Biosynthesis of murine terminal deoxynucleotidyltransferase

PATZ1 Is a DNA Damage-Responsive Transcription Factor That Inhibits p53 Function

Insults to cellular health cause p53 protein accumulation, and loss of p53 function leads to tumorigenesis. Thus, p53 has to be tightly controlled. Here we report that the BTB/POZ domain transcription factor PATZ1 (MAZR), previously known for its transcriptional suppressor functions in T lymphocytes, is a crucial regulator of p53. The novel role of PATZ1 as an inhibitor of the p53 protein marks its gene as a proto-oncogene. PATZ1-deficient cells have reduced proliferative capacity, which we assessed by transcriptome sequencing (RNA-Seq) and real-time cell growth rate analysis. PATZ1 modifies the expression of p53 target genes associated with cell proliferation gene ontology terms. Moreover, PATZ1 regulates several genes involved in cellular adhesion and morphogenesis. Significantly, treatment with the DNA damage-inducing drug doxorubicin results in the loss of the PATZ1 transcription factor as p53 accumulates. We find that PATZ1 binds to p53 and inhibits p53-dependent transcription activation. We examine the mechanism of this functional inhibitory interaction and demonstrate that PATZ1 excludes p53 from DNA binding. This study documents PATZ1 as a novel player in the p53 pathway.

Phorbol ester regulation of terminal deoxynucleotidyl transferase, proliferation, and TcR alpha in a pre-T cell line

Terminal deoxynucleotidyl transferase (TdT) is a template-independent DNA polymerase that is transiently expressed during the normal development of T and B lymphocytes. Phorbol 12-myristate 13-acetate (PMA) has been reported to induce maturation-like changes, including the loss of TdT, in many leukemic cell lines. We investigated the mechanism of TdT repression by PMA in an early thymocyte-like cell line, RPMI 8402. At a concentration of 8 nM, PMA caused both repression of TdT synthesis and arrest of proliferation. At greater concentrations of PMA, these same changes initially occurred, but then cell proliferation resumed, and TdT was reexpressed. At both 8 and 160 nM PMA, TdT biosynthesis and TdT mRNA became undetectable within 8 hours, while cell proliferation and DNA synthesis were not significantly reduced until 16 hours. Growth arrest induced by serum starvation did not result in a similar reduction of TdT RNA even after 48 hours. With 160 nM PMA, TdT mRNA could be detected again by 24 hours, and proliferation resumed. Transcription run-off assays indicated that TdT RNA synthesis ceased within 1 hour after exposure to both 8 and 160 nM PMA. T cell receptor alpha (TcR alpha) RNA was induced when TdT RNA was repressed. TcR beta RNA levels were unchanged, and TcR gamma RNA was up-regulated. TdT gene repression and modulation of cell proliferation as well as induction of TcR gene expression are normal events during intrathymic T cell maturation. This cell model provides a system for analyzing the molecular regulation of these significant developmental events.

Expression of transferrin and vitamin D-binding protein genes in an osteogenic sarcoma cell line

Expression of genes encoding transferrin and the vitamin D-binding protein is described in a cell line, U-2 OS, derived from a human osteogenic sarcoma. The mRNA transcripts of transferrin and vitamin D-binding protein were shown to be the lengths of those found in normal human liver. The cells synthesize and secrete the transferrin and vitamin D-binding proteins, in addition to human albumin and ceruloplasmin. The U-2 OS cells were successfully transfected with chimeric genes carrying 670 bp of the 5' regulatory sequence of the human transferrin gene fused to a reporter chloramphenicol acetyltransferase gene. These data indicate that the appropriate transcriptional factors required for expression of four plasma proteins are produced by U-2 OS nuclei and that the U-2 OS cell line will be useful for studies analyzing regulation of these genes.

Measurement of terminal deoxynucleotidyl transferase mRNA in clinical samples: a new parameter in analysis of leukemia cells

A 1750 base pair cDNA to human terminal deoxynucleotidyl transferase (TdT) has been cloned. This cDNA detects a dominant 2200 base pair messenger RNA species in normal and leukemic cells synthesizing the enzyme. A quantitative dot blot assay was utilized to survey a number of clinical samples from patients with TdT positive and negative leukemias as well as cells from normal volunteers. A linear relationship was detected between the amount of TdT mRNA and the amount of enzyme activity in bone marrow cells. The assay is sensitive enough to detect normal TdT levels in bone marrow, and distinguish these levels from the lack of such mRNA in peripheral blood and bone marrow of patients with myeloid leukemia. Elevated levels of mRNA were found in two cases of patients in clinical remission. The prognostic significance of these observations must await further study.

