Search PubMed⌕ Search

Biomedical subjects

R Baserga

Publications and source records attributed to R Baserga.

At least 19 recordsLinked to original sources
PubMed1992

The double life of the IGF-1 receptor.

The IGF-1 receptor is expressed in many cell types, and its activation by its ligands is a required step for the proliferation of many cells in vivo and in vitro. In most cells in culture, requiring more than one growth factor for growth, the IGF-1 receptor can be found in one of two different modes: in the first mode, although it is autophosphorylated by its ligands and induces the expression of specific genes, it does not transmit a mitogenic signal. In the alternative mode, i.e., after priming with an unrelated growth factor, the IGF-1 receptor responds to its ligands with a mitogenic stimulus. This review examines briefly the possible alternatives to explain this different behavior, which is crucial to our understanding of the control of cellular proliferation.

Animals↗
PubMed1979

A simple method for decreasing the toxicity of polyethylene glycol in mammalian cell hybridization.

The yield of hybrid colonies after fusion of mammalian cells with polyethylene glycol (PEG) is increased if the cells are fused in Ca2+-free medium, and kept in Ca2+-free medium for at least 15 min after fusion. The protective effect of Ca2+-free medium is much more obvious when Baker PEG is used than when fusion is carried out with Koch-Light PEG. The increased yield of hybrid colonies is shown to be due to a reduced toxicity rather than to an increased efficiency of cell fusion. These improvements have been found to apply to a variety of cell lines, and also when cell fusion is carried out in suspension. This technique should be particularly useful in studies on mammalian cell hybridization using cell lines that are particularly sensitive to the toxic effect of PEG.

Animals↗
PubMed1979

Cytoplasmic regulation of two G1-specific temperature-sensitive functions.

tsAF8 and ts13 cells are temperature-sensitive (ts) mutants of BHK cells that specifically arrest, at nonpermissive temperature, in the G1 phase of the cell cycle. These two mutants can complement each other. Both cell lines can be made quiescent by serum deprivation (G0). When subsequently stimulated by serum, they can enter S phase at 34 degrees C but not at 39.5 degrees-40.6 degrees C. We have used these mutants to determine whether the nucleus is needed during the G0 leads to S transition for the expression of the G1 ts functions. For this purpose, we fused cytoplasts of G0-tsAF8 with whole ts13 cells in G0, and cytoplasts of G0-ts13 with whole tsAF8 cells in G0. Serum stimulation at the nonpermissive temperature induced DNA synthesis in both types of such fusion products. No DNA synthesis was induced by serum stimulation at the nonpermissive temperature in fusion products constructed between either G0-tsAF8 cytoplasts and whole G0-tsAF8 cells or G0-ts13 cytoplasts and whole G0-ts13 cells. These results demonstrate that the information for these two ts functions, which are required for entry of serum-stimulated cells into the S phase, are already present in the cytoplasm of G0 cells--that is, before serum stimulation commits them to the transition from the nonproliferating to the proliferating state.

Animals↗
PubMed1979

Reactivation of silent rRNA genes by simian virus 40 in human-mouse hybrid cells.

Mouse-human hybrid cells were used to study the ability of simian virus 40 to regulate the expression of rRNA genes in vivo. In these hybrid cells, only the rRNA genes of the dominant species are expressed; the genes for the rRNA of the recessive species are silent. Simian virus 40 infection of these hybrids led to the production of two distinct 28S rRNA species as analyzed by agarose/2.4% polyacrylamide gel electrophoresis. These species were identified as human and mouse rRNAs. This result was confirmed by histochemical studies which indicated that the nucleolus organizer regions of both mouse and human chromosomes were actively synthesizing rRNA in the virus-infected hybrid cells. These results indicate that simian virus 40 infection can induce the expression of otherwise silent rRNA genes.

