Factors That Modulate Chemical Carcinogenesis in the Mammary Gland of the Female Rat. In: Jones TC, Mohr U, Hunt RD, editors. Integument and Mammary Glands. Berlin: Springer Berlin Heidelberg; 1989. pp. 304-14. [DOI: 10.1007/978-3-642-83749-4_42]

Effects of Rich-in-Fat Diets and Highly Selective COX-2 Inhibitors on 7,12-Dimethylbenz-(A)-Anthracene-Induced Tumor Growth

The effects of diet, of non-steroidal anti-inflammatory drugs, or of their combination on carcinogenesis continue to be a case for controversy. Diets that are high in fat have been linked to increased risk of various tumors. At the same time there is substantial, but not conclusive, evidence that the risk of breast and colon cancer correlates with total fat intake rather than a specific type of fat. On the other hand, non-steroidal anti-inflammatory drugs (NSAIDs) have been studied extensively because they appear to delay or inhibit the development of malignant and pre-malignant lesions. 7,12-Dimethylbenz-(a)-anthracene (DMBA) has been used for a long time to induce carcinogenesis in a number of rat animal models. The present study attempts to identify the effects on DMBA-induced tumor growth (a) of diets rich in fat and (b) of the highly selective COX-2 inhibitor Celecoxib, which has been claimed to offer substantial protection against carcinogenesis.

Significant Differences in the Effects of Magnetic Field Exposure on 7,12-Dimethylbenz(a)anthracene-Induced Mammary Carcinogenesis in Two Substrains of Sprague-Dawley Rats

We have shown previously (S. Thun-Battersby et al., Cancer Res., 59: 3627-3633, 1999) that power-line frequency (50-Hz) magnetic fields (MFs) at micro T-flux densities enhance mammary gland tumor development and growth in the 7,12-dimethylbenz(a)anthracene (DMBA) model of breast cancer in female Sprague-Dawley (SD) rats. We also demonstrated that MF exposure results in an enhanced proliferative activity of the mammary epithelium of SD rats (M. Fedrowitz et al., Cancer Res., 62: 1356-1363, 2002), which is a likely explanation for the cocarcinogenic or tumor-promoting effects of MF exposure in the DMBA model. However, in contrast with our data, in a similar study conducted by Battelle in the United States, no evidence for a cocarcinogenic or tumor-promoting effect of MF exposure was found in the DMBA model in SD rats (L. E. Anderson et al., Carcinogenesis, 20: 1615-1620, 1999). Probably the most important difference between our and the Battelle studies was the use of different substrains of SD rats; the United States rats were much more susceptible to DMBA than the rats used in our studies. This prompted us to compare different substrains of SD outbred rats in our laboratory in respect to MF effects on cell proliferation in the mammary gland, susceptibility to DMBA-induced mammary cancer, and MF effects on mammary tumor development and growth in the DMBA model. The SD substrain (termed "SD1") used in all of our previous studies was considered MF-sensitive and used for comparison with another substrain ("SD2") obtained from the same breeder. In contrast with SD1 rats, no enhanced cell proliferation was determined after MF exposure in SD2 rats. MF exposure significantly increased mammary tumor development and growth in SD1 but not SD2 rats. These data indicate that the genetic background plays a pivotal role in effects of MF exposure. Different strains or substrains of rats may serve to evaluate the genetic factors underlying sensitivity to cocarcinogenic or tumor-promoting effects of MF exposure.

Effects of magnetic fields on mammary tumor development induced by 7,12-dimethylbenz(a)anthracene in rats

A series of epidemiological studies have indicated associations between exposure to magnetic fields (MFs) and a variety of cancers, including breast cancer. In order to test the possibility that MF acts as a cancer promoter or copromoter, four separate experiments have been conducted in rats in which the effects of chronic exposure to MFs on the development of mammary tumors induced by 7,12-dimethylbenz(a)anthracene (DMBA) were determined. Female rats were exposed in magnetic coils for 91 days (24 h/day) to either alternating current (AC; 50 Hz)-MF or direct current (DC)-MF. Magnetic flux density of the DC-MF was 15 mT. Two AC-MF exposures used a homogeneous field with a flux density of 30 mT (rms); one used a gradient field with flux density ranging from 0.3-1 microT. DMBA (5 mg) was administered orally at the onset of MF exposure and was repeated thrice at intervals of 1 week. In each experiment, 18-36 animals were exposed in 6 magnetic coils. The same number of rats were used as sham-exposed control. These control animals were treated with DMBA and were placed in dummy coils in the same room as the MF-exposed rats. Furthermore, groups of age-matched rats (reference controls) were treated with DMBA but housed in another room to exclude any MF exposure due to the magnetic stray field from the MF produced by coils. At the end of the exposure or sham-exposure period, tumor number and weight or size of tumors were determined at necropsy. Results were as follows: In sham-exposed animals or reference controls, the tumor incidence varied between 50 and 78% in the 4 experiments. The average number of mammary tumors per tumor-bearing animal varied between 1.6 and 2.9. In none of the experiments did MFs significantly alter tumor incidence, but in one of the experiments with AC-MF exposure at 30 mT, the number of tumors per tumor-bearing animal was significantly increased. Furthermore, exposure to a DC-MF at 15 mT significantly enhanced the tumor weight. Exposure to a gradient AC-MF at 0.3-1 microT exerted no significant effects. These experiments seem to indicate that MFs at high flux densities may act as a promoter or copromoter of breast cancer. However, this interpretation must be considered only a tentative conclusion because of the limitations of this study, particularly the small sample size used for MF exposure and the lack of repetition of data.