Medical Journal of The Islamic Republic of Iran (MJIRI)

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A better understanding of the mechanism of chromosomal aberration formation could be obtained by using DNA repair inhibitors. Immortalized normal human (MRC 5 SVI) and ataxia telangiectasia ( AT 5 BIV A ) fibroblastic cell lines were treated with adenosine arabinoside (ara-A) and cytosine arabinoside (ara-C), both potent inhibitors of DNA dsb repair, alone or in combination with x-rays at G2 or S-phase of the cell cycle. The length of G2-phase for both cell lines was determined by autoradiographic labeling to be about 4.5-5 h. A similar result was obtained by scoring of chromosomally damaged cells following treatment with ara-A or ara-C for various time intervals before fixation. The results obtained in this study show that in spite of many similarities between the action of ara-A and ara-C, e.g., inhibition of DNA synthesis cIastogenic effects at G2 and S-phase and also lack of synergism as a possible consequence of these similarities, ara-A was found to have a different effect on rejoining of x-ray induced DNA lesions than that of ara-C. Ara-A caused inhibition of chromatid deletion rejoining, interpreted as inhibition of rejoining of DNA dsb at all sampling times before fixation, whereas ara-C showed a synergistic effect on radiation-induced DNA lesions, resulting in an increased frequency of chromatid deletions. Thus there appears that these inhibitors have different modes of action on x-ray induced DNA lesions, which may suggest a peculiar and important difference in the nature of these two nucIeosides.

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The hematopoietic syndrome is anticipated when a dose of radiation greater than 100 cGy is received. The resulting clinical situation is life-threatening because of opportunistic infections and gradual decline in immune competency due to irradiation. Because of evidence of a possible immunomodulatory role for cimetidine, an antagonist of histamine H2 receptors, we studied the effects of this drug on radiationinduced lymphohematopoietic changes. The results obtained in this study indicate that cimetidine is effective in the reduction of radiation-induced injuries with a dose reduction factor of greater than 1.5. Therefore, it might be useful as a radioprotector for low doses of radiation usually used for radiation therapy.

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The relationship between the way in which normal hemopoietic stem cells respond to irradiation alone or in the presence of bleomycin sulfate (BLM-S) and actinomycin 0 (ACT-D) was investigated. Single doses of BLM-S at 0.3 mg/kg and ACT-O at 0.10 mg/kg body weight were injected intravenously 1-6 hours prior to whole body irradiation and treatment was repeated twice more with time intervals. When assessed by survival of spleen colony forming units (CFU-S) of bone marrow cells (BMC), BLM-S alone caused only 10% reduction in survival compared to controls. There was not a significant difference in survival fraction (SF) when treatment with BLM-S was repeated twice more. On the other hand, ACT-O alone caused a 45% reduction in SF after the first injection and only a 10% reduction after the third injection. Increase in survival might be due to resistance induced in BMC after treatments with the drugs. The difference between the 'SF of BMC of mice exposed to doses of 1-3 Gy whole body irradiation was statistically significant with a p-value <0.05. When used in combination with radiation, neither BLM-S nor ACT-O caused a synergistic or additive effect. Although survival was seen to be lower for ACT-O treated animals, the effect was not as pronounced as expected. A significant change in the results was also not observed for fractionated doses of gamma rays in the presence of BLM-S and ACT-O injected at various time intervals. Results obtained from the administration of drugs at various time intervals before irradiation does not suggest a specific time for drug treatment prior to irradiation. These results also suggest that no potentiating effect is likely to be produced by a combination of BLM-S or ACT-O and radiation therapy in bone marrow cells. We therefore believe that these drugs induce a modest resistive response to the effects of radiation on bone marrow cells by a mechanism which is not yet understood. Therefore, using this agent repeatedly for cancer treatment might not cause severe adverse biological effects in bone marrow stem cells.

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The frequency of chromosomal aberrations was studied in the peripheral blood lymphocytes of 65 radiology technologists CRT) working at hospitals chronically exposed to x-rays. Although film dosimetry did not show the maximal annual permitted dose in any of the examined subjects, cytogenetic analysis detected fairly high levels of chromosomal aberrations in R T compared to unexposed controls. The mean frequencies of structural chromosome aberration per 100 lymphocyte metaphases of workers and the controls were 2.93 and 0.54 respectively, excluding the high level of achromatic lesions registered. The difference between them was statistically significant with a P-value of <0.05.

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Application of nuclear magnetic resonance imaging (NMRI) as a non-invasive and accurate imaging procedure has been widely used in recent years. Meanwhile, the biological effects of magnetic fields of several tesla (T) and high energy radiofrequency (RF) is not fully known yet. Because of controversy over this issue, the present research has been carried out in order to verify the effects of magnetic fields of 1.5 T and RF of 63.86 MHz on the frequency of chromosomal aberrations in human peripheral lymphocytes. Using metaphase analysis technique, the cytogenetic effects of NMRI was studied in GO and G2lymphocytes in the presence or absence of cytosine arabinoside (ara-C) as a DNA repair inhibitor. Cells were cultured using conventional methods. Results obtained indicate that exposure of lymphocytes to NMRI field at 30 and 60 minutes has no potential effects on chromosomal aberration induction. When using ara-C, although ara-C alone caused a rather high frequency of chromosomal aberrations, especially in G2 phase of the cell cycle, exposure of cells to NMRI in the presence of ara-C did not change the frequency of ara-C-induced damage significantly. Our results indicate that NMRI may not be able to produce DNA damage that could be potentiated by ara-C. Similar responses were also observed for cells exposed to NMRI either in vivo or in vitro. Nevertheless much remains unknown about the certain effects of MRI and RF.

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Ex vivo expansion of human umbilical cord blood cells (HUCBC) is explored by several investigators to enhance the repopulating potential of HUCBC. The proliferation and expansion of human hematopoietic stem cells (HSC) in ex vivo culture was examined with the goal of generating a suitable clinical protocol for expanding HSC for patient transplantation. Using primary human mesenchymal stem cells, we established a serum-free culture system to expand human primitive progenitors and transplantable stem cells. Non-enriched cord blood CD34+ cells were cultured on a monolayer of human mesenchymal stem cells in the presence of tlu-ombopoietin (TPO), flt31flk2 ligand (FL), and/or stem cell factor (SCF), interleukin 6 (IL-6), interleukin 3 (IL-3) under serum-free conditions. After I or 2 weeks of culture, cells were examined for clonogenic progenitors and percentage of CD34+ CD38- cells. In the presence of TPO, FL, and SCF, fetal MSC cells supported more than a 35- and 20-fold expansion of CD34+ cells and colonyforming units in culture after 1 and 2 weeks of incubation, respectively. In addition, LTC-IC assay were expanded more than 7- and 16-fold after 1 and 2 weeks of culture, respectively. UCB-HSC can be expanded in culture to numbers theoretically adequate for safe, rapid engraftment of adult patients. Additional studies are needed to establish the functional activity of expanded UCB-HSC. This ex vivo expansion system should prove valuable in clinical settings in which stromal cells are available from recipients or stem cell donors.