Complexity of the primary genetic response to mitogenic activation of human T cells

Binding and subcellular distribution of cyclosporine in human fibroblasts

The uptake, binding, and subcellular sites of accumulation of [3H]-cyclosporine (CS) in two human gingival fibroblast strains, GN 23 and GN 54, have been examined. GN 23 responds to CS treatment with a decrease in collagenolysis, while GN 54 does not. Binding of the drug was determined using [3H]-CS concentrations ranging from 10(-5) to 10(-8) M in the absence or presence of excess unlabeled CS (1 mM). The binding of the drug to both strains was specific and reached a plateau within 10 min, remaining at that level for up to 1 h. Scatchard analysis of the specific binding of [3H]-CS to the responsive GN 23 strain revealed two dissociation constants: KD = 5 x 10(-8) M (1.2 x 10(7) sites/cell) and KD = 1.4 x 10(-6) M (2.2 x 10(8) sites/cell). GN 54, on the other hand, had only one class of low affinity binding site (KD = 0.47 x 10(-6) M [1.2 x 10(8) sites/cell]). Unlabeled CS (0.01-1 mM) inhibited the binding of [3H]-CS in a dose-dependent manner to both strains, as did the calmodulin antagonist W-7, to a lesser extent. However, W-7 inhibited CS binding much more efficiently in GN 54 than in GN 23, suggesting that calmodulin may be the predominant CS receptor in GN 54. In both strains, 70% of the drug accumulated in the crude nuclear fraction after a 1 min incubation, with very little (< or = 4%) being membrane associated, and the remainder was in the cytosol.(ABSTRACT TRUNCATED AT 250 WORDS)

Antigen and helper T lymphocytes activate B lymphocytes by distinct signaling pathways

Resting murine B lymphocytes can be induced to proliferate by cross-linking membrane immunoglobulin, the antigen receptor, or by contact with activated helper T lymphocytes in the absence of a signal through membrane immunoglobulin. Little is known about the molecular nature of contact-dependent T cell help. To determine whether helper T cells activate B cells through different signal transduction and second messenger pathways from those used by membrane immunoglobulin, the effects of drugs which block activation of B cells through membrane immunoglobulin were measured on B cell activation by contact with anti-CD3-activated and fixed T helper cells. Cyclosporin A, phorbol esters added at the time of activation, and cAMP agonists all block activation of B cells through membrane immunoglobulin at concentrations at least 100-fold lower than those necessary to block B cell activation by contact with activated Th1 or Th2 helper T cells. Depletion of protein kinase C by pretreatment of B cells with phorbol ester inhibits the proliferative response to anti-immunoglobulin but not the response to contact with activated T cells. The B cell response to lipopolysaccharide is intermediate in sensitivity to cyclosporin A and cAMP agonists, and resembles the response to activated T cells in resistance to phorbol esters and protein kinase C depletion. Various protein kinase inhibitors did not distinguish among these B cell activation pathways, except for the tyrosine kinase inhibitor, herbimycin A, which inhibited anti-immunoglobulin responses at 3- to 5-fold lower concentrations.

A human putative lymphocyte G0/G1 switch gene homologous to a rodent gene encoding a zinc-binding potential transcription factor

G0S24 is a member of a set of genes (putative G0/G1 switch regulatory genes) that are expressed transiently within 1-2 hr of the addition of lectin or cycloheximide to human blood mononuclear cells. Comparison of a full-length cDNA sequence with the corresponding genomic sequence reveals an open reading frame of 326 amino acids, distributed across two exons. Potential phosphorylation sites include the sequence PSPTSPT, which resembles an RNA polymerase II repeat reported to be a target of the cell cycle control kinase cdc2. Comparison of the derived protein sequence with those of rodent homologs allows classification into three groups. Group 1 contains G0S24 and the rat and mouse TIS11 genes (also known as TTP, Nup475, and Zfp36). Members of this group have three tetraproline repeats. Groups 1 and 2 have a serine-rich region and an "arginine element" (RRLPIF) at the carboxyl terminus. All groups contain cysteine- and histidine-rich putative zinc finger domains and a serine-phenylalanine "SFS" domain similar to part of the large subunit of eukaryotic RNA polymerase II. Comparison of group 1 human and mouse genomic sequences shows high conservation in the 5' flank and exons. A CpG island suggests expression in the germ line. G0S24 has potential sites for transcription factors in the 5' flank and intron; these include a serum response element. Protein and genomic sequences show similarities with those of a variety of proteins involved in transcription, suggesting that the G0S24 product has a similar role.

