Stimulation of the antigen receptor complex leads to transcriptional activation of the c-myc gene in normal human T cells.
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Ethers↗
Biomedical subjects
C B Thompson
Publications and source records attributed to C B Thompson.
Sense and anti-sense transcription in the murine c-myb attenuator region.
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Animals↗
Isolation and structural characterization of the human 4F2 heavy-chain gene, an inducible gene involved in T-lymphocyte activation.
The human 4F2 cell surface antigen is a 120-kilodalton (kDa) disulfide-linked heterodimer which is composed of an 80- to 90-kDa glycosylated heavy chain (4F2HC) and a 35- to 40-kDa nonglycosylated light chain (4F2LC). 4F2 belongs to a family of inducible cell surface molecules which are involved in T-lymphocyte activation and growth. To better understand the molecular mechanism(s) that controls 4F2HC gene expression in both resting and activated T cells, a 4F2HC human genomic clone was isolated and structurally characterized. The 4F2HC gene spans 8 kilobases of chromosome 11 and is composed of nine exons. The 5' upstream region of the gene displays several properties which are characteristic of housekeeping genes. It is G+C rich and hypomethylated in peripheral blood lymphocyte DNA and contains multiple binding sites for the Sp1 transcription factor while lacking TATA or CCAAT sequences. This region of the gene also displays sequence homologies with several other inducible T-cell genes, including the interleukin-2, interleukin-2 receptor alpha chain, dihydrofolate reductase, thymidine kinase, and transferrin receptor genes. A 255-base-pair fragment of the 4F2HC gene which contains 154 base pairs of the 5' flanking sequence was able to efficiently promote expression of the bacterial chloramphenicol acetyltransferase gene in human Jurkat T cells, indicating that it contains promoter or enhancer (or both) sequences. Analyses of chromatin structure in resting and lectin-activated T cells revealed the presence of stable DNase I-hypersensitive sites within both the 5' flanking and intron 1 regions of the 4F2HC gene. Although the 4F2HC gene displayed many of the structural features characteristic of a constitutively expressed gene, lectin-mediated activation of resting peripheral blood T lymphocytes resulted in a dramatic increase in steady-state levels of 4F2HC mRNA.
Amino Acid Sequence↗
Regulation of 4F2 heavy-chain gene expression during normal human T-cell activation can be mediated by multiple distinct molecular mechanisms.
The 4F2 molecule belongs to the set of cell surface antigens which is induced following lectin- or antigen-mediated T-cell activation. The increase in 4F2 cell surface expression following lectin-mediated stimulation has been shown to be accompanied by a parallel increase in the steady-state levels of 4F2 heavy-chain (4F2HC) mRNA. The studies described in this report were designed to further elucidate the molecular mechanisms responsible for induction of 4F2HC gene expression following activation of normal resting human peripheral blood T cells. The low levels of mature 4F2HC mRNA in resting T cells were shown to be the result of a block to transcription elongation within the exon 1-intron 1 region of the 4F2HC gene rather than promoter inactivity. Phorbol myristate acetate stimulation of resting T cells resulted in a 20-fold increase in steady-state 4F2HC mRNA levels which was mediated by removal of this block to transcription elongation. The phorbol myristate acetate-induced increase in 4F2HC gene expression is distinct from previously described AP-1-mediated, phorbol ester-induced gene expression in that it requires new protein synthesis. Treatment of resting T cells with ionomycin plus PMA resulted in a 60-fold increase in 4F2HC mRNA levels. This induction was mediated by both an increase in promoter utilization and removal of the block to transcription elongation. Finally, by increasing the half-life of 4F2HC mRNA, cycloheximide treatment of resting T cells induced an approximately five fold increase in the levels of 4F2HC gene expression, although the physiologic significance of this mechanism remains unclear. These results demonstrate that the level of 4F2HC gene expression in normal peripheral blood T cells can be regulated by at least three distinct molecular pathways: (i) changes in promoter utilization, (ii) modulation of a block to transcription elongation, and (iii) alteration in mRNA stability.
