messengers after ligation of the Class II antigens. We have investigated the role of HLA Class II antigens in the activation of B cells. Both activated and resting B cells proliferate in the presence of Sepharose--conjugated anti-Class II antibodies. This proliferation was not epitope-restricted and was unaffected by low m.w. BCGF. Intracellular free calcium elevation was also examined as a marker of cellular activation. (Ca2+)i was increased after the binding and cross-linking of an anti-DR antibody. The above results further support the role of Class II antigens as signal-transducing molecules.

Involvement of second messengers in regulation of the low-density lipoprotein receptor gene

Transcription of the low-density lipoprotein receptor (LDL-R) gene in the human monocytic leukemic cell line THP-1 and in the human hepatocarcinoma cell line Hep-G2 is regulated by second messengers of the diacylglycerol-protein kinase C (DAG-PKC), inositol 1,4,5-triphosphate-Ca2+, and cyclic AMP pathways. Exogenous phospholipase C (which releases DAG and inositol 1,4,5-triphosphate), PKC activators (phorbol esters and DAG), Ca2+ ionophores, and a cyclic AMP analog all transiently induced accumulation of LDL-R mRNA. The effects of these three signal-transducing pathways were to a large extent additive. Furthermore, PKC stimulation effected an increase in LDL binding, which suggested that the increase in LDL-R mRNA resulted in an increase in functional cell surface receptor activity. These results suggest that uptake of cholesterol by these cells is under control of both intracellular cholesterol levels and external signals.

Human peripheral blood T helper cell-induced B cell activation results in B cell surface expression of the CD23 (BLAST-2) antigen

We have developed an in vitro system to assess the early stages of B cell activation induced by peripheral blood T helper cells. Peripheral blood mononuclear cells are cultured for 16 hr with anti-CD3 monoclonal antibody (mAb), T lymphocytes are then removed by sheep red blood cell rosette depletion, and expression of the B cell surface activation antigen CD23 (BLAST-2) is assessed by indirect immunofluorescence. Anti-CD3 mAb, but not a control anti-CD5 mAb, stimulates the expression of CD23 on 20-50% of peripheral blood B cells cultured with autologous T cells. T cell subset depletion studies show that the CD4+ T cell subset is responsible for anti-CD3-mediated induction of CD23 on autologous B cells. Anti-CD3-induced, T helper cell-dependent CD23 expression is not MHC-restricted, as allogeneic combinations of T and non-T cells, cultured in the presence of anti-CD3 antibody, also result in the expression of B cell CD23. Individuals whose monocyte Fc receptors bind murine IgG1 mAb poorly fail to trigger T cell proliferation in response to murine IgG1 anti-CD3 mAb and also fail to express B cell CD23 following culture of PBMC with IgG1 anti-CD3 mAb, while the usual expression of CD23 is seen after culture with IgG2a anti-CD3 mAb. The mechanism of anti-CD3-induced B cell activation was addressed in experiments using a two-chamber culture system. While little IL-4 activity was detected in anti-CD3-stimulated culture supernatants, optimal induction of CD23 was observed when T and B cells were cultured together in a single chamber. This suggests that under physiologic conditions, in which quantities of lymphokine may be limiting, close physical contact between the anti-CD3-activated Th cell and B cell may be required for CD23 expression. The anti-CD3-induced BLAST-2 assay will facilitate the analysis of Th cell-mediated B cell activation in any individual and should permit us to separately evaluate the roles of Th cells and B cells in the impaired immunoregulation characteristic of autoimmune disorders.

T cell activation via the CD2 molecule is associated with protein kinase C translocation from the cytosol to the plasma membrane

T cell activation via the CD2 molecule involves phospholipase C and phosphoinositide hydrolysis. Here we demonstrate that the triggering of subclones of the human T leukemia Jurkat cell line by anti-CD2 as well as anti-CD3 monoclonal antibodies is able to induce activation (i.e. translocation from cytosol to cell membrane) of protein kinase C (PKC), which is dependent on the formation of 1,2-diacylglycerol from inositol 4-5-bisphosphate. The kinetics of PKC translocation parallels the rise in intracellular calcium following both CD2 and CD3 stimulations. These results further demonstrate that CD2 and CD3 activation pathways use similar signal transduction mechanisms.