The candidate oncoprotein Bcl-3 is an antagonist of p50/NF-kappa B-mediated inhibition

The candidate oncogene bcl-3 was discovered as a translocation into the immunoglobulin alpha-locus in some cases of B-cell chronic lymphocytic leukaemias. The protein Bcl-3 contains seven so-called ankyrin repeats. Similar repeat motifs are found in a number of diverse regulatory proteins but the motifs of Bcl-3 are most closely related to those found in I kappa B proteins in which the ankyrin repeat domain is thought to be directly involved in inhibition of NF-kappa B activity. No biological function has yet been described for Bcl-3, but it was noted recently that Bcl-3 interferes with DNA-binding of the p50 subunit of NF-kappa B in vitro. Here we demonstrate that Bcl-3 can aid kappa B site-dependent transcription in vivo by counteracting the inhibitory effects of p50/NF-kappa B homodimers. Bcl-3 may therefore aid activation of select NF-kappa B-regulated genes, including those of the human immunodeficiency virus.

Induction of antigen-specific tolerance by cyclosporin A

Recently, we reported preliminary evidence for the induction of tolerance in vivo by cyclosporin A (CSA) during a persistent virus infection in rats. In the present communication, those observations are verified and the findings extended to the functional level of cell-mediated immunity. Mice infected intracerebrally with lymphocytic choriomeningitis virus (LCMV) normally die from a fatal immune-mediated disease after 6-8 days but they do not succumb if treated intraperitoneally with 50 mg/kg/day of CSA. Immunosuppression initiated one day before infection and continued for at least two consecutive weeks resulted in the absence of immunopathologic disease of the brain and in the survival of mice, which were found to be persistently infected virus carriers. In these animals, no cytotoxic T cell activity could be detected. The effect of CSA was not due to a toxic effect on the immune response since immune reactivity was restored as early as 4 days after discontinuation of the drug in control animals. Neither secondary in vitro nor in vivo restimulation resulted in the generation of a cellular antiviral immune response. Cytotoxic T cell reactivity to third-party antigen, however, could be detected, although somewhat delayed. Additionally, spleen cells from CSA-treated mice did not clear the virus from LCMV-infected recipients upon adoptive transfer, whereas spleen cells from LCMV immune mice completely eliminated virus infection in carrier mice. However, mice immunosuppressed with CSA and infected with vesicular stomatitis virus (VSV) did not generate a primary immune response but were immunologically fully reactive to challenge infection, providing evidence for the absence of tolerance and the presence of antigen-specific temporal unresponsiveness. Thus, as exemplified by VSV infection in which the virus does not replicate to considerable titers in mice and viral antigens do not persist, the presence of the foreign antigen for prolonged periods of time could be shown to be a conditio sine qua non for CSA-induced tolerance.

