Molecular Mechanism Of Angptl8 Resetting The Hepatic Biological Clock In Response To Food Signals

Mammalian circadian clocks precisely control the rhythms of behavior and physiology,including activity cycles,body temperature,hormone secretion,blood pressure,and metabolism,leading to diurnal fluctuations of these processes.Mammalian clock system is consisted of the central clock located in the hypothalamus suprachiasmatic nucleus(SCN)and the peripheral clock widely distributed in the diverse tissue.With the earth auto-rotation,photic and no-photic signals were delivered into the central clock,and further generating central clock signals in SCN.This master clock signals drive slave oscillators distributed in various peripheral tissues through behavioral and neuroendocrine transmitters.Conversingly,food signal is another Zeitgeber for the peripheral clock function.Under physiological conditions,the master and the perpheral clocks are tightly coupled,such integration is essential to optimize our energy utilization and maintain body metabolic homeostasis.Epidemiological investigations have indicated that prolonged light exposure or rotating shift work is closely correlated to an elevated risk for developing various diseases,including cancer,cardiovascular disease,immune deficiency,and metabolic disorders.These observations suggest that altered sleep/wake schedules may disrupt endogenous circadian clock systems through mistimed resetting of certain time givers.Hence,the external light-dark cycle is the most dominant Zeitgeber to reset the central circadian pacemaker.On the other hand,food signal is an important synchronizer in the regulation of the peripheral clock.For example,time-restricted feeding(e.g.food access is limited within the normal rest phase of an organism)makes peripheral clocks uncoupled from the SCN and adapt to the new timing of food availability.Such an uncoupling is believed to contribute importantly to the pathogenesis of metabolic diseases found in people with abnormal life routine styles.Besides,animal studies also demonstrated that food intake during the normal rest phase promotes obesity in rodents.However,the mechanisms of food-dependent peripheral clock resetting remain largely unknown.As the largest metabolic organ,the liver sensitively responds to the food signals and secrets hepatokines,leading to the robust regulation of metabolic and clock processes.For example,fibroblast growth factor(FGF)21 is a liver-secreted,insulin-responsive factor that regulates glucose and lipid metabolism.The overexpression of FGF21 in mice suppresses physical activity and alters circadian behavior.It should be noted that food entrainment of liver clock is independent of the SCN,and does not fully depend on these factors,these remaining questions drive us to identify new hepatokines which mediate the food-driven resetting of the liver clock independent of the master clock.To address the above question,we clustered high-throughput RNA sequencing results from liver samples collected in mice subjected to an overnight fasting or constant darkness,and found that the fasting/refeeding-regualted hepatokine Angptl8 showed a daily fluctuation both in the liver and serum of mice.Consistently,the oscillation of Angptl8 levels was also reversed by restricted feeding.Injection of Angptl8 purified protein in mice at ZT2 advanced the oscillation phases of major clock and metabolic genes in liver.In addition,“Angptl8-shock” induced the rhythmic expression of clock genes in mouse Hepa1c1c-7 hepatoma cells comparable to a “Serum-shock” model.Mechanistically,the resetting function of Angptl8 is dependent on the signal relay of the membrane receptor Pir B,phosphorylation of kinases and transcriptional factors,and consequently transient activation of the central clock gene Per1.Importantly,either liver-specific Angptl8 knockdown or neutralization of Angptl8 partially attenuated food-entrained resetting of liver clock in mice.In conclusion,we show that the resetting function of Angptl8 is dependent on signal relay of the membrane receptor Pir B and transient activation of the key clock gene Per1.These interesting findings suggest that Angptl8 is a novel factor in orchestration of liver clock,in addition to its diverse effects on metabolic regulation.Our study now provides the new drug target to treat metabolic disorders arising from circadian clock disruption from the view of chronotherapy.