Activation of Cellular Oncogenes in Human and Mouse Leukemia-Lymphomas: Spontaneous and Induced Oncogene Expression in Murine B Lymphocytic Neoplasms

Chromosomal and extrachromosomal instability of the cyclin D2 gene is induced by Myc overexpression

We examined the expression of cyclins D1, D2, D3, and E in mouse B-lymphocytic tumors. Cyclin D2 mRNA was consistently elevated in plasmacytomas, which characteristically contain Myc-activating chromosome translocations and constitutive c-Myc mRNA and protein expression. We examined the nature of cyclin D2 overexpression in plasmacytomas and other tumors. Human and mouse tumor cell lines that exhibited c-Myc dysregulation displayed instability of the cyclin D2 gene, detected by Southern blot, fluorescent in situ hybridization (FISH), and in extrachromosomal preparations (Hirt extracts). Cyclin D2 instability was not seen in cells with low levels of c-Myc protein. To unequivocally demonstrate a role of c-Myc in the instability of the cyclin D2 gene, a Myc-estrogen receptor chimera was activated in two mouse cell lines. After 3 to 4 days of Myc-ER activation, instability at the cyclin D2 locus was seen in the form of extrachromosomal elements, determined by FISH of metaphase and interphase nuclei and of purified extrachromosomal elements. At the same time points, Northern and Western blot analyses detected increased cyclin D2 mRNA and protein levels. These data suggest that Myc-induced genomic instability may contribute to neoplasia by increasing the levels of a cell cycle-regulating protein, cyclin D2, via intrachromosomal amplification of its gene or generation of extrachromosomal copies.

Ultraviolet-light-inactivated Cas-Br-M murine leukemia virus induces a protective CD8+ cytotoxic T lymphocyte response in newborn mice

Newborn NFS/N mice are susceptible to the neurological disease induced by infection with Cas-Br-M murine leukemia virus (Cas), and do not develop a protective cytotoxic T cell (CTL)-mediated response to Cas infection. Here we demonstrate that whole UV light-inactivated Cas (UV-Cas), inoculated in newborn NFS/N mice, induced a strong, Cas-specific CTL response detectable 2 weeks postinoculation and persisting in vivo for > or = 36 weeks. The magnitude of the UV-Cas-induced splenic CTL response, mediated by CD8+ T cells, inversely correlated with the level of proviral cas env sequences detectable in the spleen of the UV-Cas-inoculated mice, as revealed by PCR amplification of tissue DNA. The transfer of UV-Cas-primed splenocytes, with Cas-specific CTL activity, protected 100% of recipient newborn mice from the development of neurological disease induced by infection with live Cas, for more than 28 weeks, and reduced the level of viral replication in the recipients.

Association of elevated levels of protein kinase C-zeta mRNA and protein with murine B-lymphocytic neoplasia

Expression of mRNA for protein kinase C (PKC)-alpha, -beta, -gamma, -delta, -epsilon, -zeta, and -eta has been shown, by polymerase chain reaction-generated isozyme-specific probes, to be cell-type -and differentiation-stage-specific in mouse hemopoietic cells. Recently, we cloned a 2.2-kb mouse PKC -zeta cDNA. In this study, we used the nearly full-length cDNA PKC-zeta probe to demonstrate that expression of PKC-zeta was significantly elevated in lymphocytic neoplasms at both the mRNA and protein levels. Normal brain, kidney, and liver contain 2.4- and 4.4-kb mRNAs, whereas normal lymphoid organs (spleen, thymus, and lymph nodes) express barely detectable amounts of PKC-zeta. These vanishingly small levels of PKC-zeta mRNA did not increase when polyclonal spleen B-cell proliferation and differentiation were induced in vivo with anti-immunoglobulin D antiserum or in vitro with lipopolysaccharide. In contrast, 2.4-kb transcripts of PKC-zeta are abundant in virtually all neoplastic B-lymphocytic cell lines. Furthermore, additional transcripts of a novel size, about 7 and 8 kb, were found in several mature B-cell lymphomas and plasma cell tumors. Western blot analysis of protein extracts from normal B cells and hemopoietic tumors confirmed that these quantitative differences in PKC-zeta mRNA also exist at the protein level. That is, only trace amounts of PKC-zeta protein were detectable in pro-B cells and pre-B cells, but abundant amounts of this isoform were found in protein extracts from most B-cell lymphomas and plasma cell tumors. These findings suggest that this atypical member of the PKC multigene family participate in the multistep process of malignant transformation of lymphocytes.

Unique expression pattern of protein kinase C-theta: high mRNA levels in normal mouse testes and in T-lymphocytic cells and neoplasms

A 2.2-kb cDNA that contains the entire coding region of mouse protein kinase C-theta (PKC-theta) was cloned from skeletal muscle mRNA using reverse transcription and the polymerase chain reaction (PCR). This clone was used as a probe to study the expression of this PKC isoform in normal and transformed hemopoietic cells and other normal tissues. By far the highest steady-state level of PKC-theta mRNA was found as a 2.8-kb transcript on a Northern blot of poly(A)+ RNA from testes. High levels were also found in skeletal muscle, spleen, T lymphomas and purified normal T lymphocytes, but these tissues and cells expressed two transcripts, 3.3 kb and 3.8 kb. Lower levels of similar size transcripts were found in normal brain, B lymphocytes and B-lymphocytic tumors and cell lines.

