Circadian dysfunction and obesity: is Leptin the missing link?

Abstract

While accumulating evidence suggests that circadian desynchrony is linked to obesity and metabolic syndrome, the underlining mechanism is still poorly understood. In this issue, Kettner and colleagues demonstrate that Leptin resistance, induced by circadian clock deficiency or chronic jet lag, may represent this missing link.

Because we are living in a rhythmic environment imposed by rotation of the earth around the sun, nearly all aspects of our behavior and metabolism are coordinated in a rhythmic fashion with a period of around 24 hours. Such rhythmicity is set by a net of endogenous self-sustained cell-autonomous oscillators, dubbed the circadian clock. At the level of the whole organism, the circadian system is organized in a hierarchical manner with a master clock within the suprachiasmatic nuclei (SCN) of the hypothalamus receiving light input via the retina and transmitting timing signals to slave oscillators in peripheral organs, thus ensuring phase coherence within the body. In turn, peripheral clocks impact critically on coordinating the function of each respective organ. As a consequence, perturbation of these rhythms by experimental or social conditions, for example during shift work, is associated with diverse pathologies including obesity and type 2 diabetes (Dibner and Schibler, 2015). However, a precise mechanism underlying such phenomenon has yet to be unraveled.

The article by Kettner et al. (Kettner et al., 2015) published in this issue proposes a new hypothesis that links circadian clock, obesity, and Leptin resistance. Using wild type or genetically modified mice with distinct circadian clock deficiencies, under normal diurnal rhythms or chronic jet lag, the authors show that energy balance and body weight are perturbed in different ways, depending on the circadian disruption model. Intriguingly, while Per1/Per2 KO mice gained weight under a normal light-dark cycle, Cry1/Cry2 KO exhibited a dramatic reduction in body weight, and Bmal1 KO mice were similar in weight compared to their WT counterparts. Furthermore, jet lag conditions significantly altered the metabolic balance of WT and clock-deficient mice, with the exception of Bmal1 KO mice, which were insensitive to chronic alterations of the light-dark cycle. Thus, distinct genetic and environmental disruptions of circadian homeostasis lead to different disruption patterns of energy homeostasis. The authors propose that the direct circadian control of Leptin expression in white adipocyte cells through BMAL1/CLOCK-modulated C/EBPα-mediated Leptin transcription, might account for such regulation, independently of food intake (Figure 1). Consequently, Leptin levels in the blood are exhibiting oscillatory pattern as well (Figure 1). Indeed, the Leptin peptide, secreted by white adipose tissue, has a broad impact on metabolism through its action on peripheral organs or in the central nervous system, and more particularly in the hypothalamus where Leptin acts as a strong appetite inhibitor. However, in obese people Leptin signaling is generally perturbed: they present a high level of Leptin due to the increase in white adipose tissue but its effect is counteracted by a phenomenon called Leptin resistance (Martínez de Morentin et al., 2010). Importantly, Kettner et al. show that Leptin signaling in POMC neurons is a subject for circadian control, and that circadian dysfunction, including chronic jet lag, leads by itself to Leptin resistance (Figure 1). Circadian patterns of Leptin signaling and POMC expression were differentially disturbed in the studied models of dysfunctional clock (see Figure 1 legend for details), providing a plausible explanation for the distinct metabolic phenotypes observed in these models. The key finding of this study on the circadian clock disruption leading to Leptin resistance is in resonance with a recent publication showing that shift work or circadian misalignment leads to perturbed Leptin secretion (Scheer et al., 2009), whereas mouse weight homeostasis is dependent on synchronization between feeding and Leptin rhythms (Arble et al., 2011). It is thus likely that this circadian misalignment-induced Leptin resistance is important in the understanding of associated metabolic disease.

Figure 1. Link between circadian clock, Leptin signaling and body energy balance (Kettner et al, 2015).

The adipose clock is directly regulating Leptin transcription, with BMAL1/CLOCK modulated C/EBPα binding to the Leptin promotor, leading to rhythmic transcriptional activation. This circadian pattern of Leptin synthesis, characterized by a pronounced peak in the middle of the activity phase (ZT18, Zeitgeber time (ZT) corresponding to environmental entrainment time, with ZT0 means light turns on, and ZT12, light turns off), is abolished by clock disruption, introduced by either genetic alterations of core-clock components, or by chronic environmental changes (jet lag). Consequently, plasma Leptin levels exhibit a circadian pattern with a peak at ZT18 in the presence of a functional clock, which is abolished upon genetic clock disruption, or by chronic jet lag. Following Leptin binding to its receptor (LEPR) on ARC POMC neurons, STAT3 undergoes phosphorylation in a circadian manner, with the peak at ZT2, preceding the peak of POMC expression at ZT10. Circadian patterns of both STAT3 phosphorylation and POMC expression were differentially disrupted in different clock deficient models. Phosphorylation of STAT3 and POMC expression were non-circadian and low in jet lagged and Per1/Per2 KO mice exhibiting high plasma Leptin levels. On the contrary, phosphorylation of STAT3 and POMC expression were high in Cry1/Cry2 KO mice with low plasma Leptin levels, in accordance with the distinct metabolic phenotypes observed in these models. Importantly, Per1/Per2 KO and jet lagged mice exhibited Leptin resistance due to loss of STAT3 activation in POMC, thus perturbing the energy balance at the organismal level.

However, the question how circadian misalignment leads to Leptin resistance stays largely unexplored. Leptin resistance could be induced by different factors including defective Leptin transport, genetic variation in Leptin or its receptor, alteration of Leptin signaling, and endoplasmic reticulum (ER) stress (Pan et al., 2014). If the first two hypotheses appear unlikely in the present case, using inbred mouse lines with a similar genetic background, the two other hypotheses merit our attention. Indeed, a link between Leptin signaling and the circadian clock has already been described (Fu et al., 2005). In this article, Fu et al. have shown that clock genes mediate Leptin-dependent sympathetic regulation of bone formation. They could demonstrate that Leptin determines the extent of bone formation by modulating, via sympathetic signaling, osteoblast proliferation in part through signaling pathways that involve the molecular clock. Whereas the mechanisms seem different, this study highlights the interconnection between the circadian clock and Leptin signaling. On the other hand, increasing evidences suggest that ER stress plays a fundamental function in the induction of Leptin resistance in obesity and energy imbalance (Pan et al., 2014). ER stress involves a complex network of signaling pathways that are activated when ER homeostasis is perturbed. ER stress activation is also a hallmark of a perturbed lipid metabolism as lipid homeostasis is an important regulator of ER stress (Volmer and Ron, 2015). In particular, obesity is correlated with strong ER stress activation in different tissues including hypothalamus. Indeed, a perturbed lipid metabolism in the hypothalamus is a strong inducer of ER stress where it provokes Leptin resistance (Martínez de Morentin et al., 2010). Genetically induced circadian clock deficiency and its associated disturbed lipid homeostasis has also been linked to ER stress activation in the liver (Cretenet et al., 2010). To what extends could this result be extrapolated to the hypothalamus? Recent genomic results suggest that lipid metabolism in the hypothalamus presents a rhythmic pattern (Zhang et al., 2014). The hypothesis that circadian clock dysfunction is associated with a perturbed lipid metabolism that leads to ER stress and Leptin resistance in the hypothalamus constitutes therefore a plausible mechanism that merits consideration.