Activation of distinct protein kinase C isozymes by phorbol esters: correlation with induction of interleukin 1 beta gene expression

Treatment of human promyelocytic leukemia cells U937 with phorbol 12-myristate 13-acetate (TPA) induces them to differentiate into monocytic cells [Harris, P., & Ralph, P. (1985) J. Leukocyte Biol. 37, 407-422]. Here we investigated the effects of TPA on interleukin 1 gene expression and the possible role of protein kinase C (PKC) in this process. Addition of TPA to serum-starved U937 cells induced the expression of the interleukin 1 beta (IL-1 beta) gene. This effect was apparent as early as 2 h and peaked at 24 h in the presence of 5 X 10(-8) M TPA. Higher concentrations of TPA, which partially or totally depleted protein kinase C levels in the cells (10(-9)-2 X 10(-5) M), had an inhibitory effect on IL-1 beta mRNA expression. Cell-permeable 1,2-dioctanoyl-sn-glycerol (diC8), a diacylglycerol that activates PKC in intact cells and cell-free systems, did not mimic the effect of TPA on the IL-1 beta mRNA induction. To determine the protein kinase C isozymes present in the control and TPA- (5 X 10(-8) M) treated U937 cells, we prepared antipeptide antibodies that specifically recognize the alpha, beta, and gamma isoforms of protein kinase C in rat brain cytosol and U937 cell extracts. In "control" U937 cells, 30% of PKC alpha was particulate, and PKC beta was cytosolic, while there was no detectable PKC gamma.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiproliferative mechanism of anti-class II monoclonal antibodies

The function of Class II molecules in proliferation was explored by treating human cell lines with three distinct anti-DR monoclonal antibodies (MABs). Dose-dependent, specific inhibition of eight DR+ cell lines of different origin and lineage was found. Inhibition was durable (i.e., cells did not become resistant to the anti-DR MABs despite prolonged treatment) yet reversible. The mechanism of inhibition was not due to differentiation or killing but was cytostatic. Inhibition was temporally associated with decreases in nuclear size and irregularity and appeared to be due to a non-phase-specific cell cycle arrest.

Increase of vertebral density by combination therapy with pulsatile 1-38hPTH and sequential addition of calcitonin nasal spray in osteoporotic patients

Combination therapy with the biologically active (1-38) human parathyroid hormone peptide and calcitonin using pulsatile and sequential activation of the skeleton for 14 months in patients with low-turnover osteoporosis resulted in an increase in trabecular bone mass. These favorable responses were observed without any significant changes in cortical (forearm) bone mass content.

Interleukin 1 and cyclic AMP induce kappa immunoglobulin light-chain expression via activation of an NF-kappa B-like DNA-binding protein

Interleukin 1 (IL-1) induces the synthesis of kappa immunoglobulin light chains and the expression of surface immunoglobulin in the murine pre-B-cell line 70Z/3 (J. G. Giri, P. W. Kincade, and S. B. Mizel, J. Immunol. 132:223-228, 1984). In the present study, we found that these effects of IL-1 are mimicked by cyclic AMP (cAMP) analogs and cAMP-elevating drugs. The induction of kappa immunoglobulin light-chain gene expression by IL-1 was associated with an increase in intracellular cAMP levels. Incubation of 70Z/3 cells with IL-1 or cAMP resulted in the activation of the kappa immunoglobulin enhancer, as detected by the induction of chloramphenicol acetyltransferase (CAT) in cells transfected with a kappa enhancer-CAT expression plasmid. In contrast, CAT plasmids lacking a kappa immunoglobulin enhancer were inactive in the presence of IL-1 or cAMP. Furthermore, IL-1 and cAMP analogs and inducers were found to induce the activation of a NF-kappa B-like DNA-binding protein that exhibited specificity for the kappa immunoglobulin enhancer. These results suggest that cAMP may play an important role as a second messenger for IL-1 in the induction of kappa immunoglobulin light-chain synthesis in pre-B cells via the activation of a DNA-binding protein that is similar or identical to NF-kappa B.

Transient nuclear location of the IL-2 receptor during T cell activation

Binding of interleukin-2 (IL-2) to its membrane receptor (IL-2R) on target cells is followed by internalization of the IL-2R. The subsequent intracellular fate of IL-2R is not known. This paper describes the intracellular location of the p55 subunit of the IL-2R during IL-2 mediated T cell activation and growth of two mouse T helper clones. IL-2R was visualized by immunohistochemistry using two rat monoclonal antibodies (5A2 and 7D4). Immunostaining shows that the p55 subunit of the IL-2R is transiently present in the nucleus of activated T cells. The intranuclear location of the IL-2R suggests that the p55 subunit, either alone or in conjunction with the IL-2 or the p70 subunit, may be implicated in the regulation of gene expression involved in T cell proliferation.

