Stable Peptide of the Endogenous Opioid Enkephalin Precursor and Breast Cancer Risk

Abstract

Purpose In experimental studies, enkephalins (ENKs) and related opioids have been implicated as negative regulators of breast cancer development by enhancing immune-mediated tumoral defense as well as directly inhibiting cancer cells. We hypothesized that plasma levels of ENKs are predictive of the long-term breast cancer risk. Therefore, our objective was to measure pro-ENK A, a surrogate for mature ENK, and evaluate its predictive value for the development of breast cancer in a large population of middle-aged women and an independent study population. Patients and Methods We related pro-ENK in fasting plasma samples from 1,929 healthy women (mean age, 57.6 ± 5.9 years) of the Malmö Diet and Cancer study to breast cancer incidence (n = 123) during a median follow-up of 14.7 years. For replication, pro-ENK was related to risk of breast cancer (n = 130) in an older independent sample from the Malmö Preventive Project consisting of 1,569 women (mean age, 70.0 ± 4.4 years). Results In the Malmö Diet and Cancer study, pro-ENK was inversely related to the risk of incident breast cancer, with a hazard ratio per each standard deviation increment of logarithm-transformed pro-ENK of 0.72 (95% CI, 0.62 to 0.85; P < .001). The linear elevation of risk over pro-ENK quartiles 3, 2, and 1, with the fourth quartile as a reference, was 1.38 (95% CI, 0.73 to 2.64), 2.29 (95% CI, 1.26 to 4.15), and 3.16 (95% CI, 1.78 to 5.60; for the trend, P < .001), respectively. These results were replicated in the Malmö Preventive Project, where the continuous odds ratio for incident breast cancer was 0.63 (95% CI, 0.52 to 0.76; P < .001) and the risk over pro-ENK quartiles 3, 2, and 1, where the fourth quartile was the reference, was 2.48 (95% CI, 1.25 to 4.94), 2.94 (95% CI, 1.50 to 5.77), and 4.81 (95% CI, 2.52 to 9.18; for the trend, P < .001), respecitvely. Conclusion Low fasting plasma concentration of the opioid precursor peptide pro-ENK is associated with an increased risk of future breast cancer in middle-aged and postmenopausal women.