Radiation leukemia virus and X-irradiation induce in C57BL/6 mice two distinct T-cell neoplasms: a growth factor-dependent lymphoma and a growth factor-independent lymphoma

Two different classes of neoplastic T cells were isolated from radiation leukemia virus (RadLV)-inoculated and from X-ray-treated C57BL/6 mice. One consisted of growth factor-dependent T-cell lymphoma (FD-TCL) lines which were established from the spleens and thymuses of treated mice within a day of lymphoma detection. FD-TCL cells were often eudiploid and could be grown in pure culture only at high concentrations, or on stromal feeder layers. Non-thymic, factor-dependent TCL cells produced interleukin-2 upon lectin stimulation, and were autostimulatory because they secreted growth factor(s) constitutively. Single cell cloning of FD-TCL cells in semisolid medium required the addition of exogenous conditioned medium. In vivo, FD-TCL cells that were injected intraperitoneally or intravenously homed to the spleen, proliferated in it and killed the injected mice. FD-TCL cells did not produce local tumors at the site of subcutaneous injection. The isolation and study of FD-TCL cells was facilitated by their cultivation on stromal hematopoietic monolayers in supplemented "lymphocyte medium", until an autostimulating, self-sustaining concentration of FD-TCL cells was obtained. FD-TCL cells could not be grown from lymphoid tissue of normal, control mice. In contrast, T-cell lymphoma (TCL) lines, which were established from virus-induced thymomas which had been kept in situ for 4-6 weeks after detection, consisted of factor-independent cells that possessed an aneuploid karyotype (in some cases trisomic for chromosome No. 15), and produced local tumors at the site of subcutaneous injection. These cells could be cloned in semisolid medium without addition of exogenous factor(s). The phenotypic markers of TCL cells differed from those of FD-TCL cells, suggesting heterogeneity in the stages of differentiation at which cells can give rise to growth factor-independent (TCL) and to growth factor-dependent (FD-TCL) lines.

Molecular biology of terminal transferase

Terminal transferase is an unusual deoxynucleotide polymerizing enzyme found only in prelymphocytes. The protein was purified to homogeneity from calf thymus glands in 1971 as a 32 kDa protein with a two peptide structure. Subsequent biochemical and immunological analyses of terminal transferase protein in crude extracts from a number of animal species showed a single peptide with a molecular weight of about 58,000. The two peptide structure found earlier was caused by proteolysis. Homogeneous 58 kDa terminal transferase has now been produced from human lymphoblastoid cells and calf thymus glands by immunoaffinity chromatography. In vitro phosphorylation studies showed that the terminal transferase protein contains one phosphorylation site near one end of the polypeptide chain, and the phosphorylation of the enzyme has been confirmed by in vivo labeling experiments. Unambiguous demonstration of the molecular weight of the human terminal transferase was obtained by translation of the cloned human terminal transferase DNA sequence to a 58,308 Da protein. The translated amino acid sequence also provided a possible phosphorylation site near the amino-terminus of the protein. Preliminary analysis of the genomic structure shows a simple intron/exon pattern with the total human terminal transferase gene spanning at least 65 Kb.

Fluorimetric assay for terminal deoxynucleotidyl transferase activity

A fluorimetric assay for measuring terminal deoxynucleotidyl transferase activity in purified and crude enzyme preparations has been developed. Etheno-substituted deoxynucleotides are shown to be substrates of the enzyme. The assay involves polymerization of the fluorescent nucleotide 1,N6-ethenodeoxyadenosine triphosphate (epsilon dATP) on an oligodeoxynucleotide initiator, [poly(deoxyadenylic acid) with an average chain length of 50 residues] under the reaction conditions used in the standard radiometric assay. The incorporation of epsilon dATP into polymer is quantitated by fluorescence after isolation and nuclease digestion of the product. The enzymological properties of etheno substrates were also determined. Epsilon dATP binds about twofold tighter than dATP to terminal transferase, but has a twofold-lower catalytic rate. However etheno substitution does not affect initiator binding. The fluorimetric assay is suitable for clinical analysis of terminal transferase in human leukemias, and may be a useful adjunct to recently developed immunochemical methods which detect protein, not activity.

Insertion of N regions into heavy-chain genes is correlated with expression of terminal deoxytransferase in B cells

The variable regions of immunoglobulin heavy chains are encoded in the germ line by three discrete DNA segments: VH (variable) elements, D (diversity) elements and JH (joining) elements. During the differentiation of B lymphocytes, individual segments from each group are brought together by recombination to form the complete VHDJH variable region. To understand these processes better, we have now isolated and sequenced molecular clones representing intermediates (DJH fusions) and final products (VH-to-DJH joins) of heavy-chain gene rearrangement in two cell lines that represent analogues of cells at early stages of B-lymphocyte differentiation. Heavy-chain gene assembly in one cell line but not in the other is accompanied by the appearance of short nucleotide insertions at the recombinational junctions. The generation of such insertions is positively correlated with the expression of terminal deoxynucleotidyl transferase in these lines.

