Night-shift work duration and risk of colorectal cancer according to IRS1 and IRS2 expression

Background: We hypothesized that the risk of colorectal cancer in night-shift workers might be different according to insulin receptor substrate status. Methods: Among 77,470 eligible women having night work assessed in the Nurses’ Health Study, we documented a total of 1,397 colorectal cancer cases, of which 304 or 308 had available data on IRS1 and IRS2, respectively. We used duplication-method Cox proportional hazards regression analysis for competing risks to calculate HRs and 95% confidence intervals (CI) for each colorectal cancer subtype. We measured tumor IRS1 or IRS2 expression by immunohistochemistry (IHC). Results: Compared with women who never worked night shifts, those working ≥15 years night shifts had a marginal trend of increased overall risk of colorectal cancer (P_trend = 0.06; multivariable HR = 1.20; 95% CI, 0.99–1.45). Longer duration of night-shift work was associated with a higher risk of IRS2-positive tumors (multivariable HR = 2.69; 95% CI, 1.48–4.89; P_trend = 0.001, ≥15 years night shifts vs. never) but not with IRS2-negative tumors (multivariable HR = 0.90; 95% CI, 0.54–1.51; P_trend = 0.72; P_{heterogeneity} for IRS2 = 0.008). Similarly, the corresponding multivariable HRs were 1.81 for IRS1-positive tumors (95% CI, 0.94–3.48; P_trend = 0.06) and 1.13 for IRS1-negative tumors (95% CI, 0.71–1.80; P_trend = 0.56; P_{heterogeneity} for IRS1 = 0.02). Conclusions: Our molecular pathologic epidemiology data suggest a potential role of IRS in mediating carcinogenesis induced by night-shift work. Impact: Although these findings need validation, rotating night shift might increase colorectal cancer risk in women with abnormal insulin receptor pathways.