Biochemical and Biophysical Research Communications
The circadian rhythm controls telomeres and telomerase activity
Introduction
Circadian rhythms exist in organisms from archaea to humans [1]. In mammals, the central clock of the hypothalamic suprachiasmatic nucleus and peripheral clocks in tissues coordinate multiple aspects of behavior and physiology [2]. The oscillatory rhythms regulate cell growth, hormonal homeostasis, electrolyte balance, energy metabolism, cardiovascular physiology, and sleep, with external environmental changes synchronizing with the rotation of the earth [3]. Disturbance of circadian rhythms often results in abnormal energy metabolism, increased carcinogenesis, cardiovascular diseases, sleep disturbances, and early aging [4]. Recent advances have supported evidence for a link between circadian rhythms and the physiology changes in aging. Flies with disturbed light/dark cycles or mutations in core circadian genes have reduced life span [5]. Mice deficient in BMAL1, CLOCK, or PER1/2 proteins develop early aging phenotypes [6]. In human, disturbances in circadian timing in shift workers is associated with an increased risk of coronary artery diseases and cancer [7].
Telomeres are DNA–protein complexes that protect chromosome ends and maintain genomic integrity. Telomeres shorten with cell division and impose a replicative limit on the growth of primary cells in culture. Telomere shortening is mainly compensated by the enzyme telomerase, which adds back telomeric DNA [8]. The regulation of telomerase activity requires integration at multiple levels and includes TERT expression control, enzyme phosphorylation, telomere complex subunit assembly, and transport [10].
The aging process in humans is associated with changes in circadian rhythm patterns [11]. Aged mice have decreased sensitivity to the effects of light entrainment and show reduced amplitude of circadian gene expression [4]. Cellular senescence impairs circadian rhythmicity, and introduction of telomerase rescues clock genes expression that has decreased due to senescence [12]. Cancer cells also have oscillatory rhythms in DNA synthesis and telomerase activity [13]. These findings suggest potentially important links between circadian rhythms and telomeres or telomerase. However, the exact mechanisms and potential interactions between circadian rhythms and telomere are still unclear.
Section snippets
Cell culture and antibody
Multiple and independent mouse embryonic fibroblasts (MEFs) from wild-type and Clock−/− knockout mice were derived and assayed. To assess the oscillation of gene expression, cells were synchronized using serum shock. Antibodies against mTERT (Santa Cruz), Per2 (Millipore), Rev-Erbα (Cell Signaling), and Lamin A/C (Abcam) were used for Western blot.
Animals
Clock−/− mice on a C57BL/6J background were obtained from Dr. Reppert and Dr. Weaver (University of Massachusetts Medical School, Worcester, MA) [14]
Circadian oscillation of telomerase activity
To determine whether telomerase activity exhibits circadian rhythms, we assessed telomerase activity at 4-h intervals in the livers and lungs of wild-type mice. The mice were analyzed on the first day under constant darkness (DD) after entrainment for 14 d under a 12 h light/12 h dark (LD) cycle. Under these conditions, telomerase activity was rhythmic and exhibited significant circadian rhythmicity (Fig. 1A and B). The peak telomerase activity was noted at circadian time (CT) 17 and the trough at
Discussion
Both human and mice TERT promoters have 2 classic CLOCK–BMAL1 binding E-boxes with CACGTG sequences [28]. Our data suggest that CLOCK–BMAL1 does not bind to the E-box near the transcription start site, which is more evolutionarily conserved, but binds to the E-box at −165 bp in humans or −808 bp in mice upstream. We believe that this observation suggests that despite humans and mice utilizing similar circadian machinery for telomerase control, there are still some differences between humans and
Acknowledgments
C.Y.W. received support from the National Health Research Institute (NHRI-EX100-9925SC), National Science Council (101-2314-B-182-100-MY3, 101-2314-B-182A-009), and Chang Gung Memorial Hospital (CMRPG3B1642, CMRPG3D1001, CMRPG3D0581, and CMRPG3C1761). M.S.W. received support from National Science Council and Chang Gung Memorial Hospital (CMRPG3A1073). S.S.S. received support from Chang Gung Memorial Hospital (CMRPG3C0721 and CMRPG3C0722).
We appreciate the technical assistance from Mei-Hsiu Lin,
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W.D.C. and M.S.W. contributed equally to this work.