Activation of cell growth by binding of Friend spleen focus-forming virus gp55 glycoprotein to the erythropoietin receptor

Friend spleen focus-forming virus (SFFV) is a defective murine C-type retrovirus which causes a multi-stage erythroleukaemia in mice and erythroblastosis in bone marrow cultures. The SFFV env gene encodes a membrane glycoprotein, gp55, which is located on the cell surface and in the rough endoplasmic reticulum and is essential both for the induction of leukaemia in vivo and erythroblast proliferation in vitro. The mechanism by which gp55 causes increased erythroblastosis and ultimately leukaemia is unknown, but a reasonable suggestion is that gp55 can mimic the action of erythropoietin by binding to its receptor (Epo-R), thereby triggering prolonged proliferation of erythroid cells. To test this possibility, we have co-expressed gp55 and the murine Epo-R in a fibroblast cell line. We show here that in such cells, the SFFV glycoprotein binds directly to Epo-R. Furthermore, when an interleukin-3 (IL-3)-dependent lymphoid cell line was co-infected by SFFV and a virus that carries the Epo-R gene, it could grow without IL-3. We suggest that through direct binding to Epo-R, gp55 can stimulate the receptor and by-pass the normal requirement for Epo, causing prolonged proliferation of infected erythroid cells. This could be the first step of leukaemogenesis induced by Friend virus.

Inhibition of receptor binding and neutralization of bioactivity by anti-erythropoietin monoclonal antibodies

We have generated four high affinity monoclonal antibodies (MoAbs) to recombinant human erythropoietin (EPO). All four MoAbs immunoprecipitate radioiodinated native EPO, and the concentrations of MoAbs required for maximum binding range from 10 nmol/L to 100 nmol/L. Two MoAbs, designated Group I MoAbs, bind to an epitope within the N-terminal 20 amino acids of EPO and also immunoprecipitate sodium dodecyl sulfate (SDS)-denatured EPO. Two other MoAbs (Group II MoAbs) do not immunoprecipitate SDS-denatured EPO and do not bind to any of the eight endo C fragments of EPO. We first used murine erythroleukemia (MEL) cells to test the MoAbs for inhibition of EPO-receptor binding. MEL cells, although unresponsive to EPO, express 760 high affinity receptors for EPO per cell (Kd = 0.24 nmol/L). To assay our MoAbs, MEL cells were grown as monolayers on fibronectin-coated Petri dishes and incubated at 4 degrees C with radioiodinated EPO. Group I MoAbs do not inhibit binding of radioiodinated EPO to the MEL EPO-receptor, but Group II MoAbs do inhibit binding in a dose-dependent manner. We next examined the neutralization of EPO bioactivity by our MoAbs, using EPO-dependent cell line. Only Group II MoAbs inhibit a newly developed EPO-dependent cell growth, demonstrating that inhibition of EPO-receptor binding correlates with neutralization of EPO bioactivity.

Cloning of cDNA for the major DNA-binding protein of the erythroid lineage through expression in mammalian cells

Genes expressed in erythroid cells contain binding sites for a cell-specific factor believed to be an important regulator for this haematopoietic lineage. Using high-level transient expression in mammalian cells, we have identified complementary DNA encoding the murine protein. The factor, a new member of the zinc-finger family of DNA-binding proteins, is restricted to erythroid cells at the level of RNA expression and is closely homologous between mouse and man.

Expression cloning of the murine erythropoietin receptor

Two independent cDNA clones encoding the erythropoietin receptor (EPO-R) were isolated from a pXM expression library made from uninduced murine erythroleukemia (MEL) cells. The clones were identified by screening COS cell transfectants for binding and uptake of radioiodinated recombinant human erythropoietin. As inferred from the cDNA sequence, the murine erythropoietin receptor is a 507 amino acid polypeptide with a single membrane-spanning domain. It shows no similarities to known proteins or nucleic acid sequences in the data bases. Although the MEL cell EPO-R has a single affinity with a dissociation constant of approximately 240 pM, the EPO-R cDNA, expressed in COS cells, generates both a high-affinity (30 pM) and a low-affinity (210 pM) receptor.