Antigens, Surface↗
The pathophysiology and clinical relevance of platelet heterogeneity.
Recent studies on platelet heterogeneity support the hypothesis that platelet production is regulated to maintain a constant functional platelet mass. In concept this form of regulation is analogous to the manner by which RBC production is controlled to maintain the oxygen-carrying capacity of blood. The platelet mass appears to correlate more closely with platelet function than the platelet count alone, since several factors in addition to the platelet count have been shown to influence the platelets' hemostatic function. These factors include platelet size, density, age, and previous hemostatic interactions. Application of these concepts to clinical problems has provided important insights into platelet physiology and reactivity. Failure to account for differences in platelet heterogeneity among individuals may introduce significant errors in the interpretation of data from laboratory and clinical investigations. However, despite advances, a number of practical issues remain to be resolved before measurements of platelet heterogeneity become accepted as routine clinical tests and are used in the diagnosis of pathologic states.
Animals↗
The effect of alterations in myc gene expression on B cell development in the bursa of Fabricius.
Infection of 18-day embryonic bursal lymphocytes with a v-myc-containing retrovirus leads directly to a polyclonal proliferation of surface immunoglobulin-positive (slg+) cells in the bursa of Fabricius detected four weeks after hatching. These v-myc-expressing bursal cells repopulate the follicles of chemically ablated bursae more efficiently than total normal 18-day embryonic bursal cells. In contrast, comparable normal bursal cells lose the ability to repopulate follicles by four weeks. Bursal lymphocytes expressing either a retroviral v-myc or a c-myc gene deregulated by adjacent retroviral integration retain the ability of embryonic bursal lymphocytes to diversify their immunoglobulin light chain genes. These results suggest that retroviral deregulation of myc expression during avian B cell development induces outgrowth of a population of cells with the cardinal phenotypic characteristics of bursal stem cells.
Animals↗
Differential expression of c-myb mRNA in murine B lymphomas by a block to transcription elongation.
Expression of c-myb proto-oncogene messenger RNA (mRNA) and protein has been detected principally in tumors and in normal tissue of hematopoietic origin. In each hematopoietic lineage examined, expression of the c-myb gene is markedly downregulated during hematopoietic maturation. However, the mechanism by which differential expression of the c-myb gene is regulated is not known. In murine B-lymphoid tumor cell lines, the amount of steady-state c-myb mRNA is 10 to more than 100 times greater in pre-B cell lymphomas than in B cell lymphomas and plasmacytomas. The downregulation of c-myb mRNA correlates with events at the pre-B cell-B cell junction. Differential expression of c-myb mRNA levels detected between a pre-B cell lymphoma and a mature B cell lymphoma is now shown to be mediated by a block to transcription elongation in the first intron of the c-myb locus. In addition, this developmentally regulated difference in transcriptional activity is correlated with alterations in higher order chromatin structure as reflected by changes in the patterns of hypersensitivity to deoxyribonuclease I at the 5' end of the c-myb transcription unit. Regulation of transcription elongation may provide a more sensitive mechanism for rapidly increasing and decreasing mRNA levels in response to external stimuli than regulation of the initiation of transcription.
Animals↗
Biotransformation of the antiestrogen clomiphene to chemically reactive metabolites in the immature female rat.
Novel metabolites of clomiphene (CLO), an antiestrogen effective in experimental breast cancer models, were characterized in studies using immature female rats. After i.p. administration, CLO was eliminated unchanged in feces and as two components which were chromatographically identical to synthetic CLO analogues bearing a m-methoxy-p-hydroxyphenyl (guaiacol) moiety in place of one or the other of its phenyl rings. These components were also found in liver tissue. In the presence of liver microsomal homogenate, CLO underwent p-hydroxylation of either of its phenyl rings, affording 4-hydroxy-CLO and 4'-hydroxy-CLO. These in turn underwent liver microsomal mediated conversion to the respective guaiacol metabolites. 4'-Hydroxy-CLO and its 3'-methoxy analogue, but not positional isomers of these, had arylating ability as shown by chemical and spectral studies, apparently due to spontaneous conversion to electrophilic allene-quinones. Reproductive tract abnormalities produced in neonatal rats by CLO were suggested not to be mediated via such metabolites, since similar such effects were caused by 4'-fluoro-CLO. However, the latent arylating potential of the 4'-hydroxylated metabolites of CLO suggests that these compounds may be useful in biochemical studies of breast cancer cell growth inhibition.
