Low intensity and frequency pulsed electromagnetic fields selectively impair breast cancer cell viability

Abstract

Introduction: A common drawback of many anticancer therapies is non-specificity in action of killing. We investigated the potential of ultra-low intensity and frequency pulsed electromagnetic fields (PEMFs) to kill breast cancer cells. Our criteria to accept this technology as a potentially valid therapeutic approach were: 1) cytotoxicity to breast cancer cells and; 2) that the designed fields proved innocuous to healthy cell classes that would be exposed to the PEMFs during clinical treatment.

Methods: MCF7 breast cancer cells and their normal counterparts, MCF10 cells, were exposed to PEMFs and cytotoxic indices measured in order to design PEMF paradigms that best kill breast cancer cells. The PEMF parameters tested were: 1) frequencies ranging from 20 to 50 Hz; 2) intensities ranging from 2 mT to 5 mT and; 3) exposure durations ranging from 30 to 90 minutes per day for up to three days to determine the optimum parameters for selective cancer cell killing.

Results: We observed a discrete window of vulnerability of MCF7 cells to PEMFs of 20 Hz frequency, 3 mT magnitude and exposure duration of 60 minutes per day. The cell damage accrued in response to PEMFs increased with time and gained significance after three days of consecutive daily exposure. By contrast, the PEMFs parameters determined to be most cytotoxic to breast cancer MCF-7 cells were not damaging to normal MCF-10 cells.

Conclusion: Based on our data it appears that PEMF-based anticancer strategies may represent a new therapeutic approach to treat breast cancer without affecting normal tissues in a manner that is non-invasive and can be potentially combined with existing anti-cancer treatments.

Conflict of interest statement

Competing Interests: One of the authors, Grit Shade, is an employee of Amphasys, the company that provided the authors with the prototype of the Impedance Flow Cytometer utilized to conduct some of the experiments in the manuscript. GS provided technical support only. There are no patents, products in development or marketed products to declare. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials.

Figures

Figure 1. Trypan blue detection of dead cells after exposure to PEMFs for 3 consecutive days.

(A) The percentage dead MCF-7 and MCF-10 cells after exposure to 2, 3 or 5 mT PEMFs at a frequency of 20 Hz for 60 minutes a day for three days. MCF7 breast cancer cell viability was significantly reduced by exposure to PEMFs relative to unexposed samples (controls) or MCF-10 cells (P-values, left to right: 0.02857, 0.00004, 0.02857). (B) Cells treated with PEMFs (3 mT at 20 Hz) for 30, 60 or 90 minutes per day for 3 consecutive days. The histogram depicts the percentage of dead cancer cells relative to unexposed (control) samples (((PEMFs exposed trypan blue positive cells - unexposed trypan blue positive cells)/unexposed trypan blue positive cells))/total cells). Sixty minutes exposures to 3 mT PEMFs significantly increased MCF7 cancer cell death, whereas shorter (30 minutes) or longer (90 minutes) exposure durations exerted smaller effects (P-values, left to right: 0.03175, 0.00004, 0.00015). Values represent the averages of at least 4 independent experiments (n = 4, 12, 4 for 2, 3 and 5 mT, respectively; n =  5, 12, 8 for 30, 60 and 90 minutes, respectively) for MCF7 cells (average ± SD). A total of 5 independent experiments (average ± SD) is provided for MCF-10 cells for all conditions. MCF10 were unresponsive to PEMFs (3 mT, 60 minutes per day for three days) (also see Figure S3). 50 Hz PEMFs (3 mT for 60 minutes a day for three days) was less effective at killing MCF-7 cells (see Figure S2). The potential recovery of MCF-7 cancer cells following PEMF treatment is addressed in Figure S6.

Figure 2. Time course in the development of cell death in response to PEMF exposure.

Histograms showing the total number of cells (dark grey) and the total number of dead cells (trypan blue positive, light grey) after 1, 2 or 3 days of daily PEMF exposure (B, D) or in unexposed (control) cultures (A, C). (A, C) Unexposed cultures exhibited a steady increase in bulk cell number during 3 days in culture. (B) Exposure to 3 mT PEMFs for 60 min/day abrogated the typical monotonic increase in total cell number (dark grey) observed in unexposed samples (A) concomitant with an increase in the amount of trypan blue positive cells (light grey) that increased in significance with consecutive daily exposures to PEMFs. The total number of cells in treated samples showed a 40% (+/– 6%) decrease relative to control, whereas trypan blue positive cells increased by 20% (+/– 13%), (total cells in control sample – total cell in treated sample)/total cells in control sample) and (dead cells in control sample – dead cell in treated sample)/dead cells in control sample), respectively. (D) Exposure to 3 mT PEMFs for 90 min/day slowed the increase in total cell number (dark grey) typical of control samples in combination with an increase in the amount of trypan blue positive cells (light grey) that increased in significance with consecutive daily exposures to PEMFs. The total amount of cells in treated sample showed a 20% (+/– 4%) decrease relative to control, whereas trypan blue positive cells increased by 36% (+/– 10%), (total cells in control sample – total cell in treated sample)/total cells in control sample) and (dead cells in control sample – dead cell in treated sample)/dead cells in control sample), respectively. All the values represent the averages of 4 independent experiments with 3 replicates/experiment (n = 12) for the 60-min/day time points and 2 replicates/experiments (n = 8) for 90-min/day time points. P-values, left to right: 0.3246, 0.02032, 0.00004 for 60min/day of exposure and 0.2595, 0.02953, 0.00015 for 90 min/day of exposure.

