Glucocorticoid signaling synchronizes the liver circadian transcriptome.

UNLABELLED: Circadian control of physiology is mediated by local, tissue-based clocks, synchronized to each other and to solar time by signals from the suprachiasmatic nuclei (SCN), the master oscillator in the hypothalamus. These local clocks coordinate the transcription of key pathways to establish tissue-specific daily metabolic programs. How local transcriptomes are synchronized across the organism and their relative contribution to circadian output remain unclear. In the present study we showed that glucocorticoids alone are able to synchronize expression of about 60% of the circadian transcriptome. We propose that synchronization occurs directly by the action of glucocorticoids on a diverse range of downstream targets and indirectly by regulating the core clock genes mPer1, Bmal1, mCry1, and Dbp. We have identified the pivotal liver transcription factor, HNF4alpha, as a mediator of circadian and glucocorticoid-regulated transcription, showing that it is a key conduit for downstream targeting. CONCLUSION: We have demonstrated that by orchestrating transcriptional cascades, glucocorticoids are able to direct synchronization of a diverse range of functionally important circadian genes.

Effects of chronic jet lag on tumor progression in mice.

Frequent transmeridian flights or predominant work at night can increase cancer risk. Altered circadian rhythms also predict for poor survival in cancer patients, whereas physical destruction of the suprachiasmatic nuclei (SCN), the hypothalamic circadian pacemaker, accelerates tumor growth in mice. Here we tested the effect of functional disruption of circadian system on tumor progression in a novel experimental model of chronic jet lag. B6D2F(1) mice were synchronized with 12 hours of light and 12 hours of darkness or underwent repeat 8-hour advances of the light/dark cycle every 2 days before inoculation of Glasgow osteosarcoma. The 24-hour changes were assessed for plasma corticosterone, clock protein mPER1 expression in the SCN, and mRNA expression of clock genes mPer2 and mRev-erbalpha in liver and tumor. Time series were analyzed by spectral analysis and/or Cosinor. Differences were compared with analysis of variance (ANOVA). The 24-hour rest/activity cycle was ablated, and the rhythms of body temperature, serum corticosterone, and mPER1 protein expression in the SCN were markedly altered in jet-lagged mice as compared with controls (ANOVA, P < 0.001 for corticosterone and P = 0.01 for mPER1). Tumor grew faster in the jet-lagged animals as compared with controls (ANOVA, P < 0.001), whereas exposure to constant light or darkness had no effect (ANOVA, P = 0.66 and P = 0.8, respectively). The expression of mPer2 and mRev-erbalpha mRNAs in controls showed significant circadian rhythms in the liver (P = 0.006 and P = 0.003, respectively, Cosinor) and in the tumor (P = 0.04 and P < 0.001). Both rhythms were suppressed in the liver (P = 0.2 and P = 0.1, respectively, Cosinor) and in the tumor (P = 0.5) of jet-lagged mice. Altered environmental conditions can disrupt circadian clock molecular coordination in peripheral organs including tumors and play a significant role in malignant progression.

Persistent twenty-four hour changes in liver and bone marrow despite suprachiasmatic nuclei ablation in mice.

Rest-activity or cortisol rhythms can be altered in cancer patients, a condition that may impair the benefits from a timed delivery of anticancer treatments. In rodents, the circadian pattern in rest-activity is suppressed by the destruction of the suprachiasmatic nuclei (SCN) in the hypothalamus. We sought whether such ablation would result in a similar alteration of cellular rhythms known to be relevant for anticancer drug chronopharmacology. The SCN of 77 B6D2F(1) mice synchronized with 12 h of light and 12 h of darkness were destroyed by electrocoagulation [SCN(-)], while 34 animals were sham operated. Activity and body temperature were recorded by telemetry. Blood and organs were sampled at one of six circadian times for determinations of serum corticosterone concentration, blood leukocyte count, reduced glutathione (GSH), and dihydropyrimidine dehydrogenase (DPD) mRNA expression in liver and cell cycle phase distribution of bone marrow cells. Sham-operated mice displayed significant 24-h rhythms in rest-activity and body temperature, whereas such rhythms were found in none and in 15% of the SCN(-) mice, respectively. SCN lesions markedly altered the rhythmic patterns in serum corticosterone and liver GSH, which became nonsinusoidal. Liver DPD mRNA expression and bone marrow cell cycle phase distribution displayed similar 24-h sinusoidal patterns in sham-operated and SCN(-) mice. These results support the existence of another light-dark entrainable pacemaker that can coordinate cellular functions in peripheral organs. They suggest that the delivery of anticancer treatments at an optimal time of day may still be beneficial, despite suppressed rest-activity or cortisol rhythms.

