Oncogenes in X-ray-transformed C3H 10T1/2 mouse cells and in X-ray-induced mouse fibrosarcoma (RIF-1) cells

Neutron carcinogenesis: past, present, and future

An interest in the possible cancer causing ability of neutrons began soon after their discovery. Early use of neutrons from radioactive sources and from cyclotrons led to a need to define risk for such exposures. This need was soon followed by a more tangible need to define risk to the general population of high LET radiation from nuclear fall out and use of the Atomic bomb and possible use of the H-bomb. Neutrons were soon found to be very effective cell killing agents compared to conventional ionizing radiation. However High LET radiation sources and neutrons in particular, come in many different energies and from many types of sources. I will survey the differences between different energy neutrons and conventional types of radiation, particularly with respect to the dose rate of exposures and the influence of repair or lack thereof and more recently the effect of cell cycle distribution on the carcinogenic outcome. I will illustrate these ideas with examples of carcinogenicity studies and mutation studies from my own laboratory and in some cases from the work of others. Lastly I will introduce some possible avenues for molecular studies of neutron effects that might answer such vexing questions as the real risk at very low doses, is repair error free or error prone, do neutrons cause genetic instability for many cell generations after exposure, and others? There remain many questions about the biology of neutron action that require answers if we are to protect the ever increasing number of people exposed to them because of their growing use in medicine, in the military and in commercial industry.

Alteration of p53 protein in C3H/10T1/2 cells morphologically transformed by gamma-rays in stationary phase

The panel of 70 transformed clones was isolated after exposure of C3H/10T1/2 cells in stationary phase to low-moderate doses (1-4 Gy) of 137Cs gamma-rays. Two widely different dose rates were used: high (HDR, 0.66 Gy/min) and low (LDR, 4.8 x 10(-4) Gy/min). Immunohistochemical analyses were performed by cellular staining with three types of monoclonal anti-p53 antibodies, Ab-1 (PAb421), Ab-3 (PAb240) and Ab-4 (PAb246) in order to identify wild-type and altered conformation of the p53 protein in cell nuclei. The gamma-ray exposure led to induction of altered p53 protein in the majority of morphologically transformed clones. For LDR exposure the percentage of clones with changed p53 protein was 79 (11/14), 71 (12/17) and 100 (6/6) for the exposure doses of 2, 3 and 3.6 Gy, respectively. For HDR exposure the percentage of such clones was 60 (3/5), 40 (4/10) and 87 (13/15) for 1, 2 and 3 Gy, respectively. Moreover, in some transformed clones, especially in those induced by higher gamma-ray doses, p53 protein in cell nuclei was not found. It was demonstrated by lack of the staining with Ab-1 antibody which can detect both mutant and wild-type of p53 protein. An altered conformation of p53 protein was detected, using Ab-3 antibody which does not react with its native conformation, in 27% (18/67) of all radiation-induced clones. A native conformation of p53 protein was recognized by Ab-4 antibody in 33% (10/30) of clones induced by HDR, and in 22% (8/37) of clones induced by LDR exposure. Our study shows that alterations of p53 protein in cell nuclei is a frequent feature of morphological transformations induced by ionizing radiation in C3H/10T1/2 cells, and that these alterations occur independently of dose rate.

Search for oncogene mutations in X-ray-transformed mouse 10T1/2 cells by denaturing gradient gel electrophoresis blotting

We have sought evidence for possible mutations within the c-myc, c-Ha-ras and c-Ki-ras loci of X-ray-transformed mouse C3H10T1/2 cell clones using the denaturing gradient gel electrophoresis (DGGE) blot technique. This highly sensitive method was developed to detect any mutations (e.g. single base changes, small deletions) in genomic DNA, by measuring differences in the melting behaviour of short DNA fragments (50-800 bp) obtained by digestion of genomic DNA with several specific 4 bp recognition site restriction enzymes. In this study, genomic DNAs derived from 23 X-ray-transformed clones were digested with several restriction enzymes, electrophorezed on denaturing gradient gel and hybridized to c-myc, c-Ha-ras and c-Ki-ras cDNA probes. No alterations in melting patterns were observed for any of these oncogenes as compared with DNA from 18 control, non-irradiated wild-type 10T1/2 cell clones, suggesting that transformation was not associated with mutation of these genes nor with changes in their patterns of methylation. However, our screening of the large portion of exons 2 and 3 of c-myc as well as of exons 1 and 2 of c-Ha-ras gene cannot exclude the possibility that some sequence differences in the high melting domains of examined fragments were not detected by this assay.

Control of the cell cycle

Cell biology has made major progress in identifying the molecules that drive the cell cycle. The evidence accumulating from these studies indicates that derangements in the cell cycle machinery contribute to the uncontrolled cell growth of tumours. The cell cycle machinery has been found to be substantially altered in tumour cells and also may be crucial for carcinogenesis. In this context, various aspects of tumour cell growth have been studied in an effort to understand 1) why tumour cells display uncontrolled growth, 2) why radiation selectively affects growing cells, and 3) whether aspects of the cell cycle and tumour cell growth may be used in tumour diagnosis and prognosis.