Differentiation of cloned populations of immature B cells after transformation with Abelson murine leukemia virus

The nature of the target cell for Abelson virus transformation and the effect of transformation on B cell differentiation were studied with six cloned lines of nontransformed immature B lymphocytes. Three clones were at the pre-B cell stage of maturation prior to A-MuLV infection; two were at the B cell stage, and one appeared to represent a stage prior to rearrangement of the mu heavy chain gene. All six cloned lines could be transformed by Abelson virus. Many of the transformants of the pre-B cell clones underwent kappa light chain gene rearrangement and expression following viral infection. Distinct light chain gene rearrangements were segregated by further subcloning these transformed lines. Abelson virus infection of one cloned cell line believed to represent a stage of maturation prior to the pre-B cell stage produced pre-B cell transformants with a variety of heavy chain gene rearrangements. Thus B lymphoid target cells for Abelson virus are not restricted to a single developmental stage, and some transformed subclones can undergo extensive immunoglobulin gene rearrangements shortly after viral infection.

Purification of terminal deoxynucleotidyl transferase from pig thymus: identification of 42,000 and 57, 000 dalton species

Terminal deoxynucleotidyl transferase (TdT, EC 2.7.7.31) has been purified 4365-fold from pig thymus. It was further separated into two molecular forms of 57,000 and 45,000 dalton by Sephadex G-100 gel-filtration. The former sedimented at 4.2s through a sucrose gradient, while the latter, at 3.6s. By a SDS-polyacrylamide gel-electrophoresis, their molecular weights were estimated as 57,000 and 42,000 dalton, respectively. Thus each of the large and small pig TdT consists of a single polypeptide of 57,000 or 42,000 dalton and has no subunit structure. These two forms were indistinguishable in antigenicity by a neutralization assay of 42,000 dalton-TdT antibody. The enzymological properties of 42,000 dalton-TdT from pig thymus were very similar to those of calf TdT which has a two-subunits structure.

Induction of lymphocyte differentiation by epidermal cultures

Human and murine lymphoid cell populations were induced to express terminal deoxynucleotidyl transferase, a marker of early lymphoid differentiation, by exposure to allogeneic or syngeneic epidermal cells. Control growth medium, fibroblasts, or a mammary epithelial cell line did not induce this marker. These findings suggest that epidermal cells can induce lymphoid cell differentiation in vitro.

Ferritin synthesis by human T lymphocytes

Mononuclear cells from peripheral blood of normal humans, unselected spleen cells from patients with Hodgkin's disease, and selected T and non-T lymphoid cells from normal peripheral blood and from the spleens of Hodgkin's disease patients were examined for de novo synthesis and secretion of ferritin. After precipitation of labeled lysates and supernatants from unseparated and selected T cells with antiserum to human liver ferritin, two bands were visible on sodium dodecyl sulfate-polyacrylimide gel analysis. The two bands were detected in molecular weight regions 19,000 and 21,000, which are thought to represent the L and H subunits of the ferritin molecule, respectively. The slower band (subunit H) was more radioactive than the faster band (subunit L). The H subunit is found in greater amounts in the serum of some tumor patients, but its cellular origin has not been established. The present findings indicate that cells of the immune system contribute to the synthesis and secretion of a ferritin molecule with a high proportion of H subunits.

In vitro maintenance of differentiation marker synthesis by subpopulations of mouse thymocytes

Mouse thymocyte populations enriched in functionally incompetent, "immature" cells on the one hand, or in competent "mature" cell on the other hand, express different steady-state levels of certain surface antigens and marker enzymes. In the cases of the glycoproteins H-2 (K and D), Qa, and TL, and the DNA polymerase terminal deoxynucleotidyl transferase (TdT), these levels reflect different rates of de novo synthesis in the two populations. Thus each population appears to manifest a characteristic pattern of synthetic rates for the various products relative to total protein synthesis. To investigate the maintenance of these patterns, enriched pools of "immature" and "mature" thymocytes were incubated in vitro for 24 h, and the rates of product synthesis before and after culture were compared. H-2 synthesis, initially most rapid in the mature cells, continued to be made at the highest rate in this population. TdT synthesis, a characteristic activity of the immature cells, was not induced in the mature cells, but proceeded at an increased relative rate in the immature population. Therefore, the differences between the rates of H-2 and TdT synthesis were stable properties of the two thymocyte populations. Another marker of immature cells. TL, did not continue to be produced in parallel with TdT. Rather, its synthesis was selectively curtailed in relation to the continuing protein synthesis in the immature cultures. This non-coordinate regulation of TL and TdT production in immature thymocytes may be due to several mechanisms. These are discussed with regard to their implications for pathways of thymocyte maturation.