A research group led by Graduate Student Shusaku Masuda (at the time of research) and Emeritus Professor Yoshitaka Fukada of the Graduate School of Science at the University of Tokyo, and Associate Investigator Nobuhiro Kurabayashi and Project Leader Hikari Yoshitane (also Associate Professor of the Graduate School of Science at the University of Tokyo) of the Tokyo Metropolitan Institute of Medical Science has announced that they discovered a degradation regulator involved in the circadian rhythm of protein levels. They found that the clock protein DBP, which is important for circadian rhythms in physiological functions, is degraded by two E2 enzymes and the E3 ligase TRAF7. The findings are expected to contribute to the elucidation of the mechanisms underlying circadian rhythms. The results were published in the international journal Communications Biology on October 8.
The circadian clock regulates physiological functions according to various daily activities, such as eating and sleeping. It induces changes in the amounts of biological molecules involved in various functions according to time of the day. In mammals, the expression levels of 43% of genes change during the course of the day, and these genes and the proteins produced via expression thereof control the physiological functions.
DBP protein is known to exhibit a pronounced circadian rhythm in the mouse liver, in which the protein accumulates in the evening and decreases to minuscule amounts at dawn. This protein is a transcription factor that binds to a specific DNA sequence and promotes the expression of genes under its control. Oscillations of time-of-the-day dependent increases and decreases in the DBP protein level are considered to be propagated to oscillations of increases and decreases in its downstream genes. In fact, circadian variations of drug metabolism, free fatty acid metabolism, and blood pressure have been reported to be under the control of DBP protein. Regarding DBP protein levels, Dbp gene transcription was reported to be activated at certain times of the day. Meanwhile, it was unclear how the degradation of DBP protein is regulated.
In this study, the research group searched for factors involved in the regulation of DBP protein degradation. First, experiments using lysosome and proteasome inhibitors revealed that DBP protein is degraded mainly through the ubiquitin−proteasome system, in which the two ubiquitin-binding enzymes UBE2G1 and UBE2T and the ubiquitin ligase TRAF7 are involved in the ubiquitination and degradation of DBP protein. Next, to examine the function of TRAF7, they generated cultured cells deficient in TRAF7 protein. In these cells, the peak level of DBP was increased to a 2-fold higher level, suggesting that degradation of DPB by TRAF7 plays a role in regulating the peak expression level.
To investigate in vivo functions of TRAF7, they generated TRAF7-deficient mice. In these mice, while the DBP protein level in the liver was similar to that in wild-type mice, the expression patterns of several proteins under the control of DBP were altered. They tested the transcriptional activity of DBP in cultured cells and found that TRAF7 also plays a role in regulating the activity of DBP. Finally, the research group examined the effects of TRAF7 on the circadian clock oscillation. The results suggest that TRAF7 may regulate the amplitude of the circadian clock via the DBP ubiquitination.
Yoshitane said, "In this study, we identified a gene involved in the regulation of degradation of the clock protein DBP for the first time in the world. DBP is a master transcription factor, which turns on liver metabolism in particular, with a distinct expression rhythm showing increased expression while sleeping and protein degradation during wakefulness. The DBP-regulating factor is expected to be a therapeutic target for problems based on circadian rhythm disorders, such as susceptibility to weight gain after a nighttime meal and increased risk for diseases due to irregular lifestyles."
Journal Information
Publication: Communications Biology
Title: TRAF7 determines circadian period through ubiquitination and degradation of DBP
DOI: 10.1038/s42003-024-07002-x
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