Age Factors↗
Somatic diversification of the chicken immunoglobulin light chain gene is limited to the rearranged variable gene segment.
Previous studies have shown that the chicken lambda immunoglobulin light chain gene undergoes a single rearrangement that results in functional VJ joining of the unique variable (V lambda 1) and joining (J lambda) coding regions. The immunologic repertoire of lambda genes is created through extensive sequence diversification within the rearranged locus during B cell development in the bursa of Fabricius. This sequence diversification was detected only at the rearranged V lambda 1 segment and not within the 5' leader sequence, the J lambda segment, or the unrearranged V lambda 1 segment. The selective diversification of the rearranged V lambda 1 segment was associated with unique DNAase I-hypersensitive sites on the rearranged allele. While probes for V lambda 1 sequences detect multiple homologous V lambda segments, probes for both the 5' leader and J lambda segments fail to detect homologous sequences. Taken together, these results suggest that a highly selective process, possibly gene conversion, operates during B cell ontogeny to generate diversity within the lambda gene.
Alleles↗
T-cell proliferation involving the CD28 pathway is associated with cyclosporine-resistant interleukin 2 gene expression.
CD28 is a homodimeric glycoprotein expressed on the surface of a major subset of human T cells that has recently been identified as a member of the immunoglobulin supergene family. The binding of monoclonal antibodies to the CD28 antigen on purified T cells does not result in proliferation; however, previous studies have shown that the combination of CD28 stimulation and protein kinase C activation by phorbol myristate acetate (PMA) results in T-cell proliferation that is independent of both accessory cells and activation of the T-cell receptor-CD3 complex. In the present study, effects of stimulation by anti-CD28 on cell cycle progression and on the interleukin 2 (IL-2) and IL-2 receptor system have been investigated on primary cultures of purified peripheral-blood CD28+ T cells. There was no measurable effect on cell size or on DNA synthesis after stimulation of resting (G0) cells by CD28 alone. After 3 h of activation of T cells by PMA alone, a slight (8%) increase in cell volume occurred that did not progress to DNA synthesis. In contrast, T-cell stimulation by CD28 in combination with PMA resulted in a progressive increase in cell volume in approximately 100% of cells at 12 to 14 h after stimulation. Northern blot (RNA blot) analysis revealed that CD28 stimulation alone failed to cause expression of the alpha chain of the IL-2 receptor or of IL-2 mRNA, and in accord with previous studies, stimulation by PMA alone resulted in the accumulation of IL-2 receptor transcripts but no detectable IL-2 mRNA. In contrast, T-cell stimulation by the combination of CD28 and PMA resulted in the appearance of IL-2 transcripts and enhanced expression of IL-2 receptor mRNA. Functional studies revealed that the proliferation induced by CD28 and PMA stimulation was entirely resistant to cyclosporine, in contrast to T-cell activation induced by the CD3-T-cell receptor complex. Cyclosporine was found not to affect the accumulation of IL-2 mRNA after CD28 plus PMA stimulation, although there was no detectable IL-2 mRNA after stimulation by CD3 in the presence of the drug. Furthermore, stimulation by CD28 in combination with immobilized CD3 antibodies caused a striking enhancement of IL-2 mRNA expression that was, in part, resistant to the effects of cyclosporine. These studies indicate that the CD28 molecule synergizes with protein kinase C activation to induce IL-2 gene expression and demonstrate that stimulation by the CD28 pathway can cause vigorous T-cell proliferation even in the presence of cyclosporine and that cyclosporine does not prevent transcription of 16-2 mRNA, as has been suggested previously. Moreover, these findings suggest that a potential role for the CD28 molecule in vivo may be to augment IL-2 production after stimulation of the CD3-T-cell receptor molecular complex and thereby to amplify an antigen-specific immune response. Finally, these results provide further evidence that the CD28 molecule triggers T-cell proliferation in a manner that differs biochemically from CD3-T-cell receptor-induced proliferation.
