Animal studies on the role of 50/60-Hertz magnetic fields in carcinogenesis

EMF and current cancer concepts

Exposure to power frequency electric and magnetic fields (EMF) is ubiquitous, and a body of epidemiologic studies has produced evidence suggestive of a possible link between EMF exposure and cancer of several types. This paper provides a perspective that holds key findings in the EMF literature against the background of important models and established principles in cancer biology. It is intended primarily for scientists whose expertise lies outside of cancer biology and animal bioassays. Current thinking holds that carcinogenesis is a multistep process that requires at least two genotoxic events in its critical path but that is facilitated by nongenotoxic proliferative effects on target cells. EMF, which itself is not believed to be genotoxic, could influence carcinogenesis if it exerted either direct or indirect effects on target cell turnover. Such effects could operate through receptor-mediated or nonreceptor-mediated pathways. However, effects relevant to carcinogenesis have not been confirmed, and a mode of action for EMF has not been determined. Chronic bioassays in rodents are in progress to examine the potential carcinogenicity of EMFs. EMF research has the opportunity to capitalize on the recent major advances in our understanding of carcinogenic processes.

Protein kinase C activity is altered in HL60 cells exposed to 60 Hz AC electric fields

We examined the effects of electric fields (EFs) on the activity and subcellular distribution of protein kinase C (PKC) of living HL60 cells. Sixty Hertz AC sinusoidal EFs (1.5-1,000 mV/cm p-p) were applied for 1 h to cells (10(7)/ml) in Teflon chambers at 37 degrees C in the presence or absence of 2 microM phorbol 12-myristate 13-acetate (PMA). PMA stimulation alone evoked intracellular translocation of PKC from the cytosolic to particulate fractions. In cells that were exposed to EFs (100-1,000 mV/cm) without PMA, a loss of PKC activity from the cytosol, but no concomitant rise in particulate PKC activity, was observed. In the presence of PMA, EFs (33-330 mV/cm) also accentuated the expected loss of PKC activity from the cytosol and augmented the rise in PKC activity in the particulate fraction. These data show that EFs alone or combined with PMA promote down-regulation of cytosolic PKC activity similar to that evoked by mitogens and tumor promoters but that it does not elicit the concomitant rise in particulate activity seen with those agents.

Linear relationship between flux density and tumor co-promoting effect of prolonged magnetic field exposure in a breast cancer model

Previous epidemiologic studies have suggested that exposure to 50- or 60-Hz (power-frequency) magnetic fields in occupational or residential environments may increase the risk of certain cancers, including breast cancer. However, in view of the methodological problems of epidemiological studies on associations between magnetic field exposures and increased incidence of cancers, laboratory studies are necessary to determine if 50/60-Hz magnetic fields are cancer promoters or can progress cancers. The objective of the present study was to characterize the relation, if any, between dose (i.e. flux density) of 50-Hz magnetic field exposure and tumor growth in a model of breast cancer in female rats. Mammary tumors were induced by the chemical carcinogen 7, 12-dimethylbenz[a]anthracene (DMBA). The dosing protocol of DMBA chosen for the magnetic field experiments induced mammary tumors in about 50% of sham-exposed control animals within 3 months after application. Per flux density, a group of 36-99 rats was exposed to a magnetic field of 50-Hz for 24 h/day 7 days/week; another group of 36-99 rats was sham-exposed under the same environmental conditions as the MF-exposed rats. The exposure chambers were identical for MF-exposed and sham-exposed animals. DMBA was administered orally at a dose of 5 mg per rat at the first day of exposure and at weekly intervals thereafter up to a total dose of 20 g per rat. Duration of MF- or sham-exposure was 91 days. At the end of the exposure period, the animals were sacrificed for examination of the number of mammary tumors. Four flux densities were studied in a total of 666 rats (including sham-exposed controls): 0.3-1 muT, 10 muT, 50 muT, and 100 muT. At autopsy, i.e. at the end of the 13 weeks period of MF-exposure, incidence of macroscopically visible mammary tumors was significantly enhanced in the experiment with 50 muT (25.5% above control) and 100 muT (50% above control). No increase in incidence of mammary tumors was seen in the experiment with 0.3-1 muT, while a 10% (non-significant) increase was determined in the experiment with 10 muT. Linear regression analysis of the data from the four experiments indicated a highly significant linear relation between flux density and increase in tumor incidence at time of autopsy. The correlation coefficient was 0.9944 (P < 0.01). The data demonstrate that long-term exposure of DMBA-treated female rats increases the growth of mammary tumors in a highly dose-related fashion.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo exposure of rats to a weak alternating magnetic field increases ornithine decarboxylase activity in the mammary gland by a similar extent as the carcinogen DMBA