IFN-gamma production in response to neuropathogenic Cas-Br-M murine leukemia virus infection

T cell-mediated production of IFN-gamma followed infection of adult, but not neonatal NFS/N mice with Cas-Br-M murine leukemia virus (Cas). The IFN-gamma response was associated with the appearance of CTL specific for Cas and with age-dependent resistance to neurologic disease. While both immune responses were mediated by a CD8-enriched population of T cells, IFN-gamma did not play a role in the activation of the Cas-specific CTL response. However, when given exogenously, IFN-gamma delayed the onset and reduced the incidence of Cas-induced neurologic disease. These data suggest that the IFN-gamma response to Cas infection may be an important host defense mechanism whose effects on virus replication and neurologic disease expression are independent of its effect on Cas-specific CTL.

Mouse protein kinase C-delta, the major isoform expressed in mouse hemopoietic cells: sequence of the cDNA, expression patterns, and characterization of the protein

A complementary DNA (cDNA) of 2559 bp which encode all 674 amino acids of mouse protein kinase C-delta (PKC-delta) has been isolated from a cDNA library prepared from ABPL-2, a mouse myeloid tumor. The library was screened with a partial PKC-delta cDNA clone that had been created by polymerase chain reaction (PCR) amplification of ABPL-2 RNA using primers that are conserved among all rat PKC isozymes. This approach proved to be a distinct improvement over screening with synthetic oligonucleotides. Similar sets of cDNAs prepared from other hemopoietic cell lines were screened with this PKC-delta cDNA and with probes for the other PKC isoforms. These experiments revealed that the major isoform of PKC expressed in hemopoietic cells is PKC-delta. PKC-delta protein was purified from ABPL-3, a mouse myeloid tumor which expressed principally the delta isoform of PKC. The protein eluted from a hydroxylapatite column in the same position as PKC-beta and -epsilon would elute, if present. The kinase activity of purified PKC-delta showed strict dependence on the presence of phospholipids, but showed no activation by Ca2+.

Gene specific damage and repair after treatment of cells with UV and chemotherapeutical agents

We have previously demonstrated preferential DNA repair of active genes in mammalian cells. The methodology involves the use of a specific endonuclease or other more direct approaches to create nicks at sites of damage followed by quantitative Southern analysis and probing for specific genes. Initially, we used pyrimidine dimer specific endonuclease to detect pyrimidine dimers after UV irradiation. We now also use the bacterial enzyme ABC excinuclease to examine the DNA damage and repair of a number of adducts other than pyrimidine dimers in specific genes. We can detect gene specific alkylation damage by creating nicks via depurination and alkaline hydrolysis. In our assay for preferential repair, we compare the efficiency of repair in the DHFR gene to that in the 3' flanking, non-coding region to the gene. In CHO cells, UV induced pyrimidine dimers are efficiently repaired from the active DHFR gene, but not from the inactive region. We have demonstrated that the 6-4 photoproducts are also preferentially repaired and that they are removed faster from the regions studied than pyrimidine dimers. Using similar approaches, we find that DNA adducts and crosslinks caused by cisplatinum are preferentially repaired in the active gene compared to the inactive regions and to the inactive c-fos oncogene. Also, nitrogen mustard and methylnitrosurea damage is preferentially repaired whereas dimethylsulphate damage is not. NAAAF adducts do not appear to be preferentially repaired in this system.(ABSTRACT TRUNCATED AT 250 WORDS)

Extinction of expression of the translocated myc gene in somatic cell hybrids between mouse myeloma and L-cells

Most murine plasma-cell tumors show a t(12;15) reciprocal chromosomal translocation which truncates the first exon of one of the myc gene alleles and fuses it to one of the switch regions of the immunoglobulin (Ig) heavy-chain locus. This results in constitutive activation of the translocated myc gene and the production of smaller-sized mRNA molecules, which are initiated at new sites in the first myc intron. The normal myc allele is not expressed in these myeloma cells. We have studied the expression of the translocated myc gene in somatic cell hybrids between mouse myeloma and L-cells. Our previous findings show that Ig gene expression is extinguished in such hybrids. In the present work we found that the hybrids contain the normal and translocated myc genes. In contrast to the myeloma parental cells which express the translocated myc gene, the hybrids are similar to the L-cells in expressing only the normal myc allele. Our results suggest that the L-cell, fibroblast-like phenotype, is dominant in these hybrids, and show that the translocated myc gene is expressed in a tissue-specific manner in the context of the myeloma cell, and is not expressed when subjected to a fibroblast-like cellular environment.

Developmental stage specificity of the lymphoid V(D)J recombination activity

We have examined the level of immunoglobulin gene V(D)J recombination activity in a number of cell lines derived from lymphoid or nonlymphoid lineages. The assay we employed uses extrachromosomal DNA as substrate and thereby avoids difficulties associated with the use of chromosomally integrated substrates. The recombination activity decreases during B-lymphoid development. It is highest at the earliest stages of committed B-cell differentiation and then falls progressively, reaching undetectable levels at the mature B-cell stage. The activity is also present in multipotential progenitors of myeloid cells and in pre-T cells but not mature T cells. No activity was found in several nonhematopoietic cell lines. Recombination was seen only among substrate molecules which had replicated in the eukaryotic cells. Several possible interpretations of this result are discussed.