Bozidar Djordjevic

Effects of X-Irradiation on RNA and Protein Synthesis in HeLa Cells

Alterations in protein and RNA metabolism in asynchronously growing HeLa cell population following 2000 rads of x-irradiation were studied using sucrose density gradients, radioactive precursor labeling, and autoradiographic procedures. It was found that irradiation reduced protein synthesis within a 2-hour period while a depression of RNA synthesis by x-irradiation was evident somewhat later. Rapidly labeled ribosome precursor RNA was delayed in its transformation into smaller RNA molecules as revealed by pulse chase experiments at a time when inhibition of total RNA synthesis becomes noticeable after 2000 rads of x-irradiation. Specific activities of polysomal fractions from irradiated and control cells labeled with radioactive amino acids are almost identical, but the total amount of radioactivity was less in the sedimentation profile of irradiated cells than in controls. Several possible mechanisms that could result in the depression of protein synthesis are discussed.

Spontaneous Unscheduled DNA Synthesis in G1 HeLa Cells

DNA synthesis can be induced in all stages of the division cycle of mammalian cells by such agents as ultraviolet light1, X-irradiation2 and alkylating agents3. This so-called unscheduled DNA synthesis is of low magnitude compared with normal, semi-conservative DNA synthesis4, and has been equated to repair activity in damaged cells. We have observed that significant amounts of 3H-thymidine are incorporated into the DNA of HeLa cells without an extracellular stimulus, and at a time when normal, semi-conservative DNA synthesis does not occur. We report here the results of autoradiographic investigations of this spontaneous unscheduled DNA synthesis and its possible role in normal DNA metabolism.

The radiosensitivity of cultured mammalian cells exposed to single high intensity pulses of electrons in various concentrations of oxygen.

The oxygen effect in cultured HeLa cells exposed to single short (3 nsec) pulses of electrons from a field emission source has been investigated. Cellular radiosensitivity as measured by the abilit...

The Effect of X-Irradiation on Thymidine Kinase Activity in Synchronous Populations of HeLa Cells

The periodic synthesis of thymidine kinase during the cell cycle has been studied in synchronous populations of HeLa cells. There is a sharp decrease in enzyme activity after cell division followed...

CHANGES IN BIOLOGICAL EFFECTIVENESS OF A FAST NEUTRON BEAM WITH DEPTH IN TISSUE-EQUIVALENT MATERIAL.

A neutron beam of modal energy 6 MeV obtained by bombarding a beryllium target with

Effect of Non-toxic Concentrations of Acriflavine on Unscheduled DNA-synthesis and Survival of Synchronized HeLa Cells Exposed to Ultra-violet Light

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Modification of radiation response in synchronized hela cells by metabolic inhibitors: I. Effect of an inhibitor of RNA-synthesis, puromycin aminonucleoside

ions accelerated by the Institute cyclotron has been employed to study the biological effectiveness of fast neutrons with depth in tissue-equivalent material. The end point used was the reproductive integrity of cultured HeLa cells as judged by colony-forming ability after irradiation. Survival curves obtained after irradiating attached cells at different depths with fast neutrons show a complete lack of shoulder. Split-dose experiments supported this observation in that there is little recovery of radioresistance in the period immediately after the first neutron exposure. The relative biological effectiveness (RBE) of the neutron beam compared to cobalt-60 radiation decreases monotonically from about 3.4 at 2.4 cm depth to about 2.3 at 16 cm depth in the phantom.

Increasing Radiosensitivity in the Course of Fractionated X Irradiation: The Effect of Contact with Dead and Dying Cells

SummaryNon-toxic concentrations (3 × 10−6 M) of acrifavine depress the unscheduled DNA-synthesis in G1 HeLa cells at low ultra-violet (u.v.) doses (< 50 ergs/mm2), but not at higher doses. Moreover, these drug concentrations have little effect on survival at u.v. doses where the depression of unscheduled DNA-synthesis is greatest.

Potentiation of Radiation Lethality in HeLa Cells by Combined Mild Hyperthermia and Chloroquine

SummaryTreatment of irradiated HeLa cells by puromycin aminonucleoside (PAN), an inhibitor of ribosomal RNA-synthesis, markedly reduces colony-forming ability. Post-irradiation treatment by PAN in the mitotically synchronized cells shows that the highest potentiating effect of PAN is observed at the end of the S phase, normally the most radioresistant part of the cell cycle, and least effective during G-2 and G-1 phases, in that order.Based on the known mechanism of action of this drug, the possible role of certain metabolic events in radiation response of living cells is discussed.

Hypothermic potentiation of radiation lethality in HeLa cells.

We have measured survival after successive 2-Gy doses of X rays in HeLa cells and 1-Gy doses in cells of the nonimmortalized human fibroblast cell line AG15-22 under conditions where any effect of cell proliferation during multifraction X irradiation has been factored out. When HeLa cells in parallel series of (pseudo)hybrid spheroids (i.e. in agglomerates consisting of a mixture of supralethally irradiated HeLa feeder and viable HeLa cells) were exposed to n daily radiation doses and then trypsinized and exposed to the last dose, the surviving fraction at 2 Gy (SF2) declined exponentially from 0.55 +/- 0.01 to 0.31 +/- 0.01 after the fifth fraction. In monolayer HeLa cell cultures, the decline in SF2 was smaller but significant and was not influenced by the presence of feeder cells. Pure spheroids, composed entirely of viable HeLa cells, showed the same decline in SF2 as did monolayer cells. The cumulative-effect curve (i.e. the product of SF2 values) was linear-quadratic with the quadratic term increasing in the order monolayer, pure spheroids, pseudohybrid spheroids. SF2n and D0Eff (deduced from the initial SF2) severely underestimated cumulative radiosensitivity. This cumulative effect is clearly associated with the proximity of lethally irradiated cells and might be explained by differential population shifts in the course of the multifraction regimen. Similarly, AG15-22 cells irradiated with daily 1-Gy doses of X rays showed a larger increase in radiosensitivity when in hybrid spheroids than when in pure spheroids. However, for the AG15-22 cells, SF1 was twofold lower for the former than for the latter condition and remained constant for both conditions rather than decreasing with increasing fraction number. This large radiosensitizing effect remains unexplained.