Disruption of circadian coordination accelerates malignant growth in mice.

An animal model (mice B6D2F1) was developed to study the consequence of suprachiasmatic nuclei (SCN) destruction on tumor growth. SCN destruction abolished the rest-activity and body temperature rhythms and markedly altered the rhythms in serum corticosterone concentration and lymphocyte count. Tumor growth was faster in mice with lesioned SCN than in controls for both tumor models studied, Glasgow osteosarcoma (GOS) and pancreatic adenocarcinoma (P03). This shows that disruption of circadian coordination accelerates malignant growth in mice, suggesting that the host circadian clock controls tumor progression.

Host circadian clock as a control point in tumor progression.

BACKGROUND: The circadian timing system controlled by the suprachiasmatic nuclei (SCN) of the hypothalamus regulates daily rhythms of motor activity and adrenocortical secretion. An alteration in these rhythms is associated with poor survival of patients with metastatic colorectal or breast cancer. We developed a mouse model to investigate the consequences of severe circadian dysfunction upon tumor growth. METHODS: The SCN of mice were destroyed by bilateral electrolytic lesions, and body activity and body temperature were recorded with a radio transmitter implanted into the peritoneal cavity. Plasma corticosterone levels and circulating lymphocyte counts were measured (n = 75 with SCN lesions, n = 64 sham-operated). Complete SCN destruction was ascertained postmortem. Mice were inoculated with implants of Glasgow osteosarcoma (n = 16 with SCN lesions, n = 12 sham-operated) or pancreatic adenocarcinoma (n = 13 with SCN lesions, n = 13 sham-operated) tumors to determine the effects of altered circadian rhythms on tumor progression. Time series for body temperature and rest-activity patterns were analyzed by spectral analysis and cosinor analysis. Parametric data were compared by the use of analysis of variance (ANOVA) and survival curves with the log-rank test. All statistical tests were two-sided. RESULTS: The 24-hour rest-activity cycle was ablated and the daily rhythms of serum corticosterone level and lymphocyte count were markedly altered in 75 mice with complete SCN destruction as compared with 64 sham-operated mice (two-way ANOVA for corticosterone: sampling time effect P<.001, lesion effect P =.001, and time x lesion interaction P<.001; for lymphocytes P =.001,.002, and.002 respectively). Body temperature rhythm was suppressed in 60 of the 75 mice with SCN lesions (P<.001). Both types of tumors grew two to three times faster in mice with SCN lesions than in sham-operated mice (two-way ANOVA: P<.001 for lesion and for tumor effects; P =.21 for lesion x tumor effect interaction). Survival of mice with SCN lesions was statistically significantly shorter compared with that of sham-operated mice (log-rank P =.0062). CONCLUSIONS: Disruption of circadian rhythms in mice was associated with accelerated growth of malignant tumors of two types, suggesting that the host circadian clock may play an important role in endogenous control of tumor progression.

Circadian cycling of the mouse liver transcriptome, as revealed by cDNA microarray, is driven by the suprachiasmatic nucleus.

BACKGROUND: Genes encoding the circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) of mammals have recently been identified, but the molecular basis of circadian timing in peripheral tissue is not well understood. We used a custom-made cDNA microarray to identify mouse liver transcripts that show circadian cycles of abundance under constant conditions. RESULTS: Using two independent tissue sampling and hybridization regimes, we show that approximately 9% of the 2122 genes studied show robust circadian cycling in the liver. These transcripts were categorized by their phase of abundance, defining clusters of day- and night-related genes, and also by the function of their products. Circadian regulation of genes was tissue specific, insofar as novel rhythmic liver genes were not necessarily rhythmic in the brain, even when expressed in the SCN. The rhythmic transcriptome in the periphery is, nevertheless, dependent on the SCN because surgical ablation of the SCN severely dampened or destroyed completely the cyclical expression of both canonical circadian genes and novel genes identified by microarray analysis. CONCLUSIONS: Temporally complex, circadian programming of the transcriptome in a peripheral organ is imposed across a wide range of core cellular functions and is dependent on an interaction between intrinsic, tissue-specific factors and extrinsic regulation by the SCN central pacemaker.