Insulin-like growth factor peptides and melatonin among rotating shift nurses
Boehme, Kirstin Elaine
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Background: In 2007, the International Agency for Research on Cancer (IARC) classified long-term shift work as a probable human carcinogen; however, the mechanism through which shift work potentially increases cancer risk is not known. One hypothesis is that diminished melatonin production may be involved, possibly as a result of exposure to light during night work. Experimental studies suggest a link between melatonin and peptides in the insulin-like growth factor (IGF) family, also implicated in carcinogenesis. This research aimed to describe the distributions of circulating concentrations of insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-3 (IGFBP-3) and their associations with urinary melatonin as possible intermediates in the pathway between work at night and breast cancer. Methods: A cross-sectional study was conducted among 85 premenopausal nurses working a rotating shift pattern of two 12-hour days, two 12-hour nights, and five days off. Once during both the summer and winter seasons, melatonin metabolites were measured in urine samples and circulating concentrations of IGF-I and IGFBP-3 were determined from serum samples. Weight and height were measured by the study coordinator, while a questionnaire and study diaries were used to collect all other covariate information. Predictors of IGF levels were identified using multivariate mixed effects modeling and relationships between melatonin and the IGFs were investigated using Spearman’s rank correlation and multivariate mixed effects modeling. Results: Both age (β = -3.6, p < 0.0001) and current OC use (β = -40.8, p = 0.003) were associated with decreases in circulating IGF-I, while levels of IGF-I were increased in the winter months (β = 26.3, p = 0.02). A positive relationship between recent alcohol consumption and serum IGFBP-3 was also suggested (β = 197.8, p = 0.05). Neither Spearman’s rank correlations nor mixed effects modeling indicated that urinary melatonin was a determinant of serum IGFs. Conclusions: Age, season, and current OC use were observed to predict circulating IGF-I, while recent alcohol consumption was a determinant of IGFBP-3 levels. A relationship between melatonin and IGFs, theorized as a component of the mechanism linking shift work and cancer, was not supported by the results of this project.