Differential effects of cyclosporin A on diverse B cell activation phenomena triggered by crosslinking of membrane IgM

Cyclosporin A (CsA) was tested for its modulatory effects on the mIgM-mediated signaling of G0*-associated increases in class II MHC expression, G1-related RNA synthesis, and S phase-related DNA synthesis in human B cells. While CsA at concentrations as low as 10-100 ng/ml could completely ablate anti-IgM-induced DNA synthesis, earlier G1-associated RNA synthesis was only partially inhibited, and signaling of increased membrane class II MHC expression was unaffected by up to 1000 ng/ml of CsA. Similar phenomena were observed in a clonal population of leukemic B lymphocytes susceptible to anti-IgM-mediated activation in the absence of T cells and T cell factors indicating (a) that the inhibitory effects are not due to CsA-mediated suppression of cytokine production by contaminating T cells, and (b) that the varying effects of CsA on the diverse activation phenomena do not reflect B cell subpopulation diversity. Pulsing studies revealed that while maximal suppression of anti-IgM-induced G1-associated RNA synthesis required CsA at culture initiation, near maximal suppression of DNA synthesis occurred when CsA, or soluble human IgM, was added up to 30 hr after the initial exposure of resting B cells to the anti-IgM ligand. These latter findings are consistent with the possibility that the CsA-mediated suppression of S phase entry is due to the inhibition of a signaling event proximal to mIgM ligation which must be repeatedly initiated throughout the first 30 hr of activation.

SRC-related proto-oncogenes and transcription factors in primary human T cells: modulation by cyclosporin A and FK506

Activation of T lymphocytes induces transcription of genes encoding for lymphokines. Interleukin-2 (IL-2) gene expression is controlled transcriptionally by the cooperative activity of specific trans-activating factors that bind to the IL-2 enhancer. Cyclosporin A (CsA) and FK506 inhibit the production of IL-2 in T lymphocytes at the level of gene transcription. A member of the src gene family, the lymphocyte-specific protein tyrosine kinase, p56lck, has been implicated in IL-2 production. CsA was found not to inhibit lck gene expression, nor the activity of the lck gene product. However, CsA and FK506 inhibit the appearance of DNA binding activity of factors that bind to the NF-AT and AP-1 sites in the IL-2 enhancer. Since the induction of NF-AT and AP-1 is induced by the same stimuli that stimulate IL-2 production, these results indicate that the immunosuppressant action of CsA and FK506 is exerted at the level of these trans-activating factors.

B cell lymphoma-associated chromosomal translocation involves candidate oncogene lyt-10, homologous to NF-kappa B p50

A B cell lymphoma-associated chromosomal translocation, t(10;14)(q24;q32), juxtaposes the immunoglobulin C alpha 1 locus to a novel gene, lyt-10. The normal lyt-10 cDNA codes for a 98 kd protein which displays amino-terminal homology with the rel (DNA-binding) domain of the NF-kappa B-rel family of transcription factors and carboxy-terminal homology with the NF-kappa B p50 precursor protein, including the putative proteolytic cleavage domain (poly-G) and the ankyrin-like repeat domains. The lyt-10 protein can bind to kappa B sequences in vitro, although with different specificity from NF-kappa B p50, and in vitro DNA-binding is activated by removal of the ankyrin domain. Chromosomal translocation generates an lyt-10-C alpha 1 fusion gene coding for a protein that retains the rel effector domain, lacks the ankyrin regulatory domain, and binds kappa B sequences in vitro, suggesting its constitutive activation in vivo. Analogous rearrangements of the lyt-10 gene have been found in an additional three cases of lymphoid neoplasia. The lyt-10 gene defines a new subfamily (rel/poly-G/ankyrin) of NF-kappa B-rel transcription factors with potential for oncogenic activation in human cancer.

Post-transcriptional regulation of the transferrin receptor and 4F2 antigen heavy chain mRNA during growth activation of spleen cells

The expression of several cell surface proteins including the transferrin receptor and 4F2 antigen is induced when quiescent cells are activated and proliferate. We have studied this induction in mouse spleen cells after stimulation with 4 beta-phorbol 12-myristate 13-acetate (PMA), the Ca(2+)-ionophore, ionomycin and recombinant interleukin-2 (IL-2). The 4F2 antigen heavy chain and transferrin receptor mRNA were barely detectable in resting cells, but increased 60-fold within 4 h of growth stimulation. The corresponding proteins became measurable at the cell surface after 6 h, prior to the S phase. In run-on transcription assays the transferrin receptor gene was transcribed to almost the same extent in resting and growth-stimulated cell populations and the 4F2 antigen heavy chain gene was induced fivefold. This suggests that post-transcriptional control mechanisms are mainly responsible for the accumulation of the respective mRNA at the onset of cell proliferation. In the case of the transferrin receptor, the induction correlated with an activation of the mRNA-binding iron-regulatory factor which is known to increase the stability of the cytoplasmic transferrin receptor mRNA.

