Culture at a Higher Temperature Mildly Inhibits Cancer Cell Growth but Enhances Chemotherapeutic Effects by Inhibiting Cell-Cell Collaboration

Abstract

Acute febrile infections have historically been used to treat cancer. To explore the underlying mechanism, we studied chronic effects of fever on cancer cell growth and chemotherapeutic efficacy in cell culture. We found that culturing cancer cells at 39°C mildly inhibited cell growth by arresting the cells at the G1 phase of the cell cycle. When cells were seeded in culture dishes at a lower density, e.g. about 1000-2000 cells per 35-mm dish, the growth inhibition was much greater, manifested as many fewer cell colonies in the 39°C dishes, compared with the results at a higher density seeding, e.g. 20,000 cells per dish, suggesting that cell-cell collaboration as the Allee effect in cell culture is inhibited at 39°C. Withdrawal of cells from serum enhanced the G1 arrest at 39°C and, for some cell lines such as A549 lung cancer cells, serum replenishment failed to quickly drive the cells from the G1 into the S and G2-M phases. Therapeutic effects of several chemotherapeutic agents, including clove bud extracts, on several cancer cell lines were more potent at 39°C than at 37°C, especially when the cells were seeded at a low density. For some cell lines and some agents, this enhancement is long-lasting, i.e. continuing after the cessation of the treatment. Collectively these results suggest that hyperthermia may inhibit cancer cell growth by G1 arrest and by inhibition of cell-cell collaboration, and may enhance the efficacy of several chemotherapeutic agents, an effect which may persist beyond the termination of chemotherapy.

Fig 1. Cell viability detected by MTT assays.

A: The optical density of MTT is often higher in the cells at 39°C than at 37°C, even when fewer cells at 39°C are observed under microscope, indicating that cell viability may be overestimated at 39°C when MTT assay is used. B, C and D: Treatment of Hela, A549 and HCT116 cells with 1 μM cisplatin (CDDP), 4 μM 5-FU or 0.05% clove bud ethanol extract significantly decreases the cell viability compared with the untreated control at the same temperature (a in C, B and D, p<0.05; t test), whereas comparison between 37°C and 39°C is not recommended.

Fig 2. About 20,000 Hela or A549 cells were seeded in each of 35-mm dishes and allowed to grow to about 70% confluence.

The cells were then treated with 1 μM cisplatin or 2 μM 5-FU for 3 days and, after the drug was withdrawn, were continued on the culture for 3 additional days so that the cell colonies grow to visible sizes. Note that the cells of solvent-treated control also show a lower density at 39°C than at 37°C.

Fig 3. Enhanced effects of cisplatin, 5-FU or clove oil on HCT16 cells at 39°C.

Top panel: About 20,000 HCT116 cells were seeded in a 35-mm dish and allowed to grow for 3 days at 37°C or 39°C. The cells were then treated with indicated agent for 3 days, followed by staining the viable cells with crystal violet. Bottom panel: The cells were treated as above but were allowed to grow for 3 more days after the 3-day treatment with the indicated agent was terminated. Note that the difference in cell density between 37°C and 39°C was widened for cisplatin- and clove-treated cells, compared with the counterpart at the top panel. However, this post-treatment effect is not discerned in 5-FU treated cells at 39°C.

Fig 4. About 1000 Hela or A549 cells were seeded in each of 35-mm dishes and allowed to grow for 6, 9 or 12 days with medium changes.

Note that there are fewer colonies of visible sizes in the 39°C dish than in the 37°C one. A549 cells form larger colonies than Hela cells at the later time points, especially at 37°C.

Fig 5. About 1000 HCT116 cells were seeded in each of 35-mm dishes and allowed to grow for the indicated days.

Growth is much slower at 39°C than at 37°C, but the cells are still alive, as observed under the microscope (arrows in the 6-day dish).

Fig 6. Inhibited growth of Hela cells at 39°C.

Left panel: Hela cells usually grow in three dimensions in the dish to form spherical colonies which, after growing beyond 9 days, easily shed off during crystal violet staining, leaving the dish with only the periphery of the colonies, especially in the 37°C dish wherein the colonies are much larger. Right panel: Six days after the cells were seeded, individual cells seeded at a density of about 20,000 cells per 35-mm dish develop to larger colonies, compared with the colonies in the dish wherein cells were seeded at a density of 2,000 cells. Cells at 37°C develop larger colonies than at 39°C.

Fig 7. Effects of 39°C on cell morphology.

Left panel: HCT116 cells at 37°C usually show irregular shape (arrow) but become spherical at 39°C. Treatment with 0.025% clove oil kills many more cells at 39°C than at 37°C, with the remaining cells shrunken to spherical shape within 30 minutes. Right panel: Thirty minutes post treatment with 1 μM cisplatin (CDDP), many Hela cells have changed to a spindle shape (arrow) at 39°C but remain unchanged at 37°C. Two hours post a treatment with 0.025% clove oil, many more Hela cells die at 39°C than at 37°C.

Fig 8. About 2000 Hela or A549 cells were seeded in each of 35-mm dishes and given 12 hours for the cells to attach the dish.

The cells were then treated with 1 μM cisplatin, 4 μM 5-FU or 0.025% clove oil for 3 days. After the agent had been withdrawn, the cells were allowed to grow for one more week. Note that there are hardly any colonies in the 39°C dishes of the Hela and A549 cells treated with cisplatin, or of the A549 cells treated with clove oil, although one more week had been given for the growth, suggesting a post-treatment growth inhibition. However, this post-treatment effect at 39°C is not discerned in 5-FU treated Hela or clove oil treated A549 cells.

Fig 9. Effects of 39°C culture on cell cycle distribution.

Data are presented as mean of triplet dishes in each experiment, plus or minors standard deviation (SD). In some experiments, one or all groups contain only a single dish and thus the data lack SD. "Synchronized" means that the cells were deprived from serum for 24 or 48 hours and then replenished with serum for indicated time before determination of cell cycle distribution. “Exp”: number of experiments performed. “Temp”: culture temperature. a: significantly different from the 39°C counterpart (p<0.05, Wilcoxon rank-sum test of three repeats of the experiment). b: significantly different from the 39°C counterpart (p<0.05, Wilcoxon rank-sum test of triplet dishes within the same experiment). c: significantly different from the serum-replenished group at the same temperature 4 hours later (p<0.05, Wilcoxon rank-sum test of triplet dishes within the same experiment).

Fig 10. Effects of cisplatin on cell cycle distribution at 37°C and 39°C.

Exp: number of experiments performed. Treat: treatment. Temp: temperature. Con: untreated control. CDDP: cisplatin. S/G2/M is the sum of the S and G2-M fractions that differ from G1 cells by their doubled DNA content. a: significantly different from the untreated cells at 39°C and from the CDDP-treated cells at 37°C (p<0.05). b: Significantly different from CDDP-treated cells at 37°C (p<0.05). c: significantly different from untreated cells at 39°C. Wilcoxon rank-sum test was used.