Magnetic field (MF) exposure has been discussed in the process of tumor promotion as indicated by epidemiologic data as well as laboratory studies. However, the precise mechanisms of tumor promoting effects of MFs are unknown. Tumor promotion is often accompanied by an increase in the activity of the enzyme ornithine decarboxylase (ODC), i.e. a key enzyme in the biosynthesis of polyamines, which have roles in cell proliferation and control of gene expression. In the present work, we studied if exposure of female rats to a 50-Hz MF with a flux density of 50 microT influences ODC activity in different tissues, including the mamma. Rats were exposed for a period of 6 weeks either with or without oral administration of the chemical carcinogen DMBA and all data were compared with those from sham-exposed controls. Magnetic field exposure resulted in an approximate doubling of ODC in mammary tissue. A significant ODC increase was also seen in the spleen, but not in the liver, small intestine, bone marrow, and ear skin. The ODC increase produced by MF exposure in the mammae was of similar magnitude as that observed after treatment with DMBA. Combined treatment with MF and DMBA was not more effective in increasing ODC than treatment with DMBA alone, except for liver tissue. The present results on in vivo increases of ODC by MF exposure strengthen the hypothesis that weak 50-Hz MFs affect ODC activity and may thus function as a tumor-promoting or co-promoting agent.

Cytological effects of 50 Hz electromagnetic fields on human lymphocytes in vitro

Incubation of human peripheral blood cultures in the presence of an electromagnetic field (EMF) of 50 Hz and 5 mT leads to stimulation of the cell cycle of dividing lymphocytes but has no influence on the frequencies of sister-chromatid exchanges. Comparative studies with two different exposure systems and with different culture temperatures indicate that the effect on the cell cycle results from the EMF and is not a thermal effect. These data support the assumption that with respect to their suspected carcinogenic effects EMFs have no initiating but probably promoting effects.

Effects of static and time-varying (50-Hz) magnetic fields on reproduction and fetal development in rats

Mated Wistar rats were chronically exposed to a static magnetic field (MF) from day 1 to day 20 of pregnancy. Flux density of the MF was 30 mTesla (mT), i.e., within the range of exposures of operators of magnetic resonance imaging (MRI) devices. For comparison with effects of this static field, a time-varying (50-Hz) MF of the same flux density was used. In both experiments, i.e., static or 50-Hz MF exposure, sham-exposed groups of dams served as controls. On day 20, the dams were sacrificed for reproductive and teratological assessment. The litters were evaluated for numbers of implantations, fetal deaths and resorptions, gross external, visceral, and skeletal malformations, and fetal weights. No adverse effects were seen in the dams during MF exposure. The mean number of living fetuses per litter was significantly decreased in the group with static MF exposure, but not the group with 50-Hz MF exposure. Further differences between the two types of field exposure were observed with respect to minor (skeletal) fetal anomalies which were increased in the 50-Hz but not the static MF group. No serious malformations were seen in any group. In both MF-exposed groups, increased skeletal ossification was found, possibly indicating an accelerated prenatal development. In a second experiment with static MF (30 mT) in which dams were exposed for the whole period of pregnancy, and postnatal growth and development of offspring were studied, the postnatal growth was significantly enhanced in offspring following their prenatal MF exposure.(ABSTRACT TRUNCATED AT 250 WORDS)