Additive Effects of X-Rays and Methylcholanthrene in Inducing Mouse Leukemia

Abstract

Agents of three general types, one physical and two chemical, have been shown to be leukemogenic for mice. These are ionizing radiations, such as x-rays and gamma rays (1–3), carcinogenic hydrocarbons, as for example methylcholanthrene benzpyrene and 9,10-dimethyl-1,2- benzanthracene (4–7), and various estrogens (8, 9). In susceptible stocks the effectiveness of these agents is proportional to the dose used, varying with the genetic constitution of the strain of mice (3). Furth and Boon (10) reported synergistic effects of combined application of methylcholanthrene and irradiation with x-rays, but Kaplan and Kirschbaum (11) were unable to demonstrate synergism of these agents for other stocks of mice. However, in the latter experiments very potent doses of each agent were used, and it is probable that under such circumstances neither additive nor synergistic effects may be obtained. MeEndy, Boon and Furth (12) did not observe additive effects when very large doses of each agent were used. The purposes of the experiments to be reported here are the following: 1. To confirm the observation that two leukemogens may, when applied simultaneously to mice in greater than threshold doses, have additive effects. 2. To show that the phenomenon of additive effects of two leukemogens probably depends upon susceptibility of the strain of mice to the independent action of each of the leukemogens. 3. To demonstrate the importance of genetic constitution in determining leukemogenesis. Material and Methods The x-ray-treated mice were exposed to general body irradiation in cardboard boxes at 30 cm. target-skin distance, 140 kv., and 2 mm. aluminum filter, h.v.1. 4.2 mm. A1. All doses are given as roentgens in air at the surface of the mouse. Carcinogen- treated mice were skin-painted three times weekly with a 0.25 per cent solution of methylcholanthrene in benzene. A camel's-hair brush, 3/8 by 5/8 inches, was used, a different site being chosen for each successive painting to decrease the number of induced skin tumors. Four groups of strain dba mice (subline 212) are included in this study, as follows: 1. Thirty-four mice received a total of 1,100 r of x-rays in eleven weekly treatments of 100 r. The mice were ten weeks of age at the time of first exposure. 2. Thirty-four mice received eighteen skin paintings of methylcholanthrene, the first painting at fourteen weeks of age. 3. Forty-eight mice received both treatments-x-rays (total of 750–1,100 r) beginning at ten weeks and methylcholanthrene at fourteen weeks of age. 4. Ninety-two controls received neither x-radiation nor methylcholanthrene. Four groups of strain CBA mice were put on experiment as follows: 1. Twenty-one mice received 500 r in ten weeks (50 r X 10) and 5 animals received 750 r (100 r X 5 and 50 r X 5). Mice were ten weeks of age at first exposure.