Knowledge of the mechanisms that govern mammalian circadian rhythm permits the molecular manipulation of these mechanisms. A recent study showed that pharmacologic intervention results in an increase in metabolism and subsequent weight loss in overfed, obese mice.
Our physiological processes and our behaviors are generally organized on a cycle of about 24 hours. Circadian rhythms occur in almost all organisms and presumably confer an evolutionary advantage. For instance, humans are physiologically prepared for activity and food ingestion during daylight hours and for fasting and recuperation during darkness.
Circadian rhythms are endogenous, persisting in the absence of changes in the environment or behaviors across the day and night. The strength of circadian rhythmicity is best exemplified when our behaviors occur at the “wrong” time according to our internal body clock. For instance, persons with jet lag eat and attempt to sleep at unusual circadian phases, and this often causes nausea and fatigue. Night workers can have almost continuous jet-lag symptoms, potentially leading to insomnia, cardiovascular disease, hypertension, obesity, and diabetes.1-3Of interest, therefore, is a recent study by Solt et al.4 involving synthetic compounds that alter circadian rhythms. (A related study, by Cho et al.,5 was published contemporaneously.)
In mammals, a central circadian pacemaker exists in the suprachiasmatic nucleus of the hypothalamus that orchestrates the many circadian rhythms in physiological processes and behaviors. This central pacemaker is initiated and maintained within the neurons of the suprachiasmatic nucleus by a molecular clock that involves a sequence of events forming a transcription–translation feedback loop (a loop in which the translation of a protein inhibits, through indirect or direct pathways, the transcription of the gene encoding that protein). This sequence of events lasts approximately 24 hours, incorporating oscillations in the expression of key proteins such as CLOCK, BMAL1, PER, CRY, and NPAS2. Two nuclear receptors, called REV-ERB-α and REV-ERB-β, help to regulate the oscillations of BMAL1 and CLOCK and thereby modulate circadian rhythmicity. Peripheral cells and tissues possess the same molecular clock, and peripheral circadian rhythms are usually synchronized with the central pacemaker through indirect neural, humoral, and temperature-related influences. How this synchronization occurs throughout the body is not well understood.
The suprachiasmatic-nucleus clock normally adjusts to the seasonal changes in day length. This adjustment is caused by the effects of timing stimuli (called zeitgebers) such as light. However, the phase of the central pacemaker in the suprachiasmatic nucleus may take many days to adapt when these zeitgebers occur out of synchronization, such as after the rapid shifts in the light–dark schedule that occur with jet travel across time zones and night work. In addition, internal dyssynchrony can occur between the circadian phase of the suprachiasmatic nucleus and the phase of peripheral organs owing to differences in how fast the central and peripheral-tissue clocks reset after a behavioral or zeitgeber shift.
The meter of metabolism
Circadian rhythms of metabolism are among the most important rhythms in mammals.6Solt et al. based their study4on previous observations that the binding of REV-ERB by ligands results in changes in the expression of genes that regulate lipid and glucose metabolism and that REV-ERB knockout mice were hyperlipidemic. Thus, they developed compounds SR9009 and SR9011 (both of which are synthetic REV-ERB-α and REV-ERB-β agonists) to determine whether these could elicit beneficial metabolic effects in obese mice. The results were similar for both SR9009 and SR9011. In a comprehensive set of experiments, Solt et al. found that these agonists affected the molecular clock (by inhibiting the expression of BMAL1) and reduced the amplitudes of circadian-rhythm oscillations in suprachiasmatic-nucleus explants and fibroblasts in vitro. Intraperitoneal injection of either of these agonists into mice kept in constant darkness resulted in substantial suppression of locomotor activity during the subsequent biologic night, when mice would normally be active. (Both agonists could be detected in the blood and in the brain after injection.) After such injections, Solt et al. also found substantial changes in the amplitudes and phases of the circadian rhythms of various components of the transcription–translation feedback loop in the hypothalamus.
How these complex changes in the molecular clock would translate into altered behavior or function was somewhat unpredictable. The authors observed that the effects of SR9009 and SR9011 on locomotor activity and molecular-clock function in the hypothalamus were greatly attenuated when the mice were maintained on a normal light–dark schedule, a finding that suggests that light input either negates or interacts with these drug effects.
Solt et al. then administered the agonists to normal-weight mice for 7 to 10 days, which resulted in weight loss (beyond that seen with control injections) due mostly to loss of fat mass. This weight loss was probably caused by increased basal oxygen consumption; the mice showed neither an increase in activity (which decreased by 15%) nor a decrease in food intake (which increased by 10% at night).
Next, Solt et al. assayed the expression of molecular-clock and metabolic genes separately in muscle, liver, and white adipose tissue after single injections of synthetic REV-ERB-α and REV-ERB-β agonists. They observed differences in the effect of these agonists on the molecular clocks of the suprachiasmatic nucleus and of the periphery, a finding that suggests internal dyssynchrony. They observed diminished levels of expression of lipogenic genes in the liver and elevated levels of enzymes responsible for the oxidation of glucose and fatty acids and the transport of fatty acids in muscle tissue. In white adipose tissue, the expression of genes involved in lipid storage was comparatively feeble. These results are consistent with a suppression of lipogenesis and of cholesterol and bile-acid synthesis in the liver, increased lipid and glucose oxidation in skeletal muscle, and decreased triglyceride synthesis and storage in white adipose tissue. Building on these impressive findings, the authors delivered a pièce de résistance: injection of the agonists into obese mice over a period of 12 days resulted in weight reduction (60% beyond that seen with control injections), loss of fat mass, and an improved overall metabolic profile, including lower plasma levels of triglycerides, total cholesterol, nonesterified fatty acids, glucose, and insulin (Figure 1). (There was also an 80% decrease in leptin, presumably owing to the loss of fat mass.) The investigators further observed that injection of synthetic REV-ERB agonists reduced triglycerides and total cholesterol even in lean mice.
With the use of synthetic REV-ERB agonists, Solt et al. have provided evidence of the pharmacologic modulation of circadian rhythms — centrally, peripherally, and (in various organs and tissues) differentially. The physiological effects are compelling and offer potential in humans, although many questions remain. First, the dynamics of the drug effects in the various tissues await delineation. In particular, it is important to determine whether the same effects or different effects occur when injections occur at different times across the day and night. Second, knowledge about the interaction of these effects with those of other zeitgebers is rudimentary to nonexistent. Third, mice are nocturnally active and humans are diurnally active, so REV-ERB-α and REV-ERB-β agonists could conceivably have opposite effects in the two species. Fourth, what would be the symptoms and effects incurred by such compounds in humans? It is noticeable that the mice “took a night off” from their routine of wheel-running activity after injection of the synthetic REV-ERB-α and REV-ERB-β agonists, perhaps owing to debilitating symptoms or side effects. Finally, it would seem that the physiological and behavioral changes induced by the REV-ERB-α and REV-ERB-β agonists occur because of changes in the core molecular clock rhythms, but little is known about the mechanisms of transduction of signals from the core clock to the physiological and behavioral effectors. That all being said, this is an exciting, timely area of research, on which researchers are working day and night.
Steven A. Shea, Ph.D. “Obesity and pharmacologic control of the body clock” July 12, 2012
N Engl J Med 2012; 367:175-178.