The DNA synthesis of leukemic (L2C) guinea pig B lymphocytes involves a permanent activation of protein kinase C without corresponding phosphoinositide hydrolysis

L2C B lymphocytes have a constant high DNA synthesis due to their continuous proliferative state. The addition of polymyxin B (PmB), a rather selective inhibitor of protein kinase C, stopped (3H)thymidine incorporation with an IC50 of 10 microM when added 18 h before measuring DNA synthesis. Interestingly, PmB inhibition of DNA synthesis was suppressed when 4 nM 12-O-tetradecanoylphorbol-13-acetate was added along with PmB, indicating that PmB may act through inhibition of protein kinase C. In the node and spleen lymphocytes of normal guinea pigs, protein kinase C activity was entirely cytosolic and was eluted at 0.12 M NaCl when adsorbed on DEAE-cellulose. In L2C leukemic lymphocytes, total protein kinase C activity was of the same order of magnitude, but 20% of it was associated with the membrane fraction. The lipid-dependent activity, eluted at 0.12 M NaCl from cytosolic and membrane fractions, was suppressed by staurosporine with an IC50 of 10-40 nM and by polymyxin B with an IC50 of 2-6 microM. Phosphoinositide metabolism was studied in the transformed cells. Incorporation of 32Pi into polyphosphoinositides was considerable, whereas much more time was required for a tiny incorporation of inositol. We detected no release of radioactive inositol triphosphate. Taken together, these results suggest that protein kinase C function is indispensible for triggering L2C leukemic lymphocyte proliferation. The causes of this permanent activation merit further investigation.

Cholera toxin partially inhibits the T-cell response to phytohaemagglutinin through the ADP-ribosylation of a 45 kDa membrane protein

This study examines the influence of cholera toxin (CT) on T lymphocyte activation by the mitogenic lectin phytohaemagglutinin (PHA). CT suppressed lectin-induced [3H]thymidine uptake in a dose-dependent fashion and acted synergistically with PHA in the generation of intracellular cyclic AMP. The toxin was assumed to act on Gs, because it also stimulated ADP-ribosylation of a 45 kDa membrane protein in vitro; no additional substrates were seen. The inhibitory effect of the adenylate cyclase/cyclic AMP pathway was shown to be directed at a concomitant stimulatory pathway, namely inositol phospholipid turnover. Lectin-stimulated 32P incorporation into both phosphatidylinositol as well as its 4,5-biphosphate derivative was depressed in the presence of CT or exogenous dibutyryl cyclic AMP. This, in turn, was associated with reduced activation of C-kinase as determined by decreased lectin-induced translocation from the cytosol to the surface membrane. These results indicate that Gs probably acts as a transducer between the PHA receptor and adenylate cyclase and may give rise to an exaggerated adenylate cyclase response in the presence of CT. It would seem as if reduction in inositol phospholipid turnover is related to the elevation of cyclic AMP rather than a CT effect on a putative transducer which acts directly on phospholipase C. Our study does not exclude the existence of non-CT-sensitive transducers in this capacity.

Norepinephrine and serotonin content of the murine spleen: its relationship to lymphocyte beta-adrenergic receptor density and the humoral immune response in vivo and in vitro

Numerous reports in the literature describe the effects of beta-adrenergic agonists and/or their second messenger cyclic AMP on in vitro and in vivo immune responses. The fact that the murine spleen receives rich adrenergic innervation and that the pharmacologic disruption of this innervation leads to altered immune responsiveness has led some investigators to postulate that the immune system may be modulated in vivo by the sympathetic nervous system. In this report HPLC is used to quantitate the norepinephrine (NE) and serotonin (5-HT) found in the B6C3F1 spleen. These transmitters were found to be distributed nonhomogeneously among the cell, supernatant, and capsule fractions of the spleen. The majority of NE was found in the capsule while most 5-HT was found associated with the cell pellet. During an immune response to sheep red blood cells the concentration of NE in the spleen was found to be decreased. However, the total amount of splenic NE was unaltered and thus the decreased concentration may be attributed to the increased weight of immunized spleens. Simultaneously, the total amount of the dopamine metabolite 3,4-dihydroxyphenylacetic acid found in the spleen was found to be increased, a difference not explained by increased spleen size. These results suggest an antigen-induced increase in sympathetic activity in the spleen. Splenic NE could be rapidly depleted using 6-hydroxydopamine. Lymphocytes from the NE-depleted animals were found to have upregulated the number of beta-adrenergic receptors on their surfaces and demonstrated a reduced ability to respond to sheep red blood cells in vitro.

