Artificial Light at Night of Different Spectral Compositions Differentially Affects Tumor Growth in Mice: Interaction With Melatonin and Epigenetic Pathways

Abstract

Lighting technology is rapidly advancing toward shorter wavelength illuminations that offer energy-efficient properties. Along with this advantage, the increased use of such illuminations also poses some health challenges, particularly breast cancer progression. Here, we evaluated the effects of artificial light at night (ALAN) of 4 different spectral compositions (500-595 nm) at 350 Lux on melatonin suppression by measuring its urine metabolite 6-sulfatoxymelatonin, global DNA methylation, tumor growth, metastases formation, and urinary corticosterone levels in 4T1 breast cancer cell-inoculated female BALB/c mice. The results revealed an inverse dose-dependent relationship between wavelength and melatonin suppression. Short wavelength increased tumor growth, promoted lung metastases formation, and advanced DNA hypomethylation, while long wavelength lessened these effects. Melatonin treatment counteracted these effects and resulted in reduced cancer burden. The wavelength suppression threshold for melatonin-induced tumor growth was 500 nm. These results suggest that short wavelength increases cancer burden by inducing aberrant DNA methylation mediated by the suppression of melatonin. Additionally, melatonin suppression and global DNA methylation are suggested as promising biomarkers for early diagnosis and therapy of breast cancer. Finally, ALAN may manifest other physiological responses such as stress responses that may challenge the survival fitness of the animal under natural environments.

Keywords: 6-SMT; CFL; EE-halogen; GDM-levels; body mass; carbon; corticosterone; cosinor analysis; light at night; yellow-LED.

Figure 1.

Percentage change in body mass of female BALB/c mice at the end of the study relative to baseline mass prior to tumor cell inoculation (1 × 10⁶ cells). Mice were kept under short photoperiod (8 L:16D, lights were on from 8:00 to 16:00 hours) at 4 different spectral compositions with or without melatonin (MLT) supplement for 28 days. Melatonin was nocturnally administered in the drinking water (10 mg/L). Mice in all spectral groups were exposed to 1 × 30-minute/night artificial light at night at 00:00 using the same light source and spectral composition as during the day. Results are presented as mean ± standard error (SE) of n = 6-10. Different letters represent statistically significant difference among groups (Tukey P = .01).

Figure 2.

Tumor growth rates of female BALB/c mice during 4 weeks post cancer cell inoculation at 4 different spectral compositions with or without melatonin (MLT) supplement. Tumor volumes were measured by digital caliper semi-weekly. Results are mean ± standard error (SE) of n = 6-10.

Figure 3.

6-Sulfatoxymelatonin daily rhythms of female BALB/c mice under 4 different spectral compositions (carbon, compact fluorescent lamp [CFL], energy-efficient [EE]-halogen, and Yellow light-emitting diode [LED]) without melatonin treatment 4 weeks postinoculation. Results are mean ± standard error (SE) of n = 6-10. The best-fitted cosine curve and Cosinor estimates (period, P value, and percentage of the rhythm [PR]) are depicted. Gray area in each plot represents the length of the dark period. *P = .03 versus 12:00 hours (t test).

Figure 4.

Urinary corticosterone daily rhythms of female BALB/c mice under 4 different spectral compositions with melatonin (+MLT) and without melatonin (−MLT) treatments, 4 weeks post-inoculation. Results are mean ± standard error (SE) of n = 6-10. The best-fitted cosine curve and Cosinor estimates (period, P value, and percentage of the rhythm [PR]) are depicted. Gray area in each plot represents the length of the dark period. *P < .05 versus 12:00 hour and ^# P < .05 versus +MLT (t test).

Figure 5.

The effects of spectral composition and melatonin (MLT) treatment on global DNA methylation in female BALB/c mice 4 weeks post-inoculation. Results are mean ± standard error (SE) of n = 10. Different letters represent statistically significant difference among groups within the same tissue and same MLT treatment (Tukey P < .05). * P < .05 (t test).

Figure 6.

Correlation between various parameters studied in female BALB/c mice 4 weeks post-inoculation with 4T1 breast cancer cells. Regression analysis between total 24-hour content of urinary 6-sulfatoxymelatonin (6-SMT) and both tumor volume and global DNA methylation (GDM). The regression was imposed on the combined data of all spectral groups without melatonin supplement (N = 33). Black and gray solid lines represent the regression lines. The estimated equation, R ², and P value are also depicted (A). Correlation between wavelength and tumor volume (B), total 6-SMT (C), and GDM (D). Correlations with wavelength were imposed on mean values of control data (without melatonin) at each wavelength. Six wavelengths were used for this analysis: 4 (595, 569, 535, and 500 nm) from the current study and 2 (460 and 580 nm) were adopted from the previous study.

Figure 7.

Schematic representation of an epigenetic mechanism of action postulated to mediate cancer progression by artificial light at night (ALAN)-induced melatonin suppression. Photoperiod (day and night cycle) and ALAN signals are detected by intrinsically photosensitive retinal ganglion cells (ipRGCs) and then propagated to circadian clock in the hypothalamic superchiasmatic nucleus (SCN) by the retinohypothalamic tract (RTH). The circadian signals from the SCN are conveyed to the pineal gland (PG) via the superior cervical ganglion (SCG). The SCN-PG pathway regulates normal rhythms of melatonin (solid gray arrows) that are associated with normal activity of DNA methyltransferases (DNMTs), normal global methylation, and finally normal cellular development (cancer free). Conversely, ALAN exposures (solid white arrow and dotted gray arrows) particularly of short wavelengths are associated with converse actions and finally trigger cancer progression and promote metastasis formation. Exogenous melatonin treatment may rectify the ALAN maladaptive responses.