Inhibitory effect of cyclosporin A on antigen and alloantigen presenting capacity of human epidermal Langerhans cells

The effect of cyclosporin A (CyA) on the capacity of human epidermal Langerhans cells (LC) to stimulate allogeneic T cells or to present antigen to autologous T cells was investigated. Preparations of LC enriched by discontinuous density gradient centrifugation were pulsed for 2 or 16 h with graded doses (5-5000 ng/ml) of CyA prior to co-culture with T cells. Pretreatment of LC with CyA resulted in a dose-dependent decrease of the functional capacity of LC to stimulate T cells. This inhibition (up to 90%), already achieved after a pulse of 2 h, was not due to a cytotoxic effect of the drug and appeared to be reversible. The possibility that CyA exerted its effect indirectly on T cells via release of CyA from LC into the supernatant during co-culture was excluded. The suppression of immunostimulatory function was a direct effect of the drug on LC. CyA did not affect the production by LC of IL-1 or prostaglandin, nor the expression of MHC class II products HLA-D and RFD1 or adhesion molecules ICAM-1 and LFA-3. These results suggest that inhibition of contact allergic skin reactions by CyA may be due in part to an impairment of the function of LC.

Signal transduction in T cells

Rapid progress was made during the past year in the delineation of the nature of the initial biochemical events triggered by the T-cell antigen receptor. Antigen-mediated activation of phospholipase C was demonstrated to require protein tyrosine phosphorylation and, most surprising, activation of the Ras family of signal transduction molecules was shown to closely follow stimulation of the T-cell antigen receptor. Major controversy continues over which events are relevant to the various effector functions of T cells.

Evidence for protein kinase C--independent pathways mediating phorbol ester induced plasmacytoid differentiation of B chronic lymphocytic leukemia cells

The effects of phorbol esters on many cell types are known to be mediated through activation of the protein kinase C (PKC) signal transduction pathway. By using the specific inhibitor of this enzyme 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine dihydrochloride (H7) we have assessed the role of PKC activation in phorbol ester (phorbol 12-myristate 13-acetate, PMA)-induced plasmacytoid differentiation of B chronic lymphocytic leukemia cells (B-CLL) as a model of terminal differentiation of human B lymphocytes. H7 affected a dose-dependent inhibition of PMA-induced thymidine and uridine uptake with ID50 values of 41 microM and 32 microM, respectively. A comparable ID50 value (34 microM) was obtained for H7 inhibition of B-CLL PKC activity in a cell-free system. PMA-induced changes in cell morphology, expression of CD20, CD37 and FMC7 surface antigens together with increased secretion of immunoglobulin were variably abrogated by H7 suggesting that PKC activation is more important in B cell activation/DNA synthesis than in the differentiative response. Consistent with this, expression of a sizable proportion of PMA-inducible genes was not significantly affected by H7. These data are consistent with the existence of a PMA-activated, PKC-independent signal transduction pathway which may be important, though by itself apparently insufficient, for eliciting full terminal differentiation in B lymphocytes.