Cyclic AMP and phorbol ester-stimulated transcription mediated by similar DNA elements that bind distinct proteins

cAMP and phorbol esters mediate cellular metabolism by the activation of distinct signal transduction pathways consisting of a cascade of sequential protein phosphorylations. An important consequence of the activation of these pathways is the stimulation of gene transcription by way of interactions of specific proteins with DNA control elements. The 8-base-pair (bp) DNA consensus sequence TGACGTCA [cAMP response element (cAMP-RE)] has been shown to confer cAMP responsivity on transcription from various promoters, and the closely related 7-bp consensus sequence TGA-(C or G)TCA [phorbol 12-myristate 13-acetate response element (PMA-RE)] lends transcriptional responsiveness to phorbol esters. In the JEG-3 placental cell line we find that several variants of the cAMP-REs fused to a gonadotropin alpha promoter chloramphenicol acetyltransferase reporter gene mediate responsiveness to cAMP but not to phorbol esters. The PMA-RE is responsive to phorbol esters but also imparts submaximal sensitivity to cAMP in the JEG-3 cells and in the Hep G2 hepatoma cell line. The transcriptional activities of cAMP-RE and PMA-RE are markedly influenced by the composition of the neighboring bases, but different sequences are permissive for the activity of the cAMP-RE versus the PMA-RE. The two signaling agents together display a supraadditive effect on reporter genes containing active PMA-REs but not cAMP-REs. Gel-mobility-shift and UV cross-linking analyses show that distinct proteins bind to the two control elements. One protein of 38 kDa binds to the cAMP-RE and several proteins of 48-84 kDa bind to the PMA-RE.

Rapid activation of protein kinase C in isolated rat liver nuclei by prolactin, a known hepatic mitogen

Rat liver nuclei pure by enzymatic and electron microscope criteria contain protein kinase C (PKC) that can be activated several hundredfold within 3 min of addition of prolactin or phorbol 12-tetradecanoate 13-acetate. Rat prolactin stimulated PKC maximally at 10(-12) M, whereas ovine prolactin was maximally stimulatory at 10(-10) M. Activation was time and dose dependent, exhibited a biphasic pattern, and was blocked by anti-prolactin antiserum, by PKC inhibitors such as 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) and sphingosine, and by cyclosporine. Moreover, the ability of prolactin to activate nuclear PKC was inhibited totally by a monoclonal antibody to the rat liver prolactin receptor, implicating a prolactin receptor-mediated activation process. Epidermal growth factor (EGF), a liver mitogen, caused a lesser but significant activation of nuclear PKC. However, EGF and suboptimal prolactin were synergistic. Human growth hormone, which has lactogenic properties, stimulated PKC activity, whereas nonlactogenic substances such as ovine growth hormone, insulin, dexamethasone, and 8-bromo-cAMP were inactive. That this may be a general mechanism for prolactin is suggested by the ability of prolactin to stimulate PKC 140-fold in rat splenocyte nuclei. Prolactin has comitogenic properties in lymphocytes.

Insulin and glucose modulate protein kinase C activity in rat adipocytes

In the presence of 1 mM glucose, insulin (10 ng/ml) increases both catalytic and receptor-binding properties of adipocyte cytosolic protein kinase C (PKC). Preincubation of adipocytes with 10 mM glucose raises basal PKC catalytic activity and prevents further stimulation of this enzyme by insulin. The effect of hyperglycemia is likely to be mediated by direct conversion of glucose into diacylglycerol. Thus, an incorporation of 14C-glucose into diacylglycerol is enhanced 10-fold in the presence of 10 mM glucose. These observations indicate that, in normal adipocytes, both insulin and glucose activate PKC; hyperglycemia eliminates the ability of insulin to stimulate this enzyme, thereby interfering with insulin action.

DNA-binding proteins in protein kinase C preparations

Human DNA enriched in repetitive sequences specifically bound to a component(s) in purified preparations of rat brain protein kinase C (PKC). DNA which bound to protein was cloned in pUC-19 and one clone characterized as containing an approx. 140 bp insert. The band containing this insert (separated by acrylamide gel electrophoresis) was lost when the DNA was incubated with purified PKC preparations. Thus a protein in relatively pure PKC preparations is a sequence-selective DNA-binding protein. The results raise the possibility that PKC or a fragment of PKC binds selectively to specific DNA sequences.

