The effect of constant light and melatonin on urethane-induced lung carcinogenesis in mice and expression of clock genes and proteins

Abstract

Introduction. Circadian rhythms are an adaptational mechanism to day-night cycle. At the cellular level rhythms are supported by oscillations of transcription of Bmal1, 2, Clock and Npas2, Per1-3, Cry1, 2 clock genes; in the organism rhythms are synchronized with melatonin, the "hormone of darkness". Reliable data have been obtained that disruption of biological "clocks" at the cellular and/or organism level are associated with carcinogenesis, but the experimental results in this direction remain incomplete.
 Aim. Assessment of the continuous lighting and melatonin administration effects on chemically induced carcinogenesis in mice and expression of clock genes and proteins in normal and tumor lung tissue.
 Materials and methods. Lung tumors were induced by urethane in 120 male SHR mice. Animals were kept at constant (LL) or standard (LD) light; half of the animals received melatonin (MT) daily at 20 mg/l at night with water for 45 weeks of experiment. At the end of the experiment, the frequency and multiplicity of lung tumours of different size were assessed. Expression of clock genes Clock, Bmal1, Cry1 was assessed in tumor samples and normal lung tissue by real-time PCR; protein content of BMAL1, CLOCK, CRY1 and PER2 was determined by immunohistochemistry. Statistical data were processed using the programs MS Excel 2007, GraphPad Prism 6.0 with commonly used statistical criteria.
 Results. The number of animals with lung tumors was between 89 and 100% in different groups. Large tumors (>2 mm) were more frequently observed in LL group (62 tumors out of 294, 21,1%) than in LD group (59 tumors out of 405, 14,6%, p=0,0245). Administration of exogenous melatonin at constant lighting statistically significantly reduced the frequency of large tumors (28 tumors out of 320, 8.8%, p=0,0001 in comparison with LL). In normal lung tissue of animals kept at constant lighting (LL) the increase in relative expression of clock genes in comparison with LD group was revealed: Bmal1 by 3,1 times (p=0,02) and Cry1 by 3,6 times (p=0,0002). No such differences were found for Clock gene. The relative content of BMAL1 and CLOCK proteins in all experimental conditions was higher in adenomas and adenocarcinomas compared to normal tissue.
 Conclusion. Constant lighting promotes the development of chemically induced lung tumors, and melatonin administration inhibits carcinogenesis under constant lighting.
 The content of BMAL1 and CLOCK transcription activator proteins in lung tumors was found to increase in comparison with normal tissue, whereas no increase in the expression level of corresponding genes in tumors was observed.