Radiation damage repair capacity of a human germ-cell tumour cell line: inhibition by 3-aminobenzamide

The capacity of a human germ-cell tumour line to repair radiation damage has been investigated by means of a clonogenic assay. Dose-rate dependence studies, split-dose experiments and experiments designed to measure repair of potentially lethal damage have been performed. The cells showed some ability to repair radiation-induced damage in all three types of experiment. An attempt has been made to understand the possible cellular mechanisms of these repair processes by the use of 3-aminobenzamide (3-AB), an agent thought to act by inhibition of ADP-ribosylation. 3-AB added 2 h prior to and removed 18 h after irradiation at a non-toxic dose to unirradiated cells caused a small but consistent increase in cell kill with acute (150 cGy min-1) irradiation, largely involving a reduction in the shoulder region of the survival curve, but had a greater effect in increasing cell kill at a dose rate of 7.6 cGy min-1 and an even greater effect at a dose rate of 1.6 cGy min-1. When 3-Ab was present 2 h prior to the first dose and between two equal doses in a split-dose experiment, inhibition of split-dose recovery was observed. In addition, some inhibition of potentially lethal damage recovery was observed with 3-AB. A possible role for poly(ADP-ribosylation) is thus implicated in the repair of radiation-induced damage of this human tumour cell line during continuous low dose rate or fractionated radiation schedules, although other effects of 3-AB on respiratory metabolism and/or purine synthesis cannot be eliminated as the cause of the observed inhibitory effects.     

Arachidonic acid and linoleic acid supplementation increase prostanoid production in rats fed a butter-enriched diet

Male Sprague Dawley rats were fed a butter-enriched diet (50% fat) for 2 weeks and then supplemented orally with either 90 mg of ethyl arachidonate or ethyl linoleate daily for 2 weeks. For comparative reasons, one group of animals was fed standard laboratory rat chow for 4 weeks. Aortic prostacyclin (PGI2) production, platelet aggregation and thromboxane A2 (TXA2) production and plasma and aortic phospholipid (PL) fatty acids were measured. When compared to butter-fed rats, aortic PGI2 production, collagen-induced platelet aggregation and TXA2 production were significantly increased in rats supplemented with ethyl arachidonate to levels similar to those seen in chow-fed rats. Ethyl linoleate supplementation also tended to increase aortic PGI2 production, collagen-induced platelet aggregation and TXA2, but not to the same extent. These changes were accompanied by increases in the level of arachidonic acid and linoleic acid in aortic and plasma PL and a decrease in the level of eicosapentaenoic acid (EPA) and docsahexaenoic acid (DHA). These data indicate that supplementation with small doses of preformed arachidonic acid was more effective than supplementation with its precursor, linoleic acid, in reversing the effects on prostanoid production and phospholipid fatty acid composition in rats fed diets enriched with butter.     

Protection of mice against mixed fission neutron-gamma (n: gamma = 1:1) irradiation by WR-2721, 16,16-dimethyl PGE2, and the combination of both agents

The survival of mice after whole-body exposure to a modified fission neutron-gamma field (n: gamma = 1:1) was used to examine radiation protection by WR-2721, 16,16-dimethyl PGE2(DiPGE2), and the combination of both agents. Administration of WR-2721 (453 mg/kg) increased the LD50/30 from 5.24 to 7.17 Gy (DMF = 1.37), whereas pretreatment with DiPGE2 (1.6 mg/kg) increased the LD50/30 to 5.77 Gy (dose modification factor (DMF) = 1.10). The combination of 453 mg/kg WR-2721 and 0.4 mg/kg DiPGE2 resulted in an LD50/30 of 7.33 Gy, yielding a DMF of 1.39. However, no significant difference in protection was obtained with the combination of the two agents compared to that seen with WR-2721 alone.     

Characterization of a bombesin receptor on Swiss mouse 3T3 cells by affinity cross-linking

We have previously identified by chemical cross-linking a cell surface protein in Swiss 3T3 cells of apparent Mr 75,000-85,000, which may represent a major component of the receptor for peptides of the bombesin family in these cells. Because bombesin-like peptides may interact with other cell surface molecules, it was important to establish the correlation between receptor binding and functions of this complex and further characterize the Mr 75,000-85,000 cross-linked protein. Detailed time courses carried out at different temperatures demonstrated that the Mr 75,000-85,000 affinity-labelled band was the earliest cross-linked complex detected in Swiss 3T3 cells incubated with 125I-labelled gastrin-releasing peptide (125I-GRP). Furthermore, the ability of various nonradioactive bombesin agonists and antagonists to block the formation of the Mr 75,000-85,000 cross-linked complex correlated extremely well (r = 0.994) with the relative capacity of these peptides to inhibit 125I-GRP specific binding. Pretreatment with unlabelled GRP for up to 6 h caused only a slight decrease in both specific 125I-GRP binding and the affinity labelling of the Mr 75,000-85,000 protein. We also show that the cross-linked complex is a glycoprotein. First, solubilized affinity labelled Mr 75,000-85,000 complex applied to wheat germ lectin-sepharose columns was eluted by addition of 0.3 M N-acetyl-D-glucosamine. Second, treatment with endo-beta-N-acetylglucosaminidase F reduced the apparent molecular weight of the affinity-labelled band from 75,000-85,000 to 43,000, indicating the presence of N-linked oligosaccharide groups.     

ACTIVITY OF MARGINAL AND PLATE MERISTEMS DURING LEAF DEVELOPMENT OF XANTHIUM PENNSYLVANICUM

The mitotic and biosynthetic activities of the marginal and plate meristems were studied during the entire course of leaf development of Xanthium pennsylvanicum. In contrast to statements in the literature, marginal meristem activity is long in duration, as assayed by the mitotic counts and H³-thymidine incorporation. This me istem is active 23 days. The plate meristem is active for an additional 3 days after cessation of cell division in the marginal meristem, but the total duration of its mitotic activity is also approximately 23 days. Numerous periclinal cell divisions of the plate meristem form additional cell layers and contribute to the growth of the lamina in thickness. Incorporation of H³-thymidine increased during the course of leaf development. Cells between plastochronic ages 0 and 2.0 incorporated more of the radioisotopic precursor than those of younger leaf primordia. The uptake and incorporation of H³-thymidine into nuclear DNA was more sluggish during the early stages of development than in the more expanded leaves. No DNA synthesis was demonstrated after cessation of cell division in the leaf lamina. Metabolic or endomitotic DNA synthesis after leaf plastochron index (LPI) 3.0 seems improbable. No significant differences in the incorporation of H³-thymidine could be demonstrated between the marginal and plate meristems. This would indicate no distinct biosynthetic differences between the two meristems. The definitions of the marginal and plate meristems of Xanthium leaves were formulated in view of the above findings.