Adult↗
Normal and neoplastic B cell development in the bursa of fabricius.
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Animals↗
Bone marrow transplantation.
The current status of bone marrow transplantation is reviewed. The diseases that are treatable with marrow transplantation, the basic transplant procedure, and the potential complications of marrow transplantation are discussed in detail. The future application of marrow transplantation to additional disease processes is considered.
Bone Marrow Transplantation↗
Correlation of hypomagnesemia with the onset of cyclosporine-associated hypertension in marrow transplant patients.
Cyclosporine is known to cause hypertension, and we have recently reported that it causes hypomagnesemia and renal magnesium wasting in marrow transplant recipients. We performed a case-control study to ask whether hypomagnesemia might be related to this form of drug-induced hypertension. The charts of 188 patients treated with cyclosporine were evaluated for the development of hypertension. The 32 patients who became hypertensive were age, sex, and disease-matched with 32 cyclosporine-treated controls. Baseline serum Mg levels were normal in both groups. However at the time of development of hypertension, the hypertensive patients had a mean (+/- SD) Mg of 1.22 +/- 0.20 mEq/L versus controls 1.40 +/- 0.33 mEq/L (P less than 0.01). Serum calcium, albumin, creatinine, potassium, and cyclosporine concentrations were not different between the two groups. This study may indicate that hypertension and hypomagnesemia are coincident toxicities in cyclosporine-treated patients. Alternatively, our data support the hypothesis that acquired derangements in magnesium metabolism may contribute to the development of hypertension. Magnesium replacement may prove beneficial in the treatment and/or prevention of cyclosporine-associated hypertension.
Adolescent↗
The risks of central nervous system relapse and leukoencephalopathy in patients receiving marrow transplants for acute leukemia.
The records of 415 patients who received allogeneic marrow transplants for acute leukemia were reviewed to assess the risk of central nervous system (CNS) relapse and leukoencephalopathy after marrow transplantation. The Kaplan-Meier estimates of the probability of CNS relapse posttransplant were 13% for patients with acute lymphoblastic leukemia (ALL) and 2% for patients with acute nonlymphoblastic leukemia (ANL). Previous CNS disease was significantly correlated with an increased risk of CNS relapse in patients transplanted for ALL but not for ANL. In contrast, bone marrow involvement with leukemia at the time of transplant was associated with an increased risk of CNS relapse in patients with ANL but not in patients with ALL. Seventy-one patients with ALL did not receive posttransplant intrathecal methotrexate (IT-MTX) and 127 did. The probability of CNS relapse in these two groups was 38% and 7%, respectively (P less than .02). This protective benefit from IT-MTX was present in patients both with and without a history of CNS involvement or marrow involvement at the time of transplant. In patients with ANL, 116 patients did not receive posttransplant IT-MTX and 101 patients did, but no protection from CNS relapse was observed from IT-MTX irrespective of a patient's previous CNS history or marrow status at the time of transplant. Leukoencephalopathy was seen exclusively in patients who had received radiation and/or intrathecal chemotherapy to the CNS before preparation for marrow transplantation and posttransplant IT-MTX. In such patients the risk of leukoencephalopathy was 7%. From our data, it appears that posttransplant IT-MTX is a significant benefit for ALL patients in preventing CNS relapse after marrow transplantation. A similar benefit from posttransplant IT-MTX for ANL patients cannot be established from this study. In both groups, increasing total CNS therapy was associated with an increasing risk of leukoencephalopathy.
Acute Disease↗
From precursor to product: how do megakaryocytes produce platelets?