A new superfamily of lymphoid and melanoma cell proteins with extensive homology to Schistosoma mansoni antigen Sm23

A novel cDNA clone termed R2 was isolated by subtractive hybridization of a cDNA library of phytohemagglutinin (PHA)/phorbol myristate acetate-stimulated Jurkat cells and by rescreening a cDNA library of PHA-stimulated peripheral blood lymphocytes. It hybridizes to a single mRNA species of about 2.2 kb, which is inducible in lymphoid cells and codes for a protein of 267 amino acids which contains four potential transmembrane domains. A computer-aided comparison showed strong homology to four other membrane proteins, the pan B cell marker CD37, the pan leukocyte marker CD53, the melanoma antigen ME491 and, surprisingly, the Schistosoma mansoni antigen Sm23. The four human proteins share a number of additional similarities in their overall structure. These include identical spacing of the transmembrane domains, similar hydrophobicity plots, possible N-linked glycosylation sites of similar number and position as well as similar distribution of the cysteine residues. The majority of these characteristics are still conserved in the evolutionary most distant member of this family, the Schistosoma mansoni antigen Sm23. Here we introduce this new protein superfamily and characterize the inducible, lymphoid-specific member R2.

Cloning of a mitogen-inducible gene encoding a kappa B DNA-binding protein with homology to the rel oncogene and to cell-cycle motifs

We have cloned and characterized a mitogen-inducible gene isolated from human T cells that predicts a protein of 968 amino acids. The amino-terminal domain has regions homologous to the oncogene rel and to the developmentally important gene dorsal of Drosophila. The carboxy-terminal domain contains repeat structures found in a variety of proteins that are involved in cell-cycle control of yeast and in tissue differentiation in Drosophila and Ceanorhabditis elegans, as well as in the putative human oncogene bcl-3 and in the ankyrin protein. A truncated form of the product of this gene translated in vitro is a DNA-binding protein which interacts specifically with the kappa B binding site found in many inducible genes, including the enhancer in human immunodeficiency virus. This gene is yet another in a growing list of important regulatory molecules whose expression is transcriptionally induced upon cellular activation.

A set of human putative lymphocyte G0/G1 switch genes includes genes homologous to rodent cytokine and zinc finger protein-encoding genes

Lymphocyte G0/G1 switch genes (G0S genes) are potential oncogenes and may regulate, be regulated by, or be coordinately regulated with, latent lymphotropic viruses. To identify these genes, a cDNA library was prepared from blood mononuclear cells that had been cultured for 2 hr with a T-cell mitogen (lectin) and cycloheximide. Eight differentially hybridizing recombinants were characterized by RNA and DNA blotting and sequencing. One cDNA (G0S7) corresponded to the oncogene c-fos. Another cDNA (G0S19) was homologous (70%) to a cDNA encoding a murine inhibitor of stem cell proliferation (the cytokine MIP1 alpha) and, less closely, to other members of the "small inducible" secreted protein-encoding gene family. Whereas cDNA hybridization to genomic DNA blots indicated a small subfamily of G0S19 genes, simple patterns of bands indicated that most cDNAs, including G0S30 cDNA, corresponded to single-copy genes. The 3' noncoding sequence of G0S30 cDNA was homologous (87-89%) to the 3' noncoding sequences of certain rodent genes (NGFI-A, Krox24, EGR1) that encode zinc finger proteins (putative transcriptional regulators). This degree of evolutionary conservation suggests an important function for the 3' noncoding region. The 3' noncoding regions of some cDNAs contained the TTATTTAT (mRNA destabilization) element. The corresponding RNAs each formed doublets in agarose gels. Previous studies of c-fos RNA from HeLa cells indicate that this is due to cycloheximide-dependent stabilization of poly(A) tails. Our results reveal the power of cycloheximide enrichment in isolating what would appear to be significant low-abundance mRNAs.