Pineocytomas of childhood. A reappraisal of natural history and response to therapy

Pineocytomas are pineal parenchymal tumors composed of differentiated cells histologically similar to those of the mature pineal gland. Little is known about the incidence, pattern of growth, or response to treatment of pineocytomas. Between 1975 and 1985, six children with pineocytomas were treated at our institution, and pineocytomas constituted 11% of all newly diagnosed pineal region tumors. The clinical, radiographic, and pathologic features of these six patients with pineocytomas are presented. Initial treatment for these children included craniospinal plus supplemental local radiotherapy (three), local radiotherapy alone (one), or radiation therapy plus adjuvant chemotherapy (two). Four of six patients had tumor recurrence a median of two years after diagnosis. Three patients had leptomeningeal dissemination, one at the time of diagnosis and two following therapy. Our findings suggest that: biopsy is necessary to distinguish pineocytomas from other pineal region tumors; radiation therapy alone is inadequate; and these tumors are aggressive in the pediatric population, with a high propensity for leptomeningeal dissemination.

Immune-specific gamma interferon production correlates with lymphocyte blastogenesis

Gamma interferon (IFN-gamma) production, measured by a new commercially available radioimmunoassay, and lymphocyte blastogenesis were investigated in human peripheral lymphocyte cultures from healthy adults stimulated by crude cytomegalovirus (CMV) antigen. Mononuclear cells were obtained from 32 healthy adults (18 CMV seropositive [S+] and 14 CMV seronegative [S-]) by Ficoll-Hypaque gradients and cultured in microtiter plates containing CMV antigen. Lymphocyte blastogenesis ([3H]thymidine uptake) and IFN-gamma were determined on day 6. The mean stimulation index for S+ individuals was significantly greater than that for S- individuals (P less than 0.001). Similarly, the IFN-gamma stimulation index was greater for S+ individuals than for S- individuals (P less than 0.005). A significant increase in the concentration of IFN-gamma (10 NIH units/ml) was observed for S+ individuals at 24 h of antigen stimulation, with peak levels at 4 days. The radioimmunoassay for IFN-gamma production by antigen-stimulated lymphocytes in vitro (IMRX; Centocor Inc., Malvern, Pa.) is a rapid and sensitive measure of cell-mediated immunity.

High resolution analysis of the timing of replication of specific DNA sequences during S phase of mammalian cells

A new method, utilizing selective photodegradation of 5-bromo-deoxyuridine (BUdR)-substituted DNA and flow cytometry, has been developed for analyzing the timing of replication of specific DNA sequences. Chemically synchronized Chinese hamster ovary cells were given a pulse of the deoxythymidine analogue, BUdR, at different times during S phase, and flow sorted according to DNA content, before DNA isolation. Newly-replicated, unifilarly BUdR-substituted DNA was selectively degraded by treatment with 33258 Hoechst plus near UV light followed by S1 nuclease digestion; the resistant DNA was analyzed for its content of 18s and 28s rDNA or dihydrofolate reductase (DHFR) sequences via Southern blot analysis. Both the rDNA and DHFR sequences were found to replicate almost entirely during the first quarter of S phase. The approach described should have general utility for analyzing replication kinetics of specific DNA sequences in mammalian cells.

Modification of DNA by aflatoxin B1 creates alkali-labile lesions in DNA at positions of guanine and adenine

The damage to DNA by the hepatocarcinogen aflatoxin B1 was investigated. A DNA fragment of known sequence of the lactose promoter-operator region was used as a substrate for modification by aflatoxin B1. The DNA was incubated with aflatoxin B1 in crude mammalian liver extracts or with purified microsomes. Treatment of the DNA incubated in the complete system with either 1 M piperidine or 0.1 M NaOH at 90 degrees revealed alkali-labile lesions in the DNA. The exact location of the cleavage site was determined by comparison of the length of the cleavage products with the known sequence on polyacrylamide gels. The lengths of the cleavage products were the same as those produced by alkali-induced breakage of the same sequence of DNA that had been modified with dimethyl sulfate. The major cleavage products of the aflatoxin B1-modified DNA were at positions of guanine and the minor cleavage products were at positions of adenine. These studies show that modification of DNA by aflatoxin B1 creates alkali-labile sites at positions of guanine and, to a lesser extent, adenine.

Sequence specific cleavage of DNA by the antitumor antibiotics neocarzinostatin and bleomycin

We have investigated the sites of DNA damage by the antitumor antibiotics neocarzinostatin and bleomycin by using a 5'-end-labeled DNA fragment of defined sequence as a substrate. At the high drug concentrations used here, neocarzinostatin creates single-strand breaks in DNA at positions of adenine and thymine in the presence of 2-mercaptoethanol, and bleomycin cleaves DNA at GC and GT sequences and to a lesser extent at TA sequences with its degradative activity enhanced by 2-mercaptoethanol. In the presence of ferrous ions, bleomycin cleaves DNA at TT, AT, and TA, as well as at GC and GT sequences. Both antibiotics make double-strand breaks in DNA at specific sites and it is likely that these result from two independent single-strand breaks at nearby sites on opposite strands of the DNA.