Rewiring of liver diurnal transcriptome rhythms by triiodothyronine (T3) supplementation

Diurnal (i.e., 24-hour) physiological rhythms depend on transcriptional programs controlled by a set of circadian clock genes/proteins. Systemic factors like humoral and neuronal signals, oscillations in body temperature, and food intake align physiological circadian rhythms with external time. Thyroid hormones (THs) are major regulators of circadian clock target processes such as energy metabolism, but little is known about how fluctuations in TH levels affect the circadian coordination of tissue physiology. In this study, a high triiodothyronine (T3) state was induced in mice by supplementing T3 in the drinking water, which affected body temperature, and oxygen consumption in a time-of-day dependent manner. 24-hour transcriptome profiling of liver tissue identified 37 robustly and time independently T3 associated transcripts as potential TH state markers in the liver. Such genes participated in xenobiotic transport, lipid and xenobiotic metabolism. We also identified 10 – 15 % of the liver transcriptome as rhythmic in control and T3 groups, but only 4 % of the liver transcriptome (1,033 genes) were rhythmic across both conditions – amongst these several core clock genes. In-depth rhythm analyses showed that most changes in transcript rhythms were related to mesor (50%), followed by amplitude (10%), and phase (10%). Gene set enrichment analysis revealed TH state dependent reorganization of metabolic processes such as lipid and glucose metabolism. At high T3 levels, we observed weakening or loss of rhythmicity for transcripts associated with glucose and fatty acid metabolism, suggesting increased hepatic energy turnover. In sum, we provide evidence that tonic changes in T3 levels restructure the diurnal liver metabolic transcriptome independent of local molecular circadian clocks. Two to three months-old male C57BL/6J mice were housed in groups of three under a 12-hour light, 12-hour dark (LD, ~300 lux) cycle at 22 ± 2 °C and a relative humidity of 60 ± 5 % with ad-libitum access to food and water. To render mice hyperthyroid (i.e., high T3 levels) the animals received one week of 0.01 % BSA (Sigma-Aldrich, St. Louis, USA, A7906-50G) in their drinking water, followed by two weeks with water supplemented with T3 (0.5 mg/L, Sigma-Aldrich T6397, in 0.01 % of BSA). Control animals received only 0.01 % BSA in the drinking water over the whole treatment period. Methimazole (MMI) is used for hyperthyroidism treatment. At the end of three weeks, mice were sacrificed, and livers were collected every 4h for 24h. RNA was isolated and processed for microarray analysis.