Cell-autonomous circadian clock of hepatocytes drives rhythms in transcription and polyamine synthesis

Abstract

The circadian clock generates daily rhythms in mammalian liver processes, such as glucose and lipid homeostasis, xenobiotic metabolism, and regeneration. The mechanisms governing these rhythms are not well understood, particularly the distinct contributions of the cell-autonomous clock and central pacemaker to rhythmic liver physiology. Through microarray expression profiling in Met murine hepatocytes (MMH)-D3, we identified over 1,000 transcripts that exhibit circadian oscillations, demonstrating that the cell-autonomous clock can drive many rhythms, and that MMH-D3 is a valid circadian model system. The genes represented by these circadian transcripts displayed both cophasic and antiphasic organization within a protein-protein interaction network, suggesting the existence of competition for binding sites or partners by genes of disparate transcriptional phases. Multiple pathways displayed enrichment in MMH-D3 circadian transcripts, including the polyamine synthesis module of the glutathione metabolic pathway. The polyamine synthesis module, which is highly associated with cell proliferation and whose products are required for initiation of liver regeneration, includes enzymes whose transcripts exhibit circadian oscillations, such as ornithine decarboxylase and spermidine synthase. Metabolic profiling revealed that the enzymatic product of spermidine synthase, spermidine, cycles as well. Thus, the cell-autonomous hepatocyte clock can drive a significant amount of transcriptional rhythms and orchestrate physiologically relevant modules such as polyamine synthesis.networks | chronobiology | resistance distance M any aspects of mammalian physiology and behavior display circadian (∼24-h) rhythms, including the sleep/wake cycle, blood pressure, heart rate, metabolism, and liver regeneration (1, 2). These rhythms are regulated by the circadian clock, which enables consolidation and coordination of physiological events to specific phases of the 24-h cycle in anticipation of daily environmental changes. Dysfunction of the clock is associated with serious human health conditions, including shift work syndrome, sleep disorders, increased risk of cancer, cardiovascular disease, and metabolic syndrome (1, 2).The circadian clock is a self-sustaining, entrainable, cell-autonomous network of three interlocked transcriptional negative feedback loops (2). The primary loop consists of BMAL1/CLOCK transcriptional activators, which dimerize and turn on transcription of Period (Per1, Per2, and Per3) and Crytochrome (Cry1 and Cry2) genes through E-box elements. PER and CRY proteins dimerize and feed back to inhibit BMAL1/CLOCK activation. Two associate loops interlock with the core loop: the ROR/REV-ERB element (RRE) loop composed of ROR activators (RORa, RORb, and RORc) and REV-ERB repressors (REV-ERBα and REV-ERBβ), which compete for RRE transcription factor binding sites (TFBS), and the D-box loop composed of the activator DBP and repressor E4BP4, which act through D-box TFBS (2).In addition to internal regulation of clock genes,...

Interactions between the circadian clock and metabolism: there are good times and bad times

“…In humans, a number of metabolic pathways are directly controlled by the circadian clock, independent of sleep and feeding [66]. Consistently, genome-wide transcriptional profiling demonstrated that the cell-autonomous hepatocyte clock can drive rhythmic expression of a significant amount of genes [67].…”

Breast cancer among shift workers: results of the WOLF longitudinal cohort study

“…Clock genes take part in crucial functions of cell biology, eg, cell proliferation (17), DNA repair (18), and apoptosis (19). The cell division is temporally regulated, and this rhythmic control affects the rate of the DNA replication (20).…”

Monitoring Cell-autonomous Circadian Clock Rhythms of Gene Expression Using Luciferase Bioluminescence Reporters

“…By comparing in vitro and in vivo circadian rhythms, we will be able to discriminate between cell-autonomous and systemic cue-driven clock functions. Indeed, recent studies have begun to reveal a significant role for the liver clock in hepatic gene expression 6,51,52 . Future research in the field warrants development of various tissue-and cell type-specific clock models representing different physiological outputs.…”

“…In this context, the reporter approach provides a useful tool to test whether the clock mechanism exists in a particular cell type. Presence of cell-autonomous clock mechanisms warrant studies of clock outputs in these cells, as well as in tissues in vivo, including the transcriptome by microarray or RNA sequencing 51,52 , transcriptional regulatory networks, the proteome, and the metabolome. These studies examine genome-wide RNA and protein expression patterns and thus would complement cell-based luminescence reporter assays that do not necessarily reflect the endogenous expression.…”