Figure 3. Box plots depicting the increase in cell death after 1, 2 or 3 days of consecutive PEMF treatment.

(A) 3 mT PEMFs for 60 min/day impaired MCF7 cancer cell viability sufficiently to cause a time-dependent accumulation of compromised cells over the time course of 1 to 3 days. The most significant degree of cell impairment was seen after 3 days (4 independent experiments with 3 replicates/experiment (n = 12)) (p-values, left to right: 0.3246, 0.02032, 0.00004) (also see Table 1 for the mean, high value, low value and average absolute deviation from median). (B) MCF7 cancer cells treated with 3 mT PEMFs for 90 min/day for 1, 2 or 3 days. Overall, 90 min/day of exposure produced less cytotoxicity than 60 min/day. Data were generated from 4 independent experiments with 2 replicates/experiment (n = 8) (p-values, left to right: 0.2595, 0.02953, 0.00015) (also see table 2 for the mean, high value, low value and average absolute deviation from median).

Figure 4. FCM determination of PEMF-induced DNA damage in MCF7 (cancer) and MCF10 (non-tumorigenic).

(A) Overlay of MCF7 cell populations treated with 2, 3 or 5 mT PEMF amplitudes at 20 Hz for 60 minutes per day for 3 days. MCF7 cells exposed to 3 mT PEMFs showed the greatest degree of DNA strand breaks as reflected by their greater fluorescence intensity (larger FL1 values). (B) Overlay of MCF7 cell populations treated with 3 mT PEMFs (20 Hz) for 30, 60 or 90 minutes per day for three days. The highest level of PEMF-induced DNA fragmentation occurred in response to 60-minute exposures. (C) Percentage of MCF7 apoptotic cells (relative to control) detected by flow cytometry after exposure to 2, 3 or 5 mT PEMFs for 60 minutes per day for 3 days. Values represent the averages of 5 independent experiments (single replicates (n = 5)) (average ± SD); P values, left to right: 0.1, 0.02857 and 0.02857. (D) Percentage of MCF7 apoptotic cells after exposure to 3 mT (20 Hz) PEMFs for 30, 60 or 90 minutes/day for three consecutive days. Values represent the averages of 5 independent experiments (single replicates (n = 5)) (average ± SD); P values, left to right: 0.1, 0.02857 and 0.02857. (E) MCF10 normal breast cells are unharmed by the PEMF parameters shown to cause the greatest apoptosis in MCF7 cancer cells. Indeed, 3 mT PEMFs applied for 60 minutes per day for three days reduced basal apoptotic rates in MCF-10 cells, suggesting that PEMFs are protective to normal cells. The dot plots shown were generated from 1 of 5 independent experiments showing representative responses. Two different measurements obtained from 2 independents experiments were chosen for 3 mT 60 min condition for figure A and B (also see Figure S7).

Figure 5. Time course of apoptosis induction by PEMFs in MCF7 cells determined by FCM.

(A) Overlay of MCF7 cells treated with 3 mT PEMFs for 60 min/day for 1, 2 or 3 consecutive days. PEMF-induced DNA damage accrued with time yet, only obtained significance after 3 consecutive days of exposure. (B) Overlay of MCF7 cells exposed to 3 mT PEMFs for 90 min/day for 1, 2 or 3 consecutive days. As in A statistical significance was only achieved after three days. Paralleling our trypan blue (figures 1B, 2A-D and 3 A-B) and FCM (figure 4A-D) results, 90 min/day of exposure to PEMFs (3 mT) was less cytotoxic than 60 min/day. (C) Percentage of MCF7 apoptotic cells (relative to control) detected by flow cytometry after exposure to 3 mT PEMFs for 60 minutes per day for 1 day up to 3 days. Values represent the averages of 3, 3 and 5 independent experiments for 1, 2 or 3 days exposure, respectively (1 replicate/experiment (total n = 3, 3, 5, respectively)) (average ± SD); P values, left to right: 0.1, 0.1 and 0.02857. (D) Percentage of MCF7 apoptotic cells after exposure to 3 mT PEMFs for 90 minutes/day for 1, 2 or 3 consecutive days. Values represent the averages of 3, 3 and 5 independent experiments for 1, 2 or 3 days of exposure, respectively (single replicates (total n = 3, 3, 5, respectively)) (average ± SD); P values, left to right: 0.1, 0.1 and 0.02857.

Figure 6. Post-PEMF apoptosis determination by impedance flow cytometry (IFC) at 0.5 MHz.

