Radiation-Induced Cell Transformation: Transformation Efficiencies of Different Types of Ionizing Radiation and Molecular Changes in Radiation Transformants and Tumor Cell Lines

This study aims to compare the efficiencies of 5.4 keV soft X-rays, α-particles, and γ-rays in transforming C3H 10T1/2 cells and to assess the sequence of cellular and molecular changes during the process of radiation-induced transformation of Syrian hamster embryo (SHE) cells. The somewhat more densely ionizing soft X-rays are more effective than γ-rays both for cell inactivation and cell transformation. The relative biological effectiveness (RBE) appears to be independent of dose; it is approximately 1.3 for either end point. The RBE of α-particles versus γ-rays, on the other hand, increases with decreasing dose; the dose dependence is somewhat more apparent for cell transformation than for cell inactivation. SHE cells transformed by different types of ionizing radiation and related tumor cell lines isolated from nude mice tumors were found to have a distinct growth advantage compared to primary SHE cells, documented by higher plating efficiencies, shorter doubling times, and higher cloning efficiencies in semisolid medium. Most transformed and tumor cell lines that were investigated have elevated mRNA levels for the H-ras gene, some of them show restriction fragment length polymorphisms of the H-ras gene, and some exhibit a substantially amplified c-myc gene. In a sequence analysis of the Syrian hamster H-ras gene of eight tumor cell lines from radiation transformants, we have not found any mutation in codons 12, 13, 59, 61, nor in the flanking regions of these codons. The transformed and tumor cell lines tend to have lower chromosome numbers than primary SHE cells.