Animals↗
PubMed1979

DNA synthesis in temperature-sensitive mutants of the cell cycle infected by polyoma virus and adenovirus.

tsAF8 cells are a temperature-sensitive (ts) mutant of BHK cells that are arrested in G1 at the nonpermissive temperature. When made quiescent by serum restriction, they can be stimulated to enter S phase by 10% serum at 34 degrees C but not at 40.6 degrees C. The same results can be obtained if quiescent cells are infected with polyoma virus or adenovirus 12 instead of serum. However, adenovirus 2 infection stimulates DNA synthesis in tsAF8 cells at both 34 degrees C and 40.6 degrees C. The DNA synthesized after adenovirus 2 infection has been shown to be cellular DNA by CsCl density centrifugation. By density labeling it can be shown that adenovirus 2-induced DNA synthesis is due to semiconservative replication. The difference between adenovirus 2 and polyoma (or serum) is also evident with another ts mutant of BHK cells, ts13 cells. These results open the possibility of identifying the viral or cellular mechanism at the basis of this difference in the induction of host DNA synthesis between adenovirus 2 and polyoma or serum.

Adenoviruses, Human↗
PubMed1978

Stimulated DNA synthesis in frog nuclei by cytoplasmic extracts of temperature-sensitive mammalian cells.

Cytoplasmic extracts of proliferating cells stimulate DNA synthesis in isolated nuclei of Xenopus laevis liver. When tested by the same assay, cytoplasmic extracts of resting cells are completely inactive. When cytoplasmic extracts are prepared from cell cycle-specific temperature-sensitive mutants arrestd in the G1 phase of the cell cycle by the nonpermissive temperature, they also fail to stimulate DNA synthesis in frog nuclei. The results indicate that, to stimulate DNA synthesis in isolated frog nuclei, essentially all information of G1 cells must be present.

Animals↗
PubMed1978

Constancy of the shift-up point in two temperature-sensitive mammalian cell lines that arrest in G1.

Two cell cycle-specific temperature sensitive (ts) mutants of mammalian cell lines, AF8 and K12, are known to arrest in G1 when shifted to the non-permissive temperature. We have determined the entry into S of both AF8 and K12 cells in five different growth conditions, namely: (1) quiescent sparse cultures stimulated to proliferative by serum; (2) quiescent dense cultures stimulated by serum; (3) quiescent sparse cultures stimulated by trypsinization and replating; (4) quiescent, dense cultures stimulated by trypsinization and replating; and (5) mitotic cells collected by mitotic detachment. In addition, for each cell line and for each different growth condition, we have determined the shift-up time, i.e., the time at which a shift-up to the nonpermissive temperature no longer prevents the entry of cells into S. In no case did K12 or AF8 enter S at the nonpermissive temperature. At the permissive temperature, the average time of entry into S varied in different growth conditions, and so did the shift-up time. However, in both cell lines, the distance of the average shift-up time from the average time of entry into S was remarkably constant, regardless of the growth conditions. i.e., 1.8 hours in K12 and 8.6 hours in AF8.

Cell Cycle↗
PubMed1978

Failure of reactivation of chick erythrocytes after fusion with temperature-sensitive mutants of mammalian cells arrested in G1.

Two temperature-sensitive (ts) mutants of mammalian cell lines (AF8 and cs4D3) that arrest in G1 at the nonpermissive temperature were fused with chick erythrocytes and the induction of DNA synthesis was studied in the resulting heterokaryons. While both AF8 and cs4D3 could induce DNA synthesis in chick nuclei at the permissive temperature, they both failed to do so when arrested in G1 at the nonpermissive temperature. When S phase AF8 cells were fused with chick erythrocytes, chick nuclei were reactivated even if the heterokaryons were incubated at the temperature nonpermissive for AF8. A third ts mutant, ts111, that is blocked in cytokinesis but continues to synthesize DNA, reactivated chick nuclei at both permissive and nonpermissive temperature. It is concluded that chick erythrocyte reactivation depends on the presence of S phase-specific factors.