While the production of a subcellular particle that can be used as a basic building block for clot formation may be a significant adaptive advantage for mammals, little is known about the exact process by which platelets are produced. Studies of platelet heterogeneity suggest that platelet production is regulated to maintain a constant platelet mass. The number of platelets produced in a given interval is determined primarily by the number of cells differentiating along the megakaryocytic pathway and changes in the rate at which platelets are produced take several days to occur. In contrast, the mean platelet volume (MPV) appears to be determined by the conditions under which the platelets are produced and can be increased within hours of a change in platelet demand. This latter observation, coupled with the observation that megakaryocytes are active, motile cells responsive to a variety of humoral agonists suggests that platelet production is a dynamic process which is under tight regulatory control. Various issues of platelet production are considered in relationship to these observations. An approach to establishing a test model for platelet release is discussed.
Animals↗
Oncogenes with potential nuclear function: myc, myb and fos.
The protein products of the retroviral oncogenes, v-myc, v-myb and v-fos, and their normal cellular counterparts, the c-myc, c-myb and c-fos proto-oncogenes, have been found to be localized predominantly in the nucleus. In view of the oncogenicity of the retroviral forms of these genes, it is probable that they have central roles in the nuclear events involved in cellular proliferation and differentiation. To investigate these possibilities, detailed studies on the expression of these 'nuclear oncogenes' have been performed. The cellular forms of all three genes are normally expressed in a variety of cell types during proliferation and have RNA and protein products with short half-lives, which is consistent with the idea that they may have regulatory roles. Studies have shown that both c-myc and c-fos are induced during the stimulation of quiescent cells to enter the cell cycle and are continually expressed in replicating cells. In contrast, c-myb levels are greatest in cells as they prepare to enter and traverse S phase. Both c-myc and c-myb expression cease as cells terminally differentiate, whereas in some cell types c-fos is induced during differentiation. Unregulated expression of all three genes has distinct effects on cellular differentiation: myc seems to inhibit differentiation of several cell types when expressed at high levels; myb does not appear to effect differentiation in the systems in which it has been examined; and fos appears to be able to induce differentiation of some cell types. In most cell types, c-myc and c-myb expression is controlled primarily by posttranscriptional mechanisms, whereas c-fos expression is regulated primarily at the transcriptional level. The protein products of these oncogenes are all phosphorylated and probably undergo additional modifications. The nuclear association of these proteins is complex and apparently takes multiple forms. Taken together, these data suggest that all three nuclear oncogenes have distinct regulatory functions with respect to cellular proliferation and differentiation. Several models for function are discussed.
Animals↗
Selective consumption of large platelets during massive bleeding.
To see whether selective consumption of either small or large platelets occurs during haemostasis in vivo the mean platelet volume was studied in six patients who developed thrombocytopenia after trauma. The mean platelet volume at the onset of thrombocytopenia was significantly lower than that on admission (p less than 0.01): selective loss of large platelets had occurred.
Blood Platelets↗
Profound hypomagnesemia and renal magnesium wasting associated with the use of cyclosporine for marrow transplantation.
We prospectively studied 41 marrow transplant patients to evaluate the possible association of hypomagnesemia with cyclosporine immunosuppressive therapy. During the 3 months posttransplant the mean nadir serum magnesium level was 1.06 +/- 0.16 mEq/L in 24 patients treated with cyclosporine and 1.33 +/- 0.13 in 14 patients treated with methotrexate (P less than 0.0001). Eleven of 24 patients receiving cyclosporine versus 1 of 14 patients receiving methotrexate had magnesium levels less than 1 mEq/L, or were begun on replacement therapy for presumed symptomatic hypomagnesemia (P less than 0.02). Agents known to be associated with hypomagnesemia, aminoglycosides and amphotericin B, were given in both lesser quantity and frequency to cyclosporine-treated patients than to methotrexate-treated patients. Hypomagnesemic patients treated with cyclosporine had inappropriately elevated urine magnesium excretion. Renal magnesium wasting may be added to the spectrum of nephrotoxicity resulting from cyclosporine. Several adverse reactions previously attributed to cyclosporine may be secondary to magnesium deficiency.
Adolescent↗