Mammary tumors compromise time-of-day differences in hypothalamic gene expression and circadian behavior and physiology in mice.

Circadian rhythms influence various aspects of biology, including hormonal, immunological, and behavioral processes. These 24-hour oscillations are necessary to optimize cellular functions and to synchronize these processes with the environment. Breast cancer patients and survivors frequently report disruptions in circadian oscillations that adversely affect quality-of-life, including fragmented sleep-wake cycles and flattened cortisol rhythms, which are associated with negative behavioral comorbidities (e.g., fatigue). However, the potential causal role of tumor biology in circadian dysregulation has not been investigated. Here, we examined the extent to which sham surgery, non-metastatic mammary tumors, or mammary tumor removal in mice disrupts circadian rhythms in brain clock gene expression, locomotor behavior (free-running and entrained), and physiological rhythms that have been associated with cancer behavioral comorbidities. Tumors and tumor resection altered time-of-day differences in hypothalamic expression of eight circadian-regulated genes. The onset of activity in entrained running behavior was advanced in tumor-bearing mice, and the amplitude of free-running rhythms was increased in tumor-resected mice. Tumors flattened rhythms in circulating corticosterone and Ly6cHi monocytes which were largely restored by surgical tumor resection. This work implies that tumors alone may directly impact central and/or peripheral circadian rhythmicity in breast cancer patients, and that these effects may persist in cancer survivors, potentially contributing to behavioral comorbidities.