Ligand-induced desensitization of B-cell membrane immunoglobulin-mediated Ca2+ mobilization and protein kinase C translocation

Binding of ligand to B-cell membrane immunoglobulin (mIg) can lead to activation of a number of distinct biologic responses, including altered expression of genes encoding c-fos, c-myc, and Ia, as well as proliferation and immunologic tolerance. Tolerance could reflect a functional uncoupling of receptors from systems that generate intracellular second messengers (i.e., receptor desensitization). To better understand the molecular basis of immune regulation, we examined the ability of mIg to function as a signal transducer after the cell's initial contact with mIg-binding ligand. The results show that ligand binding to as little as 2-10% of mIgM or mIgD renders the cell unresponsive to ligand binding to the reciprocal isotype as judged by Ca2+ mobilization and protein kinase C translocation responses. This heterologous receptor desensitization lasts longer than 24 hr and does not reflect loss of receptor from the cell surface. Studies with the calcium ionophore ionomycin, 1,2-dioctanoyl-sn-glycerol, and the protein kinase inhibitor staurosporine indicate that both protein kinase C-dependent and protein kinase C-independent (staurosporine-insensitive) mechanisms mediate heterologous desensitization after mIg crosslinking.

Signaling to a B-cell clone by Ek, but not Ak, does not reflect alteration of Ak genes

The mouse B-cell clone, CH12.LX (Iak, Ly-1+, mu+, delta+), can be induced to differentiate and secrete antibody in an antigen-specific, H-2-restricted manner. Induction requires two signals. One must be provided by the binding of specific antigen to the membrane IgM; the other is delivered by the binding of Ek-specific T-cell hybridomas to the Ek molecules of CH12.LX (Bishop and Haughton 1986). Previous studies demonstrated that Ek-specific monoclonal antibodies (mAbs) could substitute for T cells in delivering the second differentiative signal (Bishop and Haughton 1986). Although CH12.LX cells present Ak to Ak-restricted or alloreactive T-helper cells, neither T cells nor mAbs specific for Ak induce differentiation (Bishop and Haughton 1986). However, since the Akspecific mAbs tested previously were beta-chain-specific and the Ia epitope specificity of the T cells used was unknown, it is possible that the differentiative signal delivered to the CH12.LX class II molecule is chain-specific. Here we report the effects of ten additional Iak-specific mAbs upon the differentiation of CH12.LX. In addition, a cDNA library was prepared from CH12.LX cells, clones corresponding to the alpha and beta chains of the Ak molecule were isolated, and their nucleotide sequences were determined. Finally, the Ak and Ek molecules of CH12.LX and H-2k spleen cells were compared by two-dimensional gel electrophoresis to examine possible post-translational differences in the Iak molecules of CH12.LX.

Functional differences between immunoglobulins M and D expressed on the surface of an immature B-cell line

Crosslinked IgM molecules expressed on the surface of immature B cells mediate responses that inhibit further development, in contrast to the activational and proliferative events that follow crosslinking of the mu heavy chain in mature B cells. Concomitant with this change in IgM signaling capacity is the appearance of surface IgD, which has been proposed to modulate the response elicited by the mu heavy chain. In an attempt to gain insight into the mechanism(s) by which surface IgM is able to generate such disparate responses, delta heavy chain gene transfectants of the murine B-cell lymphoma line WEHI-231 were established. WEHI-231 cells resemble phenotypically immature B cells, in addition to being highly susceptible to the growth-inhibitory effect of surface IgM cross-linking. Endogenous mu and exogenous delta heavy chains expressed on the surface of the transfectants were compared for their role in cell proliferation and on gene expression. Our results indicate that the growth-inhibitory response is associated only with the mu heavy chain and that surface IgD does not mediate such a response. Furthermore, in contrast to IgM, IgD molecules appear to have an inductive effect on the expression of Myc and the endogenous mu and exogenous delta Ig heavy chain genes but not on the expression of the housekeeping gene encoding beta 2-microglobulin. These findings suggest that IgM and IgD are functionally distinct when expressed on the surface of an immature B cell.