Animals↗
PubMed1978

Characterization of ts13 cells a temperature-sensitive mutant of the G1 phase of the cell cycle.

ts 13 cells are a temperature-sensitive (ts) mutant of BHK cells that are known to arrest in G1 when shifted to the nonpermissive temperature. We have determined the entry into S of ts13 cells in five different growth conditions, namely: 1) quiescent, sparse cultures stimulated to proliferate by serum. 2) Quiescent, dense cultures stimulated by serum. 3) Quiescent, sparse cultures stimulated by trypsinization and replating. 4) Quiescent, dense cultures stimulated by trypsinization and replating. 5) Mitotic cells collected by mitotic detachment. For each different growth condition we have also determined the execution point of the mutant function, i.e. the time at which a shift-up to the nonpermissive temperature no longer prevents the entry of cells into S. The median time of entry into S and the execution point varied in different growth conditions, but the distance between the median execution point and the median time of entry into S was remarkably constant, i.e. 3.2 hr. In addition we have fused ts 13 cells cells with chick erythrocytes and studied the ability of ts13 cells in heterokaryon formation to induce DNA synthesis in chick nuclei. Although ts13 cells can induce DNA synthesis in chick nuclei at the permissive temperature, they fail to do so when fused and stimulated at the nonpermissive temperature of 39.5 degrees C.

Animals↗
PubMed1978

Effect of 5-bromodeoxyuridine on deoxyribonucleic acid-protein adducts induced by ultraviolet light on chromatin cells.

The formation of DNA-protein adducts induced by ultraviolet irradiation (254-nm. wavelength) has been analyzed by cesium chloride equilibrium sedimentation. The formation of ultraviolet-induced DNA-protein adducts is increased in 5-bromodeoxyuridine-substituted chromatin. Cross-linking is dependent in part upon the extent of 5-bromodeoxyuridine substitution, and is detectable either in chromatin irradiated in vitro or in chromatin from cells irradiated in vivo prior to the isolation of chromatin. As much as 80% of the DNA can be cross-linked to proteins by ultraviolet irradiation of 5-bromodeoxyuridine-substituted chromatin at a fluence of 2,928 Jm-2. In unsubstituted chromatin 11,712 Jm-2 are required to obtain to obtain the same effect. Under the same conditions of 5-bromodeoxyuridine replacement and ultraviolet irradiation only 10% of the total chromosomal proteins can be cross-linked to DNA, whether in vivo or in vitro. Approximately two-thirds of the cross-linked proteins chromatograph as nonhistone proteins, the remaining one-third as histones.

Bromodeoxyuridine↗
PubMed1978

A comparison of cell cycle-related changes in postmitotic and quiescent AF8 cells as measured by cytofluorometry after acridine orange staining.

AF8 cells were collected by mitotic detachment or made quiescent by serum restriction. Replated mitotic cells or serum-stimulated quiescent cells were then compared by flow cytofluorometry, when the use of acridine orange staining. Red fluorescence intensity (F greater than 600) was the same in quiescent cells and in cells immediately after mitosis. However, F greater than 600 increased very rapidly in postmitotic cells, while there was a delay in serum-stimulated quiescent cells. F greater than 600 reached a peak at 4 hr in postmitotic cells and between 16 and 19 hr in serum-stimulated quiescent cells. A similar delay in the time of entry into S phase occurred after serum stimulation of resting cell populations. The results are compatible with the hypothesis that cells after mitosis may enter a state that is different from the state of cells made quiescent by serum restriction.

Acridines↗
PubMed1977

Composition and template activity of chromatin fractionated by isoelectric focusing.

HeLa cell interphase chromatin has been sheared and fractionated by isoelectric focusing. Chromatin fractions are obtained with a wide range of isoelectric points. No free DNA is observed. While protein/DNA rations are similar in the various fractions, they appear to contain different nonhistone chromosomal proteins. A minor chromatin fraction with isoelectric point congruent to 7.0 does not contain histone H1. This fraction is considerably more active as template with different RNA polymerases than the other fractions. Kinetic studies, in which RNA polymerase activity is assayed at various concentrations of chromatin, indicate that the greater activity of Escherichia coli RNA polymerase is due to an increased rate of transcription at saturating concentrations of template (Vmax) and is not due to a lower concentration required for half-maximal rate of transciption (Km). In contrast, the increased rate of transcription by calf-thymus RNA polymerases II and III is due to a decrease in chromatin concentration required for half-maximal rate of transcription rather than an increased rate of transcription at saturating concentrations of template. These results suggest that chromatin with isoelectric point congruent to 7 offers a greater frequency of binding sites for mammalian RNA polymerases, as would be expected for a "transcriptionally active" fraction.

Chromatin↗