"Objective Impairment of clock gene expression and changes in melatonin and 17-?-estradiol levels may constitute biological alterations underlying the increased risk of breast cancer among shift workers. The aim of this study was to compare levels of selected core clock gene expression, 6-sulfatoxymelatonin (aMT6s), and 17-?-estradiol between rotational shift work (SW) and daytime (DT) workers after a day off.Methods The cross-sectional study comprised 60 nurses with ?2 years of SW and 56 permanent DT nurses. Transcript levels of circadian genes BMAL1, CLOCK, NPAS2, CRY1, CRY2, PER1, PER2, PER3, and REVERB? were determined by quantitative real-time polymerase chain reaction (PCR) in lymphocytes. All participants were tested in the early follicular phase of the menstrual cycle. Samples were collected at the beginning of the morning-shift after a regular night's sleep on a day off. Chronotype and sociodemographic characteristics were also evaluated. Results We found a significantly higher expression of BMAL1, CLOCK, NPAS2, PER1, PER2, and REVERB? and a lower expression of PER3, CRY1 and CRY2 among SW compared to DT nurses. SW participants did not demonstrate a significant difference in aMT6s levels, but they did show significantly higher 17-?-estradiol levels compared to DT nurses. Multiple linear regression analysis confirmed the role of SW on expression of BMAL1 (? 0.21, P=0.040), CLOCK (? 0.35, P=0.008), NPAS2 (? 0.30, P=0.012), PER1 (? 0.33, P=0.008), PER2 (? 0.19, P=0.047), PER3 (? -0.27, P=0.012), CRY1 (? -0.33, P=0.002), CRY2 (? -0.31, P=0.005), REVERB? (? 0.19, P=0.045), and on 17-?-estradiol levels (? 0.32, P=0.003). The analysis also confirmed the role of chronotype as an independent factor for PER1 (? 0.48, P=0.001) and PER2 (? -0.22, P=0.022) expression, and 17-?-estradiol levels (? 0.26, P=0.011).Conclusions Rotating SW nurses show alterations in peripheral clock gene expression and 17-?-estradiol levels at the beginning of the morning shift after a day off."

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