Cell-type-specific early response gene expression during plasmacytoid differentiation of human B lymphocytic leukemia cells

Monoclonal B lymphocytes from B cell chronic lymphocytic leukemia (B-CLL) can be induced to undergo plasmacytoid differentiation in vitro by the phorbol ester, phorbol 12-myristate 13-acetate (PMA). By differential screening of a cDNA library derived from cells treated with phorbol ester we have isolated and characterised a set of early response genes (ERGs) displaying rapid transient up-regulation of expression in response to PMA. Cross-hybridisation studies showed that PMA probably induces the expression of over one hundred distinct genes, implying an ERG complexity comparable to that activated by mitogenic stimulation of fibroblasts and normal T lymphocytes. Of 13 genes analysed in detail, most were induced by PMA without a requirement for de novo protein synthesis, whilst nuclear run-on analysis showed that at least some of the more abundant classes of ERG were up-regulated through transcriptional mechanisms. In a proliferating variant B-CLL population, few differences in ERG expression were seen, suggesting that these genes are part of a gene regulatory pathway coupled to the differentiative rather than the proliferative response of B-CLL cells. However, studies in a range of cell types revealed a surprisingly diverse pattern of PMA-induced expression where most ERGs were relatively B-CLL-specific. This implies an extreme diversity of gene regulatory pathways activated in the primary response by phorbol ester generally and suggests that the onset of PMA-induced plasmacytoid differentiation of B-CLL cells is preceded by activation of a complex gene regulatory program that is largely unique to this maturation-arrested B cell.

Molecular oncogenetics of metastasis

Metastasis is a complex non-stochastic process that is most likely the result of genetic and epigenetic interactions of a wide variety of genes. The search for a single gene which can encompass such a pleiotropic response as to account for the observed phenotypic characteristics of metastatic tumour populations has been unsuccessful. Particular studies involving gene transfection, subtractive hybridisation and cell fusion are beginning to identify specific genes which contribute to metastasis in some cell types. However, such analyses are complicated by the inherent genetic instability and phenotypic heterogeneity present in tumour populations. A more detailed understanding of the metastatic process may require an abandoning of current generalised approaches to metastasis in favour of concentrating on key components of the metastatic cascade such as adhesion and invasion.

Oncogenes, growth, and the cell cycle: an overview

In spite of the complexity of the network of regulatory factors which control the balance between the cell cycle and quiescence, a picture is emerging, if only in outline. Several dozens of protooncogenes participate in growth signal transduction and integration, and, when expressed inappropriately, generate growth signals that may override other cellular controls. Some of these controls are provided by the negatively regulating growth factors, and when these are lost (e.g. by chromosomal deletion), or inactivated (e.g. by binding to an inactive analogue or a DNA viral oncoprotein), cell cycle activity is favoured over quiescence. Embryonic tissues are rapidly growing, so their cells are actively cycling and expression of proto-oncogenes is usually observed (Schuuring et al., 1989). As embryonic and stem cells in adult tissues mature, expression of the active proto-oncogenes is generally lost, but other proto-oncogenes may now be expressed (e.g. Muller et al., 1982). These changes in proto-oncogene expression are not achieved by modulation of transcriptional rates alone; transcriptional attenuation, message processing and stability, and post-translational protein modifications are all known to be important for the regulation of proto-oncogene expression during the transition from growth to the differentiated state. When quiescent cells re-enter the cell cycle approximately 60 genes become up-regulated, including proto-oncogene c-fos, the jun family, and c-myc (Zipfel et al., 1989). Evidence is strong that fos and jun proteins are transcriptional regulators. Terminal differentiation, on the other hand, is sometimes accompanied by the up-regulation of the ras gene family, as well as of several other proto-oncogenes. Proto-oncogene function is essential to the cell cycle traverse, but the genes involved are different in various cell types, and the precise order of oncogene expression may not turn out to be important. This is because cell cycle traverse appears to be more dependent on a critical threshold of growth signals propagated by parallel pathways, rather than on a strict order of predetermined steps. The participation of proto-oncogenes in growth signal transduction offers opportunities for errors, and abnormal growth may result from aberrant oncogene products generating a persistent or excessive growth signal, which shifts the balance of input to the integrating genes from quiescence to an active cell cycle. Thus, cancer may result from an entirely normal processing of growth signals that are abnormal.(ABSTRACT TRUNCATED AT 400 WORDS)