Cyclic AMP has the ability to influence multiple events during B cell stimulation

Negative regulators of cellular proliferation are important in maintaining a balanced growth control. In this study we have examined the effects of the diterpene forskolin on various parameters of B cell activation. Forskolin is known to elevate intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels and thereby to influence B cell stimulation. We found that forskolin exerted an inhibitory effect on early as well as late events during stimulation of resting normal human B cells. Cells were activated either by antibodies to surface immunoglobulins (anti-mu), by the monoclonal antibody 1F5 reactive with the CD20 antigen or by 12-O-tetradecanoylphorbol 13-acetate. While anti-mu stimulation induces increased phosphatidylinositol (PI) turnover and [Ca2+]i fluxes, the latter two reagents confer an activation of B cells independent of the PI/Ca2+ pathway. We found a clear inhibitory effect of forskolin on the anti-mu-induced PI turnover and [Ca2+]i fluxes as well as on later parameters of cell activation. There was also a clear inhibition of G1 entry and DNA synthesis when PI/Ca2+-independent activation was employed, indicating that cAMP interferes with B lymphocyte stimulation in several ways. Importantly, forskolin maintained its inhibitory effect when added late after anti-mu stimulation, implying an effect also at multiple stages of activation. When examining the inhibitory effect of forskolin on neoplastic B cells, we found essentially no differences from the responses in normal cells.

Phosphorylation-induced binding and transcriptional efficacy of nuclear factor CREB

A nuclear protein, CREB, has been isolated from rat brain and shown to stimulate transcription of the cyclic AMP-responsive gene somatostatin as a dimer. Biochemical analysis suggests that dimerization and transcriptional efficacy of CREB protein in vitro are regulated by phosphorylation. These findings demonstrate that cellular signals can modulate gene expression by regulating the covalent modification of pre-existing nuclear factors.

Membrane receptors in the gastrointestinal tract

This review focusses on the roles that membrane receptors and their transducers play in the physiology and pathology of the gastrointestinal tract. The multifactorial regulation of [correction] mucosal growth and function is discussed in relation to the heterogeneity of exocrine and endocrine populations that originate from progenitor cells in stomach and intestine.

Nuclear location of the p55 subunit of the IL-2 receptor following activation of the HT-2 T helper cell line

After mitogenic or antigenic stimulation, T cells express interleukin-2 receptors (IL-2R). The mechanism and control of signal transduction following binding of IL-2 to IL-2R are poorly understood. Using two rat monoclonal antibodies (5A2 and 7D4) specific for two distinct epitopes of the p55 subunit of mouse IL-2R, we have studied the cellular localization of this molecule by immunocytochemistry during the IL-2-mediated activation of mouse T helper cell clone HT-2. During the activation cycle, nuclear staining for the p55 subunit of the IL-2 receptor was transiently observed. It is suggested that the transient nuclear location of the IL-2R may play a critical role in the control of T-cell activation, proliferation and/or differentiation.

Cooperative interactions of protein kinase C and cAMP-dependent protein kinase systems in human promyelocytic leukemia HL60 cells

Interactions of protein kinase C (PKC) and cAMP-dependent protein kinase (PKA) systems were investigated in HL60 cells. It was found that the differentiating effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) were potentiated by dibutyryl cAMP (dbcAMP) or prostaglandin E2 (PGE2). In addition, dbcAMP or PGE2 inhibited TPA-induced binding of PKC to plasma membrane, leading to decreased protein phosphorylation, and promoted subsequent redistribution of enzyme to the nuclear membrane region. The findings are consistent with the hypothesis that PKC and PKA systems regulate cooperatively the phenotypical differentiation of leukemic cells.

Nuclear protein kinase substrates in AtT-20 cells: translocation of a 14 kDa phosphoprotein

In this work we characterize some acidic nuclear substrates of protein kinase C (PKC) and cyclic AMP-dependent protein kinase (PKA), using intact anterior pituitary corticotrophic tumor cells (AtT-20/D16-16). It was found that, as in the cytosolic fraction, substrates for both PKC and PKA exist in the nucleus and that changes in the phosphorylation states of a few of these phosphoproteins are mediated by both kinases. One of the phosphoproteins examined, a 14 kDa phosphoprotein (pp14) described previously, exhibited a phorbol-ester induced translocation from nucleus to cytosol in pulse-chase experiments utilizing 35S-methionine labeling. These results suggest that pp14 may be involved in signal transduction in AtT-20 cells. Although its identity remains to be determined, a 14 kDa DNA-binding protein was also seen in nuclear extracts of AtT-20 cells.