ABSTRACT
The proper balance and transition between cellular quiescence and proliferation are critical to tissue homeostasis, and their deregulations are commonly found in many human diseases, including cancer and aging. Recent studies showed that the reentry of quiescent cells to the cell cycle is subjected to circadian regulation. However, the underlying mechanisms are largely unknown. Here, we report that two circadian proteins, Cryptochrome (Cry) and Rev-erb, deepen cellular quiescence in rat embryonic fibroblasts, resulting in stronger serum stimulation required for cells to exit quiescence and reenter the cell cycle. This finding was opposite from what we expected from the literature. By modeling a library of possible regulatory topologies linking Cry and Rev-erb to a bistable Rb-E2f gene network switch that controls the quiescence-to-proliferation transition and by experimentally testing model predictions, we found Cry and Rev-erb converge to downregulate Cyclin D/Cdk4,6 activity, leading to an ultrasensitive increase of the serum threshold to activate the Rb-E2f bistable switch. Our findings suggest a mechanistic role of circadian proteins in modulating the depth of cellular quiescence, which may have implications in the varying potentials of tissue repair and regeneration at different times of the day.
Significance Statement Tissue homeostasis requires a proper balance between quiescence and proliferation. The quiescence-to-proliferation transition appears affected by the circadian clock: the rates to activate stem and progenitor cells to proliferate can vary diurnally. But the underlying molecular mechanisms are largely unknown. Here we show increasing circadian proteins Cry and Rev-erb pushed cells to deeper quiescence and become harder to transition into proliferation. Through exploratory model searches and experimental tests, we found Cry and Rev-erb converge to downregulate Cyclin D/Cdk4,6, which increases the activation threshold of a bistable Rb-E2f gene switch underlying quiescence exit. Our findings suggest novel and converging mechanistic roles of circadian proteins in regulating cellular quiescence depth and potentially varying abilities for tissue regeneration around the circadian clock.
Competing Interest Statement
The authors have declared no competing interest.