(A) Dot plots generated from MCF7 cell exposed to 2, 3 or 5 mT amplitude PEMFs for 60 minutes per day for three days. The histograms above and to the right of each dot plot show the apoptotic cell subpopulation shaded in black. MCF-7 cancer cells treated with 3 mT PEMFs exhibited the greatest separation between viable (right) and non-viable (left) cell populations as well as a higher overall percentage of dead cells. (B) Viability of MCF7 cells after exposure to 3 mT (20 Hz) for 30, 60 or 90 minutes per day for three days. (C) Percentage of MCF7 apoptotic cells detected by IFC in response to 2, 3 or 5 mT PEMFs normalized to its respective control. Each value represents the average of 4 independent experiments (1 replicate/experiment, n = 4) (± SD); P values, left to right: 0.4818, 0.0004552 and 0.1818. (D) Percentage of MCF7 dead cells in each treated sample normalized to its respective control in response to 30, 60 or 90 minutes exposures to PEMFs. Each value represents the average of 4 independent experiments (1 replicate/experiment, n = 4) (± SD). P-values, left to right: 0.1905, 0.0004552 and 0.3929. (E) MCF10 cells treated with PEMFs (3 mT, 20 Hz) for 60 minutes/day for three days. The dot plots shown were generated from cells of the same experimental date and are representative of cells responses observed in all of the independent experiments with identical conditions. Two different replicates obtained from 2 independents experiments were chosen for the 3 mT, 60 minute condition for figure A and B. Also see Figure S8 for the spread of individual measurements.

Figure 7. MCF7 and MCF10 cell metabolic status analyzed by IFC at 9 MHz.

(A) Dot plots generated from MCF7 cells after exposure to PEMFs of 2, 3 or 5 mTs and in control (non-exposed) samples and analyzed at a scan frequency of 9 MHz. Exposed samples exhibited a larger right-side population, particularly after exposure to 3 mT PEMFs. (B) Dot plots of MCF7 cells after exposure to 30, 60 or 90 minutes of PEMFs (3 mT, 20 Hz) per day for 3 days; the right-side population was preferentially enhanced in response to 60 minutes exposures. (C) Histograms depicting the percentage increase in the size of the right population normalized to controls after exposure to 2, 3 or 5 mT PEMFs for 60 minutes. Each value is the average of 4 independent experiments (1 replicate/experiment, n = 4) (± SD). P-values, left to right: 0.00879, 0.0017 and 0.07033. (D) Size of right population as a function of exposure duration and normalized to each respective control (unexposed) sample; the right-side population was preferentially enhanced in response to 60 minutes exposures. Each value is the average of 4 independent experiments (1 replicate/experiment, n = 4) (± SD). P-values, left to right: 0.6786, 0.0017 and 1. (E) Dot plots generated from MCF10 cells exposed to 3 mT PEMFs (20 Hz) for 60 minutes/day for three days and in control (unexposed) samples, revealing essentially no change in response to treatment. The dot plots shown were generated from cells of the same experimental date and are representative of cells responses observed in all of the independent experiments with identical conditions. Also see Figure S8, for the spread of individual measurements.

Figure 8. Independent corroboration that IFC detects impaired cells at 0.5 MHz and 9 MHz.

(A) Comparison of the dot plots (left) and amplitude histograms (left (above, inset) and right (vertically expanded)) generated from MCF7 cells exposed to PEMFs or H2O2 at 0,5 MHz. PEMFs produce similar population displacements to the lower left quadrant of the dot plot (lower phase and magnitude values) as in H2O2 treated samples: untreated cells (gray), cells exposed to PEMFs (3 mT, 20 Hz for 60 min/day for 3 days; 25% dead cells: black) and cells incubated overnight with H2O2 (1 mM; 87% dead cells: light blue). (B) Amplitude histograms correspondent to dot plots generated from MCF7 cells and analyzed by IFC at 9 MHz. H2O2 treatment (1 mM; producing 87% cell death) caused the displacement of the entire cell population to the right; horizontal bars indicate inclusion gates. The shift to the right upon death induction is clearly shown in the overlay of controls (untreated; gray), PEMF-exposed cells (black) and H2O2 treated samples (light blue) in panel C. The dot plots were generated from cells of the same experimental date and are representative of cells responses observed in all of the independent experiments with identical conditions. Trypan blue inclusion was used to quantify the percentage cell death in the H2O2 treated samples.

Figure 9. Assessment of PEMF-induced apoptosis by Annexin V assay.

MCF7 (cancer) and MCF10 (non-tumorigenic) cells were treated with the PEMF paradigms producing the greatest amount of cell death in MCF7 (3 mT for 60 min/day for 3 consecutive days). (A) Dot plots generated by FCM analyses of MCF7 cells show greater increases in the proportions of cells in early (Annexin V+/PI-) and later stages of apoptosis (Annexin V+/PI+) in treated samples (left) relative to control (unexposed) samples (right). (B) MCF10 (non-tumorigenic) cells appear to be unharmed by PEMFs as underscored by the similar amounts of viable cells in treated (89%) versus unexposed (80%) cultures.